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Internet Engineering Task Force SIP WG
Internet Draft J. Rosenberg
dynamicsoft
H. Schulzrinne
Columbia U.
G. Camarillo
Ericsson
A. Johnston
Worldcom
J. Peterson
Neustar
R. Sparks
dynamicsoft
M. Handley
ACIRI
E. Schooler
AT&T
draft-ietf-sip-rfc2543bis-08.txt
draft-ietf-sip-rfc2543bis-09.txt
February 21, 27, 2002
Expires: Aug 2002
SIP: Session Initiation Protocol
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract
The Session Initiation Protocol (SIP) is an application-layer control
J. Rosenberg et. al. [Page 1]
Internet Draft SIP February 21, 27, 2002
(signaling) protocol for creating, modifying, and terminating
sessions with one or more participants. These sessions include
Internet telephone calls, multimedia distribution, and multimedia
conferences.
SIP invitations used to create sessions carry session descriptions
that allow participants to agree on a set of compatible media types.
SIP makes use of elements called proxy servers to help route requests
to the user's current location, authenticate and authorize users for
services, implement provider call-routing policies, and provide
features to users. SIP also provides a registration function that
allows users to upload their current locations for use by proxy
servers. SIP runs on top of several different transport protocols.
J. Rosenberg et. al. [Page 2]
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Table of Contents
1 Introduction ........................................ 3 10
2 Overview of SIP Functionality ....................... 3 10
3 Terminology ......................................... 4 11
4 Overview of Operation ............................... 5 12
5 Structure of the Protocol ........................... 12 19
6 Definitions ......................................... 14 21
7 SIP Messages ........................................ 21 28
7.1 Requests ............................................ 21 28
7.2 Responses ........................................... 22 29
7.3 Header Fields ....................................... 23 30
7.3.1 Header Field Format ................................. 24 31
7.3.2 Header Field Classification ......................... 27 34
7.3.3 Compact Form ........................................ 27 34
7.4 Bodies .............................................. 27 34
7.4.1 Message Body Type ................................... 27 34
7.4.2 Message Body Length ................................. 28 35
7.5 Framing SIP messages ................................ 28 35
8 General User Agent Behavior ......................... 28 35
8.1 UAC Behavior ........................................ 29 36
8.1.1 Generating the Request .............................. 29 36
8.1.1.1 Request-URI ......................................... 29 36
8.1.1.2 To .................................................. 30 37
8.1.1.3 From ................................................ 31 38
8.1.1.4 Call-ID ............................................. 32 39
8.1.1.5 CSeq ................................................ 32 39
8.1.1.6 Max-Forwards ........................................ 33 40
8.1.1.7 Via ................................................. 33 40
8.1.1.8 Contact ............................................. 34 41
8.1.1.9 Supported and Require ............................... 35 42
8.1.1.10 Additional Message Components ....................... 35 42
8.1.2 Sending the Request ................................. 35 42
8.1.3 Processing Responses ................................ 36 43
8.1.3.1 Transaction Layer Errors ............................ 36 43
8.1.3.2 Unrecognized Responses .............................. 37 44
8.1.3.3 Vias ................................................ 37 44
8.1.3.4 Processing 3xx Responses ............................ 37 44
8.1.3.5 Processing 4xx Responses ............................ 39 46
8.2 UAS Behavior ........................................ 40 47
8.2.1 Method Inspection ................................... 40 47
8.2.2 Header Inspection ................................... 41 48
8.2.2.1 To and Request-URI .................................. 41 48
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8.2.2.2 Merged Requests ..................................... 41 48
8.2.2.3 Require ............................................. 42 49
8.2.3 Content Processing .................................. 43 50
8.2.4 Applying Extensions ................................. 43 50
8.2.5 Processing the Request .............................. 43 51
8.2.6 Generating the Response ............................. 44 51
8.2.6.1 Sending a Provisional Response ...................... 44 51
8.2.6.2 Headers and Tags .................................... 44 51
8.2.7 Stateless UAS Behavior .............................. 45 52
8.3 Redirect Servers .................................... 45 52
9 Canceling a Request ................................. 47 54
9.1 Client Behavior ..................................... 48 55
9.2 Server Behavior ..................................... 49 56
10 Registrations ....................................... 50 57
10.1 Overview ............................................ 50 57
10.2 Constructing the REGISTER Request ................... 51 58
10.2.1 Adding Bindings ..................................... 54 61
10.2.1.1 Setting the Expiration Interval of Contact
Addresses ...................................................... 54 62
10.2.1.2 Preferences among Contact Addresses ................. 55 62
10.2.2 Removing Bindings ................................... 55 62
10.2.3 Fetching Bindings ................................... 56 63
10.2.4 Refreshing Bindings ................................. 56 63
10.2.5 Setting the Internal Clock .......................... 56 63
10.2.6 Discovering a Registrar ............................. 56 63
10.2.7 Transmitting a Request .............................. 57 64
10.2.8 Error Responses ..................................... 57 64
10.3 Processing REGISTER Requests ........................ 57 64
11 Querying for Capabilities ........................... 60 68
11.1 Construction of OPTIONS Request ..................... 61 68
11.2 Processing of OPTIONS Request ....................... 62 69
12 Dialogs ............................................. 63 70
12.1 Creation of a Dialog ................................ 64 71
12.1.1 UAS behavior ........................................ 65 72
12.1.2 UAC Behavior ........................................ 66 73
12.2 Requests within a Dialog ............................ 67 74
12.2.1 UAC Behavior ........................................ 67 74
12.2.1.1 Generating the Request .............................. 67 74
12.2.1.2 Processing the Responses ............................ 70 77
12.2.2 UAS Behavior ........................................ 70 77
12.3 Termination of a Dialog ............................. 72 79
13 Initiating a Session ................................ 72 79
13.1 Overview ............................................ 72 79
13.2 UAC Processing ...................................... 73 80
13.2.1 Creating the Initial INVITE ......................... 73 80
13.2.2 Processing INVITE Responses ......................... 75 82
13.2.2.1 1xx responses ....................................... 75 82
13.2.2.2 3xx responses ....................................... 75 83
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13.2.2.3 4xx, 5xx and 6xx responses .......................... 75 83
13.2.2.4 2xx responses ....................................... 76 83
13.3 UAS Processing ...................................... 77 84
13.3.1 Processing of the INVITE ............................ 77 84
13.3.1.1 Progress ............................................ 78 85
13.3.1.2 The INVITE is redirected ............................ 79 86
13.3.1.3 The INVITE is rejected .............................. 79 86
13.3.1.4 The INVITE is accepted .............................. 79 86
14 Modifying an Existing Session ....................... 80 87
14.1 UAC Behavior ........................................ 81 88
14.2 UAS Behavior ........................................ 82 89
15 Terminating a Session ............................... 83 91
15.1 Terminating a Session with a BYE Request ............ 84 92
15.1.1 UAC Behavior ........................................ 84 92
15.1.2 UAS Behavior ........................................ 85 92
16 Proxy Behavior ...................................... 85 93
16.1 Overview ............................................ 85 93
16.2 Stateful Proxy ...................................... 86 94
16.3 Request Validation .................................. 88 95
16.4 Route Information Preprocessing ..................... 90 97
16.5 Determining request targets ......................... 91 98
16.6 Request Forwarding .................................. 93 100
16.7 Response Processing ................................. 101 108
16.8 Processing Timer C .................................. 109 117
16.9 Handling Transport Errors ........................... 110 117
16.10 CANCEL Processing ................................... 110 117
16.11 Stateless Proxy ..................................... 111 118
16.12 Summary of Proxy Route Processing ................... 113 120
16.12.1 Examples ............................................ 113 121
16.12.1.1 Basic SIP Trapezoid ................................. 113 121
16.12.1.2 Traversing a strict-routing proxy ................... 115 123
16.12.1.3 Rewriting Record-Route header field values .......... 117 125
17 Transactions ........................................ 118 126
17.1 Client Transaction .................................. 120 128
17.1.1 INVITE Client Transaction ........................... 121 128
17.1.1.1 Overview of INVITE Transaction ...................... 121 129
17.1.1.2 Formal Description .................................. 121 129
17.1.1.3 Construction of the ACK Request ..................... 125 132
17.1.2 Non-INVITE Client Transaction ....................... 126 133
17.1.2.1 Overview of the non-INVITE Transaction .............. 126 133
17.1.2.2 Formal Description .................................. 126 134
17.1.3 Matching Responses to Client Transactions ........... 127 135
17.1.4 Handling Transport Errors ........................... 129 135
17.2 Server Transaction .................................. 129 137
17.2.1 INVITE Server Transaction ........................... 129 137
17.2.2 Non-INVITE Server Transaction ....................... 132 140
17.2.3 Matching Requests to Server Transactions ............ 133 141
17.2.4 Handling Transport Errors ........................... 135 143
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18 Transport ........................................... 135 143
18.1 Clients ............................................. 136 144
18.1.1 Sending Requests .................................... 136 144
18.1.2 Receiving Responses ................................. 138 146
18.2 Servers ............................................. 139 146
18.2.1 Receiving Requests .................................. 139 146
18.2.2 Sending Responses ................................... 140 148
18.3 Framing ............................................. 141 149
18.4 Error Handling ...................................... 141 149
19 Common Message Components ........................... 142 149
19.1 SIP and SIPS Uniform Resource Indicators ............ 142 149
19.1.1 SIP and SIPS URI Components ......................... 142 150
19.1.2 Character Escaping Requirements ..................... 146 153
19.1.3 Example SIP and SIPS URIs ........................... 147 155
19.1.4 URI Comparison ...................................... 148 155
19.1.5 Forming Requests from a URI ......................... 151 158
19.1.6 Relating SIP URIs and tel URLs ...................... 152 160
19.2 Option Tags ......................................... 154 161
19.3 Tags ................................................ 154 162
20 Header Fields ....................................... 155 162
20.1 Accept .............................................. 158 164
20.2 Accept-Encoding ..................................... 159 166
20.3 Accept-Language ..................................... 159 167
20.4 Alert-Info .......................................... 160 167
20.5 Allow ............................................... 160 168
20.6 Authentication-Info ................................. 161 168
20.7 Authorization ....................................... 161 168
20.8 Call-ID ............................................. 161 169
20.9 Call-Info ........................................... 162 169
20.10 Contact ............................................. 162 170
20.11 Content-Disposition ................................. 163 171
20.12 Content-Encoding .................................... 164 172
20.13 Content-Language .................................... 165 173
20.14 Content-Length ...................................... 165 173
20.15 Content-Type ........................................ 165 173
20.16 CSeq ................................................ 166 174
20.17 Date ................................................ 166 174
20.18 Error-Info .......................................... 166 174
20.19 Expires ............................................. 167 175
20.20 From ................................................ 167 175
20.21 In-Reply-To ......................................... 168 176
20.22 Max-Forwards ........................................ 168 176
20.23 Min-Expires ......................................... 169 177
20.24 MIME-Version ........................................ 169 177
20.25 Organization ........................................ 169 177
20.26 Priority ............................................ 170 178
20.27 Proxy-Authenticate .................................. 170 178
20.28 Proxy-Authorization ................................. 171 179
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20.29 Proxy-Require ....................................... 171 179
20.30 Record-Route ........................................ 172 179
20.31 Reply-To ............................................ 172 180
20.32 Require ............................................. 172 180
20.33 Retry-After ......................................... 173 181
20.34 Route ............................................... 173 181
20.35 Server .............................................. 173 181
20.36 Subject ............................................. 174 182
20.37 Supported ........................................... 174 182
20.38 Timestamp ........................................... 174 182
20.39 To .................................................. 175 183
20.40 Unsupported ......................................... 175 183
20.41 User-Agent .......................................... 176 183
20.42 Via ................................................. 176 184
20.43 Warning ............................................. 177 185
20.44 WWW-Authenticate .................................... 179 186
21 Response Codes ...................................... 179 187
21.1 Provisional 1xx ..................................... 179 187
21.1.1 100 Trying .......................................... 179 187
21.1.2 180 Ringing ......................................... 179 187
21.1.3 181 Call Is Being Forwarded ......................... 179 187
21.1.4 182 Queued .......................................... 180 188
21.1.5 183 Session Progress ................................ 180 188
21.2 Successful 2xx ...................................... 180 188
21.2.1 200 OK .............................................. 180 188
21.3 Redirection 3xx ..................................... 180 188
21.3.1 300 Multiple Choices ................................ 180 188
21.3.2 301 Moved Permanently ............................... 181 189
21.3.3 302 Moved Temporarily ............................... 181 189
21.3.4 305 Use Proxy ....................................... 181 189
21.3.5 380 Alternative Service ............................. 182 189
21.4 Request Failure 4xx ................................. 182 190
21.4.1 400 Bad Request ..................................... 182 190
21.4.2 401 Unauthorized .................................... 182 190
21.4.3 402 Payment Required ................................ 182 190
21.4.4 403 Forbidden ....................................... 182 190
21.4.5 404 Not Found ....................................... 182 190
21.4.6 405 Method Not Allowed .............................. 182 190
21.4.7 406 Not Acceptable .................................. 183 190
21.4.8 407 Proxy Authentication Required ................... 183 191
21.4.9 408 Request Timeout ................................. 183 191
21.4.10 410 Gone ............................................ 183 191
21.4.11 413 Request Entity Too Large ........................ 183 191
21.4.12 414 Request-URI Too Long ............................ 183 191
21.4.13 415 Unsupported Media Type .......................... 184 191
21.4.14 416 Unsupported URI Scheme .......................... 184 192
21.4.15 420 Bad Extension ................................... 184 192
21.4.16 421 Extension Required .............................. 184 192
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21.4.17 423 Interval Too Brief .............................. 184 192
21.4.18 480 Temporarily Unavailable ......................... 184 192
21.4.19 481 Call/Transaction Does Not Exist ................. 185 193
21.4.20 482 Loop Detected ................................... 185 193
21.4.21 483 Too Many Hops ................................... 185 193
21.4.22 484 Address Incomplete .............................. 185 193
21.4.23 485 Ambiguous ....................................... 185 193
21.4.24 486 Busy Here ....................................... 186 194
21.4.25 487 Request Terminated .............................. 186 194
21.4.26 488 Not Acceptable Here ............................. 186 194
21.4.27 491 Request Pending ................................. 186 194
21.4.28 493 Undecipherable .................................. 187 195
21.5 Server Failure 5xx .................................. 187 195
21.5.1 500 Server Internal Error ........................... 187 195
21.5.2 501 Not Implemented ................................. 187 195
21.5.3 502 Bad Gateway ..................................... 187 195
21.5.4 503 Service Unavailable ............................. 187 195
21.5.5 504 Server Time-out ................................. 188 196
21.5.6 505 Version Not Supported ........................... 188 196
21.5.7 513 Message Too Large ............................... 188 196
21.6 Global Failures 6xx ................................. 188 196
21.6.1 600 Busy Everywhere ................................. 188 196
21.6.2 603 Decline ......................................... 189 196
21.6.3 604 Does Not Exist Anywhere ......................... 189 197
21.6.4 606 Not Acceptable .................................. 189 197
22 Usage of HTTP Authentication ........................ 189 197
22.1 Framework ........................................... 190 198
22.2 User-to-User Authentication ......................... 192 200
22.3 Proxy-to-User Authentication ........................ 193 201
22.4 The Digest Authentication Scheme .................... 196 204
23 S/MIME .............................................. 198 206
23.1 S/MIME Certificates ................................. 198 206
23.2 S/MIME Key Exchange ................................. 199 207
23.3 Securing MIME bodies ................................ 202 210
23.4 SIP Header Privacy and Integrity using S/MIME:
Tunneling SIP .................................................. 203 211
23.4.1 Integrity and Confidentiality Properties of SIP
Headers ........................................................ 204 212
23.4.1.1 Integrity ........................................... 204 212
23.4.1.2 Confidentiality ..................................... 204 212
23.4.2 Tunneling Integrity and Authentication .............. 205 213
23.4.3 Tunneling Encryption ................................ 207 215
24 Examples ............................................ 209 217
24.1 Registration ........................................ 210 218
24.2 Session Setup ....................................... 211 219
25 Augmented BNF for the SIP Protocol ................. 216 224
25.1 Basic Rules ......................................... 217 225
26 Security Considerations: Threat Model and Security
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Usage Recommendations .......................................... 234 242
26.1 Attacks and Threat Models ........................... 234 242
26.1.1 Registration Hijacking .............................. 235 243
26.1.2 Impersonating a Server .............................. 235 243
26.1.3 Tampering with Message Bodies ....................... 236 244
26.1.4 Tearing Down Sessions ............................... 237 245
26.1.5 Denial of Service and Amplification ................. 237 245
26.2 Security Mechanisms ................................. 238 246
26.2.1 Transport and Network Layer Security ................ 239 247
26.2.2 SIPS URI Scheme ..................................... 240 248
26.2.3 HTTP Authentication ................................. 241 249
26.2.4 S/MIME .............................................. 241 249
26.3 Implementing Security Mechanisms .................... 242 250
26.3.1 Requirements for Implementers of SIP ................ 242 250
26.3.2 Security Solutions .................................. 243 251
26.3.2.1 Registration ........................................ 243 251
26.3.2.2 Interdomain Requests ................................ 244 252
26.3.2.3 Peer to Peer Requests ............................... 247 255
26.3.2.4 DoS Protection ...................................... 247 255
26.4 Limitations ......................................... 248 256
26.4.1 HTTP Digest ......................................... 248 256
26.4.2 S/MIME .............................................. 249 257
26.4.3 TLS ................................................. 250 258
26.4.4 SIPS URIs ........................................... 251 259
26.5 Privacy ............................................. 252 260
27 IANA Considerations ................................. 253 261
27.1 Option Tags ......................................... 253 261
27.2 Warn-Codes .......................................... 253 262
27.3 Header Field Names .................................. 254 262
27.4 Method and Response Codes ........................... 254 262
27.5 The "message/sip" MIME type. ....................... 255 263
27.6 New Content-Disposition Parameter Registrations
................................................................ 255 264
28 Changes From RFC 2543 ............................... 256 264
28.1 Major Functional Changes ............................ 256 265
28.2 Minor Functional Changes ............................ 260 269
29 Acknowledgments ..................................... 261 270
30 Authors' Addresses .................................. 262 270
31 Normative References ................................ 263 271
32 Non-Normative Informative References ............................ 265 .............................. 273
A Table of Timer Values ............................... 266 275
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1 Introduction
There are many 9]
Internet Draft SIP February 27, 2002
1 Introduction
There are many applications of the Internet that require the creation
and management of a session, where a session is considered an
exchange of data between an association of participants. The
implementation of these applications is complicated by the practices
of participants: users may move between endpoints, they may be
addressable by multiple names, and they may communicate in several
different media - sometimes simultaneously. Numerous protocols have
been authored that carry various forms of real-time multimedia
session data such as voice, video, or text messages. SIP works in
concert with these protocols by enabling Internet endpoints (called
user agents ) to discover one another and to agree on a
characterization of a session they would like to share. For locating
prospective session participants, and for other functions, SIP
enables creation of an infrastructure of network hosts (called proxy
servers ) to which user agents can send registrations, invitations to
sessions, and other requests. SIP is an agile, general-purpose tool
for creating, modifying, and terminating sessions that works
independently of underlying transport protocols and without
dependency on the type of session that is being established.
2 Overview of SIP Functionality
SIP is an application-layer control protocol that can establish,
modify, and terminate multimedia sessions (conferences) such as
Internet telephony calls. SIP can also invite participants to already
existing sessions, such as multicast conferences. Media can be added
to (and removed from) an existing session. SIP transparently supports
name mapping and redirection services, which supports personal
mobility [26] [27] - users can maintain a single externally visible
identifier regardless of their network location.
SIP supports five facets of establishing and terminating multimedia
communications:
User location: determination of the end system to be used for
communication;
User availability: determination of the willingness of the
called party to engage in communications;
User capabilities: determination of the media and media
parameters to be used;
Session setup: "ringing", establishment of session parameters at
both called and calling party;
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Internet Draft SIP February 21, 27, 2002
Session management: including transfer and termination of
sessions, modifying session parameters, and invoking
services.
SIP is not a vertically integrated communications system. SIP is
rather a component that can be used with other IETF protocols to
build a complete multimedia architecture. Typically, these
architectures will include protocols such as the real-time transport
protocol (RTP) (RFC 1889 [27]) [28]) for transporting real-time data and
providing QoS feedback, the real-time streaming protocol (RTSP) (RFC
2326 [28]) [29]) for controlling delivery of streaming media, the Media
Gateway Control Protocol (MEGACO) (RFC 3015 [29]) [30]) for controlling
gateways to the Public Switched Telephone Network (PSTN), and the
session description protocol (SDP) (RFC 2327 [1]) for describing
multimedia sessions. Therefore, SIP should be used in conjunction
with other protocols in order to provide complete services to the
users. However, the basic functionality and operation of SIP does not
depend on any of these protocols.
SIP does not provide services. SIP rather provides primitives that
can be used to implement different services. For example, SIP can
locate a user and deliver an opaque object to his current location.
If this primitive is used to deliver a session description written in
SDP, for instance, the endpoints can agree on the parameters of a
session. If the same primitive is used to deliver a photo of the
caller as well as the session description, a "caller ID" service can
be easily implemented. As this example shows, a single primitive is
typically used to provide several different services.
SIP does not offer conference control services such as floor control
or voting and does not prescribe how a conference is to be managed.
SIP can be used to initiate a session that uses some other conference
control protocol. Since SIP messages and the sessions they establish
can pass through entirely different networks, SIP cannot, and does
not, provide any kind of network resource reservation capabilities.
The nature of the services provided make security particularly
important. To that end, SIP provides a suite of security services,
which include denial-of-service prevention, authentication (both user
to user and proxy to user), integrity protection, and encryption and
privacy services.
SIP works with both IPv4 and IPv6.
3 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
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Internet Draft SIP February 21, 27, 2002
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [2] and indicate requirement levels for
compliant SIP implementations.
4 Overview of Operation
This section introduces the basic operations of SIP using simple
examples. This section is tutorial in nature and does not contain any
normative statements.
The first example shows the basic functions of SIP: location of an
end point, signal of a desire to communicate, negotiation of session
parameters to establish the session, and teardown of the session once
established.
Figure 1 shows a typical example of a SIP message exchange between
two users, Alice and Bob. (Each message is labeled with the letter
"F" and a number for reference by the text.) In this example, Alice
uses a SIP application on her PC (referred to as a softphone) to call
Bob on his SIP phone over the Internet. Also shown are two SIP proxy
servers that act on behalf of Alice and Bob to facilitate the session
establishment. This typical arrangement is often referred to as the
"SIP trapezoid" as shown by the geometric shape of the dashed lines
in Figure 1.
Alice "calls" Bob using his SIP identity, a type of Uniform Resource
Identifier (URI) called a SIP URI and which is defined in Section
19.1. It has a similar form to an email address, typically containing
a username and a host name. In this case, it is sip:bob@biloxi.com,
where biloxi.com is the domain of Bob's SIP service provider (which
can be an enterprise, retail provider, etc). Alice also has a SIP URI
of sip:alice@atlanta.com. Alice might have typed in Bob's URI or
perhaps clicked on a hyperlink or an entry in an address book. SIP
also provides a secure URI, called a SIPS URI. An example would be
sips:bob@biloxi.com. A call made to a SIPS URI guarantees that
secure, encrypted transport (namely TLS) is used to carry all SIP
messages from the caller to the domain of the callee. From there,
the request is sent securely to the callee, but with security
mechanisms that depend on the policy of the domain of the callee.
SIP is based on an HTTP-like request/response transaction model. Each
transaction consists of a request that invokes a particular method ,
or function, on the server and at least one response. In this
example, the transaction begins with Alice's softphone sending an
INVITE request addressed to Bob's SIP URI. INVITE is an example of a
SIP method that specifies the action that the requestor (Alice) wants
the server (Bob) to take. The INVITE request contains a number of
J. Rosenberg et. al. [Page 5] 12]
Internet Draft SIP February 21, 27, 2002
atlanta.com . . . biloxi.com
. proxy proxy .
. .
Alice's . . . . . . . . . . . . . . . . . . . . Bob's
softphone SIP Phone
| | | |
| INVITE F1 | | |
|--------------->| INVITE F2 | |
| 100 Trying F3 |--------------->| INVITE F4 |
|<---------------| 100 Trying F5 |--------------->|
| |<-------------- | 180 Ringing F6 |
| | 180 Ringing F7 |<---------------|
| 180 Ringing F8 |<---------------| 200 OK F9 |
|<---------------| 200 OK F10 |<---------------|
| 200 OK F11 |<---------------| |
|<---------------| | |
| ACK F12 |
|------------------------------------------------->|
| Media Session |
|<================================================>|
| BYE F13 |
|<-------------------------------------------------|
| 200 OK F14 |
|------------------------------------------------->|
| |
Figure 1: SIP session setup example with SIP trapezoid
header fields. Header fields are named attributes that provide
additional information about a message. The ones present in an INVITE
include a unique identifier for the call, the destination address,
Alice's address, and information about the type of session that Alice
wishes to establish with Bob. The INVITE (message F1 in Figure 1)
might look like this:
INVITE sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
Max-Forwards: 70
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.com
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CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.com>
Content-Type: application/sdp
Content-Length: 142
(Alice's SDP not shown)
The first line of the text-encoded message contains the method name
(INVITE). The lines that follow are a list of header fields. This
example contains a minimum required set. The header fields are
briefly described below:
Via contains the address (pc33.atlanta.com) at which Alice is
expecting to receive responses to this request. It also contains a
branch parameter that contains an identifier for this transaction.
To contains a display name (Bob) and a SIP or SIPS URI
(sip:bob@biloxi.com) towards which the request was originally
directed. Display names are described in RFC 2822 [3].
From also contains a display name (Alice) and a SIP or SIPS URI
(sip:alice@atlanta.com) that indicate the originator of the request.
This header field also has a tag parameter containing a pseudorandom
string (1928301774) that was added to the URI by the softphone. It is
used for identification purposes.
Call-ID contains a globally unique identifier for this call,
generated by the combination of a pseudorandom string and the
softphone's IP address. The combination of the To tag, From tag, and
Call-ID completely define a peer-to-peer SIP relationship between
Alice and Bob and is referred to as a dialog
CSeq or Command Sequence contains an integer and a method name. The
CSeq number is incremented for each new request within a dialog and
is a traditional sequence number.
Contact contains a SIP or SIPS URI that represents a direct route to
contact Alice, usually composed of a username at a fully qualified
domain name (FQDN). While an FQDN is preferred, many end systems do
not have registered domain names, so IP addresses are permitted.
While the Via header field tells other elements where to send the
response, the Contact header field tells other elements where to send
future requests.
Max-Forwards serves to limit the number of hops a request can make on
the way to its destination. It consists of an integer that is
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decremented by one at each hop.
Content-Type contains a description of the message body (not shown).
Content-Length contains an octet (byte) count of the message body.
The complete set of SIP header fields is defined in Section 20.
The details of the session, type of media, codec, sampling rate, etc.
are not described using SIP. Rather, the body of a SIP message
contains a description of the session, encoded in some other protocol
format. One such format is the Session Description Protocol (SDP)
(RFC 2327 [1]). This SDP message (not shown in the example) is
carried by the SIP message in a way that is analogous to a document
attachment being carried by an email message, or a web page being
carried in an HTTP message.
Since the softphone does not know the location of Bob or the SIP
server in the biloxi.com domain, the softphone sends the INVITE to
the SIP server that serves Alice's domain, atlanta.com. The address
of the atlanta.com SIP server could have been configured in Alice's
softphone, or it could have been discovered by DHCP, for example.
The atlanta.com SIP server is a type of SIP server known as a proxy
server. A proxy server receives SIP requests and forwards them on
behalf of the requestor. In this example, the proxy server receives
the INVITE request and sends a 100 (Trying) response back to Alice's
softphone. The 100 (Trying) response indicates that the INVITE has
been received and that the proxy is working on her behalf to route
the INVITE to the destination. Responses in SIP use a three-digit
code followed by a descriptive phrase. This response contains the
same To, From, Call-ID,CSeq and branch parameter in the Via as the
INVITE, which allows Alice's softphone to correlate this response to
the sent INVITE. The atlanta.com proxy server locates the proxy
server at biloxi.com, possibly by performing a particular type of DNS
(Domain Name Service) lookup to find the SIP server that serves the
biloxi.com domain. This is described in [4]. As a result, it obtains
the IP address of the biloxi.com proxy server and forwards, or
proxies, the INVITE request there. Before forwarding the request, the
atlanta.com proxy server adds an additional Via header field value
that contains its own address (the INVITE already contains Alice's
address in the first Via). The biloxi.com proxy server receives the
INVITE and responds with a 100 (Trying) response back to the
atlanta.com proxy server to indicate that it has received the INVITE
and is processing the request. The proxy server consults a database,
generically called a location service, that contains the current IP
address of Bob. (We shall see in the next section how this database
can be populated.) The biloxi.com proxy server adds another Via
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header field value with its own address to the INVITE and proxies it
to Bob's SIP phone.
Bob's SIP phone receives the INVITE and alerts Bob to the incoming
call from Alice so that Bob can decide whether to answer the call,
that is, Bob's phone rings. Bob's SIP phone indicates this in a 180
(Ringing) response, which is routed back through the two proxies in
the reverse direction. Each proxy uses the Via header field to
determine where to send the response and removes its own address from
the top. As a result, although DNS and location service lookups were
required to route the initial INVITE, the 180 (Ringing) response can
be returned to the caller without lookups or without state being
maintained in the proxies. This also has the desirable property that
each proxy that sees the INVITE will also see all responses to the
INVITE.
When Alice's softphone receives the 180 (Ringing) response, it passes
this information to Alice, perhaps using an audio ringback tone or by
displaying a message on Alice's screen.
In this example, Bob decides to answer the call. When he picks up the
handset, his SIP phone sends a 200 (OK) response to indicate that the
call has been answered. The 200 (OK) contains a message body with the
SDP media description of the type of session that Bob is willing to
establish with Alice. As a result, there is a two-phase exchange of
SDP messages: Alice sent one to Bob, and Bob sent one back to Alice.
This two-phase exchange provides basic negotiation capabilities and
is based on a simple offer/answer model of SDP exchange. If Bob did
not wish to answer the call or was busy on another call, an error
response would have been sent instead of the 200 (OK), which would
have resulted in no media session being established. The complete
list of SIP response codes is in Section 21. The 200 (OK) (message F9
in Figure 1) might look like this as Bob sends it out:
SIP/2.0 200 OK
Via: SIP/2.0/UDP server10.biloxi.com;branch=z9hG4bKnashds8
;received=192.0.2.3
Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
;received=192.0.2.2
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
;received=192.0.2.1
To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Contact: <sip:bob@192.0.2.4>
Content-Type: application/sdp
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Content-Length: 131
(Bob's SDP not shown)
The first line of the response contains the response code (200) and
the reason phrase (OK). The remaining lines contain header fields.
The Via, To, From, Call-ID, and CSeq header fields are copied from
the INVITE request. (There are three Via header field values - one
added by Alice's SIP phone, one added by the atlanta.com proxy, and
one added by the biloxi.com proxy.) Bob's SIP phone has added a tag
parameter to the To header field. This tag will be incorporated by
both endpoints into the dialog and will be included in all future
requests and responses in this call. The Contact header field
contains a URI at which Bob can be directly reached at his SIP phone.
The Content-Type and Content-Length refer to the message body (not
shown) that contains Bob's SDP media information.
In addition to DNS and location service lookups shown in this
example, proxy servers can make flexible "routing decisions" to
decide where to send a request. For example, if Bob's SIP phone
returned a 486 (Busy Here) response, the biloxi.com proxy server
could proxy the INVITE to Bob's voicemail server. A proxy server can
also send an INVITE to a number of locations at the same time. This
type of parallel search is known as forking
In this case, the 200 (OK) is routed back through the two proxies and
is received by Alice's softphone, which then stops the ringback tone
and indicates that the call has been answered. Finally, Alice's
softphone sends an acknowledgement message, ACK to Bob's SIP phone to
confirm the reception of the final response (200 (OK)). In this
example, the ACK is sent directly from Alice's softphone to Bob's SIP
phone, bypassing the two proxies. This occurs because the endpoints
have learned each other's address from the Contact header fields
through the INVITE/200 (OK) exchange, which was not known when the
initial INVITE was sent. The lookups performed by the two proxies are
no longer needed, so the proxies drop out of the call flow. This
completes the INVITE/200/ACK three-way handshake used to establish
SIP sessions. Full details on session setup are in Section 13.
Alice and Bob's media session has now begun, and they send media
packets using the format to which they agreed in the exchange of SDP.
In general, the end-to-end media packets take a different path from
the SIP signaling messages.
During the session, either Alice or Bob may decide to change the
characteristics of the media session. This is accomplished by sending
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a re-INVITE containing a new media description. This re-INVITE
references the existing dialog so that the other party knows that it
is to modify an existing session instead of establishing a new
session. The other party sends a 200 (OK) to accept the change. The
requestor responds to the 200 (OK) with an ACK. If the other party
does not accept the change, he sends an error response such as 406
(Not Acceptable), which also receives an ACK. However, the failure of
the re-INVITE does not cause the existing call to fail - the session
continues using the previously negotiated characteristics. Full
details on session modification are in Section 14.
At the end of the call, Bob disconnects (hangs up) first and
generates a BYE message. This BYE is routed directly to Alice's
softphone, again bypassing the proxies. Alice confirms receipt of the
BYE with a 200 (OK) response, which terminates the session and the
BYE transaction. No ACK is sent - an ACK is only sent in response to
a response to an INVITE request. The reasons for this special
handling for INVITE will be discussed later, but relate to the
reliability mechanisms in SIP, the length of time it can take for a
ringing phone to be answered, and forking. For this reason, request
handling in SIP is often classified as either INVITE or non-INVITE,
referring to all other methods besides INVITE. Full details on
session termination are in Section 15.
Full details of all the messages shown in the example of Figure 1 are
shown in Section 24.2.
In some cases, it may be useful for proxies in the SIP signaling path
to see all the messaging between the endpoints for the duration of
the session. For example, if the biloxi.com proxy server wished to
remain in the SIP messaging path beyond the initial INVITE, it would
add to the INVITE a required routing header field known as Record-
Route that contained a URI resolving to the hostname or IP address of
the proxy. This information would be received by both Bob's SIP phone
and (due to the Record-Route header field being passed back in the
200 (OK)) Alice's softphone and stored for the duration of the
dialog. The biloxi.com proxy server would then receive and proxy the
ACK, BYE, and 200 (OK) to the BYE. Each proxy can independently
decide to receive subsequent messaging, and that messaging will go
through all proxies that elect to receive it. This capability is
frequently used for proxies that are providing mid-call features.
Registration is another common operation in SIP. Registration is one
way that the biloxi.com server can learn the current location of Bob.
Upon initialization, and at periodic intervals, Bob's SIP phone sends
REGISTER messages to a server in the biloxi.com domain known as a SIP
registrar. The REGISTER messages associate Bob's SIP or SIPS URI
(sip:bob@biloxi.com) with the machine into which he is currently
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logged (conveyed as a SIP or SIPS URI in the Contact header field).
The registrar writes this association, also called a binding, to a
database, called the location service , where it can be used by the
proxy in the biloxi.com domain. Often, a registrar server for a
domain is co-located with the proxy for that domain. It is an
important concept that the distinction between types of SIP servers
is logical, not physical.
Bob is not limited to registering from a single device. For example,
both his SIP phone at home and the one in the office could send
registrations. This information is stored together in the location
service and allows a proxy to perform various types of searches to
locate Bob. Similarly, more than one user can be registered on a
single device at the same time.
The location service is just an abstract concept. It generally
contains information that allows a proxy to input a URI and receive a
set of zero or more URIs that tell the proxy where to send the
request. Registrations are one way to create this information, but
not the only way. Arbitrary mapping functions can be configured at
the discretion of the administrator.
Finally, it is important to note that in SIP, registration is used
for routing incoming SIP requests and has no role in authorizing
outgoing requests. Authorization and authentication are handled in
SIP either on a request-by-request basis with a challenge/response
mechanism, or by using a lower layer scheme as discussed in Section
26.
The complete set of SIP message details for this registration example
is in Section 24.1.
Additional operations in SIP, such as querying for the capabilities
of a SIP server or client using OPTIONS, or canceling a pending
request using CANCEL, will be introduced in later sections.
5 Structure of the Protocol
SIP is structured as a layered protocol, which means that its
behavior is described in terms of a set of fairly independent
processing stages with only a loose coupling between each stage. The
protocol behavior is described as layers for the purpose of
presentation, allowing the description of functions common across
elements in a single section. It does not dictate an implementation
in any way. When we say that an element "contains" a layer, we mean
it is compliant to the set of rules defined by that layer.
Not every element specified by the protocol contains every layer.
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Furthermore, the elements specified by SIP are logical elements, not
physical ones. A physical realization can choose to act as different
logical elements, perhaps even on a transaction-by-transaction basis.
The lowest layer of SIP is its syntax and encoding. Its encoding is
specified using an augmented Backus-Naur Form grammar (BNF). The
complete BNF is specified in Section 25; an overview of a SIP
message's structure can be found in Section 7.
The second layer is the transport layer. It defines how a client
sends requests and receives responses and how a server receives
requests and sends responses over the network. All SIP elements
contain a transport layer. The transport layer is described in
Section 18.
The third layer is the transaction layer. Transactions are a
fundamental component of SIP. A transaction is a request sent by a
client transaction (using the transport layer) to a server
transaction, along with all responses to that request sent from the
server transaction back to the client. The transaction layer handles
application-layer retransmissions, matching of responses to requests,
and application-layer timeouts. Any task that a user agent client
(UAC) accomplishes takes place using a series of transactions.
Discussion of transactions can be found in Section 17. User agents
contain a transaction layer, as do stateful proxies. Stateless
proxies do not contain a transaction layer. The transaction layer has
a client component (referred to as a client transaction) and a server
component (referred to as a server transaction), each of which are
represented by a finite state machine that is constructed to process
a particular request.
The layer above the transaction layer is called the transaction user
(TU). Each of the SIP entities, except the stateless proxy, is a
transaction user. When a TU wishes to send a request, it creates a
client transaction instance and passes it the request along with the
destination IP address, port, and transport to which to send the
request. A TU that creates a client transaction can also cancel it.
When a client cancels a transaction, it requests that the server stop
further processing, revert to the state that existed before the
transaction was initiated, and generate a specific error response to
that transaction. This is done with a CANCEL request, which
constitutes its own transaction, but references the transaction to be
cancelled (Section 9).
The SIP elements, that is, user agent clients and servers, stateless
and stateful proxies and registrars, contain a core that
distinguishes them from each other. Cores, except for the stateless
proxy, are transaction users. While the behavior of the UAC and UAS
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cores depends on the method, there are some common rules for all
methods (Section 8). For a UAC, these rules govern the construction
of a request; for a UAS, they govern the processing of a request and
generating a response. Since registrations play an important role in
SIP, a UAS that handles a REGISTER is given the special name
registrar. Section 10 describes UAC and UAS core behavior for the
REGISTER method. Section 11 describes UAC and UAS core behavior for
the OPTIONS method, used for determining the capabilities of a UA.
Certain other requests are sent within a dialog. A dialog is a peer-
to-peer SIP relationship between two user agents that persists for
some time. The dialog facilitates sequencing of messages and proper
routing of requests between the user agents. The INVITE method is the
only way defined in this specification to establish a dialog. When a
UAC sends a request that is within the context of a dialog, it
follows the common UAC rules as discussed in Section 8 but also the
rules for mid-dialog requests. Section 12 discusses dialogs and
presents the procedures for their construction and maintenance, in
addition to construction of requests within a dialog.
The most important method in SIP is the INVITE method, which is used
to establish a session between participants. A session is a
collection of participants, and streams of media between them, for
the purposes of communication. Section 13 discusses how sessions are
initiated, resulting in one or more SIP dialogs. Section 14
discusses how characteristics of that session are modified through
the use of an INVITE request within a dialog. Finally, section 15
discusses how a session is terminated.
The procedures of Sections 8, 10, 11, 12, 13, 14, and 15 deal
entirely with the UA core (Section 9 describes cancellation, which
applies to both UA core and proxy core). Section 16 discusses the
proxy element, which facilitates routing of messages between user
agents.
6 Definitions
This specification uses a number of terms to refer to the roles
played by participants in SIP communications. The terms and generic
syntax of URI and URL are defined in RFC 2396 [5]. The following
terms have special significance for SIP.
Address-of-Record: An address-of-record (AOR) is a SIP or SIPS
URI that points to a domain with a location service that
can map the URI to another URI where the user might be
available. Typically, the location service is populated
through registrations. An AOR is frequently thought of as
the "public address" of the user.
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Back-to-Back User Agent: A back-to-back user agent (B2BUA) is a
logical entity that receives a request and processes it as
an user agent server (UAS). In order to determine how the
request should be answered, it acts as an user agent client
(UAC) and generates requests. Unlike a proxy server, it
maintains dialog state and must participate in all requests
sent on the dialogs it has established. Since it is a
concatenation of a UAC and UAS, no explicit definitions are
needed for its behavior.
Call: A call is an informal term that refers to some
communication between peers generally set up for the
purposes of a multimedia conversation.
Call Leg: Another name for a dialog [30]; [31]; no longer used in this
specification.
Call Stateful: A proxy is call stateful if it retains state for
a dialog from the initiating INVITE to the terminating BYE
request. A call stateful proxy is always transaction
stateful, but the converse is not necessarily true.
Client: A client is any network element that sends SIP requests
and receives SIP responses. Clients may or may not interact
directly with a human user. User agent clients and proxies
are clients.
Conference: A multimedia session (see below) that contains
multiple participants.
Core: Core designates the functions specific to a particular
type of SIP entity, i.e., specific to either a stateful or
stateless proxy, a user agent or registrar. All cores
except those for the stateless proxy are transaction users.
Dialog: A dialog is a peer-to-peer SIP relationship between two
UAs that persists for some time. A dialog is established by
SIP messages, such as a 2xx response to an INVITE request.
A dialog is identified by a call identifier, local tag, and
a remote tag. A dialog was formerly known as a call leg in
RFC 2543.
Downstream: A direction of message forwarding within a
transaction that refers to the direction that requests flow
from the user agent client to user agent server.
Final Response: A response that terminates a SIP transaction, as
opposed to a provisional response that does not. All 2xx,
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3xx, 4xx, 5xx and 6xx responses are final.
Header: A header is a component of a SIP message that conveys
information about the message. It is structured as a
sequence of header fields.
Header field: A header field is a component of the SIP message
header. It consists of one or more header field values
separated by comma or having the same header field name.
Header field value: A header field value consists of a field
name and is a field singular value, separated by
which can be one of many in a colon. header field.
Home Domain: The domain providing service to a SIP user.
Typically, this is the domain present in the URI in the
address-of-record of a registration.
Informational Response: Same as a provisional response.
Initiator, Calling Party, Caller: The party initiating a session
(and dialog) with an INVITE request. A caller retains this
role from the time it sends the initial INVITE that
established a dialog until the termination of that dialog.
Invitation: An INVITE request.
Invitee, Invited User, Called Party, Callee: The party that
receives an INVITE request for the purposes of establishing
a new session. A callee retains this role from the time it
receives the INVITE until the termination of the dialog
established by that INVITE.
Location Service: A location service is used by a SIP redirect
or proxy server to obtain information about a callee's
possible location(s). It contains a list of bindings of
address-of-record keys to zero or more contact addresses.
The bindings can be created and removed in many ways; this
specification defines a REGISTER method that updates the
bindings.
Loop: A request that arrives at a proxy, is forwarded, and later
arrives back at the same proxy. When it arrives the second
time, its Request-URI is identical to the first time, and
other header fields that affect proxy operation are
unchanged, so that the proxy would make the same processing
decision on the request it made the first time. Looped
requests are errors, and the procedures for detecting them
and handling them are described by the protocol.
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Loose Routing: A proxy is said to be loose routing if it follows
the procedures defined in this specification for processing
of the Route header field. These procedures separate the
destination of the request (present in the Request-URI)
from the set of proxies that need to be visited along the
way (present in the Route header field). A proxy compliant
to these mechanisms is also known as a loose router.
Message: Data sent between SIP elements as part of the protocol.
SIP messages are either requests or responses.
Method: The method is the primary function that a request is
meant to invoke on a server. The method is carried in the
request message itself. Example methods are INVITE and BYE.
Outbound Proxy: A proxy that receives requests from a client,
even though it may not be the server resolved by the
Request-URI. Typically, a UA is manually configured with
an outbound proxy, or can learn about one through auto-
configuration protocols.
Parallel Search: In a parallel search, a proxy issues several
requests to possible user locations upon receiving an
incoming request. Rather than issuing one request and then
waiting for the final response before issuing the next
request as in a sequential search , a parallel search
issues requests without waiting for the result of previous
requests.
Provisional Response: A response used by the server to indicate
progress, but that does not terminate a SIP transaction.
1xx responses are provisional, other responses are
considered final. Provisional responses are not sent
reliably.
Proxy, Proxy Server: An intermediary entity that acts as both a
server and a client for the purpose of making requests on
behalf of other clients. A proxy server primarily plays the
role of routing, which means its job is to ensure that a
request is sent to another entity "closer" to the targeted
user. Proxies are also useful for enforcing policy (for
example, making sure a user is allowed to make a call). A
proxy interprets, and, if necessary, rewrites specific
parts of a request message before forwarding it.
Recursion: A client recurses on a 3xx response when it generates
a new request to one or more of the URIs in the Contact
header field in the response.
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Redirect Server: A redirect server is a user agent server that
generates 3xx responses to requests it receives, directing
the client to contact an alternate set of URIs.
Registrar: A registrar is a server that accepts REGISTER
requests and places the information it receives in those
requests into the location service for the domain it
handles.
Regular Transaction: A regular transaction is any transaction
with a method other than INVITE, ACK, or CANCEL.
Request: A SIP message sent from a client to a server, for the
purpose of invoking a particular operation.
Response: A SIP message sent from a server to a client, for
indicating the status of a request sent from the client to
the server.
Ringback: Ringback is the signaling tone produced by the calling
party's application indicating that a called party is being
alerted (ringing).
Route Set: A route set is a collection of ordered SIP or SIPS
URI which represent a list of proxies that must be
traversed when sending a particular request. A route set
can be learned, through headers like Record-Route, or it
can be configured.
Server: A server is a network element that receives requests in
order to service them and sends back responses to those
requests. Examples of servers are proxies, user agent
servers, redirect servers, and registrars.
Sequential Search: In a sequential search, a proxy server
attempts each contact address in sequence, proceeding to
the next one only after the previous has generated a final
response. A 2xx or 6xx class final response always
terminates a sequential search.
Session: From the SDP specification: "A multimedia session is a
set of multimedia senders and receivers and the data
streams flowing from senders to receivers. A multimedia
conference is an example of a multimedia session." (RFC
2327 [1]) (A session as defined for SDP can comprise one or
more RTP sessions.) As defined, a callee can be invited
several times, by different calls, to the same session. If
SDP is used, a session is defined by the concatenation of
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the SDP user name , session id , network type , address
type , and address elements in the origin field.
SIP Transaction: A SIP transaction occurs between a client and a
server and comprises all messages from the first request
sent from the client to the server up to a final (non-1xx)
response sent from the server to the client. If the
request is INVITE and the final response is a non-2xx, the
transaction also includes an ACK to the response. The ACK
for a 2xx response to an INVITE request is a separate
transaction.
Spiral: A spiral is a SIP request that is routed to a proxy,
forwarded onwards, and arrives once again at that proxy,
but this time differs in a way that will result in a
different processing decision than the original request.
Typically, this means that the request's Request-URI
differs from its previous arrival. A spiral is not an error
condition, unlike a loop. A typical cause for this is call
forwarding. A user calls joe@example.com. The example.com
proxy forwards it to Joe's PC, which in turn, forwards it
to bob@example.com. This request is proxied back to the
example.com proxy. However, this is not a loop. Since the
request is targeted at a different user, it is considered a
spiral, and is a valid condition.
Stateful Proxy: A logical entity that maintains the client and
server transaction state machines defined by this
specification during the processing of a request. Also
known as a transaction stateful proxy. The behavior of a
stateful proxy is further defined in Section 16. A
(transaction) stateful proxy is not the same as a call
stateful proxy.
Stateless Proxy: A logical entity that does not maintain the
client or server transaction state machines defined in this
specification when it processes requests. A stateless proxy
forwards every request it receives downstream and every
response it receives upstream.
Strict Routing: A proxy is said to be strict routing if it
follows the Route processing rules of RFC 2543 and many
prior Internet Draft versions of this RFC. That rule caused
proxies to destroy the contents of the Request-URI when a
Route header field was present. Strict routing behavior is
not used in this specification, in favor of a loose routing
behavior. Proxies that perform strict routing are also
known as strict routers.
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Target Refresh Request: A target refresh request sent within a
dialog is defined as a request that can modify the remote
target of the dialog.
Transaction User (TU): The layer of protocol processing that
resides above the transaction layer. Transaction users
include the UAC core, UAS core, and proxy core.
Upstream: A direction of message forwarding within a transaction
that refers to the direction that responses flow from the
user agent server back to the user agent client.
URL-encoded: A character string encoded according to RFC 1738,
Section 2.2 [6].
User Agent Client (UAC): A user agent client is a logical entity
that creates a new request, and then uses the client
transaction state machinery to send it. The role of UAC
lasts only for the duration of that transaction. In other
words, if a piece of software initiates a request, it acts
as a UAC for the duration of that transaction. If it
receives a request later, it assumes the role of a user
agent server for the processing of that transaction.
UAC Core: The set of processing functions required of a UAC that
reside above the transaction and transport layers.
User Agent Server (UAS): A user agent server is a logical entity
that generates a response to a SIP request. The response
accepts, rejects, or redirects the request. This role lasts
only for the duration of that transaction. In other words,
if a piece of software responds to a request, it acts as a
UAS for the duration of that transaction. If it generates a
request later, it assumes the role of a user agent client
for the processing of that transaction.
UAS Core: The set of processing functions required at a UAS that
reside above the transaction and transport layers.
User Agent (UA): A logical entity that can act as both a user
agent client and user agent server.
The role of UAC and UAS as well as proxy and redirect servers are
defined on a transaction-by-transaction basis. For example, the user
agent initiating a call acts as a UAC when sending the initial INVITE
request and as a UAS when receiving a BYE request from the callee.
Similarly, the same software can act as a proxy server for one
request and as a redirect server for the next request.
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Proxy, location, and registrar servers defined above are logical
entities; implementations MAY combine them into a single application.
7 SIP Messages
SIP is a text-based protocol and uses the ISO 10646 character set in UTF-8 encoding charset (RFC 2279
[7]).
A SIP message is either a request from a client to a server, or a
response from a server to a client.
Both Request (section 7.1) and Response (section 7.2) messages use
the basic format of RFC 2822 [3], even though the syntax differs in
character set and syntax specifics. (SIP allows header fields that
would not be valid RFC 2822 header fields, for example.) Both types
of messages consist of a start-line, one or more header fields, an
empty line indicating the end of the header fields, and an optional
message-body.
generic-message = start-line
*message-header
CRLF
[ message-body ]
start-line = Request-Line / Status-Line
The start-line, each message-header line, and the empty line MUST be
terminated by a carriage-return line-feed sequence (CRLF). Note that
the empty line MUST be present even if the message-body is not.
Except for the above difference in character sets, much of SIP's
message and header field syntax is identical to HTTP/1.1. Rather than
repeating the syntax and semantics here, we use [HX.Y] to refer to
Section X.Y of the current HTTP/1.1 specification (RFC 2616 [8]).
However, SIP is not an extension of HTTP.
7.1 Requests
SIP requests are distinguished by having a Request-Line for a start-
line. A Request-Line contains a method name, a Request-URI, and the
protocol version separated by a single space (SP) character.
The Request-Line ends with CRLF. No CR or LF are allowed except in
the end-of-line CRLF sequence. No linear whitespace (LWS) is allowed
in any of the elements.
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Request-Line = Method SP Request-URI SP SIP-Version CRLF
Method:
This specification defines six methods: REGISTER for
registering contact information, INVITE, ACK, and CANCEL
for setting up sessions, BYE for terminating sessions, and
OPTIONS for querying servers about their capabilities. SIP
extensions, documented in standards track RFCs, may define
additional methods.
Request-URI: The Request-URI is a SIP or SIPS URI as described
in Section 19.1 or a general URI (RFC 2396 [5]). It
indicates the user or service to which this request is
being addressed. The Request-URI MUST NOT contain unescaped
spaces or control characters and MUST NOT be enclosed in
"<>".
SIP elements MAY support Request-URIs with schemes other
than "sip" and "sips", for example the "tel" URI scheme of
RFC 2806 [9]. SIP elements MAY translate non-SIP URIs using
any mechanism at their disposal, resulting in either SIP
URI, SIPS URI, or some other scheme.
SIP-Version: Both request and response messages include the
version of SIP in use, and follow [H3.1] (with HTTP
replaced by SIP, and HTTP/1.1 replaced by SIP/2.0)
regarding version ordering, compliance requirements, and
upgrading of version numbers. To be compliant with this
specification, applications sending SIP messages MUST
include a SIP-Version of "SIP/2.0". The SIP-Version string
is case-insensitive, but implementations MUST send upper-
case.
Unlike HTTP/1.1, SIP treats the version number as a
literal string. In practice, this should make no
difference.
7.2 Responses
SIP responses are distinguished from requests by having a Status-Line
as their start-line. A Status-Line consists of the protocol version
followed by a numeric Status-Code and its associated textual phrase,
with each element separated by a single SP character.
No CR or LF is allowed except in the final CRLF sequence.
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Status-Line = SIP-Version SP Status-Code SP Reason-Phrase CRLF
The Status-Code is a 3-digit integer result code that indicates the
outcome of an attempt to understand and satisfy a request. The
Reason-Phrase is intended to give a short textual description of the
Status-Code. The Status-Code is intended for use by automata, whereas
the Reason-Phrase is intended for the human user. A client is not
required to examine or display the Reason-Phrase.
While this specification suggests specific wording for the reason
phrase, implementations MAY choose other text, for example, in the
language indicated in the Accept-Language header field of the
request.
The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. For this reason,
any response with a status code between 100 and 199 is referred to as
a "1xx response", any response with a status code between 200 and 299
as a "2xx response", and so on. SIP/2.0 allows six values for the
first digit:
1xx: Provisional -- request received, continuing to process the
request;
2xx: Success -- the action was successfully received,
understood, and accepted;
3xx: Redirection -- further action needs to be taken in order to
complete the request;
4xx: Client Error -- the request contains bad syntax or cannot
be fulfilled at this server;
5xx: Server Error -- the server failed to fulfill an apparently
valid request;
6xx: Global Failure -- the request cannot be fulfilled at any
server.
Section 21 defines these classes and describes the individual codes.
7.3 Header Fields
SIP header fields are similar to HTTP header fields in both syntax
and semantics. In particular, SIP header fields follow the [H4.2]
definitions of syntax for message-header and the rules for extending
header fields over multiple lines. However, the latter is specified
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in HTTP with implicit whitespace and folding. This specification
conforms with RFC 2234 [10] and uses only explicit whitespace and
folding as an integral part of the grammar.
[H4.2] also specifies that multiple header fields of the same field
name whose value is a comma-separated list can be combined into one
header field. That applies to SIP as well, but the specific rule is
different because of the different grammars. Specifically, any SIP
header whose grammar is of the form:
header = "header-name" HCOLON header-value *(COMMA header-value)
allows for combining header fields of the same name into a comma-
separated list. This is also true for the Contact header, as long as
none of the header field values are "*".
7.3.1 Header Field Format
Header fields follow the same generic header format as that given in
Section 2.2 of RFC 2822 [3]. Each header field consists of a field
name followed by a colon (":") and the field value.
field-name: field-value
The formal grammar for a message-header specified in Section 25
allows for an arbitrary amount of whitespace on either side of the
colon; however, implementations should avoid spaces between the field
name and the colon and use a single space (SP) between the colon and
the field-value. Thus,
Subject: lunch
Subject : lunch
Subject :lunch
Subject: lunch
are all valid and equivalent, but the last is the preferred form.
Header fields can be extended over multiple lines by preceding each
extra line with at least one SP or horizontal tab (HT). The line
break and the whitespace at the beginning of the next line are
treated as a single SP character. Thus, the following are equivalent:
Subject: I know you're there, pick up the phone and talk to me!
Subject: I know you're there,
pick up the phone
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and talk to me!
The relative order of header fields with different field names is not
significant. However, it is RECOMMENDED that header fields which are
needed for proxy processing (Via, Route, Record-Route, Proxy-Require,
Max-Forwards, and Proxy-Authorization, for example) appear towards
the top of the message to facilitate rapid parsing. The relative
order of header field rows with the same field name is important.
Multiple header field rows with the same field-name MAY be present in
a message if and only if the entire field-value for that header field
is defined as a comma-separated list (that is, if follows the grammar
defined in Section 7.3). It MUST be possible to combine the multiple
header field rows into one "field-name: field-value" pair, without
changing the semantics of the message, by appending each subsequent
field-value to the first, each separated by a comma. The exceptions
to this rule are the WWW-Authenticate, Authorization, Proxy-
Authenticate, and Proxy-Authorization header fields. Multiple header
field rows with these names MAY be present in a message, but since
their grammar does not follow the general form listed in Section 7.3,
they MUST NOT be combined into a single header field row.
Implementations MUST be able to process multiple header field rows
with the same name in any combination of the single-value-per-line or
comma-separated value forms.
The following groups of header field rows are valid and equivalent:
Route: <sip:alice@atlanta.com>
Subject: Lunch
Route: <sip:bob@biloxi.com>
Route: <sip:carol@chicago.com>
Route: <sip:alice@atlanta.com>, <sip:bob@biloxi.com>
Route: <sip:carol@chicago.com>
Subject: Lunch
Subject: Lunch
Route: <sip:alice@atlanta.com>, <sip:bob@biloxi.com>, <sip:carol@chicago.com>
Each of the following blocks is valid but not equivalent to the
others:
Route: <sip:alice@atlanta.com>
Route: <sip:bob@biloxi.com>
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Route: <sip:carol@chicago.com>
Route: <sip:bob@biloxi.com>
Route: <sip:alice@atlanta.com>
Route: <sip:carol@chicago.com>
Route: <sip:alice@atlanta.com>,<sip:carol@chicago.com>,<sip:bob@biloxi.com>
The format of a header field-value is defined per header-name. It
will always be either an opaque sequence of TEXT-UTF8 octets, or a
combination of whitespace, tokens, separators, and quoted strings.
Many existing header fields will adhere to the general form of a
value followed by a semi-colon separated sequence of parameter-name,
parameter-value pairs:
field-name: field-value *(;parameter-name=parameter-value)
Even though an arbitrary number of parameter pairs may be attached to
a header field value, any given parameter-name MUST NOT appear more
than once.
When comparing header fields, field names are always case-
insensitive. Unless otherwise stated in the definition of a
particular header field, field values, parameter names, and parameter
values are case-insensitive. Tokens are always case-insensitive.
Unless specified otherwise, values expressed as quoted strings are
case-sensitive.
For example,
Contact: <sip:alice@atlanta.com>;expires=3600
is equivalent to
CONTACT: <sip:alice@atlanta.com>;ExPiReS=3600
and
Content-Disposition: session;handling=optional
is equivalent to
content-disposition: Session;HANDLING=OPTIONAL
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The following two header fields are not equivalent:
Warning: 370 devnull "Choose a bigger pipe"
Warning: 370 devnull "CHOOSE A BIGGER PIPE"
7.3.2 Header Field Classification
Some header fields only make sense in requests or responses. These
are called request header fields and response header fields,
respectively. If a header field appears in a message not matching
its category (such as a request header field in a response), it MUST
be ignored. Section 20 defines the classification of each header
field.
7.3.3 Compact Form
SIP provides a mechanism to represent common header field names in an
abbreviated form. This may be useful when messages would otherwise
become too large to be carried on the transport available to it
(exceeding the maximum transmission unit (MTU) when using UDP, for
example). These compact forms are defined in Section 20. A compact
form MAY be substituted for the longer form of a header field name at
any time without changing the semantics of the message. A header
field name MAY appear in both long and short forms within the same
message. Implementations MUST accept both the long and short forms of
each header name.
7.4 Bodies
Requests, including new requests defined in extensions to this
specification, MAY contain message bodies unless otherwise noted.
The interpretation of the body depends on the request method.
For response messages, the request method and the response status
code determine the type and interpretation of any message body. All
responses MAY include a body.
7.4.1 Message Body Type
The Internet media type of the message body MUST be given by the
Content-Type header field. If the body has undergone any encoding
such as compression, then this MUST be indicated by the Content-
Encoding header field; otherwise, Content-Encoding MUST be omitted.
If applicable, the character set of the message body is indicated as
part of the Content-Type header-field value.
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The "multipart" MIME type defined in RFC 2046 [11] MAY be used within
the body of the message. Implementations that send requests
containing multipart message bodies MUST send a session description
as a non-multipart message body if the remote implementation requests
this through an Accept header field that does not contain multipart.
Note that SIP messages MAY contain binary bodies or body parts.
7.4.2 Message Body Length
The body length in bytes is provided by the Content-Length header
field. Section 20.14 describes the necessary contents of this header
field in detail.
The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used for SIP.
(Note: The chunked encoding modifies the body of a message in order
to transfer it as a series of chunks, each with its own size
indicator.)
7.5 Framing SIP messages
Unlike HTTP, SIP implementations can use UDP or other unreliable
datagram protocols. Each such datagram carries one request or
response. See Section 18 on constraints on usage of unreliable
transports.
Implementations processing SIP messages over stream-oriented
transports MUST ignore any CRLF appearing before the start-line
[H4.1].
The Content-Length header field value is used to locate the
end of each SIP message in a stream. It will always be
present when SIP messages are sent over stream-oriented
transports.
8 General User Agent Behavior
A user agent represents an end system. It contains a user agent
client (UAC), which generates requests, and a user agent server
(UAS), which responds to them. A UAC is capable of generating a
request based on some external stimulus (the user clicking a button,
or a signal on a PSTN line) and processing a response. A UAS is
capable of receiving a request and generating a response based on
user input, external stimulus, the result of a program execution, or
some other mechanism.
When a UAC sends a request, the request passes through some number of
proxy servers, which forward the request towards the UAS. When the
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UAS generates a response, the response is forwarded towards the UAC.
UAC and UAS procedures depend strongly on two factors. First, based
on whether the request or response is inside or outside of a dialog,
and second, based on the method of a request. Dialogs are discussed
thoroughly in Section 12; they represent a peer-to-peer relationship
between user agents and are established by specific SIP methods, such
as INVITE.
In this section, we discuss the method-independent rules for UAC and
UAS behavior when processing requests that are outside of a dialog.
This includes, of course, the requests which themselves establish a
dialog.
Security procedures for requests and responses outside of a dialog
are described in Section 26. Specifically, mechanisms exist for the
UAS and UAC to mutually authenticate. A limited set of privacy
features are also supported through encryption of bodies using
S/MIME.
8.1 UAC Behavior
This section covers UAC behavior outside of a dialog.
8.1.1 Generating the Request
A valid SIP request formulated by a UAC MUST at a minimum contains
the following header fields: To, From, CSeq, Call-ID, Max-Forwards,
and Via; all of these header fields are mandatory in all SIP
messages. These six header fields are the fundamental building blocks
of a SIP message, as they jointly provide for most of the critical
message routing services including the addressing of messages, the
routing of responses, limiting message propagation, ordering of
messages, and the unique identification of transactions. These header
fields are in addition to the mandatory request line, which contains
the method, Request-URI, and SIP version.
Examples of requests sent outside of a dialog include an INVITE to
establish a session (Section 13) and an OPTIONS to query for
capabilities (Section 11).
8.1.1.1 Request-URI
The initial Request-URI of the message SHOULD be set to the value of
the URI in the To field. One notable exception is the REGISTER
method; behavior for setting the Request-URI of REGISTER is given in
Section 10. It may also be undesirable for privacy reasons or
convenience to set these fields to the same value (especially if the
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originating UA expects that the Request-URI will be changed during
transit).
In some special circumstances, the presence of a pre-existing route
set can affect the Request-URI of the message. A pre-existing route
set is an ordered set of URIs that identify a chain of servers, to
which a UAC will send outgoing requests that are outside of a dialog.
Commonly, they are configured on the UA by a user or service provider
manually, or through some other non-SIP mechanism. When a provider
wishes to configure a UA with an outbound proxy, it is RECOMMENDED
that this be done by providing it with a pre-existing route set with
a single URI, that of the outbound proxy.
When a pre-existing route set is present, the procedures for
populating the Request-URI and Route header field detailed in Section
12.2.1.1 MUST be followed, even though there is no dialog.
8.1.1.2 To
The To header field first and foremost specifies the desired
"logical" recipient of the request, or the address-of-record of the
user or resource that is the target of this request. This may or may
not be the ultimate recipient of the request. The To header field MAY
contain a SIP or SIPS URI, but it may also make use of other URI
schemes (the tel URL (RFC 2806 [9]), for example) when appropriate.
All SIP implementations MUST support the SIP and URI scheme. Any
implementation that supports TLS MUST support the SIPS URI scheme.
The To header field allows for a display name.
A UAC may learn how to populate the To header field for a particular
request in a number of ways. Usually the user will suggest the To
header field through a human interface, perhaps inputting the URI
manually or selecting it from some sort of address book. Frequently,
the user will not enter a complete URI, but rather a string of digits
or letters (for example, "bob"). It is at the discretion of the UA to
choose how to interpret this input. Using the string to form the user
part of a SIP URI implies that the UA wishes the name to be resolved
in the domain to the right-hand side (RHS) of the at-sign in the SIP
URI (for instance, sip:bob@example.com). Using the string to form
the user part of a SIPS URI implies that the UA wishes to communicate
securely, and that the name is to be resolved in the domain to the
RHS of the at-sign. The RHS will frequently be the home domain of
the user, which allows for the home domain to process the outgoing
request. This is useful for features like "speed dial" that require
interpretation of the user part in the home domain. The tel URL may
be used when the UA does not wish to specify the domain that should
interpret a telephone number that has been inputted by the user.
Rather, each domain through which the request passes would be given
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that opportunity. As an example, a user in an airport might log in
and send requests through an outbound proxy in the airport. If they
enter "411" (this is the phone number for local directory assistance
in the United States), that needs to be interpreted and processed by
the outbound proxy in the airport, not the user's home domain. In
this case, tel:411 would be the right choice.
A request outside of a dialog MUST NOT contain a tag; the tag in the
To field of a request identifies the peer of the dialog. Since no
dialog is established, no tag is present.
For further information on the To header field, see Section 20.39.
The following is an example of valid To header field:
To: Carol <sip:carol@chicago.com>
8.1.1.3 From
The From header field indicates the logical identity of the initiator
of the request, possibly the user's address-of-record. Like the To
header field, it contains a URI and optionally a display name. It is
used by SIP elements to determine which processing rules to apply to
a request (for example, automatic call rejection). As such, it is
very important that the From URI not contain IP addresses or the FQDN
of the host on which the UA is running, since these are not logical
names.
The From header field allows for a display name. A UAC SHOULD use the
display name "Anonymous", along with a syntactically correct, but
otherwise meaningless URI (like sip:thisis@anonymous.invalid), if the
identity of the client is to remain hidden.
Usually the value that populates the From header field in requests
generated by a particular UA is pre-provisioned by the user or by the
administrators of the user's local domain. If a particular UA is used
by multiple users, it might have switchable profiles that include a
URI corresponding to the identity of the profiled user. Recipients of
requests can authenticate the originator of a request in order to
ascertain that they are who their From header field claims they are
(see Section 22 for more on authentication).
The From field MUST contain a new "tag" parameter, chosen by the UAC.
See Section 19.3 for details on choosing a tag.
For further information on the From header field, see Section 20.20.
Examples:
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From: "Bob" <sips:bob@biloxi.com> ;tag=a48s
From: sip:+12125551212@phone2net.com;tag=887s
From: Anonymous <sip:c8oqz84zk7z@privacy.org>;tag=hyh8
8.1.1.4 Call-ID
The Call-ID header field acts as a unique identifier to group
together a series of messages. It MUST be the same for all requests
and responses sent by either UA in a dialog. It SHOULD be the same in
each registration from a UA.
In a new request created by a UAC outside of any dialog, the Call-ID
header field MUST be selected by the UAC as a globally unique
identifier over space and time unless overridden by method-specific
behavior. All SIP UAs must have a means to guarantee that the Call-ID
header fields they produce will not be inadvertently generated by any
other UA. Note that when requests are retried after certain failure
responses that solicit an amendment to a request (for example, a
challenge for authentication), these retried requests are not
considered new requests, and therefore do not need new Call-ID header
fields; see Section 8.1.3.5.
Use of cryptographically random identifiers (RFC 1750 [12]) in the
generation of Call-IDs is RECOMMENDED. Implementations MAY use the
form "localid@host". Call-IDs are case-sensitive and are simply
compared byte-by-byte.
Using cryptographically random identifiers provides some
protection against session hijacking and reduces the
likelihood of unintentional Call-ID collisions.
No provisioning or human interface is required for the selection of
the Call-ID header field value for a request.
For further information on the Call-ID header field, see Section
20.8.
Example:
Call-ID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6@foo.bar.com
8.1.1.5 CSeq
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The CSeq header field serves as a way to identify and order
transactions. It consists of a sequence number and a method. The
method MUST match that of the request. For non-REGISTER requests
outside of a dialog, the sequence number value is arbitrary. The
sequence number value MUST be expressible as a 32-bit unsigned
integer and MUST be less than 2**31. As long as it follows the above
guidelines, a client may use any mechanism it would like to select
CSeq header field values.
Section 12.2.1.1 discusses construction of the CSeq for requests
within a dialog.
Example:
CSeq: 4711 INVITE
8.1.1.6 Max-Forwards
The Max-Forwards header field serves to limit the number of hops a
request can transit on the way to its destination. It consists of an
integer that is decremented by one at each hop. If the Max-Forwards
value reaches 0 before the request reaches its destination, it will
be rejected with a 483(Too Many Hops) error response.
A UAC MUST insert a Max-Forwards header field into each request it
originates with a value which SHOULD be 70. This number was chosen to
be sufficiently large to guarantee that a request would not be
dropped in any SIP network when there were no loops, but not so large
as to consume proxy resources when a loop does occur. Lower values
should be used with caution and only in networks where topologies are
known by the UA.
8.1.1.7 Via
The Via header field indicates the transport used for the transaction
and identifies the location where the response is to be sent. A Via
header field value is added only after the transport that will be
used to reach the next hop has been selected (which may involve the
usage of the procedures in [4]).
When the UAC creates a request, it MUST insert a Via into that
request. The protocol name and protocol version in the header field
MUST be SIP and 2.0, respectively. The Via header field value MUST
contain a branch parameter. This parameter is used to identify the
transaction created by that request. This parameter is used by both
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the client and the server.
The branch parameter value MUST be unique across space and time for
all requests sent by the UA. The exceptions to this rule are CANCEL
and ACK for non-2xx responses. As discussed below, a CANCEL request
will have the same value of the branch parameter as the request it
cancels. As discussed in Section 17.1.1.3, an ACK for a non-2xx
response will also have the same branch ID as the INVITE whose
response it acknowledges.
The uniqueness property of the branch ID parameter, to
facilitate its use as a transaction ID, was not part of RFC
2543
The branch ID inserted by an element compliant with this
specification MUST always begin with the characters "z9hG4bK". These
7 characters are used as a magic cookie (7 is deemed sufficient to
ensure that an older RFC 2543 implementation would not pick such a
value), so that servers receiving the request can determine that the
branch ID was constructed in the fashion described by this
specification (that is, globally unique). Beyond this requirement,
the precise format of the branch token is implementation-defined.
The Via header maddr, ttl, and sent-by components will be set when
the request is processed by the transport layer (Section 18).
Via processing for proxies is described in Section 16.6 Item 8 and
Section 16.7 Item 3.
8.1.1.8 Contact
The Contact header field provides a SIP URI that can be used to
contact that specific instance of the UA for subsequent requests. The
Contact header field MUST be present and contain exactly one SIP or
SIPS URI in any request that can result in the establishment of a
dialog. For the methods defined in this specification, that includes
only the INVITE request. For these requests, the scope of the
Contact is global. That is, the Contact header field value contains
the URI at which the UA would like to receive requests, and this URI
MUST be valid even if used in subsequent requests outside of any
dialogs.
If the Request-URI or top Route header field value contains a SIPS
URI, the Contact header field MUST contain a SIPS URI as well.
For further information on the Contact header field, see Section
20.10.
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8.1.1.9 Supported and Require
If the UAC supports extensions to SIP that can be applied by the
server to the response, the UAC SHOULD include a Supported header
field in the request listing the option tags (Section 19.2) for those
extensions.
The option tags listed MUST only refer to extensions defined in
standards-track RFCs. This is to prevent servers from insisting that
clients implement non-standard, vendor-defined features in order to
receive service. Extensions defined by experimental and informational
RFCs are explicitly excluded from usage with the Supported header
field in a request, since they too are often used to document
vendor-defined extensions.
If the UAC wishes to insist that a UAS understand an extension that
the UAC will apply to the request in order to process the request, it
MUST insert a Require header field into the request listing the
option tag for that extension. If the UAC wishes to apply an
extension to the request and insist that any proxies that are
traversed understand that extension, it MUST insert a Proxy-Require
header field into the request listing the option tag for that
extension.
As with the Supported header field, the option tags in the Require
and Proxy-Require header fields MUST only refer to extensions defined
in standards-track RFCs.
8.1.1.10 Additional Message Components
After a new request has been created, and the header fields described
above have been properly constructed, any additional optional header
fields are added, as are any header fields specific to the method.
SIP requests MAY contain a MIME-encoded message-body. Regardless of
the type of body that a request contains, certain header fields must
be formulated to characterize the contents of the body. For further
information on these header fields, see Sections 20.11 through 20.15.
8.1.2 Sending the Request
The destination for the request is then computed. Unless there is
local policy specifying otherwise, then the destination MUST be
determined by applying the DNS procedures described in [4] as
follows. If the first element in the route set indicated a strict
router (resulting in forming the request as described in Section
12.2.1.1), the procedures MUST be applied to the Request-URI of the
request. Otherwise, the procedures are applied to the first Route
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header field value in the request (if one exists), or to the
request's Request-URI if there is no Route header field present.
These procedures yield an ordered set of address, port, and
transports to attempt. Independent of which URI is used as input to
the procedures of [4], if the Request-URI specifies a SIPS resource,
the UAC MUST follow the procedures of [4] as if the input URI were a
SIPS URI.
Local policy MAY specify an alternate set of destinations to attempt.
If the Request-URI contains a SIPS URI, any alternate destinations
MUST be contacted with TLS. Beyond that, there are no restrictions on
the alternate destinations if the request contains no Route header
field. This provides a simple alternative to a pre-existing route set
as a way to specify an outbound proxy. However, that approach for
configuring an outbound proxy is NOT RECOMMENDED; a pre-existing
route set with a single URI SHOULD be used instead. If the request
contains a Route header field, the request SHOULD be sent to the
locations derived from its topmost value, but MAY be sent to any
server that the UA is certain will honor the Route and Request-URI
policies specified in this document (as opposed to those in RFC
2543). In particular, a UAC configured with an outbound proxy SHOULD
attempt to send the request to the location indicated in the first
Route header field value instead of adopting the policy of sending
all messages to the outbound proxy.
This ensures that outbound proxies that do not add Record-
Route header field values will drop out of the path of
subsequent requests. It allows endpoints that cannot
resolve the first Route URI to delegate that task to an
outbound proxy.
The UAC SHOULD follow the procedures defined in [4] for stateful
elements, trying each address until a server is contacted. Each try
constitutes a new transaction, and therefore each carries a different
topmost Via header field value with a new branch parameter.
Furthermore, the transport value in the Via header field is set to
whatever transport was determined for the target server.
8.1.3 Processing Responses
Responses are first processed by the transport layer and then passed
up to the transaction layer. The transaction layer performs its
processing and then passes the response up to the TU. The majority
of response processing in the TU is method specific. However, there
are some general behaviors independent of the method.
8.1.3.1 Transaction Layer Errors
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In some cases, the response returned by the transaction layer will
not be a SIP message, but rather a transaction layer error. When a
timeout error is received from the transaction layer, it MUST be
treated as if a 408 (Request Timeout) status code has been received.
If a fatal transport error is reported by the transport layer
(generally, due to fatal ICMP errors in UDP or connection failures in
TCP), the condition MUST be treated as a 503 (Service Unavailable)
status code.
8.1.3.2 Unrecognized Responses
A UAC MUST treat any final response it does not recognize as being
equivalent to the x00 response code of that class, and MUST be able
to process the x00 response code for all classes. For example, if a
UAC receives an unrecognized response code of 431, it can safely
assume that there was something wrong with its request and treat the
response as if it had received a 400 (Bad Request) response code. A
UAC MUST treat any provisional response different than 100 that it
does not recognize as 183 (Session Progress). A UAC MUST be able to
process 100 and 183 responses.
8.1.3.3 Vias
If more than one Via header field value is present in a response, the
UAC SHOULD discard the message.
The presence of additional Via header field values that
precede the originator of the request suggests that the
message was misrouted or possibly corrupted.
8.1.3.4 Processing 3xx Responses
Upon receipt of a redirection response (for example, a 301 response
status code), clients SHOULD use the URI(s) in the Contact header
field to formulate one or more new requests based on the redirected
request. This process is similar to that of a proxy recursing on a
3xx class response as detailed in Sections 16.5 and 16.6. A client
starts with an initial target set containing exactly one URI, the
Request-URI of the original request. If a client wishes to formulate
new requests based on a 3xx class response to that request, it places
the URIs to try into the target set. Subject to the restrictions in
this specification, a client can choose which Contact URIs it places
into the target set. As with proxy recursion, a client processing 3xx
class responses MUST NOT add any given URI to the target set more
than once. If the original request had a SIPS URI in the Request-
URI, the client MAY choose to recurse to a non-SIPS URI, but SHOULD
inform the user of the redirection to an insecure URI.
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Any new request may receive 3xx responses themselves
containing the original URI as a contact. Two locations can
be configured to redirect to each other. Placing any given
URI in the target set only once prevents infinite
redirection loops.
As the target set grows, the client MAY generate new requests to the
URIs in any order. A common mechanism is to order the set by the "q"
parameter value from the Contact header field value. Requests to the
URIs MAY be generated serially or in parallel. One approach is to
process groups of decreasing q-values serially and process the URIs
in each q-value group in parallel. Another is to perform only serial
processing in decreasing q-value order, arbitrarily choosing between
contacts of equal q-value.
If contacting an address in the list results in a failure, as defined
in the next paragraph, the element moves to the next address in the
list, until the list is exhausted. If the list is exhausted, then the
request has failed.
Failures SHOULD be detected through failure response codes (codes
greater than 399); for network errors the client transaction will
report any transport layer failures to the transaction user. Note
that some response codes (detailed in 8.1.3.5) indicate that the
request can be retried; requests that are reattempted should not be
considered failures.
When a failure for a particular contact address is received, the
client SHOULD try the next contact address. This will involve
creating a new client transaction to deliver a new request.
In order to create a request based on a contact address in a 3xx
response, a UAC MUST copy the entire URI from the target set into the
Request-URI, except for the "method-param" and "header" URI
parameters (see Section 19.1.1 for a definition of these parameters).
It uses the "header" parameters to create header field values for the
new request, overwriting header field values associated with the
redirected request in accordance with the guidelines in Section
19.1.5.
Note that in some instances, header fields that have been
communicated in the contact address may instead append to existing
request header fields in the original redirected request. As a
general rule, if the header field can accept a comma-separated list
of values, then the new header field value MAY be appended to any
existing values in the original redirected request. If the header
field does not accept multiple values, the value in the original
redirected request MAY be overwritten by the header field value
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communicated in the contact address. For example, if a contact
address is returned with the following value:
sip:user@host?Subject=foo&Call-Info=<http://www.foo.com>
Then any Subject header field in the original redirected request is
overwritten, but the HTTP URL is merely appended to any existing
Call-Info header field values.
It is RECOMMENDED that the UAC reuse the same To, From, and Call-ID
used in the original redirected request, but the UAC MAY also choose
to update the Call-ID header field value for new requests, for
example.
Finally, once the new request has been constructed, it is sent using
a new client transaction, and therefore MUST have a new branch ID in
the top Via field as discussed in Section 8.1.1.7.
In all other respects, requests sent upon receipt of a redirect
response SHOULD re-use the header fields and bodies of the original
request.
In some instances, Contact header field values may be cached at UAC
temporarily or permanently depending on the status code received and
the presence of an expiration interval; see Sections 21.3.2 and
21.3.3.
8.1.3.5 Processing 4xx Responses
Certain 4xx response codes require specific UA processing,
independent of the method.
If a 401 (Unauthorized) or 407 (Proxy Authentication Required)
response is received, the UAC SHOULD follow the authorization
procedures of Section 22.2 and Section 22.3 to retry the request with
credentials.
If a 413 (Request Entity Too Large) response is received (Section
21.4.11), the request contained a body that was longer than the UAS
was willing to accept. If possible, the UAC SHOULD retry the request,
either omitting the body or using one of a smaller length.
If a 415 (Unsupported Media Type) response is received (Section
21.4.13), the request contained media types not supported by the UAS.
The UAC SHOULD retry sending the request, this time only using
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content with types listed in the Accept header field in the response,
with encodings listed in the Accept-Encoding header field in the
response, and with languages listed in the Accept-Language in the
response.
If a 416 (Unsupported URI Scheme) response is received (Section
21.4.14), the Request-URI used a URI scheme not supported by the
server. The client SHOULD retry the request, this time, using a SIP
URI.
If a 420 (Bad Extension) response is received (Section 21.4.15), the
request contained a Require or Proxy-Require header field listing an
option-tag for a feature not supported by a proxy or UAS. The UAC
SHOULD retry the request, this time omitting any extensions listed in
the Unsupported header field in the response.
In all of the above cases, the request is retried by creating a new
request with the appropriate modifications. This new request SHOULD
have the same value of the Call-ID, To, and From of the previous
request, but the CSeq should contain a new sequence number that is
one higher than the previous.
With other 4xx responses, including those yet to be defined, a retry
may or may not be possible depending on the method and the use case.
8.2 UAS Behavior
When a request outside of a dialog is processed by a UAS, there is a
set of processing rules that are followed, independent of the method.
Section 12 gives guidance on how a UAS can tell whether a request is
inside or outside of a dialog.
Note that request processing is atomic. If a request is accepted, all
state changes associated with it MUST be performed. If it is
rejected, all state changes MUST NOT be performed.
UASs SHOULD process the requests in the order of the steps that
follow in this section (that is, starting with authentication, then
inspecting the method, the header fields, and so on throughout the
remainder of this section).
8.2.1 Method Inspection
Once a request is authenticated (or authentication is skipped), the
UAS MUST inspect the method of the request. If the UAS recognizes but
does not support the method of a request, it MUST generate a 405
(Method Not Allowed) response. Procedures for generating responses
are described in Section 8.2.6. The UAS MUST also add an Allow header
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field to the 405 (Method Not Allowed) response. The Allow header
field MUST list the set of methods supported by the UAS generating
the message. The Allow header field is presented in Section 20.5.
If the method is one supported by the server, processing continues.
8.2.2 Header Inspection
If a UAS does not understand a header field in a request (that is,
the header field is not defined in this specification or in any
supported extension), the server MUST ignore that header field and
continue processing the message. A UAS SHOULD ignore any malformed
header fields that are not necessary for processing requests.
8.2.2.1 To and Request-URI
The To header field identifies the original recipient of the request
designated by the user identified in the From field. The original
recipient may or may not be the UAS processing the request, due to
call forwarding or other proxy operations. A UAS MAY apply any policy
it wishes to determine whether to accept requests when the To header
field is not the identity of the UAS. However, it is RECOMMENDED that
a UAS accept requests even if they do not recognize the URI scheme
(for example, a tel: URI) in the To header field, or if the To header
field does not address a known or current user of this UAS. If, on
the other hand, the UAS decides to reject the request, it SHOULD
generate a response with a 403 (Forbidden) status code and pass it to
the server transaction for transmission.
However, the Request-URI identifies the UAS that is to process the
request. If the Request-URI uses a scheme not supported by the UAS,
it SHOULD reject the request with a 416 (Unsupported URI Scheme)
response. If the Request-URI does not identify an address that the
UAS is willing to accept requests for, it SHOULD reject the request
with a 404 (Not Found) response. Typically, a UA that uses the
REGISTER method to bind its address-of-record to a specific contact
address will see requests whose Request-URI equals that contact
address. Other potential sources of received Request-URIs include the
Contact header fields of requests and responses sent by the UA that
establish or refresh dialogs.
8.2.2.2 Merged Requests
If the request has no tag in the To header field, the UAS core MUST
check the request against ongoing transactions. If the To tag, From
tag, Call-ID, CSeq exactly match (including tags) those associated with an ongoing
transaction, but the branch-ID in the topmost Via request does not match, match that transaction (based
on the matching rules in Section 17.2.3), the UAS core SHOULD generate a 482 (Loop Detected)
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generate a 482 (Loop Detected) response and pass it to the server
transaction.
The same request has arrived at the UAS more than once,
following different paths, most likely due to forking. The
UAS processes the first such request received and responds
with a 482 (Loop Detected) to the rest of them.
8.2.2.3 Require
Assuming the UAS decides that it is the proper element to process the
request, it examines the Require header field, if present.
The Require header field is used by a UAC to tell a UAS about SIP
extensions that the UAC expects the UAS to support in order to
process the request properly. Its format is described in Section
20.32. If a UAS does not understand an option-tag listed in a Require
header field, it MUST respond by generating a response with status
code 420 (Bad Extension). The UAS MUST add an Unsupported header
field, and list in it those options it does not understand amongst
those in the Require header field of the request.
Note that Require and Proxy-Require MUST NOT be used in a SIP CANCEL
request, or in an ACK request sent for a non-2xx response. These
header fields MUST be ignored if they are present in these requests.
An ACK request for a 2xx response MUST contain only those Require and
Proxy-Require values that were present in the initial request.
Example:
UAC->UAS: INVITE sip:watson@bell-telephone.com SIP/2.0
Require: 100rel
UAS->UAC: SIP/2.0 420 Bad Extension
Unsupported: 100rel
This behavior ensures that the client-server interaction
will proceed without delay when all options are understood
by both sides, and only slow down if options are not
understood (as in the example above). For a well-matched
client-server pair, the interaction proceeds quickly,
saving a round-trip often required by negotiation
mechanisms. In addition, it also removes ambiguity when the
client requires features that the server does not
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client requires features that the server does not
understand. Some features, such as call handling fields,
are only of interest to end systems.
8.2.3 Content Processing
Assuming the UAS understands any extensions required by the client,
the UAS examines the body of the message, and the header fields that
describe it. If there are any bodies whose type (indicated by the
Content-Type), language (indicated by the Content-Language) or
encoding (indicated by the Content-Encoding) are not understood, and
that body part is not optional (as indicated by the Content-
Disposition header field), the UAS MUST reject the request with a 415
(Unsupported Media Type) response. The response MUST contain an
Accept header field listing the types of all bodies it understands,
in the event the request contained bodies of types not supported by
the UAS. If the request contained content encodings not understood by
the UAS, the response MUST contain an Accept-Encoding header field
listing the encodings understood by the UAS. If the request contained
content with languages not understood by the UAS, the response MUST
contain an Accept-Language header field indicating the languages
understood by the UAS. Beyond these checks, body handling depends on
the method and type. For further information on the processing of
content-specific header fields, see Section 7.4 as well as Section
20.11 through 20.15.
8.2.4 Applying Extensions
A UAS that wishes to apply some extension when generating the
response MUST NOT do so unless support for that extension is
indicated in the Supported header field in the request. If the
desired extension is not supported, the server SHOULD rely only on
baseline SIP and any other extensions supported by the client. In
rare circumstances, where the server cannot process the request
without the extension, the server MAY send a 421 (Extension Required)
response. This response indicates that the proper response cannot be
generated without support of a specific extension. The needed
extension(s) MUST be included in a Require header field in the
response. This behavior is NOT RECOMMENDED, as it will generally
break interoperability.
Any extensions applied to a non-421 response MUST be listed in a
Require header field included in the response. Of course, the server
MUST NOT apply extensions not listed in the Supported header field in
the request. As a result of this, the Require header field in a
response will only ever contain option tags defined in standards-
track RFCs.
8.2.5 Processing the Request
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8.2.5 Processing the Request
Assuming all of the checks in the previous subsections are passed,
the UAS processing becomes method-specific. Section 10 covers the
REGISTER request, section 11 covers the OPTIONS request, section 13
covers the INVITE request, and section 15 covers the BYE request.
8.2.6 Generating the Response
When a UAS wishes to construct a response to a request, it follows
the general procedures detailed in the following subsections.
Additional behaviors specific to the response code in question, which
are not detailed in this section, may also be required.
Once all procedures associated with the creation of a response have
been completed, the UAS hands the response back to the server
transaction from which it received the request.
8.2.6.1 Sending a Provisional Response
One largely non-method-specific guideline for the generation of
responses is that UASs SHOULD NOT issue a provisional response for a
non-INVITE request. Rather, UASs SHOULD generate a final response to
a non-INVITE request as soon as possible.
When a 100 (Trying) response is generated, any Timestamp header field
present in the request MUST be copied into this 100 (Trying)
response. If there is a delay in generating the response, the UAS
SHOULD add a delay value into the Timestamp value in the response.
This value MUST contain the difference between time of sending of the
response and receipt of the request, measured in seconds.
8.2.6.2 Headers and Tags
The From field of the response MUST equal the From header field of
the request. The Call-ID header field of the response MUST equal the
Call-ID header field of the request. The CSeq header field of the
response MUST equal the CSeq field of the request. The Via header
field values in the response MUST equal the Via header field values
in the request and MUST maintain the same ordering.
If a request contained a To tag in the request, the To header field
in the response MUST equal that of the request. However, if the To
header field in the request did not contain a tag, the URI in the To
header field in the response MUST equal the URI in the To header
field; additionally, the UAS MUST add a tag to the To header field in
the response (with the exception of the 100 (Trying) response, in
which a tag MAY be present). This serves to identify the UAS that is
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responding, possibly resulting in a component of a dialog ID. The
same tag MUST be used for all responses to that request, both final
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and provisional (again excepting the 100 (Trying)). Procedures for
generation of tags are defined in Section 19.3.
8.2.7 Stateless UAS Behavior
A stateless UAS is a UAS that does not maintain transaction state. It
replies to requests normally, but discards any state that would
ordinarily be retained by a UAS after a response has been sent. If a
stateless UAS receives a retransmission of a request, it regenerates
the response and resends it, just as if it were replying to the first
instance of the request. Stateless UASs do not use a transaction
layer; they receive requests directly from the transport layer and
send responses directly to the transport layer.
The stateless UAS role is needed primarily to handle unauthenticated
requests for which a challenge response is issued. If unauthenticated
requests were handled statefully, then malicious floods of
unauthenticated requests could create massive amounts of transaction
state that might slow or completely halt call processing in a UAS,
effectively creating a denial of service condition; for more
information see Section 26.1.5.
The most important behaviors of a stateless UAS are the following:
o A stateless UAS MUST NOT send provisional (1xx) responses.
o A stateless UAS MUST NOT retransmit responses.
o A stateless UAS MUST ignore ACK requests.
o A stateless UAS MUST ignore CANCEL requests.
o To header tags MUST be generated for responses in a stateless
manner - in a manner that will generate the same tag for the
same request consistently. For information on tag
construction see Section 19.3.
In all other respects, a stateless UAS behaves in the same manner as
a stateful UAS. A UAS can operate in either a stateful or stateless
mode for each new request.
8.3 Redirect Servers
In some architectures it may be desirable to reduce the processing
load on proxy servers that are responsible for routing requests, and
improve signaling path robustness, by relying on redirection.
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Redirection allows servers to push routing information for a request
back in a response to the client, thereby taking themselves out of
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the loop of further messaging for this transaction while still aiding
in locating the target of the request. When the originator of the
request receives the redirection, it will send a new request based on
the URI(s) it has received. By propagating URIs from the core of the
network to its edges, redirection allows for considerable network
scalability.
A redirect server is logically constituted of a server transaction
layer and a transaction user that has access to a location service of
some kind (see Section 10 for more on registrars and location
services). This location service is effectively a database containing
mappings between a single URI and a set of one or more alternative
locations at which the target of that URI can be found.
A redirect server does not issue any SIP requests of its own. After
receiving a request other than CANCEL, the server either refuses the
request or gathers the list of alternative locations from the
location service and returns a final response of class 3xx. For
well-formed CANCEL requests, it SHOULD return a 2xx response. This
response ends the SIP transaction. The redirect server maintains
transaction state for an entire SIP transaction. It is the
responsibility of clients to detect forwarding loops between redirect
servers.
When a redirect server returns a 3xx response to a request, it
populates the list of (one or more) alternative locations into the
Contact header field. An "expires" parameter to the Contact header
field values may also be supplied to indicate the lifetime of the
Contact data.
The Contact header field contains URIs giving the new locations or
user names to try, or may simply specify additional transport
parameters. A 301 (Moved Permanently) or 302 (Moved Temporarily)
response may also give the same location and username that was
targeted by the initial request but specify additional transport
parameters such as a different server or multicast address to try, or
a change of SIP transport from UDP to TCP or vice versa.
However, redirect servers MUST NOT redirect a request to a URI equal
to the one in the Request-URI; instead, provided that the URI does
not point to itself, the redirect server SHOULD MAY proxy the request to the
destination URI. URI, or MAY reject it with a 404.
If a client is using an outbound proxy, and that proxy
actually redirects requests, a potential arises for
infinite redirection loops.
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Note that a Contact header field value MAY also refer to a different
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resource than the one originally called. For example, a SIP call
connected to PSTN gateway may need to deliver a special informational
announcement such as "The number you have dialed has been changed."
A Contact response header field can contain any suitable URI
indicating where the called party can be reached, not limited to SIP
URIs. For example, it could contain URIs for phones, fax, or irc (if
they were defined) or a mailto: (RFC 2368 [31]) [32]) URL. Section 26.4.4
discusses implications and limitations of redirecting a SIPS URI to a
non-SIPS URI.
The "expires" parameter of a Contact header field value indicates how
long the URI is valid. The value of the parameter is a number
indicating seconds. If this parameter is not provided, the value of
the Expires header field determines how long the URI is valid.
Malformed values SHOULD be treated as equivalent to 3600.
This provides a modest level of backwards compatibility
with RFC 2543, which allowed absolute times in this header
field. If an absolute time is received, it will be treated
as malformed, and then default to 3600.
Redirect servers MUST ignore features that are not understood
(including unrecognized header fields, any unknown option tags in
Require, or even method names) and proceed with the redirection of
the request in question.
9 Canceling a Request
The previous section has discussed general UA behavior for generating
requests and processing responses for requests of all methods. In
this section, we discuss a general purpose method, called CANCEL.
The CANCEL request, as the name implies, is used to cancel a previous
request sent by a client. Specifically, it asks the UAS to cease
processing the request and to generate an error response to that
request. CANCEL has no effect on a request to which a UAS has already
given a final response. Because of this, it is most useful to CANCEL
requests to which it can take a server long time to respond. For this
reason, CANCEL is best for INVITE requests, which can take a long
time to generate a response. In that usage, a UAS that receives a
CANCEL request for an INVITE, but has not yet sent a final response,
would "stop ringing", and then respond to the INVITE with a specific
error response (a 487).
CANCEL requests can be constructed and sent by both proxies and user
agent clients. Section 15 discusses under what conditions a UAC
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agent clients. Section 15 discusses under what conditions a UAC
would CANCEL an INVITE request, and Section 16.10 discusses proxy
usage of CANCEL.
A stateful proxy responds to a CANCEL, rather than simply forwarding
a response it would receive from a downstream element. For that
reason, CANCEL is referred to as a "hop-by-hop" request, since it is
responded to at each stateful proxy hop.
9.1 Client Behavior
A CANCEL request SHOULD NOT be sent to cancel a request other than
INVITE.
Since requests other than INVITE are responded to
immediately, sending a CANCEL for a non-INVITE request
would always create a race condition.
The following procedures are used to construct a CANCEL request. The
Request-URI, Call-ID, To, the numeric part of CSeq, and From header
fields in the CANCEL request MUST be identical to those in the
request being cancelled, including tags. A CANCEL constructed by a
client MUST have only a single Via header field value matching the
top Via value in the request being cancelled. Using the same values
for these header fields allows the CANCEL to be matched with the
request it cancels (Section 9.2 indicates how such matching occurs).
However, the method part of the CSeq header field MUST have a value
of CANCEL. This allows it to be identified and processed as a
transaction in its own right (See Section 17).
If the request being cancelled contains a Route header field, the
CANCEL request MUST include that Route header field's values.
This is needed so that stateless proxies are able to route
CANCEL requests properly.
The CANCEL request MUST NOT contain any Require or Proxy-Require
header fields.
Once the CANCEL is constructed, the client SHOULD check whether it
has received any response (provisional or final) for the request
being cancelled (herein referred to as the "original request").
If no provisional response has been received, the CANCEL request MUST
NOT be sent; rather, the client MUST wait for the arrival of a
provisional response before sending the request. If the original
request has generated a final response, the CANCEL SHOULD NOT be
sent, as it is an effective no-op, since CANCEL has no effect on
requests that have already generated a final response. When the
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requests that have already generated a final response. When the
client decides to send the CANCEL, it creates a client transaction
for the CANCEL and passes it the CANCEL request along with the
destination address, port, and transport. The destination address,
port, and transport for the CANCEL MUST be identical to those used to
send the original request.
If it was allowed to send the CANCEL before receiving a
response for the previous request, the server could receive
the CANCEL before the original request.
Note that both the transaction corresponding to the original request
and the CANCEL transaction will complete independently. However, a
UAC canceling a request cannot rely on receiving a 487 (Request
Terminated) response for the original request, as an RFC 2543-
compliant UAS will not generate such a response. If there is no final
response for the original request in 64*T1 seconds (T1 is defined in
Section 17.1.1.1), the client SHOULD then consider the original
transaction cancelled and SHOULD destroy the client transaction
handling the original request.
9.2 Server Behavior
The CANCEL method requests that the TU at the server side cancel a
pending transaction. The TU determines the transaction to be
cancelled by taking the CANCEL request, and then assuming that the
request method is anything but CANCEL and applying the transaction
matching procedures of Section 17.2.3. The matching transaction is
the one to be cancelled.
The processing of a CANCEL request at a server depends on the type of
server. A stateless proxy will forward it, a stateful proxy might
respond to it and generate some CANCEL requests of its own, and a UAS
will respond to it. See Section 16.10 for proxy treatment of CANCEL.
A UAS first processes the CANCEL request according to the general UAS
processing described in Section 8.2. However, since CANCEL requests
are hop-by-hop and cannot be resubmitted, they cannot be challenged
by the server in order to get proper credentials in an Authorization
header field. Note also that CANCEL requests do not contain a Require
header field.
If the UAS did not find a matching transaction for the CANCEL
according to the procedure above, it SHOULD respond to the CANCEL
with a 481 (Call Leg/Transaction Does Not Exist). If the transaction
for the original request still exists, the behavior of the UAS on
receiving a CANCEL request depends on whether it has already sent a
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final response for the original request. If it has, the CANCEL
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request has no effect on the processing of the original request, no
effect on any session state, and no effect on the responses generated
for the original request. If the UAS has not issued a final response
for the original request, its behavior depends on the method of the
original request. If the original request was an INVITE, the UAS
SHOULD immediately respond to the INVITE with a 487 (Request
Terminated). The behavior upon reception of a CANCEL request for any
other method defined in this specification is effectively no-op.
Regardless of the method of the original request, as long as the
CANCEL matched an existing transaction, the UAS answers the CANCEL
request itself with a 200 (OK) response. This response is
constructed following the procedures described in Section 8.2.6
noting that the To tag of the response to the CANCEL and the To tag
in the response to the original request SHOULD be the same. The
response to CANCEL is passed to the server transaction for
transmission.
10 Registrations
10.1 Overview
SIP offers a discovery capability. If a user wants to initiate a
session with another user, SIP must discover the current host(s) at
which the destination user is reachable. This discovery process is
frequently accomplished by SIP network elements such as proxy servers
and redirect servers which are responsible for receiving a request,
determining where to send it based on knowledge of the location of
the user, and then sending it there. To do this, SIP network
elements consult an abstract service known as a location service ,
which provides address bindings for a particular domain. These
address bindings map an incoming SIP or SIPS URI, sip:bob@biloxi.com
, for example, to one or more URIs that are somehow "closer" to the
desired user, sip:bob@engineering.biloxi.com , for example.
Ultimately, a proxy will consult a location service that maps a
received URI to the user agent(s) at which the desired recipient is
currently residing.
Registration creates bindings in a location service for a particular
domain that associate an address-of-record URI with one or more
contact addresses. Thus, when a proxy for that domain receives a
request whose Request-URI matches the address-of-record, the proxy
will forward the request to the contact addresses registered to that
address-of-record. Generally, it only makes sense to register an
address-of-record at a domain's location service when requests for
that address-of-record would be routed to that domain. In most cases,
this means that the domain of the registration will need to match the
domain in the URI of the address-of-record.
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domain in the URI of the address-of-record.
There are many ways by which the contents of the location service can
be established. One way is administratively. In the above example,
Bob is known to be a member of the engineering department through
access to a corporate database. However, SIP provides a mechanism for
a UA to create a binding explicitly. This mechanism is known as
registration.
Registration entails sending a REGISTER request to a special type of
UAS known as a registrar. A registrar acts as the front end to the
location service for a domain, reading and writing mappings based on
the contents of REGISTER requests. This location service is then
typically consulted by a proxy server that is responsible for routing
requests for that domain.
An illustration of the overall registration process is given in 2.
Note that the registrar and proxy server are logical roles that can
be played by a single device in a network; for purposes of clarity
the two are separated in this illustration. Also note that UAs may
send requests through a proxy server in order to reach a registrar if
the two are separate elements.
SIP does not mandate a particular mechanism for implementing the
location service. The only requirement is that a registrar for some
domain MUST be able to read and write data to the location service,
and a proxy or redirect server for that domain MUST be capable of
reading that same data. A registrar MAY be co-located with a
particular SIP proxy server for the same domain.
10.2 Constructing the REGISTER Request
REGISTER requests add, remove, and query bindings. A REGISTER request
can add a new binding between an address-of-record and one or more
contact addresses. Registration on behalf of a particular address-
of-record can be performed by a suitably authorized third party. A
client can also remove previous bindings or query to determine which
bindings are currently in place for an address-of-record.
Except as noted, the construction of the REGISTER request and the
behavior of clients sending a REGISTER request is identical to the
general UAC behavior described in Section 8.1 and Section 17.1.
A REGISTER request does not establish a dialog. A UAC MAY include a
Route header field in a REGISTER request based on a pre-existing
route set as described in Section 8.1. The Record-Route header field
has no meaning in REGISTER requests or responses, and MUST be ignored
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if present. In particular, the UAC MUST NOT create a new route set
based on the presence or absence of a Record-Route header field in
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any response to a REGISTER request.
The following header fields, except Contact, MUST be included in a
REGISTER request. A Contact header field MAY be included:
Request-URI: The Request-URI names the domain of the location
service for which the registration is meant (for example,
"sip:chicago.com"). The "userinfo" and "@" components of
the SIP URI MUST NOT be present.
To: The To header field contains the address of record whose
registration is to be created, queried, or modified. The To
header field and the Request-URI field typically differ, as
the former contains a user name. This address-of-record
MUST be a SIP URI or SIPS URI.
From: The From header field contains the address-of-record of
the person responsible for the registration. The value is
the same as the To header field unless the request is a
third-party registration.
Call-ID: All registrations from a UAC SHOULD use the same Call-
ID header field value for registrations sent to a
particular registrar.
If the same client were to use different Call-ID
values, a registrar could not detect whether a delayed
REGISTER request might have arrived out of order.
CSeq: The CSeq value guarantees proper ordering of REGISTER
requests. A UA MUST increment the CSeq value by one for
each REGISTER request with the same Call-ID.
Contact: REGISTER requests MAY contain a Contact header field
with zero or more values containing address bindings.
UAs MUST NOT send a new registration (that is, containing new Contact
header field values, as opposed to a retransmission) until they have
received a final response from the registrar for the previous one or
the previous REGISTER request has timed out.
The following Contact header parameters have a special meaning in
REGISTER requests:
action: The "action" parameter from RFC 2543 has been
deprecated. UACs SHOULD NOT use the "action" parameter.
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bob
+----+
| UA |
| |
+----+
|
|3)INVITE
| carol@chicago.com
chicago.com +--------+ V
+---------+ 2)Store|Location|4)Query +-----+
|Registrar|=======>| Service|<=======|Proxy|sip.chicago.com
+---------+ +--------+=======>+-----+
A 5)Resp |
| |
| |
1)REGISTER| |
| |
+----+ |
| UA |<-------------------------------+
cube2214a| | 6)INVITE
+----+ carol@cube2214a.chicago.com
carol
Figure 2: REGISTER example
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deprecated. UACs SHOULD NOT use the "action" parameter.
expires: The "expires" parameter indicates how long the UA would
like the binding to be valid. The value is a number
indicating seconds. If this parameter is not provided, the
value of the Expires header field is used instead.
Implementations MAY treat values larger than 2**32-1
(4294967295 seconds or 136 years) as equivalent to 2**32-1.
Malformed values SHOULD be treated as equivalent to 3600.
10.2.1 Adding Bindings
The REGISTER request sent to a registrar includes the contact
address(es) to which SIP requests for the address-of-record should be
forwarded. The address-of-record is included in the To header field
of the REGISTER request.
The Contact header field values of the request typically consist of
SIP or SIPS URIs that identify particular SIP endpoints (for example,
"sip:carol@cube2214a.chicago.com"), but they MAY use any URI scheme.
A SIP UA can choose to register telephone numbers (with the tel URL,
RFC 2806 [9]) or email addresses (with a mailto URL, RFC 2368[31]) 2368[32]) as
Contacts for an address-of-record, for example.
For example, Carol, with address-of-record "sip:carol@chicago.com",
would register with the SIP registrar of the domain chicago.com. Her
registrations would then be used by a proxy server in the chicago.com
domain to route requests for Carol's address-of-record to her SIP
endpoint.
Once a client has established bindings at a registrar, it MAY send
subsequent registrations containing new bindings or modifications to
existing bindings as necessary. The 2xx response to the REGISTER
request will contain, in a Contact header field, a complete list of
bindings that have been registered for this address-of-record at this
registrar.
If the address-of-record in the To header field of a REGISTER request
is a SIPS URI, then any Contact header field values in the request
SHOULD also be SIPS URIs. Clients should only register non-SIPS URIs
under a SIPS address-of-record when the security of the resource
represented by the contact address is guaranteed by other means.
This may be applicable to URIs that invoke protocols other than SIP,
or SIP devices secured by protocols other than TLS.
Registrations do not need to update all bindings. Typically, a UA
only updates its own contact addresses.
10.2.1.1 Setting the Expiration Interval of Contact Addresses
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10.2.1.1 Setting the Expiration Interval of Contact Addresses
When a client sends a REGISTER request, it MAY suggest an expiration
interval that indicates how long the client would like the
registration to be valid. (As described in Section 10.3, the
registrar selects the actual time interval based on its local
policy.)
There are two ways in which a client can suggest an expiration
interval for a binding: through an Expires header field or an
"expires" Contact header parameter. The latter allows expiration
intervals to be suggested on a per-binding basis when more than one
binding is given in a single REGISTER request, whereas the former
suggests an expiration interval for all Contact header field values
that do not contain the "expires" parameter.
If neither mechanism for expressing a suggested expiration time is
present in a REGISTER, a default suggestion of one hour SHOULD be
assumed.
10.2.1.2 Preferences among Contact Addresses
If more than one Contact is sent in a REGISTER request, the
registering UA intends to associate all of the URIs in these Contact
header field values with the address-of-record present in the To
field. This list can be prioritized with the "q" parameter in the
Contact header field. The "q" parameter indicates a relative
preference for the particular Contact header field value compared to
other bindings present in this REGISTER message or existing within
the location service of the registrar. Section 16.6 describes how a
proxy server uses this preference indication.
10.2.2 Removing Bindings
Registrations are soft state and expire unless refreshed, but can
also be explicitly removed. A client can attempt to influence the
expiration interval selected by the registrar as described in Section
10.2.1. A UA requests the immediate removal of a binding by
specifying an expiration interval of "0" for that contact address in
a REGISTER request. UAs SHOULD support this mechanism so that
bindings can be removed before their expiration interval has passed.
The REGISTER-specific Contact header field value of "*" applies to
all registrations, but it MUST NOT be used unless the Expires header
field is present with a value of "0".
Use of the "*" Contact header field value allows a
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registering UA to remove all of its bindings associated with an
address-of-record without knowing their precise values.
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10.2.3 Fetching Bindings
A success response to any REGISTER request contains the complete list
of existing bindings, regardless of whether the request contained a
Contact header field. If no Contact header field is present in a
REGISTER request, the list of bindings is left unchanged.
10.2.4 Refreshing Bindings
Each UA is responsible for refreshing the bindings that it has
previously established. A UA SHOULD NOT refresh bindings set up by
other UAs.
The 200 (OK) response from the registrar contains a list of Contact
fields enumerating all current bindings. The UA compares each contact
address to see if it created the contact address, using comparison
rules in Section 19.1.4. If so, it updates the expiration time
interval according to the expires parameter or, if absent, the
Expires field value. The UA then issues a REGISTER request for each
of its bindings before the expiration interval has elapsed. It MAY
combine several updates into one REGISTER request.
A UA SHOULD use the same Call-ID for all registrations during a
single boot cycle. Registration refreshes SHOULD be sent to the same
network address as the original registration, unless redirected.
10.2.5 Setting the Internal Clock
If the response for a REGISTER request contains a Date header field,
the client MAY use this header field to learn the current time in
order to set any internal clocks.
10.2.6 Discovering a Registrar
UAs can use three ways to determine the address to which to send
registrations: by configuration, using the address-of-record, and
multicast. A UA can be configured, in ways beyond the scope of this
specification, with a registrar address. If there is no configured
registrar address, the UA SHOULD use the host part of the address-
of-record as the Request-URI and address the request there, using the
normal SIP server location mechanisms [4]. For example, the UA for
the user "sip:carol@chicago.com" addresses the REGISTER request to
"sip:chicago.com".
Finally, a UA can be configured to use multicast. Multicast
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registrations are addressed to the well-known "all SIP servers"
multicast address "sip.mcast.net" (224.0.1.75 for IPv4). No well-
known IPv6 multicast address has been allocated; such an allocation
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will be documented separately when needed. SIP UAs MAY listen to that
address and use it to become aware of the location of other local
users (see [32]); [33]); however, they do not respond to the request.
Multicast registration may be inappropriate in some
environments, for example, if multiple businesses share the
same local area network.
10.2.7 Transmitting a Request
Once the REGISTER method has been constructed, and the destination of
the message identified, UACs follow the procedures described in
Section 8.1.2 to hand off the REGISTER to the transaction layer.
If the transaction layer returns a timeout error because the REGISTER
yielded no response, the UAC SHOULD NOT immediately re-attempt a
registration to the same registrar.
An immediate re-attempt is likely to also timeout. Waiting
some reasonable time interval for the conditions causing
the timeout to be corrected reduces unnecessary load on the
network. No specific interval is mandated.
10.2.8 Error Responses
If a UA receives a 423 (Interval Too Brief) response, it MAY retry
the registration after making the expiration interval of all contact
addresses in the REGISTER request equal to or greater than the
expiration interval within the Min-Expires header field of the 423
(Interval Too Brief) response.
10.3 Processing REGISTER Requests
A registrar is a UAS that responds to REGISTER requests and maintains
a list of bindings that are accessible to proxy servers and redirect
servers within its administrative domain. A registrar handles
requests according to Section 8.2 and Section 17.2, but it accepts
only REGISTER requests. A registrar MUST not generate 6xx responses.
A registrar MAY redirect REGISTER requests as appropriate. One common
usage would be for a registrar listening on a multicast interface to
redirect multicast REGISTER requests to its own unicast interface
with a 302 (Moved Temporarily) response.
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Registrars MUST ignore the Record-Route header field if it is
included in a REGISTER request. Registrars MUST NOT include a
Record-Route header field in any response to a REGISTER request.
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A registrar might receive a request that traversed a proxy
which treats REGISTER as an unknown request and which added
a Record-Route header field value.
A registrar has to know (for example, through configuration) the set
of domain(s) for which it maintains bindings. REGISTER requests MUST
be processed by a registrar in the order that they are received.
REGISTER requests MUST also be processed atomically, meaning that a
particular REGISTER request is either processed completely or not at
all. Each REGISTER message MUST be processed independently of any
other registration or binding changes.
When receiving a REGISTER request, a registrar follows these steps:
1. The registrar inspects the Request-URI to determine whether
it has access to bindings for the domain identified in the
Request-URI. If not, and if the server also acts as a proxy
server, the server SHOULD forward the request to the
addressed domain, following the general behavior for
proxying messages described in Section 16.
2. To guarantee that the registrar supports any necessary
extensions, the registrar MUST process the Require header
field values as described for UASs in Section 8.2.2.
3. A registrar SHOULD authenticate the UAC. Mechanisms for the
authentication of SIP user agents are described in Section
22. Registration behavior in no way overrides the generic
authentication framework for SIP. If no authentication
mechanism is available, the registrar MAY take the From
address as the asserted identity of the originator of the
request.
4. The registrar SHOULD determine if the authenticated user is
authorized to modify registrations for this address-of-
record. For example, a registrar might consult a
authorization database that maps user names to a list of
addresses-of-record for which that user has authorization
to modify bindings. If the authenticated user is not
authorized to modify bindings, the registrar MUST return a
403 (Forbidden) and skip the remaining steps.
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In architectures that support third-party
registration, one entity may be responsible for
updating the registrations associated with multiple
addresses-of-record.
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5. The registrar extracts the address-of-record from the To
header field of the request. If the address-of-record is
not valid for the domain in the Request-URI, the registrar
MUST send a 404 (Not Found) response and skip the remaining
steps. The URI MUST then be converted to a canonical form.
To do that, all URI parameters MUST be removed (including
the user-param), and any escaped characters MUST be
converted to their unescaped form. The result serves as an
index into the list of bindings.
6. The registrar checks whether the request contains the
Contact header field. If not, it skips to the last step.
If the Contact header field is present, the registrar
checks if there is one Contact field value that contains
the special value "*" and an Expires field. If the request
has additional Contact fields or an expiration time other
than zero, the request is invalid, and the server MUST
return a 400 Invalid Request and skip the remaining steps.
If not, the registrar checks whether the Call-ID agrees
with the value stored for each binding. If not, it MUST
remove the binding. If it does agree, it MUST remove the
binding only if the CSeq in the request is higher than the
value stored for that binding. Otherwise the registrar MUST
leave the binding as is. It then skips to the last step.
7. The registrar now processes each contact address in the
Contact header field in turn. For each address, it
determines the expiration interval as follows:
- If the field value has an "expires" parameter, that value
MUST be used.
- If there is no such parameter, but the request has an
Expires header field, that value MUST be used.
- If there is neither, a locally-configured default value
MUST be used.
The registrar MAY shorten the expiration interval. If and
only if the expiration interval is greater than zero AND
smaller than one hour AND less than a registrar-configured
minimum, the registrar MAY reject the registration with a
response of 423 (Registration Too Brief). This response
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MUST contain a Min-Expires header field that states the
minimum expiration interval the registrar is willing to
honor. It then skips the remaining steps.
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Allowing the registrar to set the registration
interval protects it against excessively frequent
registration refreshes while limiting the state that
it needs to maintain and decreasing the likelihood of
registrations going stale. The expiration interval of
a registration is frequently used in the creation of
services. An example is a follow-me service, where the
user may only be available at a terminal for a brief
period. Therefore, registrars should accept brief
registrations; a request should only be rejected if
the interval is so short that the refreshes would
degrade registrar performance.
For each address, the registrar then searches the list of
current bindings using the URI comparison rules. If the
binding does not exist, it is tentatively added. If the
binding does exist, the registrar checks the Call-ID value.
If the Call-ID value in the existing binding differs from
the Call-ID value in the request, the binding MUST be
removed if the expiration time is zero and updated
otherwise. If they are the same, the registrar compares
the CSeq value. If the value is higher than that of the
existing binding, it MUST update or remove the binding as
above. If not, the update MUST be aborted and the request
fails.
This algorithm ensures that out-of-order requests from
the same UA are ignored.
Each binding record records the Call-ID and CSeq values
from the request.
The binding updates MUST be committed (that is, made
visible to the proxy or redirect server) if and only if all
binding updates and additions succeed. If any one of them
fails (for example, because the back-end database commit
failed), the request MUST fail with a 500 (Server Error)
response and all tentative binding updates MUST be removed.
8. The registrar returns a 200 (OK) response. The response
MUST contain Contact header field values enumerating all
current bindings. Each Contact value MUST feature an
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"expires" parameter indicating its expiration interval
chosen by the registrar. The response SHOULD include a
Date header field.
11 Querying for Capabilities
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The SIP method OPTIONS allows a UA to query another UA or a proxy
server as to its capabilities. This allows a client to discover
information about the supported methods, content types, extensions,
codecs, etc. without "ringing" the other party. For example, before a
client inserts a Require header field into an INVITE listing an
option that it is not certain the destination UAS supports, the
client can query the destination UAS with an OPTIONS to see if this
option is returned in a Supported header field. All UAs MUST support
the OPTIONS method.
The target of the OPTIONS request is identified by the Request-URI,
which could identify another UA or a SIP server. If the OPTIONS is
addressed to a proxy server, the Request-URI is set without a user
part, similar to the way a Request-URI is set for a REGISTER request.
Alternatively, a server receiving an OPTIONS request with a Max-
Forwards header field value of 0 MAY respond to the request
regardless of the Request-URI.
This behavior is common with HTTP/1.1. This behavior can be
used as a "traceroute" functionality to check the
capabilities of individual hop servers by sending a series
of OPTIONS requests with incremented Max-Forwards values.
As is the case for general UA behavior, the transaction layer can
return a timeout error if the OPTIONS yields no response. This may
indicate that the target is unreachable and hence unavailable.
An OPTIONS request MAY be sent as part of an established dialog to
query the peer on capabilities that may be utilized later in the
dialog.
11.1 Construction of OPTIONS Request
An OPTIONS request is constructed using the standard rules for a SIP
request as discussed Section 8.1.1.
A Contact header field MAY be present in an OPTIONS.
An Accept header field SHOULD be included to indicate the type of
message body the UAC wishes to receive in the response. Typically,
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this is set to a format that is used to describe the media
capabilities of a UA, such as SDP (application/sdp).
The response to an OPTIONS request is assumed to be scoped to the
Request-URI in the original request. However, only when an OPTIONS is
sent as part of an established dialog is it guaranteed that future
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requests will be received by the server that generated the OPTIONS
response.
Example OPTIONS request:
OPTIONS sip:carol@chicago.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKhjhs8ass877
Max-Forwards: 70
To: <sip:carol@chicago.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 63104 OPTIONS
Contact: <sip:alice@pc33.atlanta.com>
Accept: application/sdp
Content-Length: 0
11.2 Processing of OPTIONS Request
The response to an OPTIONS is constructed using the standard rules
for a SIP response as discussed in Section 8.2.6. The response code
chosen MUST be the same that would have been chosen had the request
been an INVITE. That is, a 200 (OK) would be returned if the UAS is
ready to accept a call, a 486 (Busy Here) would be returned if the
UAS is busy, etc. This allows an OPTIONS request to be used to
determine the basic state of a UAS, which can be an indication of
whether the UAC UAS will accept an INVITE request.
An OPTIONS request received within a dialog generates a 200 (OK)
response that is identical to one constructed outside a dialog and
does not have any impact on the dialog.
This use of OPTIONS has limitations due the differences in proxy
handling of OPTIONS and INVITE requests. While a forked INVITE can
result in multiple 200 (OK) responses being returned, a forked
OPTIONS will only result in a single 200 (OK) response, since it is
treated by proxies using the non-INVITE handling. See Section 16.7
for the normative details.
If the response to an OPTIONS is generated by a proxy server, the
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proxy returns a 200 (OK) listing the capabilities of the server. The
response does not contain a message body.
Allow, Accept, Accept-Encoding, Accept-Language, and Supported header
fields SHOULD be present in a 200 (OK) response to an OPTIONS
request. If the response is generated by a proxy, the Allow header
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field SHOULD be omitted as it is ambiguous since a proxy is method
agnostic. Contact header fields MAY be present in a 200 (OK) response
and have the same semantics as in a 3xx response. That is, they may
list a set of alternative names and methods of reaching the user. A
Warning header field MAY be present.
A message body MAY be sent, the type of which is determined by the
Accept header field in the OPTIONS request (application/sdp is the
default if the Accept header field is not present). If the types
include one that can describe media capabilities, the UAS SHOULD
include a body in the response for that purpose. Details on
construction of such a body in the case of application/sdp are
described in [13].
Example OPTIONS response generated by a UAS (corresponding to the
request in Section 11.1):
SIP/2.0 200 OK
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKhjhs8ass877
;received=192.0.2.4
To: <sip:carol@chicago.com>;tag=93810874
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 63104 OPTIONS
Contact: <sip:carol@chicago.com>
Contact: <mailto:carol@chicago.com>
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE
Accept: application/sdp
Accept-Encoding: gzip
Accept-Language: en
Supported: foo
Content-Type: application/sdp
Content-Length: 274
(SDP not shown)
12 Dialogs
A key concept for a user agent is that of a dialog. A dialog
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represents a peer-to-peer SIP relationship between two user agents
that persists for some time. The dialog facilitates sequencing of
messages between the user agents and proper routing of requests
between both of them. The dialog represents a context in which to
interpret SIP messages. Section 8 discussed method independent UA
processing for requests and responses outside of a dialog. This
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section discusses how those requests and responses are used to
construct a dialog, and then how subsequent requests and responses
are sent within a dialog.
A dialog is identified at each UA with a dialog ID, which consists of
a Call-ID value, a local tag and a remote tag. The dialog ID at each
UA involved in the dialog is not the same. Specifically, the local
tag at one UA is identical to the remote tag at the peer UA. The tags
are opaque tokens that facilitate the generation of unique dialog
IDs.
A dialog ID is also associated with all responses and with any
request that contains a tag in the To field. The rules for computing
the dialog ID of a message depend on whether the SIP element is a UAC
or UAS. For a UAC, the Call-ID value of the dialog ID is set to the
Call-ID of the message, the remote tag is set to the tag in the To
field of the message, and the local tag is set to the tag in the From
field of the message (these rules apply to both requests and
responses). As one would expect, for a UAS, the Call-ID value of the
dialog ID is set to the Call-ID of the message, the remote tag is set
to the tag in the From field of the message, and the local tag is set
to the tag in the To field of the message.
A dialog contains certain pieces of state needed for further message
transmissions within the dialog. This state consists of the dialog
ID, a local sequence number (used to order requests from the UA to
its peer), a remote sequence number (used to order requests from its
peer to the UA), a local URI, a remote URI, the Contact URI of the
peer, a boolean flag called "secure", and a route set, which is an
ordered list of URIs. The route set is the list of servers that need
to be traversed to send a request to the peer. A dialog can also be
in the "early" state, which occurs when it is created with a
provisional response, and then transition to the "confirmed" state
when a 2xx final response arrives. For other responses, or if no
response arrives at all on that dialog, the early dialog terminates.
12.1 Creation of a Dialog
Dialogs are created through the generation of non-failure responses
to requests with specific methods. Within this specification, only
2xx and 101-199 responses with a To tag to INVITE establish a dialog.
A dialog established by a non-final response to a request is in the
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"early" state and it is called an early dialog. Extensions MAY define
other means for creating dialogs. Section 13 gives more details that
are specific to the INVITE method. Here, we describe the process for
creation of dialog state that is not dependent on the method.
UAs MUST assign values to the dialog ID components as described
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below.
12.1.1 UAS behavior
When a UAS responds to a request with a response that establishes a
dialog (such as a 2xx to INVITE), the UAS MUST copy all Record-Route
header field values from the request into the response (including the
URIs, URI parameters, and any Record-Route header field parameters,
whether they are known or unknown to the UAS) and MUST maintain the
order of those values. The UAS MUST add a Contact header field to the
response. The Contact header field contains an address where the UAS
would like to be contacted for subsequent requests in the dialog
(which includes the ACK for a 2xx response in the case of an INVITE).
Generally, the host portion of this URI is the IP address or FQDN of
the host. The URI provided in the Contact header field MUST be a SIP
or SIPS URI. If the request that initiated the dialog contained a
SIPS URI in the Request-URI or in the top Record-Route header field
value, if there was any, or the Contact header field if there was no
Record-Route header field, the Contact header field in the response
MUST be a SIPS URI. The URI SHOULD have global scope (that is, the
same URI can be used in messages outside this dialog). The same way,
the scope of the URI in the Contact header field of the INVITE is not
limited to this dialog either. It can therefore be used in messages
to the UAC even outside this dialog.
The UAS then constructs the state of the dialog. This state MUST be
maintained for the duration of the dialog.
If the request arrived over TLS, and the Request-URI contained a SIPS
URI, the "secure" flag is set to TRUE.
The route set MUST be set to the list of URIs in the Record-Route
header field from the request, taken in order and preserving all URI
parameters. If no Record-Route header field is present in the
request, the route set MUST be set to the empty set. This route set,
even if empty, overrides any pre-existing route set for future
requests in this dialog. The remote target MUST be set to the URI
from the Contact header field of the request.
The remote sequence number MUST be set to the value of the sequence
number in the CSeq header field of the request. The local sequence
number MUST be empty. The call identifier component of the dialog ID
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MUST be set to the value of the Call-ID in the request. The local tag
component of the dialog ID MUST be set to the tag in the To field in
the response to the request (which always includes a tag), and the
remote tag component of the dialog ID MUST be set to the tag from the
From field in the request. A UAS MUST be prepared to receive a
request without a tag in the From field, in which case the tag is
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considered to have a value of null.
This is to maintain backwards compatibility with RFC 2543,
which did not mandate From tags.
The remote URI MUST be set to the URI in the From field, and the
local URI MUST be set to the URI in the To field.
12.1.2 UAC Behavior
When a UAC sends a request that can establish a dialog (such as an
INVITE) it MUST provide a SIP or SIPS URI with global scope (i.e.,
the same SIP URI can be used in messages outside this dialog) in the
Contact header field of the request. If the request has a Request-URI
or a topmost Route header field value with a SIPS URI, the Contact
header field MUST contain a SIPS URI.
When a UAC receives a response that establishes a dialog, it
constructs the state of the dialog. This state MUST be maintained for
the duration of the dialog.
If the request was sent over TLS, and the Request-URI contained a
SIPS URI, the "secure" flag is set to TRUE.
The route set MUST be set to the list of URIs in the Record-Route
header field from the response, taken in reverse order and preserving
all URI parameters. If no Record-Route header field is present in the
response, the route set MUST be set to the empty set. This route set,
even if empty, overrides any pre-existing route set for future
requests in this dialog. The remote target MUST be set to the URI
from the Contact header field of the response.
The local sequence number MUST be set to the value of the sequence
number in the CSeq header field of the request. The remote sequence
number MUST be empty (it is established when the remote UA sends a
request within the dialog). The call identifier component of the
dialog ID MUST be set to the value of the Call-ID in the request. The
local tag component of the dialog ID MUST be set to the tag in the
From field in the request, and the remote tag component of the dialog
ID MUST be set to the tag in the To field of the response. A UAC
MUST be prepared to receive a response without a tag in the To field,
in which case the tag is considered to have a value of null.
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This is to maintain backwards compatibility with RFC 2543,
which did not mandate To tags.
The remote URI MUST be set to the URI in the To field, and the local
URI MUST be set to the URI in the From field.
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12.2 Requests within a Dialog
Once a dialog has been established between two UAs, either of them
MAY initiate new transactions as needed within the dialog. The UA
sending the request will take the UAC role for the transaction. The
UA receiving the request will take the UAS role. Note that these may
be different roles than the UAs held during the transaction that
established the dialog.
Requests within a dialog MAY contain Record-Route and Contact header
fields. However, these requests do not cause the dialog's route set
to be modified, although they may modify the remote target URI.
Specifically, requests that are not target refresh requests do not
modify the dialog's remote target URI, and requests that are target
refresh requests do. For dialogs that have been established with an
INVITE, the only target refresh request defined is re-INVITE (see
Section 14). Other extensions may define different target refresh
requests for dialogs established in other ways.
Note that an ACK is NOT a target refresh request.
Target refresh requests only update the dialog's remote target URI,
and not the route set formed from Record-Route. Updating the latter
would introduce severe backwards compatibility problems with RFC
2543-compliant systems.
12.2.1 UAC Behavior
12.2.1.1 Generating the Request
A request within a dialog is constructed by using many of the
components of the state stored as part of the dialog.
The URI in the To field of the request MUST be set to the remote URI
from the dialog state. The tag in the To header field of the request
MUST be set to the remote tag of the dialog ID. The From URI of the
request MUST be set to the local URI from the dialog state. The tag
in the From header field of the request MUST be set to the local tag
of the dialog ID. If the value of the remote or local tags is null,
the tag parameter MUST be omitted from the To or From header fields,
respectively.
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Usage of the URI from the To and From fields in the
original request within subsequent requests is done for
backwards compatibility with RFC 2543, which used the URI
for dialog identification. In this specification, only the
tags are used for dialog identification. It is expected
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that mandatory reflection of the original To and From URI
in mid-dialog requests will be deprecated in a subsequent
revision of this specification.
The Call-ID of the request MUST be set to the Call-ID of the dialog.
Requests within a dialog MUST contain strictly monotonically
increasing and contiguous CSeq sequence numbers (increasing-by-one)
in each direction (excepting ACK and CANCEL of course, whose numbers
equal the requests being acknowledged or cancelled). Therefore, if
the local sequence number is not empty, the value of the local
sequence number MUST be incremented by one, and this value MUST be
placed into the CSeq header field. If the local sequence number is
empty, an initial value MUST be chosen using the guidelines of
Section 8.1.1.5. The method field in the CSeq header field value MUST
match the method of the request.
With a length of 32 bits, a client could generate, within a
single call, one request a second for about 136 years
before needing to wrap around. The initial value of the
sequence number is chosen so that subsequent requests
within the same call will not wrap around. A non-zero
initial value allows clients to use a time-based initial
sequence number. A client could, for example, choose the 31
most significant bits of a 32-bit second clock as an
initial sequence number.
The UAC uses the remote target and route set to build the Request-URI
and Route header field of the request.
If the route set is empty, the UAC MUST place the remote target URI
into the Request-URI. The UAC MUST NOT add a Route header field to
the request.
If the route set is not empty, and the first URI in the route set
contains the lr parameter (see Section 19.1.1), the UAC MUST place
the remote target URI into the Request-URI and MUST include a Route
header field containing the route set values in order, including all
parameters.
If the route set is not empty, and its first URI does not contain the
lr parameter, the UAC MUST place the first URI from the route set
into the Request-URI, stripping any parameters that are not allowed
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in a Request-URI. The UAC MUST add a Route header field containing
the remainder of the route set values in order, including all
parameters. The UAC MUST then place the remote target URI into the
Route header field as the last value.
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For example, if the remote target is sip:user@remoteua and the route
set contains
<sip:proxy1>,<sip:proxy2>,<sip:proxy3;lr>,<sip:proxy4>
The request will be formed with the following Request-URI and Route
header field:
METHOD sip:proxy1
Route: <sip:proxy2>,<sip:proxy3;lr>,<sip:proxy4>,<sip:user@remoteua>
If the first URI of the route set does not contain the lr
parameter, the proxy indicated does not understand the
routing mechanisms described in this document and will act
as specified in RFC 2543, replacing the Request-URI with
the first Route header field value it receives while
forwarding the message. Placing the Request-URI at the end
of the Route header field preserves the information in that
Request-URI across the strict router (it will be returned
to the Request-URI when the request reaches a loose-
router).
A UAC SHOULD include a Contact header field in any target refresh
requests within a dialog, and unless there is a need to change it,
the URI SHOULD be the same as used in previous requests within the
dialog. If the "secure" flag is true, that URI MUST be a SIPS URI. As
discussed in Section 12.2.2, a Contact header field in a target
refresh request updates the remote target URI. This allows a UA to
provide a new contact address, should its address change during the
duration of the dialog.
However, requests that are not target refresh requests do not affect
the remote target URI for the dialog.
The rest of the request is formed as described in Section 8.1.1.
Once the request has been constructed, the address of the server is
computed and the request is sent, using the same procedures for
requests outside of a dialog (Section 8.1.2).
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The procedures in Section 8.1.2 will normally result in the
request being sent to the address indicated by the topmost
Route header field value or the Request-URI if no Route
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header field is present. Subject to certain restrictions,
they allow the request to be sent to an alternate address
(such as a default outbound proxy not represented in the
route set).
12.2.1.2 Processing the Responses
The UAC will receive responses to the request from the transaction
layer. If the client transaction returns a timeout this is treated as
a 408 (Request Timeout) response.
The behavior of a UAC that receives a 3xx response for a request sent
within a dialog is the same as if the request had been sent outside a
dialog. This behavior is described in Section 8.1.3.4.
Note, however, that when the UAC tries alternative
locations, it still uses the route set for the dialog to
build the Route header of the request.
When a UAC receives a 2xx response to a target refresh request, it
MUST replace the dialog's remote target URI with the URI from the
Contact header field in that response, if present.
If the response for a request within a dialog is a 481
(Call/Transaction Does Not Exist) or a 408 (Request Timeout), the UAC
SHOULD terminate the dialog. A UAC SHOULD also terminate a dialog if
no response at all is received for the request (the client
transaction would inform the TU about the timeout.)
For INVITE initiated dialogs, terminating the dialog
consists of sending a BYE.
12.2.2 UAS Behavior
Requests sent within a dialog, as any other requests, are atomic. If
a particular request is accepted by the UAS, all the state changes
associated with it are performed. If the request is rejected, none of
the state changes is performed.
Note that some requests such as INVITEs affect several
pieces of state.
The UAS will receive the request from the transaction layer. If the
request has a tag in the To header field, the UAS core computes the
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dialog identifier corresponding to the request and compares it with
existing dialogs. If there is a match, this is a mid-dialog request.
In that case, the UAS first applies the same processing rules for
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requests outside of a dialog, discussed in Section 8.2.
If the request has a tag in the To header field, but the dialog
identifier does not match any existing dialogs, the UAS may have
crashed and restarted, or it may have received a request for a
different (possibly failed) UAS (the UASs can construct the To tags
so that a UAS can identify that the tag was for a UAS for which it is
providing recovery). Another possibility is that the incoming request
has been simply misrouted. Based on the To tag, the UAS MAY either
accept or reject the request. Accepting the request for acceptable To
tags provides robustness, so that dialogs can persist even through
crashes. UAs wishing to support this capability must take into
consideration some issues such as choosing monotonically increasing
CSeq sequence numbers even across reboots, reconstructing the route
set, and accepting out-of-range RTP timestamps and sequence numbers.
If the UAS wishes to reject the request, because it does not wish to
recreate the dialog, it MUST respond to the request with a 481
(Call/Transaction Does Not Exist) status code and pass that to the
server transaction.
Requests that do not change in any way the state of a dialog may be
received within a dialog (for example, an OPTIONS request). They are
processed as if they had been received outside the dialog.
If the remote sequence number is empty, it MUST be set to the value
of the sequence number in the CSeq header field value in the request.
If the remote sequence number was not empty, but the sequence number
of the request is lower than the remote sequence number, the request
is out of order and MUST be rejected with a 500 (Server Internal
Error) response. If the remote sequence number was not empty, and the
sequence number of the request is greater than the remote sequence
number, the request is in order. It is possible for the CSeq sequence
number to be higher than the remote sequence number by more than one.
This is not an error condition, and a UAS SHOULD be prepared to
receive and process requests with CSeq values more than one higher
than the previous received request. The UAS MUST then set the remote
sequence number to the value of the sequence number in the CSeq
header field value in the request.
If a proxy challenges a request generated by the UAC, the
UAC has to resubmit the request with credentials. The
resubmitted request will have a new CSeq number. The UAS
will never see the first request, and thus, it will notice
a gap in the CSeq number space. Such a gap does not
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represent any error condition.
When a UAS receives a target refresh request, it MUST replace the
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dialog's remote target URI with the URI from the Contact header field
in that request, if present.
12.3 Termination of a Dialog
Independent of the method, if a request outside of a dialog generates
a non-2xx final response, any early dialogs created through
provisional responses to that request are terminated. The mechanism
for terminating confirmed dialogs is method specific. In this
specification, the BYE method terminates a session and the dialog
associated with it. See Section 15 for details.
13 Initiating a Session
13.1 Overview
When a user agent client desires to initiate a session (for example,
audio, video, or a game), it formulates an INVITE request. The INVITE
request asks a server to establish a session. This request may be
forwarded by proxies, eventually arriving at one or more UAS that can
potentially accept the invitation. These UASs will frequently need to
query the user about whether to accept the invitation. After some
time, those UAS can accept the invitation (meaning the session is to
be established) by sending a 2xx response. If the invitation is not
accepted, a 3xx, 4xx, 5xx or 6xx response is sent, depending on the
reason for the rejection. Before sending a final response, the UAS
can also send provisional responses (1xx) to advise the UAC of
progress in contacting the called user.
After possibly receiving one or more provisional responses, the UAC
will get one or more 2xx responses or one non-2xx final response.
Because of the protracted amount of time it can take to receive final
responses to INVITE, the reliability mechanisms for INVITE
transactions differ from those of other requests (like OPTIONS). Once
it receives a final response, the UAC needs to send an ACK for every
final response it receives. The procedure for sending this ACK
depends on the type of response. For final responses between 300 and
699, the ACK processing is done in the transaction layer and follows
one set of rules (See Section 17). For 2xx responses, the ACK is
generated by the UAC core.
A 2xx response to an INVITE establishes a session, and it also
creates a dialog between the UA that issued the INVITE and the UA
that generated the 2xx response. Therefore, when multiple 2xx
responses are received from different remote UAs (because the INVITE
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forked), each 2xx establishes a different dialog. All these dialogs
are part of the same call.
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This section provides details on the establishment of a session using
INVITE. A UA that supports INVITE MUST also support ACK, CANCEL and
BYE.
13.2 UAC Processing
13.2.1 Creating the Initial INVITE
Since the initial INVITE represents a request outside of a dialog,
its construction follows the procedures of Section 8.1.1. Additional
processing is required for the specific case of INVITE.
An Allow header field (Section 20.5) SHOULD be present in the INVITE.
It indicates what methods can be invoked within a dialog, on the UA
sending the INVITE, for the duration of the dialog. For example, a UA
capable of receiving INFO requests within a dialog [33] [34] SHOULD
include an Allow header field listing the INFO method.
A Supported header field (Section 20.37) SHOULD be present in the
INVITE. It enumerates all the extensions understood by the UAC.
An Accept (Section 20.1) header field MAY be present in the INVITE.
It indicates which Content-Types are acceptable to the UA, in both
the response received by it, and in any subsequent requests sent to
it within dialogs established by the INVITE. The Accept header field
is especially useful for indicating support of various session
description formats.
The UAC MAY add an Expires header field (Section 20.19) to limit the
validity of the invitation. If the time indicated in the Expires
header field is reached and no final answer for the INVITE has been
received the UAC core SHOULD generate a CANCEL request for the
INVITE, as per Section 9.
A UAC MAY also find it useful to add, among others, Subject (Section
20.36), Organization (Section 20.25) and User-Agent (Section 20.41)
header fields. They all contain information related to the INVITE.
The UAC MAY choose to add a message body to the INVITE. Section
8.1.1.10 deals with how to construct the header fields -- Content-
Type among others -- needed to describe the message body.
There are special rules for message bodies that contain a session
description - their corresponding Content-Disposition is "session".
SIP uses an offer/answer model where one UA sends a session
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description, called the offer, which contains a proposed description
of the session. The offer indicates the desired communications means
(audio, video, games), parameters of those means (such as codec
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types) and addresses for receiving media from the answerer. The other
UA responds with another session description, called the answer,
which indicates which communications means are accepted, the
parameters that apply to those means, and addresses for receiving
media from the offerer. The offer/answer model defines restrictions
on when offers and answers can be made. This results in restrictions
on where the offers and answers can appear in SIP messages. In this
specification, offers and answers can only appear in INVITE requests
and responses, and ACK. The usage of offers and answers is further
restricted. For the initial INVITE transaction, the rules are:
o The initial offer MUST be in either an INVITE or, if not
there, in the first reliable non-failure message from the UAS
back to the UAC. In this specification, that is the final 2xx
response.
o If the initial offer is in an INVITE, the answer MUST be in a
reliable non-failure message from UAS back to UAC which is
correlated to that INVITE. For this specification, that is
only the final 2xx response to that INVITE. That same exact
answer MAY also be placed in any provisional responses sent
prior to the answer. The UAC MUST treat the first session
description it receives as the answer, and MUST ignore any
session descriptions in subsequent responses to the initial
INVITE.
o If the initial offer is in the first reliable non-failure
message from the UAS back to UAC, the answer MUST be in the
acknowledgement for that message (in this specification, ACK
for a 2xx response).
o After having sent or received an answer to the first offer,
the UAC MAY generate subsequent offers in requests, but only
if it has received answers to any previous offers, and has not
sent any offers to which it hasn't gotten an answer.
o Once the UAS has sent or received an answer to the initial
offer, it MUST NOT generate subsequent offers in any responses
to the initial INVITE. This means that a UAS based on this
specification alone can never generate subsequent offers until
completion of the initial transaction.
Concretely, the above rules specify two exchanges - the offer is in
the INVITE, and the answer in the 2xx, 2xx (and possibly in a 1xx as well,
with the same value), or the offer is in the 2xx, and the answer is
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in the ACK. All user agents that support INVITE MUST support these
two exchanges.
The Session Description Protocol (SDP) (RFC 2327 [1]) MUST be
supported by all user agents as a means to describe sessions, and its
usage for constructing offers and answers MUST follow the procedures
defined in [13].
The restrictions of the offer-answer model just described only apply
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to bodies whose Content-Disposition header field value is "session".
Therefore, it is possible that both the INVITE and the ACK contain a
body message (for example, the INVITE carries a photo (Content-
Disposition: render) and the ACK a session description (Content-
Disposition: session)).
If the Content-Disposition header field is missing, bodies
of Content-Type application/sdp imply the disposition
"session", while other content types imply "render".
Once the INVITE has been created, the UAC follows the procedures
defined for sending requests outside of a dialog (Section 8). This
results in the construction of a client transaction that will
ultimately send the request and deliver responses to the UAC.
13.2.2 Processing INVITE Responses
Once the INVITE has been passed to the INVITE client transaction, the
UAC waits for responses for the INVITE. If the INVITE client
transaction returns a timeout rather than a response the TU acts as
if a 408 (Request Timeout) response had been received, as described
in Section 8.1.3.
13.2.2.1 1xx responses
Zero, one or multiple provisional responses may arrive before one or
more final responses are received. Provisional responses for an
INVITE request can create "early dialogs". If a provisional response
has a tag in the To field, and if the dialog ID of the response does
not match an existing dialog, one is constructed using the procedures
defined in Section 12.1.2.
The early dialog will only be needed if the UAC needs to send a
request to its peer within the dialog before the initial INVITE
transaction completes. Header fields present in a provisional
response are applicable as long as the dialog is in the early state
(for example, an Allow header field in a provisional response
contains the methods that can be used in the dialog while this is in
the early state).
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13.2.2.2 3xx responses
A 3xx response may contain one or more Contact header field values
providing new addresses where the callee might be reachable.
Depending on the status code of the 3xx response (see Section 21.3)
the UAC MAY choose to try those new addresses.
13.2.2.3 4xx, 5xx and 6xx responses
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A single non-2xx final response may be received for the INVITE. 4xx,
5xx and 6xx responses may contain a Contact header field value
indicating the location where additional information about the error
can be found. Subsequent final responses (which would only arrive
under error conditions) MUST be ignored.
All early dialogs are considered terminated upon reception of the
non-2xx final response.
After having received the non-2xx final response the UAC core
considers the INVITE transaction completed. The INVITE client
transaction handles generation of ACKs for the response (see Section
17).
13.2.2.4 2xx responses
Multiple 2xx responses may arrive at the UAC for a single INVITE
request due to a forking proxy. Each response is distinguished by the
tag parameter in the To header field, and each represents a distinct
dialog, with a distinct dialog identifier.
If the dialog identifier in the 2xx response matches the dialog
identifier of an existing dialog, the dialog MUST be transitioned to
the "confirmed" state, and the route set for the dialog MUST be
recomputed based on the 2xx response using the procedures of Section
12.2.1.2. Otherwise, a new dialog in the "confirmed" state MUST be
constructed using the procedures of Section 12.1.2.
Note that the only piece of state that is recomputed is the
route set. Other pieces of state such as the highest
sequence numbers (remote and local) sent within the dialog
are not recomputed. The route set only is recomputed for
backwards compatibility. RFC 2543 did not mandate
mirroring of the Record-Route header field in a 1xx, only
2xx. However, we cannot update the entire state of the
dialog, since mid-dialog requests may have been sent within
the early dialog, modifying the sequence numbers, for
example.
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The UAC core MUST generate an ACK request for each 2xx received from
the transaction layer. The header fields of the ACK are constructed
in the same way as for any request sent within a dialog (see Section
12) with the exception of the CSeq and the header fields related to
authentication. The sequence number of the CSeq header field MUST be
the same as the INVITE being acknowledged, but the CSeq method MUST
be ACK. The ACK MUST contain the same credentials as the INVITE. If
the 2xx contains an offer (based on the rules above), the ACK MUST
carry an answer in its body. If the offer in the 2xx response is not
acceptable, the UAC core MUST generate a valid answer in the ACK and
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then send a BYE immediately.
Once the ACK has been constructed, the procedures of [4] are used to
determine the destination address, port and transport. However, the
request is passed to the transport layer directly for transmission,
rather than a client transaction. This is because the UAC core
handles retransmissions of the ACK, not the transaction layer. The
ACK MUST be passed to the client transport every time a
retransmission of the 2xx final response that triggered the ACK
arrives.
The UAC core considers the INVITE transaction completed 64*T1 seconds
after the reception of the first 2xx response. At this point all the
early dialogs that have not transitioned to established dialogs are
terminated. Once the INVITE transaction is considered completed by
the UAC core, no more new 2xx responses are expected to arrive.
If, after acknowledging any 2xx response to an INVITE, the UAC does
not want to continue with that dialog, then the UAC MUST terminate
the dialog by sending a BYE request as described in Section 15.
13.3 UAS Processing
13.3.1 Processing of the INVITE
The UAS core will receive INVITE requests from the transaction layer.
It first performs the request processing procedures of Section 8.2,
which are applied for both requests inside and outside of a dialog.
Assuming these processing states complete without generating a
response, the UAS core performs the additional processing steps:
1. If the request is an INVITE that contains an Expires header
field the UAS core sets a timer for the number of seconds
indicated in the header field value. When the timer fires,
the invitation is considered to be expired. If the
invitation expires before the UAS has generated a final
response, a 487 (Request Terminated) response SHOULD be
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generated.
2. If the request is a mid-dialog request, the method-
independent processing described in Section 12.2.2 is first
applied. It might also modify the session; Section 14
provides details.
3. If the request has a tag in the To header field but the
dialog identifier does not match any of the existing
dialogs, the UAS may have crashed and restarted, or may
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have received a request for a different (possibly failed)
UAS. Section 12.2.2 provides guidelines to achieve a robust
behavior under such a situation.
Processing from here forward assumes that the INVITE is outside of a
dialog, and is thus for the purposes of establishing a new session.
The INVITE may contain a session description, in which case the UAS
is being presented with an offer for that session. It is possible
that the user is already a participant in that session, even though
the INVITE is outside of a dialog. This can happen when a user is
invited to the same multicast conference by multiple other
participants. If desired, the UAS MAY use identifiers within the
session description to detect this duplication. For example, SDP
contains a session id and version number in the origin (o) field. If
the user is already a member of the session, and the session
parameters contained in the session description have not changed, the
UAS MAY silently accept the INVITE (that is, send a 2xx response
without prompting the user).
If the INVITE does not contain a session description, the UAS is
being asked to participate in a session, and the UAC has asked that
the UAS provide the offer of the session. It MUST provide the offer
in its first non-failure reliable message back to the UAC. In this
specification, that is a 2xx response to the INVITE.
The UAS can indicate progress, accept, redirect, or reject the
invitation. In all of these cases, it formulates a response using the
procedures described in Section 8.2.6.
13.3.1.1 Progress
If the UAS is not able to answer the invitation immediately, it can
choose to indicate some kind of progress to the UAC (for example, an
indication that a phone is ringing). This is accomplished with a
provisional response between 101 and 199. These provisional responses
establish early dialogs and therefore follow the procedures of
Section 12.1.1 in addition to those of Section 8.2.6. A UAS MAY send
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as many provisional responses as it likes. Each of these MUST
indicate the same dialog ID. However, these will not be delivered
reliably.
If the UAS desires an extended period of time to answer the INVITE,
it will need to ask for an "extension" in order to prevent proxies
from canceling the transaction. A proxy has the option of canceling a
transaction when there is a gap of 3 minutes between messages in a
transaction. To prevent cancellation, the UAS MUST send a non-100
provisional response at every minute, to handle the possibility of
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lost provisional responses.
An INVITE transaction can go on for extended durations when
the user is placed on hold, or when interworking with PSTN
systems which allow communications to take place without
answering the call. The latter is common in Interactive
Voice Response (IVR) systems.
13.3.1.2 The INVITE is redirected
If the UAS decides to redirect the call, a 3xx response is sent. A
300 (Multiple Choices), 301 (Moved Permanently) or 302 (Moved
Temporarily) response SHOULD contain a Contact header field
containing one or more URIs of new addresses to be tried. The
response is passed to the INVITE server transaction, which will deal
with its retransmissions.
13.3.1.3 The INVITE is rejected
A common scenario occurs when the callee is currently not willing or
able to take additional calls at this end system. A 486 (Busy Here)
SHOULD be returned in such scenario. If the UAS knows that no other
end system will be able to accept this call a 600 (Busy Everywhere)
response SHOULD be sent instead. However, it is unlikely that a UAS
will be able to know this in general, and thus this response will not
usually be used. The response is passed to the INVITE server
transaction, which will deal with its retransmissions.
A UAS rejecting an offer contained in an INVITE SHOULD return a 488
(Not Acceptable Here) response. Such a response SHOULD include a
Warning header field value explaining why the offer was rejected.
13.3.1.4 The INVITE is accepted
The UAS core generates a 2xx response. This response establishes a
dialog, and therefore follows the procedures of Section 12.1.1 in
addition to those of Section 8.2.6.
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A 2xx response to an INVITE SHOULD contain the Allow header field and
the Supported header field, and MAY contain the Accept header field.
Including these header fields allows the UAC to determine the
features and extensions supported by the UAS for the duration of the
call, without probing.
If the INVITE request contained an offer, and the UAS had not yet
sent an answer, the 2xx MUST contain an answer. If the INVITE did not
contain an offer, the 2xx MUST contain an offer if the UAS had not
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yet sent an offer.
Once the response has been constructed it is passed to the INVITE
server transaction. Note, however, that the INVITE server transaction
will be destroyed as soon as it receives this final response and
passes it to the transport. Therefore, it is necessary to pass
periodically the response directly to the transport until the ACK
arrives. The 2xx response is passed to the transport with an interval
that starts at T1 seconds and doubles for each retransmission until
it reaches T2 seconds (T1 and T2 are defined in Section 17). Response
retransmissions cease when an ACK request for the response is
received. This is independent of whatever transport protocols are
used to send the response.
Since 2xx is retransmitted end-to-end, there may be hops
between UAS and UAC that are UDP. To ensure reliable
delivery across these hops, the response is retransmitted
periodically even if the transport at the UAS is reliable.
If the server retransmits the 2xx response for 64*T1 seconds without
receiving an ACK, the dialog is confirmed, but the session SHOULD be
terminated. This is accomplished with a BYE as described in Section
15.
14 Modifying an Existing Session
A successful INVITE request (see Section 13) establishes both a
dialog between two user agents and a session using the offer-answer
model. Section 12 explains how to modify an existing dialog using a
target refresh request (for example, changing the remote target URI
of the dialog). This section describes how to modify the actual
session. This modification can involve changing addresses or ports,
adding a media stream, deleting a media stream, and so on. This is
accomplished by sending a new INVITE request within the same dialog
that established the session. An INVITE request sent within an
existing dialog is known as a re-INVITE.
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Note that a single re-INVITE can modify the dialog and the
parameters of the session at the same time.
Either the caller or callee can modify an existing session.
The behavior of a UA on detection of media failure is a matter of
local policy. However, automated generation of re-INVITE or BYE is
NOT RECOMMENDED to avoid flooding the network with traffic when there
is congestion. In any case, if these messages are sent automatically,
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they SHOULD be sent after some randomized interval.
Note that the paragraph above refers to automatically
generated BYEs and re-INVITEs. If the user hangs up upon
media failure the UA would send a BYE request as usual.
14.1 UAC Behavior
The same offer-answer model that applies to session descriptions in
INVITEs (Section 13.2.1) applies to re-INVITEs. As a result, a UAC
that wants to add a media stream, for example, will create a new
offer that contains this media stream, and send that in an INVITE
request to its peer. It is important to note that the full
description of the session, not just the change, is sent. This
supports stateless session processing in various elements, and
supports failover and recovery capabilities. Of course, a UAC MAY
send a re-INVITE with no session description, in which case the first
reliable non-failure response to the re-INVITE will contain the offer
(in this specification, that is a 2xx response).
If the session description format has the capability for version
numbers, the offerer SHOULD indicate that the version of the session
description has changed.
The To, From, Call-ID, CSeq, and Request-URI of a re-INVITE are set
following the same rules as for regular requests within an existing
dialog, described in Section 12.
A UAC MAY choose not to add an Alert-Info header field or a body with
Content-Disposition "alert" to re-INVITEs because UASs do not
typically alert the user upon reception of a re-INVITE.
Unlike an INVITE, which can fork, a re-INVITE will never fork, and
therefore, only ever generate a single final response. The reason a
re-INVITE will never fork is that the Request-URI identifies the
target as the UA instance it established the dialog with, rather than
identifying an address-of-record for the user.
Note that a UAC MUST NOT initiate a new INVITE transaction within a
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dialog while another INVITE transaction is in progress in either
direction.
1. If there is an ongoing INVITE client transaction, the TU
MUST wait until the transaction reaches the completed or
terminated state before initiating the new INVITE.
2. If there is an ongoing INVITE server transaction, the TU
MUST wait until the transaction reaches the confirmed or
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terminated state before initiating the new INVITE.
However, a UA MAY initiate a regular transaction while an INVITE
transaction is in progress. A UA MAY also initiate an INVITE
transaction while a regular transaction is in progress.
If a UA receives a non-2xx final response to a re-INVITE, the session
parameters MUST remain unchanged, as if no re-INVITE had been issued.
Note that, as stated in Section 12.2.1.2, if the non-2xx final
response is a 481 (Call/Transaction Does Not Exist), or a 408
(Request Timeout), or no response at all is received for the re-
INVITE (that is, a timeout is returned by the INVITE client
transaction), the UAC will terminate the dialog.
If a UAC receives a 491 response to a re-INVITE, it SHOULD start a
timer with a value T chosen as follows:
1. If the UAC is the owner of the Call-ID of the dialog ID
(meaning it generated the value), T has a randomly chosen
value between 2.1 and 4 seconds in units of 10 ms.
2. If the UAC is not the owner of the Call-ID of the dialog
ID, T has a randomly chosen value of between 0 and 2
seconds in units of 10 ms.
When the timer fires, the UAC SHOULD attempt the re-INVITE once more,
if it still desires for that session modification to take place. For
example, if the call was already hung up with a BYE, the re-INVITE
would not take place.
The rules for transmitting a re-INVITE and for generating an ACK for
a 2xx response to re-INVITE are the same as for the initial INVITE
(Section 13.2.1).
14.2 UAS Behavior
Section 13.3.1 describes the procedure for distinguishing incoming
re-INVITEs from incoming initial INVITEs and handling a re-INVITE for
an existing dialog.
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A UAS that receives a second INVITE before it sends the final
response to a first INVITE with a lower CSeq sequence number on the
same dialog MUST return a 500 (Server Internal Error) response to the
second INVITE and MUST include a Retry-After header field with a
randomly chosen value of between 0 and 10 seconds.
A UAS that receives an INVITE on a dialog while an INVITE it had sent
on that dialog is in progress MUST return a 491 (Request Pending)
response to the received INVITE and MUST include a Retry-After header
field with a value chosen as follows:
1. If the UAS is the owner of the Call-ID of the dialog ID
(meaning it generated the value), the Retry-After header
field has a randomly chosen value of between 2.1 and 4
seconds in units of 10 ms.
2. If the UAS is not the owner of the Call-ID of the dialog
ID, the Retry-After header field has a randomly chosen
value of between 0 and 2 seconds in units of 10 ms. INVITE.
If a UA receives a re-INVITE for an existing dialog, it MUST check
any version identifiers in the session description or, if there are
no version identifiers, the content of the session description to see
if it has changed. If the session description has changed, the UAS
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MUST adjust the session parameters accordingly, possibly after asking
the user for confirmation.
Versioning of the session description can be used to
accommodate the capabilities of new arrivals to a
conference, add or delete media, or change from a unicast
to a multicast conference. If the new session description
is not acceptable, the UAS can reject it by returning a 488
(Not Acceptable Here) response for the re-INVITE. This
response SHOULD include a Warning header field.
If a UAS generates a 2xx response and never receives an ACK, it
SHOULD generate a BYE to terminate the dialog.
A UAS MAY choose not to generate 180 (Ringing) responses for a re-
INVITE because UACs do not typically render this information to the
user. For the same reason, UASs MAY choose not to use an Alert-Info
header field or a body with Content-Disposition "alert" in responses
to a re-INVITE.
A UAS providing an offer in a 2xx (because the INVITE did not contain
an offer) SHOULD construct the offer as if the UAS were making a
brand new call, subject to the constraints of sending an offer that
updates an existing session, as described in [13] in the case of SDP.
Specifically, this means that it SHOULD include as many media formats
and media types that the UA is willing to support. The UAS MUST
ensure that the session description overlaps with its previous
session description in media formats, transports, or other parameters
that require support from the peer. This is to avoid the need for the
peer to reject the session description. If, however, it is
unacceptable to the UAC, the UAC SHOULD generate an answer with a
valid session description, and then send a BYE to terminate the
session.
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15 Terminating a Session
This section describes the procedures for terminating a session
established by SIP. The state of the session and the state of the
dialog are very closely related. When a session is initiated with an
INVITE, each 1xx or 2xx response from a distinct UAS creates a
dialog, and if that response completes the offer/answer exchange, it
also creates a session. As a result, each session is "associated"
with a single dialog - the one which resulted in its creation. If an
initial INVITE generates a non-2xx final response, that terminates
all sessions (if any) and all dialogs (if any) that were created
through responses to the request. By virtue of completing the
transaction, a non-2xx final response also prevents further sessions
from being created as a result of the INVITE. The BYE request is used
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to terminate a specific session or attempted session. In this case,
the specific session is the one with the peer UA on the other side of
the dialog. When a BYE is received on a dialog, any session
associated with that dialog SHOULD terminate. A UA MUST NOT send a
BYE outside of a dialog. The caller's UA MAY send a BYE for either
confirmed or early dialogs, and the callee's UA MAY send a BYE on
confirmed dialogs, but MUST NOT send a BYE on early dialogs. However,
the callee's UA MUST NOT send a BYE on a confirmed dialog until it
has received an ACK for its 2xx response or until the server
transaction times out. If no SIP extensions have defined other
application layer state associated with the dialog, the BYE also
terminates the dialog.
The impact of a non-2xx final response to INVITE on dialogs and
sessions makes the use of CANCEL attractive. The CANCEL attempts to
force a non-2xx response to the INVITE (in particular, a 487).
Therefore, if a UAC wishes to give up on its call attempt entirely,
it can send a CANCEL. If the INVITE results in 2xx final response(s)
to the INVITE, this means that a UAS accepted the invitation while
the CANCEL was in progress. The UAC MAY continue with the sessions
established by any 2xx responses, or MAY terminate them with BYE.
The notion of "hanging up" is not well defined within SIP.
It is specific to a particular, albeit common, user
interface. Typically, when the user hangs up, it indicates
a desire to terminate the attempt to establish a session,
and to terminate any sessions already created. For the
caller's UA, this would imply a CANCEL request if the
initial INVITE has not generated a final response, and a
BYE to all confirmed dialogs after a final response. For
the callee's UA, it would typically imply a BYE;
presumably, when the user picked up the phone, a 2xx was
generated, and so hanging up would result in a BYE after
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the ACK is received. This does not mean a user cannot hang
up before receipt of the ACK, it just means that the
software in his phone needs to maintain state for a short
while in order to clean up properly. If the particular UI
allows for the user to reject a call before its answered, a
403 (Forbidden) is a good way to express that. As per the
rules above, a BYE can't be sent.
15.1 Terminating a Session with a BYE Request
15.1.1 UAC Behavior
A BYE request is constructed as would any other request within a
dialog, as described in Section 12.
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Once the BYE is constructed, the UAC core creates a new non-INVITE
client transaction, and passes it the BYE request. The UAC MUST
consider the session terminated (and therefore stop sending or
listening for media) as soon as the BYE request is passed to the
client transaction. If the response for the BYE is a 481
(Call/Transaction Does Not Exist) or a 408 (Request Timeout) or no
response at all is received for the BYE (that is, a timeout is
returned by the client transaction), the UAC MUST consider the
session and the dialog terminated.
15.1.2 UAS Behavior
A UAS first processes the BYE request according to the general UAS
processing described in Section 8.2. A UAS core receiving a BYE
request checks if it matches an existing dialog. If the BYE does not
match an existing dialog, the UAS core SHOULD generate a 481
(Call/Transaction Does Not Exist) response and pass that to the
server transaction.
This rule means that a BYE sent without tags by a UAC will
be rejected. This is a change from RFC 2543, which allowed
BYE without tags.
A UAS core receiving a BYE request for an existing dialog MUST follow
the procedures of Section 12.2.2 to process the request. Once done,
the UAS SHOULD terminate the session (and therefore stop sending and
listening for media). The only case where it can elect not to are
multicast sessions, where participation is possible even if the other
participant in the dialog has terminated its involvement in the
session. Whether or not it ends its participation on the session, the
UAS core MUST generate a 2xx response to the BYE, and MUST pass that
to the server transaction for transmission.
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The UAS MUST still respond to any pending requests received for that
dialog. It is RECOMMENDED that a 487 (Request Terminated) response is
generated to those pending requests.
16 Proxy Behavior
16.1 Overview
SIP proxies are elements that route SIP requests to user agent
servers and SIP responses to user agent clients. A request may
traverse several proxies on its way to a UAS. Each will make routing
decisions, modifying the request before forwarding it to the next
element. Responses will route through the same set of proxies
traversed by the request in the reverse order.
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Being a proxy is a logical role for a SIP element. When a request
arrives, an element that can play the role of a proxy first decides
if it needs to respond to the request on its own. For instance, the
request may be malformed or the element may need credentials from the
client before acting as a proxy. The element MAY respond with any
appropriate error code. When responding directly to a request, the
element is playing the role of a UAS and MUST behave as described in
Section 8.2.
A proxy can operate in either a stateful or stateless mode for each
new request. When stateless, a proxy acts as a simple forwarding
element. It forwards each request downstream to a single element
determined by making a targeting and routing decision based on the
request. It simply forwards every response it receives upstream. A
stateless proxy discards information about a message once the message
has been forwarded. A stateful proxy remembers information
(specifically, transaction state) about each incoming request and any
requests it sends as a result of processing the incoming request. It
uses this information to affect the processing of future messages
associated with that request. A stateful proxy MAY choose to "fork" a
request, routing it to multiple destinations. Any request that is
forwarded to more than one location MUST be handled statefully.
In some circumstances, a proxy MAY forward requests using stateful
transports (such as TCP) without being transaction-stateful. For
instance, a proxy MAY forward a request from one TCP connection to
another transaction statelessly as long as it places enough
information in the message to be able to forward the response down
the same connection the request arrived on. Requests forwarded
between different types of transports where the proxy's TU must take
an active role in ensuring reliable delivery on one of the transports
MUST be forwarded transaction statefully.
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A stateful proxy MAY transition to stateless operation at any time
during the processing of a request, so long as it did not do anything
that would otherwise prevent it from being stateless initially
(forking, for example, or generation of a 100 response). When
performing such a transition, all state is simply discarded. The
proxy SHOULD NOT initiate a CANCEL request.
Much of the processing involved when acting statelessly or statefully
for a request is identical. The next several subsections are written
from the point of view of a stateful proxy. The last section calls
out those places where a stateless proxy behaves differently.
16.2 Stateful Proxy
When stateful, a proxy is purely a SIP transaction processing engine.
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Its behavior is modeled here in terms of the server and client
transactions defined in Section 17. A stateful proxy has a server
transaction associated with one or more client transactions by a
higher layer proxy processing component (see figure 3), known as a
proxy core. An incoming request is processed by a server transaction.
Requests from the server transaction are passed to a proxy core. The
proxy core determines where to route the request, choosing one or
more next-hop locations. An outgoing request for each next-hop
location is processed by its own associated client transaction. The
proxy core collects the responses from the client transactions and
uses them to send responses to the server transaction.
A stateful proxy creates a new server transaction for each new
request received. Any retransmissions of the request will then be
handled by that server transaction per Section 17. The proxy core
MUST behave as a UAS with respect to sending an immediate provisional
on that server transaction (such as 100 Trying) as described in
Section 8.2.6. Thus, a stateful proxy SHOULD NOT generate 100 Trying
responses to non-INVITE requests.
This is a model of proxy behavior, not of software. An implementation
is free to take any approach that replicates the external behavior
this model defines.
For all new requests, including any with unknown methods, an element
intending to proxy the request MUST:
1. Validate the request (Section 16.3)
2. Preprocess routing information (Section 16.4)
3. Determine target(s) for the request (Section 16.5)
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+--------------------+
| | +---+
| | | C |
| | | T |
| | +---+
+---+ | Proxy | +---+ CT = Client Transaction
| S | | "Higher" Layer | | C |
| T | | | | T | ST = Server Transaction
+---+ | | +---+
| | +---+
| | | C |
| | | T |
| | +---+
+--------------------+
Figure 3: Stateful Proxy Model
For all new requests, including any with unknown methods, an element
intending to proxy the request MUST:
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1. Validate the request (Section 16.3)
2. Preprocess routing information (Section 16.4)
3. Determine target(s) for the request (Section 16.5)
4. Forward the request to each target (Section 16.6)
5. Process
4. Forward the request to each target (Section 16.6)
5. Process all responses (Section 16.7)
16.3 Request Validation
Before an element can proxy a request, it MUST verify the message's
validity. A valid message must pass the following checks:
1. Reasonable Syntax
2. URI scheme
3. Max-Forwards
4. (Optional) Loop Detection
5. Proxy-Require
6. Proxy-Authorization
If any of these checks fail, the element MUST behave as a user agent
server (see Section 8.2) and respond with an error code.
Notice that a proxy is not required to detect merged requests and
MUST NOT treat merged requests as an error condition. The endpoints
receiving the requests will resolve the merge as described in Section
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8.2.2.2.
1. Reasonable syntax check
The request MUST be well-formed enough to be handled with a
server transaction. Any components involved in the
remainder of these Request Validation steps or the Request
Forwarding section MUST be well-formed. Any other
components, well-formed or not, SHOULD be ignored and
remain unchanged when the message is forwarded. For
instance, an element would not reject a request because of
a malformed Date header field. Likewise, a proxy would not
remove a malformed Date header field before forwarding a
request.
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This protocol is designed to be extended. Future extensions
may define new methods and header fields at any time. An
element MUST NOT refuse to proxy a request because it
contains a method or header field it does not know about.
2. URI scheme check
If the Request-URI has a URI whose scheme is not understood
by the proxy, the proxy SHOULD reject the request with a
416 (Unsupported URI Scheme) response.
3. Max-Forwards check
The Max-Forwards header field (Section 20.22) is used to
limit the number of elements a SIP request can traverse.
If the request does not contain a Max-Forwards header
field, this check is passed.
If the request contains a Max-Forwards header field with a
field value greater than zero, the check is passed.
If the request contains a Max-Forwards header field with a
field value of zero (0), the element MUST NOT forward the
request. If the request was for OPTIONS, the element MAY
act as the final recipient and respond per Section 11.
Otherwise, the element MUST return a 483 (Too many hops)
response.
4. Optional Loop Detection check
An element MAY check for forwarding loops before forwarding
a request. If the request contains a Via header field with
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a sent-by value that equals a value placed into previous
requests by the proxy, the request has been forwarded by
this element before. The request has either looped or is
legitimately spiraling through the element. To determine if
the request has looped, the element MAY perform the branch
parameter calculation described in Step 8 of Section 16.6
on this message and compare it to the parameter received in
that Via header field. If the parameters match, the request
has looped. If they differ, the request is spiraling, and
processing continues. If a loop is detected, the element
MAY return a 482 (Loop Detected) response.
5. Proxy-Require check
Future extensions to this protocol may introduce features
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that require special handling by proxies. Endpoints will
include a Proxy-Require header field in requests that use
these features, telling the proxy not to process the
request unless the feature is understood.
If the request contains a Proxy-Require header field
(Section 20.29) with one or more option-tags this element
does not understand, the element MUST return a 420 (Bad
Extension) response. The response MUST include an
Unsupported (Section 20.40) header field listing those
option-tags the element did not understand.
6. Proxy-Authorization check
If an element requires credentials before forwarding a
request, the request MUST be inspected as described in
Section 22.3. That section also defines what the element
must do if the inspection fails.
16.4 Route Information Preprocessing
The proxy MUST inspect the Request-URI of the request. If the
Request-URI of the request contains a value this proxy previously
placed into a Record-Route header field (see Section 16.6 item 4),
the proxy MUST replace the Request-URI in the request with the last
value from the Route header field, and remove that value from the
Route header field. The proxy MUST then proceed as if it received
this modified request.
This will only happen when the element sending the request
to the proxy (which may have been an endpoint) is a strict
router. This rewrite on receive is necessary to enable
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backwards compatibility with those elements. It also allows
elements following this specification to preserve the
Request-URI through strict-routing proxies (see Section
12.2.1.1).
This requirement does not obligate a proxy to keep state in
order to detect URIs it previously placed in Record-Route
header fields. Instead, a proxy need only place enough
information in those URIs to recognize them as values it
provided when they later appear.
If the Request-URI contains an maddr parameter, the proxy MUST check
to see if its value is in the set of addresses or domains the proxy
is configured to be responsible for. If the Request-URI has an maddr
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parameter with a value the proxy is responsible for, and the request
was received using the port and transport indicated (explicitly or by
default) in the Request-URI, the proxy MUST strip the maddr and any
non-default port or transport parameter and continue processing as if
those values had not been present in the request.
A request may arrive with an maddr matching the proxy, but
on a port or transport different from that indicated in the
URI. Such a request needs to be forwarded to the proxy
using the indicated port and transport.
If the first value in the Route header field indicates this proxy,
the proxy MUST remove that value from the request.
16.5 Determining request targets
Next, the proxy calculates the target(s) of the request. The set of
targets will either be predetermined by the contents of the request
or will be obtained from an abstract location service. Each target in
the set is represented as a URI.
If the Request-URI of the request contains an maddr parameter, the
Request-URI MUST be placed into the target set as the only target
URI, and the proxy MUST proceed to Section 16.6.
If the domain of the Request-URI indicates a domain this element is
not responsible for, the Request-URI MUST be placed into the target
set as the only target, and the element MUST proceed to the task of
Request Forwarding (Section 16.6).
There are many circumstances in which a proxy might receive
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a request for a domain it is not responsible for. A
firewall proxy handling outgoing calls (the way HTTP
proxies handle outgoing requests) is an example of where
this is likely to occur.
If the target set for the request has not been predetermined as
described above, this implies that the element is responsible for the
domain in the Request-URI, and the element MAY use whatever mechanism
it desires to determine where to send the request. Any of these
mechanisms can be modeled as accessing an abstract Location Service.
This may consist of obtaining information from a location service
created by a SIP Registrar, reading a database, consulting a presence
server, utilizing other protocols, or simply performing an
algorithmic substitution on the Request-URI. When accessing the
location service constructed by a registrar, the Request-URI MUST
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first be canonicalized as described in Section 10.3 before being used
as an index. The output of these mechanisms is used to construct the
target set.
If the Request-URI does not provide sufficient information for the
proxy to determine the target set, it SHOULD return a 485 (Ambiguous)
response. This response SHOULD contain a Contact header field
containing URIs of new addresses to be tried. For example, an INVITE
to sip:John.Smith@company.com may be ambiguous at a proxy whose
location service has multiple John Smiths listed. See Section 21.4.23
for details.
Any information in or about the request or the current environment of
the element MAY be used in the construction of the target set. For
instance, different sets may be constructed depending on contents or
the presence of header fields and bodies, the time of day of the
request's arrival, the interface on which the request arrived,
failure of previous requests, or even the element's current level of
utilization.
As potential targets are located through these services, their URIs
are added to the target set. Targets can only be placed in the
target set once. If a target URI is already present in the set (based
on the definition of equality for the URI type), it MUST NOT be added
again.
A proxy MUST NOT add additional targets to the target set if the
Request-URI of the original request does not indicate a resource this
proxy is responsible for.
A proxy can only change the Request-URI of a request during
forwarding if it is responsible for that URI. If the proxy
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is not responsible for that URI, it will not recurse on 3xx
or 416 responses as described below.
If the Request-URI of the original request indicates a resource this
proxy is responsible for, the proxy MAY continue to add targets to
the set after beginning Request Forwarding. It MAY use any
information obtained during that processing to determine new targets.
For instance, a proxy may choose to incorporate contacts obtained in
a redirect response (3xx) into the target set. If a proxy uses a
dynamic source of information while building the target set (for
instance, if it consults a SIP Registrar), it SHOULD monitor that
source for the duration of processing the request. New locations
SHOULD be added to the target set as they become available. As above,
any given URI MUST NOT be added to the set more than once.
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Allowing a URI to be added to the set only once reduces
unnecessary network traffic, and in the case of
incorporating contacts from redirect requests prevents
infinite recursion.
For example, a trivial location service is a "no-op", where the
target URI is equal to the incoming request URI. The request is sent
to a specific next hop proxy for further processing. During request
forwarding of Section 16.6, Item 6, the identity of that next hop,
expressed as a SIP or SIPS URI, is inserted as the top-most Route
header field value into the request.
If the Request-URI indicates a resource at this proxy that does not
exist, the proxy MUST return a 404 (Not Found) response.
If the target set remains empty after applying all of the above, the
proxy MUST return an error response, which SHOULD be the 480
(Temporarily Unavailable) response.
16.6 Request Forwarding
As soon as the target set is non-empty, a proxy MAY begin forwarding
the request. A stateful proxy MAY process the set in any order. It
MAY process multiple targets serially, allowing each client
transaction to complete before starting the next. It MAY start client
transactions with every target in parallel. It also MAY arbitrarily
divide the set into groups, processing the groups serially and
processing the targets in each group in parallel.
A common ordering mechanism is to use the qvalue parameter of targets
obtained from Contact header fields (see Section 20.10). Targets are
processed from highest qvalue to lowest. Targets with equal qvalues
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may be processed in parallel.
A stateful proxy must have a mechanism to maintain the target set as
responses are received and associate the responses to each forwarded
request with the original request. For the purposes of this model,
this mechanism is a "response context" created by the proxy layer
before forwarding the first request.
For each target, the proxy forwards the request following these
steps:
1. Make a copy of the received request
2. Update the Request-URI
3. Update the Max-Forwards header field
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4. Optionally add a Record-route header field value
5. Optionally add additional header fields
6. Postprocess routing information
7. Determine the next-hop address, port, and transport
8. Add a Via header field value
9. Add a Content-Length header field if necessary
10. Forward the new request
11. Set timer C
Each of these steps is detailed below:
1. Copy request
The proxy starts with a copy of the received request. The
copy MUST initially contain all of the header fields from
the received request. Fields not detailed in the
processing described below MUST NOT be removed. The copy
SHOULD maintain the ordering of the header fields as in the
received request. The proxy MUST NOT reorder field values
with a common field name (See Section 7.3.1). The proxy
MUST NOT add to, modify, or remove the message body.
An actual implementation need not perform a copy; the
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primary requirement is that the processing for each
next hop begin with the same request.
2. Request-URI
The Request-URI in the copy's start line MUST be replaced
with the URI for this target. If the URI contains any
parameters not allowed in a Request-URI, they MUST be
removed.
This is the essence of a proxy's role. This is the
mechanism through which a proxy routes a request toward its
destination.
In some circumstances, the received Request-URI is placed
into the target set without being modified. For that
target, the replacement above is effectively a no-op.
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3. Max-Forwards
If the copy contains a Max-Forwards header field, the proxy
MUST decrement its value by one (1).
If the copy does not contain a Max-Forwards header field,
the proxy MUST add one with a field value which SHOULD be
70.
Some existing UAs will not provide a Max-Forwards
header field in a request.
4. Record-Route
If this proxy wishes to remain on the path of future
requests in a dialog created by this request (assuming the
request creates a dialog), it MUST insert a Record-Route
header field value into the copy before any existing
Record-Route header field values, even if a Route header
field is already present.
Requests establishing a dialog may contain a preloaded
Route header field.
If this request is already part of a dialog, the proxy
SHOULD insert a Record-Route header field value if it
wishes to remain on the path of future requests in the
dialog. In normal endpoint operation as described in
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Section 12 these Record-Route header field values will not
have any effect on the route sets used by the endpoints.
The proxy will remain on the path if it chooses to not
insert a Record-Route header field value into requests
that are already part of a dialog. However, it would
be removed from the path when an endpoint that has
failed reconstitutes the dialog.
A proxy MAY insert a Record-Route header field value into
any request. If the request does not initiate a dialog, the
endpoints will ignore the value. See Section 12 for details
on how endpoints use the Record-Route header field values
to construct Route header fields.
Each proxy in the path of a request chooses whether to add
a Record-Route header field value independently - the
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presence of a Record-Route header field in a request does
not obligate this proxy to add a value.
The URI placed in the Record-Route header field value MUST
be a SIP URI. This URI MUST contain an lr parameter (see
Section 19.1.1). This URI MAY be different for each
destination the request is forwarded to. The URI SHOULD NOT
contain the transport parameter unless the proxy has
knowledge (such as in a private network) that the next
downstream element that will be in the path of subsequent
requests supports that transport.
The URI this proxy provides will be used by some other
element to make a routing decision. This proxy, in
general, has no way to know what the capabilities of
that element are, so it must restrict itself to the
mandatory elements of a SIP implementation: SIP URIs
and either the TCP or UDP transports.
The URI placed in the Record-Route header field MUST
resolve to the element inserting it (or a suitable stand-
in) when the server location procedures of [4] are applied
to it, so that subsequent requests reach the same SIP
element. If the Request-URI contains a SIPS URI, or the
topmost Route header field value (after the post processing
of bullet 6 6) contains a SIPS URI, the URI placed into the
Record-Route header field MUST be a SIPS URI. Furthermore,
if the request was not received over TLS, the proxy MUST
insert a Record-Route header field. In a similar fashion, a
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proxy that receives a request over TLS, but generates a
request without a SIPS URI in the Request-URI or topmost
Record-Route
Route header field value, value (after the post processing of
bullet 6), MUST insert a Record-Route header field that is
not a SIPS URI.
A proxy at a security perimeter must remain on the
perimeter throughout the dialog.
If the URI placed in the Record-Route header field needs to
be rewritten when it passes back through in a response, the
URI MUST be distinct enough to locate at that time. (The
request may spiral through this proxy, resulting in more
than one Record-Route header field value being added).
Item 8 of Section 16.7 recommends a mechanism to make the
URI sufficiently distinct.
The proxy MAY include parameters in the Record-Route header
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field value. These will be echoed in some responses to the
request such as the 200 (OK) responses to INVITE. Such
parameters may be useful for keeping state in the message
rather than the proxy.
If a proxy needs to be in the path of any type of dialog
(such as one straddling a firewall), it SHOULD add a
Record-Route header field value to every request with a
method it does not understand since that method may have
dialog semantics.
The URI a proxy places into a Record-Route header field is
only valid for the lifetime of any dialog created by the
transaction in which it occurs. A dialog-stateful proxy,
for example, MAY refuse to accept future requests with that
value in the Request-URI after the dialog has terminated.
Non-dialog-stateful proxies, of course, have no concept of
when the dialog has terminated, but they MAY encode enough
information in the value to compare it against the dialog
identifier of future requests and MAY reject requests not
matching that information. Endpoints MUST NOT use a URI
obtained from a Record-Route header field outside the
dialog in which it was provided. See Section 12 for more
information on an endpoint's use of Record-Route header
fields.
Record-routing may be required by certain services where
the proxy needs to observe all messages in a dialog.
However, it slows down processing and impairs scalability
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and thus proxies should only record-route if required for a
particular service.
The Record-Route process is designed to work for any SIP
request that initiates a dialog. INVITE is the only such
request in this specification, but extensions to the
protocol MAY define others.
5. Add Additional Header Fields
The proxy MAY add any other appropriate header fields to
the copy at this point.
6. Postprocess routing information
A proxy MAY have a local policy that mandates that a
request visit a specific set of proxies before being
delivered to the destination. A proxy MUST ensure that all
such proxies are loose routers. Generally, this can only be
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known with certainty if the proxies are within the same
administrative domain. This set of proxies is represented
by a set of URIs (each of which contains the lr parameter).
This set MUST be pushed into the Route header field of the
copy ahead of any existing values, if present. If the Route
header field is absent, it MUST be added, containing that
list of URIs.
If the proxy has a local policy that mandates that the
request visit one specific proxy, an alternative to pushing
a Route value into the Route header field is to bypass the
forwarding logic of item 10 below, and instead just send
the request to the address, port, and transport for that
specific proxy. If the request has a Route header field,
this alternative MUST NOT be used unless it is known that
next hop proxy is a loose router. Otherwise, this approach
MAY be used, but the Route insertion mechanism above is
preferred for its robustness, flexibility, generality and
consistency of operation. Furthermore, if the Request-URI
contains a SIPS URI, TLS MUST be used to communicate with
that proxy.
If the copy contains a Route header field, the proxy MUST
inspect the URI in its first value. If that URI does not
contain a lr parameter, the proxy MUST modify the copy as
follows:
- The proxy MUST place the Request-URI into the Route
header field as the last value.
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- The proxy MUST then place the first Route header field
value into the Request-URI and remove that value from the
Route header field.
Appending the Request-URI to the Route header field is
part of a mechanism used to pass the information in
that Request-URI through strict-routing elements.
"Popping" the first Route header field value into the
Request-URI formats the message the way a strict-
routing element expects to receive it (with its own
URI in the Request-URI and the next location to visit
in the first Route header field value).
7. Determine Next-Hop Address, Port, and Transport
The proxy MAY have a local policy to send the request to a
specific IP address, port, and transport, independent of
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the values of the Route and Request-URI. Such a policy MUST
NOT be used if the proxy is not certain that the IP
address, port, and transport correspond to a server that is
a loose router. However, this mechanism for sending the
request through a specific next hop is NOT RECOMMENDED;
instead a Route header field should be used for that
purpose as described above.
In the absence of such an overriding mechanism, the proxy
applies the procedures listed in [4] as follows to
determine where to send the request. If the proxy has
reformatted the request to send to a strict-routing element
as described in step 6 above, the proxy MUST apply those
procedures to the Request-URI of the request. Otherwise,
the proxy MUST apply the procedures to the first value in
the Route header field, if present, else the Request-URI.
The procedures will produce an ordered set of (address,
port, transport) tuples. Independently of which URI is
being used as input to the procedures of [4], if the
Request-URI specifies a SIPS resource, the proxy MUST
follow the procedures of [4] as if the input URI were a
SIPS URI.
As described in [4], the proxy MUST attempt to deliver the
message to the first tuple in that set, and proceed through
the set in order until the delivery attempt succeeds.
For each tuple attempted, the proxy MUST format the message
as appropriate for the tuple and send the request using a
new client transaction as detailed in steps 8 through 10.
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Since each attempt uses a new client transaction, it
represents a new branch. Thus, the branch parameter
provided with the Via header field inserted in step 8 MUST
be different for each attempt.
If the client transaction reports failure to send the
request or a timeout from its state machine, the proxy
continues to the next address in that ordered set. If the
ordered set is exhausted, the request cannot be forwarded
to this element in the target set. The proxy does not need
to place anything in the response context, but otherwise
acts as if this element of the target set returned a 408
(Request Timeout) final response.
8. Add a Via header field value
The proxy MUST insert a Via header field value into the
copy before the existing Via header field values. The
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construction of this value follows the same guidelines of
Section 8.1.1.7. This implies that the proxy will compute
its own branch parameter, which will be globally unique for
that branch, and contain the requisite magic cookie.
Proxies choosing to detect loops have an additional
constraint in the value they use for construction of the
branch parameter. A proxy choosing to detect loops SHOULD
create a branch parameter separable into two parts by the
implementation. The first part MUST satisfy the constraints
of Section 8.1.1.7 as described above. The second is used
to perform loop detection and distinguish loops from
spirals.
Loop detection is performed by verifying that, when a
request returns to a proxy, those fields having an impact
on the processing of the request have not changed. The
value placed in this part of the branch parameter SHOULD
reflect all of those fields (including any Route, Proxy-
Require and Proxy-Authorization header fields). This is to
ensure that if the request is routed back to the proxy and
one of those fields changes, it is treated as a spiral and
not a loop (Section 16.3 A common way to create this value
is to compute a cryptographic hash of the To tag, From tag,
Call-ID header field, the Request-URI of the request
received (before translation) and the sequence number from
the CSeq header field, in addition to any Proxy-Require and
Proxy-Authorization header fields that may be present. The
algorithm used to compute the hash is implementation-
dependent, but MD5 (RFC 1321 [34]), [35]), expressed in
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hexadecimal, is a reasonable choice. (Base64 is not
permissible for a token.)
If a proxy wishes to detect loops, the "branch"
parameter it supplies MUST depend on all information
affecting processing of a request, including the
incoming Request-URI and any header fields affecting
the request's admission or routing. This is necessary
to distinguish looped requests from requests whose
routing parameters have changed before returning to
this server.
The request method MUST NOT be included in the calculation
of the branch parameter. In particular, CANCEL and ACK
requests (for non-2xx responses) MUST have the same branch
value as the corresponding request they cancel or
acknowledge. The branch parameter is used in correlating
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those requests at the server handling them (see Sections
17.2.3 and 9.2).
9. Add a Content-Length header field if necessary
If the request will be sent to the next hop using a
stream-based transport and the copy contains no Content-
Length header field, the proxy MUST insert one with the
correct value for the body of the request (see Section
20.14).
10. Forward Request
A stateful proxy MUST create a new client transaction for
this request as described in Section 17.1 and instructs the
transaction to send the request using the address, port and
transport determined in step 7.
11. Set timer C
In order to handle the case where an INVITE request never
generates a final response, the TU uses a timer which is
called timer C. Timer C MUST be set for each client
transaction when an INVITE request is proxied. The timer
MUST be larger than 3 minutes. Section 16.7 bullet 2
discusses how this timer is updated with provisional
responses, and Section 16.8 discusses processing when it
fires.
16.7 Response Processing
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When a response is received by an element, it first tries to locate a
client transaction (Section 17.1.3) matching the response. If none is
found, the element MUST process the response (even if it is an
informational response) as a stateless proxy (described below). If a
match is found, the response is handed to the client transaction.
Forwarding responses for which a client transaction (or
more generally any knowledge of having sent an associated
request) is not found improves robustness. In particular,
it ensures that "late" 2xx responses to INVITE requests are
forwarded properly.
As client transactions pass responses to the proxy layer, the
following processing MUST take place:
1. Find the appropriate response context
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2. Update timer C for provisional responses
3. Remove the topmost Via
4. Add the response to the response context
5. Check to see if this response should be forwarded
immediately
6. When necessary, choose the best final response from the
response context
If no final response has been forwarded after every client
transaction associated with the response context has been
terminated, the proxy must choose and forward the "best"
response from those it has seen so far.
The following processing MUST be performed on each response
that is forwarded. It is likely that more than one response
to each request will be forwarded: at least each
provisional and one final response.
7. Aggregate authorization header field values if necessary
8. Optionally rewrite Record-Route header field values
9. Forward the response
10. Generate any necessary CANCEL requests
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Each of the above steps are detailed below:
1. Find Context
The proxy locates the "response context" it created before
forwarding the original request using the key described in
Section 16.6. The remaining processing steps take place in
this context.
2. Update timer C for provisional responses
For an INVITE transaction, if the response is a provisional
response with status codes 101 to 199 inclusive (i.e.,
anything but 100), the proxy MUST reset timer C for that
client transaction. The timer MAY be reset to a different
value, but this value MUST be greater than 3 minutes.
3. Via
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The proxy removes the topmost Via header field value from
the response.
If no Via header field values remain in the response, the
response was meant for this element and MUST NOT be
forwarded. The remainder of the processing described in
this section is not performed on this message, the UAC
processing rules described in Section 8.1.3 are followed
instead (transport layer processing has already occurred).
This will happen, for instance, when the element generates
CANCEL requests as described in Section 10.
4. Add response to context
Final responses received are stored in the response context
until a final response is generated on the server
transaction associated with this context. The response may
be a candidate for the best final response to be returned
on that server transaction. Information from this response
may be needed in forming the best response even if this
response is not chosen.
If the proxy chooses to recurse on any contacts in a 3xx
response by adding them to the target set, it MUST remove
them from the response before adding the response to the
response context. However, a proxy SHOULD NOT recurse to a
non-SIPS URI if the Request-URI of the original request was
a SIPS URI. If the proxy recurses on all of the contacts
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in a 3xx response, the proxy SHOULD NOT add the resulting
contactless response to the response context.
Removing the contact before adding the response to the
response context prevents the next element upstream
from retrying a location this proxy has already
attempted.
3xx responses may contain a mixture of SIP, SIPS, and non-
SIP URIs. A proxy may choose to recurse on the SIP and SIPS
URIs and place the remainder into the response context to
be returned potentially in the final response.
If a proxy receives a 416 (Unsupported URI Scheme) response
to a request whose Request-URI scheme was not SIP, but the
scheme in the original received request was SIP or SIPS
(that is, the proxy changed the scheme from SIP or SIPS to
something else when it proxied a request), the proxy SHOULD
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add a new URI to the target set. This URI SHOULD be a SIP
URI version of the non-SIP URI that was just tried. In the
case of the tel URL, this is accomplished by placing the
telephone-subscriber part of the tel URL into the user part
of the SIP URI, and setting the hostpart to the domain
where the prior request was sent. See Section 19.1.6 for
more detail on forming SIP URIs from tel URLs.
As with a 3xx response, if a proxy "recurses" on the 416 by
trying a SIP or SIPS URI instead, the 416 response SHOULD
NOT be added to the response context.
5. Check response for forwarding
Until a final response has been sent on the server
transaction, the following responses MUST be forwarded
immediately:
- Any provisional response other than 100 (Trying)
- Any 2xx response
If a 6xx response is received, it is not immediately
forwarded, but the stateful proxy SHOULD cancel all client
pending transactions as described in Section 10, and it
MUST NOT create any new branches in this context.
This is a change from RFC 2543, which mandated that
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the proxy was to forward the 6xx response immediately.
For an INVITE transaction, this approach had the
problem that a 2xx response could arrive on another
branch, in which case the proxy would have to forward
the 2xx. The result was that the UAC could receive a
6xx response followed by a 2xx response, which should
never be allowed to happen. Under the new rules, upon
receiving a 6xx, a proxy will issue a CANCEL request,
which will generally result in 487 responses from all
outstanding client transactions, and then at that
point the 6xx is forwarded upstream.
After a final response has been sent on the server
transaction, the following responses MUST be forwarded
immediately:
- Any 2xx response to an INVITE request
A stateful proxy MUST NOT immediately forward any other
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responses. In particular, a stateful proxy MUST NOT forward
any 100 (Trying) response. Those responses that are
candidates for forwarding later as the "best" response have
been gathered as described in step "Add Response to
Context".
Any response chosen for immediate forwarding MUST be
processed as described in steps "Aggregate Authorization
Header Field Values" through "Record-Route".
This step, combined with the next, ensures that a stateful
proxy will forward exactly one final response to a non-
INVITE request, and either exactly one non-2xx response or
one or more 2xx responses to an INVITE request.
6. Choosing the best response
A stateful proxy MUST send a final response to a response
context's server transaction if no final responses have
been immediately forwarded by the above rules and all
client transactions in this response context have been
terminated.
The stateful proxy MUST choose the "best" final response
among those received and stored in the response context.
If there are no final responses in the context, the proxy
MUST send a 408 (Request Timeout) response to the server
transaction.
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Otherwise, the proxy MUST forward a response from the
responses stored in the response context. It MUST choose
from the 6xx class responses if any exist in the context.
If no 6xx class responses are present, the proxy SHOULD
choose from the lowest response class stored in the
response context. The proxy MAY select any response within
that chosen class. The proxy SHOULD give preference to
responses that provide information affecting resubmission
of this request, such as 401, 407, 415, 420, and 484 if the
4xx class is chosen.
A proxy which receives a 503 (Service Unavailable) response
SHOULD NOT forward it upstream unless it can determine that
any subsequent requests it might proxy will also generate a
503. In other words, forwarding a 503 means that the proxy
knows it cannot service any requests, not just the one for
the Request-URI in the request which generated the 503.
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the only response that was received is a 503, the proxy
SHOULD generate a 500 response and forward that upstream.
The forwarded response MUST be processed as described in
steps "Aggregate Authorization Header Field Values" through
"Record-Route".
For example, if a proxy forwarded a request to 4 locations,
and received 503, 407, 501, and 404 responses, it may
choose to forward the 407 (Proxy Authentication Required)
response.
1xx and 2xx responses may be involved in the establishment
of dialogs. When a request does not contain a To tag, the
To tag in the response is used by the UAC to distinguish
multiple responses to a dialog creating request. A proxy
MUST NOT insert a tag into the To header field of a 1xx or
2xx response if the request did not contain one. A proxy
MUST NOT modify the tag in the To header field of a 1xx or
2xx response.
Since a proxy may not insert a tag into the To header field
of a 1xx response to a request that did not contain one, it
cannot issue non-100 provisional responses on its own.
However, it can branch the request to a UAS sharing the
same element as the proxy. This UAS can return its own
provisional responses, entering into an early dialog with
the initiator of the request. The UAS does not have to be a
discreet process from the proxy. It could be a virtual UAS
implemented in the same code space as the proxy.
3-6xx responses are delivered hop-hop. When issuing a
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3-6xx responses are delivered hop-hop. When issuing a 3-6xx
response, the element is effectively acting as a UAS,
issuing its own response, usually based on the responses
received from downstream elements. An element SHOULD
preserve the To tag when simply forwarding a 3-6xx response
to a request that did not contain a To tag.
A proxy MUST NOT modify the To tag in any forwarded
response to a request that contains a To tag.
While it makes no difference to the upstream elements
if the proxy replaced the To tag in a forwarded 3-6xx
response, preserving the original tag may assist with
debugging.
When the proxy is aggregating information from several
responses, choosing a To tag from among them is arbitrary,
and generating a new To tag may make debugging easier. This
happens, for instance, when combining 401 (Unauthorized)
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and 407 (Proxy Authentication Required) challenges, or
combining Contact values from unencrypted and
unauthenticated 3xx responses.
7. Aggregate Authorization Header Field Values
If the selected response is a 401 (Unauthorized) or 407
(Proxy Authentication Required), the proxy MUST collect any
WWW-Authenticate and Proxy-Authenticate header field values
from all other 401 (Unauthorized) and 407 (Proxy
Authentication Required) responses received so far in this
response context and add them to this response without
modification before forwarding. The resulting 401
(Unauthorized) or 407 (Proxy Authentication Required)
response could have several WWW-Authenticate AND Proxy-
Authenticate header field values.
This is necessary because any or all of the destinations
the request was forwarded to may have requested
credentials. The client needs to receive all of those
challenges and supply credentials for each of them when it
retries the request. Motivation for this behavior is
provided in Section 26.
8. Record-Route
If the selected response contains a Record-Route header
field value originally provided by this proxy, the proxy
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MAY choose to rewrite the value before forwarding the
response. This allows the proxy to provide different URIs
for itself to the next upstream and downstream elements. A
proxy may choose to use this mechanism for any reason. For
instance, it is useful for multi-homed hosts.
If the proxy received the request over TLS, and sent it out
over a non-TLS connection, the proxy MUST rewrite the URI
in the Record-Route header field to be a SIPS URI. If the
proxy received the request over a non-TLS connection, and
sent it out over TLS, the proxy MUST rewrite the URI in the
Record-Route header field to be a SIP URI.
The new URI provided by the proxy MUST satisfy the same
constraints on URIs placed in Record-Route header fields in
requests (see Step 4 of Section 16.6) with the following
modifications:
The URI SHOULD NOT contain the transport parameter unless
the proxy has knowledge that the next upstream (as opposed
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to downstream) element that will be in the path of
subsequent requests supports that transport.
When a proxy does decide to modify the Record-Route header
field in the response, one of the operations it performs is
locating the Record-Route value that it had inserted. If
the request spiraled, and the proxy inserted a Record-Route
value in each iteration of the spiral, locating the correct
value in the response (which must be the proper iteration
in the reverse direction) is tricky. The rules above
recommend that a proxy wishing to rewrite Record-Route
header field values insert sufficiently distinct URIs into
the Record-Route header field so that the right one may be
selected for rewriting. A RECOMMENDED mechanism to achieve
this is for the proxy to append a unique identifier for the
proxy instance to the user portion of the URI.
When the response arrives, the proxy modifies the first
Record-Route whose identifier matches the proxy instance.
The modification results in a URI without this piece of
data appended to the user portion of the URI. Upon the next
iteration, the same algorithm (find the topmost Record-
Route header field value with the parameter) will correctly
extract the next Record-Route header field value inserted
by that proxy.
Not every response to a request to which a proxy adds
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a Record-Route header field value will contain a
Record-Route header field. If the response does
contain a Record-Route header field, it will contain
the value the proxy added.
9. Forward response
After performing the processing described in steps
"Aggregate Authorization Header Field Values" through
"Record-Route", the proxy MAY perform any feature specific
manipulations on the selected response. The proxy MUST NOT
add to, modify, or remove the message body. Unless
otherwise specified, the proxy MUST NOT remove any header
field values other than the Via header field value
discussed in Section 16.7 Item 3. In particular, the proxy
MUST NOT remove any "received" parameter it may have added
to the next Via header field value while processing the
request associated with this response. The proxy MUST pass
the response to the server transaction associated with the
response context. This will result in the response being
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sent to the location now indicated in the topmost Via
header field value. If the server transaction is no longer
available to handle the transmission, the element MUST
forward the response statelessly by sending it to the
server transport. The server transaction might indicate
failure to send the response or signal a timeout in its
state machine. These errors would be logged for diagnostic
purposes as appropriate, but the protocol requires no
remedial action from the proxy.
The proxy MUST maintain the response context until all of
its associated transactions have been terminated, even
after forwarding a final response.
10. Generate CANCELs
If the forwarded response was a final response, the proxy
MUST generate a CANCEL request for all pending client
transactions associated with this response context. A proxy
SHOULD also generate a CANCEL request for all pending
client transactions associated with this response context
when it receives a 6xx response. A pending client
transaction is one that has received a provisional
response, but no final response (it is in the proceeding
state) and has not had an associated CANCEL generated for
it. Generating CANCEL requests is described in Section
9.1.
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The requirement to CANCEL pending client transactions upon
forwarding a final response does not guarantee that an
endpoint will not receive multiple 200 (OK) responses to an
INVITE. 200 (OK) responses on more than one branch may be
generated before the CANCEL requests can be sent and
processed. Further, it is reasonable to expect that a
future extension may override this requirement to issue
CANCEL requests.
16.8 Processing Timer C
If timer C should fire, the proxy MUST either reset the timer with
any value it chooses, or terminate the client transaction. If the
client transaction has received a provisional response, the proxy
MUST generate a CANCEL request matching that transaction. If the
client transaction has not received a provisional response, the proxy
MUST behave as if the transaction received a 408 (Request Timeout)
response.
Allowing the proxy to reset the timer allows the proxy to dynamically
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extend the transaction's lifetime based on current conditions (such
as utilization) when the timer fires.
16.9 Handling Transport Errors
If the transport layer notifies a proxy of an error when it tries to
forward a request (see Section 18.4), the proxy MUST behave as if the
forwarded request received a 400 (Bad Request) response.
If the proxy is notified of an error when forwarding a response, it
drops the response. The proxy SHOULD NOT cancel any outstanding
client transactions associated with this response context due to this
notification.
If a proxy cancels its outstanding client transactions, a
single malicious or misbehaving client can cause all
transactions to fail through its Via header field.
16.10 CANCEL Processing
A stateful proxy MAY generate a CANCEL to any other request it has
generated at any time (subject to receiving a provisional response to
that request as described in section 9.1). A proxy MUST cancel any
pending client transactions associated with a response context when
it receives a matching CANCEL request.
A stateful proxy MAY generate CANCEL requests for pending INVITE
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client transactions based on the period specified in the INVITE's
Expires header field elapsing. However, this is generally unnecessary
since the endpoints involved will take care of signaling the end of
the transaction.
While a CANCEL request is handled in a stateful proxy by its own
server transaction, a new response context is not created for it.
Instead, the proxy layer searches its existing response contexts for
the server transaction handling the request associated with this
CANCEL. If a matching response context is found, the element MUST
immediately return a 200 (OK) response to the CANCEL request. In this
case, the element is acting as a user agent server as defined in
Section 8.2. Furthermore, the element MUST generate CANCEL requests
for all pending client transactions in the context as described in
Section 16.7 step 10.
If a response context is not found, the element does not have any
knowledge of the request to apply the CANCEL to. It MUST statelessly
forward the CANCEL request (it may have statelessly forwarded the
associated request previously).
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16.11 Stateless Proxy
When acting statelessly, a proxy is a simple message forwarder. Much
of the processing performed when acting statelessly is the same as
when behaving statefully. The differences are detailed here.
A stateless proxy does not have any notion of a transaction, or of
the response context used to describe stateful proxy behavior.
Instead, the stateless proxy takes messages, both requests and
responses, directly from the transport layer (See section 18). As a
result, stateless proxies do not retransmit messages on their own.
They do, however, forward all retransmission they receive (they do
not have the ability to distinguish a retransmission from the
original message). Furthermore, when handling a request statelessly,
an element MUST NOT generate its own 100 (Trying) or any other
provisional response.
A stateless proxy MUST validate a request as described in Section
16.3
A stateless proxy MUST follow the request processing steps described
in Sections 16.4 through 16.5 with the following exception:
o A stateless proxy MUST choose one and only one target from the
target set. This choice MUST only rely on fields in the
message and time-invariant properties of the server. In
particular, a retransmitted request MUST be forwarded to the
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same destination each time it is processed. Furthermore,
CANCEL and non-Routed ACK requests MUST generate the same
choice as their associated INVITE.
A stateless proxy MUST follow the request processing steps described
in Section 16.6 with the following exceptions:
o The requirement for unique branch IDs across space and time
applies to stateless proxies as well. However, a stateless
proxy cannot simply use a random number generator to compute
the first component of the branch ID, as described in Section
16.6 bullet 8. This is because retransmissions of a request
need to have the same value, and a stateless proxy cannot tell
a retransmission from the original request. Therefore, the
component of the branch parameter that makes it unique MUST be
the same each time a retransmitted request is forwarded. Thus
for a stateless proxy, the branch parameter MUST be computed
as a combinatoric function of message parameters which are
invariant on retransmission.
The stateless proxy MAY use any technique it likes to
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guarantee uniqueness of its branch IDs across transactions.
However, the following procedure is RECOMMENDED. The proxy
examines the branch ID in the topmost Via header field of the
received request. If it begins with the magic cookie, the
first component of the branch ID of the outgoing request is
computed as a hash of the received branch ID. Otherwise, the
first component of the branch ID is computed as a hash of the
topmost Via, the tag in the To header field, the tag in the
From header field, the Call-ID header field, the CSeq number
(but not method), and the Request-URI from the received
request. One of these fields will always vary across two
different transactions.
o All other message transformations specified in Section 16.6
MUST result in the same transformation of a retransmitted
request. In particular, if the proxy inserts a Record-Route
value or pushes URIs into the Route header field, it MUST
place the same values in retransmissions of the request. As
for the Via branch parameter, this implies that the
transformations MUST be based on time-invariant configuration
or retransmission-invariant properties of the request.
o A stateless proxy determines where to forward the request as
described for stateful proxies in Section 16.6 Item 10. The
request is sent directly to the transport layer instead of
through a client transaction.
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Since a stateless proxy must forward retransmitted requests
to the same destination and add identical branch parameters
to each of them, it can only use information from the
message itself and time-invariant configuration data for
those calculations. If the configuration state is not
time-invariant (for example, if a routing table is updated)
any requests that could be affected by the change may not
be forwarded statelessly during an interval equal to the
transaction timeout window before or after the change. The
method of processing the affected requests in that interval
is an implementation decision. A common solution is to
forward them transaction statefully.
Stateless proxies MUST NOT perform special processing for CANCEL
requests. They are processed by the above rules as any other
requests. In particular, a stateless proxy applies the same Route
header field processing to CANCEL requests that it applies to any
other request.
Response processing as described in Section 16.7 does not apply to a
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proxy behaving statelessly. When a response arrives at a stateless
proxy, the proxy MUST inspect the sent-by value in the first
(topmost) Via header field value. If that address matches the proxy
(it equals a value this proxy has inserted into previous requests)
the proxy MUST remove that header field value from the response and
forward the result to the location indicated in the next Via header
field value. The proxy MUST NOT add to, modify, or remove the
message body. Unless specified otherwise, the proxy MUST NOT remove
any other header field values. If the address does not match the
proxy, the message MUST be silently discarded.
16.12 Summary of Proxy Route Processing
In the absence of local policy to the contrary, the processing a
proxy performs on a request containing a Route header field can be
summarized in the following steps.
1. The proxy will inspect the Request-URI. If it indicates a
resource owned by this proxy, the proxy will replace it
with the results of running a location service. Otherwise,
the proxy will not change the Request-URI.
2. The proxy will inspect the URI in the topmost Route header
field value. If it indicates this proxy, the proxy removes
it from the Route header field (this route node has been
reached).
3. The proxy will forward the request to the resource
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indicated by the URI in the topmost Route header field
value or in the Request-URI if no Route header field is
present. The proxy determines the address, port and
transport to use when forwarding the request by applying
the procedures in [4] to that URI.
If no strict-routing elements are encountered on the path of the
request, the Request-URI will always indicate the target of the
request.
16.12.1 Examples
16.12.1.1 Basic SIP Trapezoid
This scenario is the basic SIP trapezoid, U1 -> P1 -> P2 -> U2, with
both proxies record-routing. Here is the flow.
U1 sends:
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INVITE sip:callee@domain.com SIP/2.0
Contact: sip:caller@u1.example.com
to P1. P1 is an outbound proxy. P1 is not responsible for domain.com,
so it looks it up in DNS and sends it there. It also adds a Record-
Route header field value:
INVITE sip:callee@domain.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p1.example.com;lr>
P2 gets this. It is responsible for domain.com so it runs a location
service and rewrites the Request-URI. It also adds a Record-Route
header field value. There is no Route header field, so it resolves
the new Request-URI to determine where to send the request:
INVITE sip:callee@u2.domain.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p2.domain.com;lr>
Record-Route: <sip:p1.example.com;lr>
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The callee at u2.domain.com gets this and responds with a 200 OK:
SIP/2.0 200 OK
Contact: sip:callee@u2.domain.com
Record-Route: <sip:p2.domain.com;lr>
Record-Route: <sip:p1.example.com;lr>
The callee at u2 also sets its dialog state's remote target URI to
sip:caller@u1.example.com and its route set to
(<sip:p2.domain.com;lr>,<sip:p1.example.com;lr>)
This is forwarded by P2 to P1 to U1 as normal. Now, U1 sets its
dialog state's remote target URI to sip:callee@u2.domain.com and its
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route set to
(<sip:p1.example.com;lr>,<sip:p2.domain.com;lr>)
Since all the route set elements contain the lr parameter, U1
constructs the following BYE request:
BYE sip:callee@u2.domain.com SIP/2.0
Route: <sip:p1.example.com;lr>,<sip:p2.domain.com;lr>
As any other element (including proxies) would do, it resolves the
URI in the topmost Route header field value using DNS to determine
where to send the request. This goes to P1. P1 notices that it is
not responsible for the resource indicated in the Request-URI so it
doesn't change it. It does see that it is the first value in the
Route header field, so it removes that value, and forwards the
request to P2:
BYE sip:callee@u2.domain.com SIP/2.0
Route: <sip:p2.domain.com;lr>
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P2 also notices it is not responsible for the resource indicated by
the Request-URI (it is responsible for domain.com, not
u2.domain.com), so it doesn't change it. It does see itself in the
first Route header field value, so it removes it and forwards the
following to u2.domain.com based on a DNS lookup against the
Request-URI:
BYE sip:callee@u2.domain.com SIP/2.0
16.12.1.2 Traversing a strict-routing proxy
In this scenario, a dialog is established across four proxies, each
of which adds Record-Route header field values. The third proxy
implements the strict-routing procedures specified in RFC 2543 and
the bis drafts up to bis-05.
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U1->P1->P2->P3->P4->U2
The INVITE arriving at U2 contains
INVITE sip:callee@u2.domain.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p4.domain.com;lr>
Record-Route: <sip:p3.middle.com>
Record-Route: <sip:p2.example.com;lr>
Record-Route: <sip:p1.example.com;lr>
Which U2 responds to with a 200 OK. Later, U2 sends the following BYE
request to P4 based on the first Route header field value.
BYE sip:caller@u1.example.com SIP/2.0
Route: <sip:p4.domain.com;lr>
Route: <sip:p3.middle.com>
Route: <sip:p2.example.com;lr>
Route: <sip:p1.example.com;lr>
P4 is not responsible for the resource indicated in the Request-URI
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so it will leave it alone. It notices that it is the element in the
first Route header field value so it removes it. It then prepares to
send the request based on the now first Route header field value of
sip:p3.middle.com, but it notices that this URI does not contain the
lr parameter, so before sending, it reformats the request to be:
BYE sip:p3.middle.com SIP/2.0
Route: <sip:p2.example.com;lr>
Route: <sip:p1.example.com;lr>
Route: <sip:caller@u1.example.com>
P3 is a strict router, so it forwards the following to P2:
BYE sip:p2.example.com;lr SIP/2.0
Route: <sip:p1.example.com;lr>
Route: <sip:caller@u1.example.com>
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P2 sees the request-URI is a value it placed into a Record-Route
header field, so before further processing, it rewrites the request
to be
BYE sip:caller@u1.example.com SIP/2.0
Route: <sip:p1.example.com;lr>
P2 is not responsible for u1.example.com so it sends the request to
P1 based on the resolution of the Route header field value.
P1 notices itself in the topmost Route header field value, so it
removes it, resulting in:
BYE sip:caller@u1.example.com SIP/2.0
Since P1 is not responsible for u1.example.com and there is no Route
header field, P1 will forward the request to u1.example.com based on
the Request-URI.
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16.12.1.3 Rewriting Record-Route header field values
In this scenario, U1 and U2 are in different private namespaces and
they enter a dialog through a proxy P1, which acts as a gateway
between the namespaces.
U1->P1->U2
U1 sends:
INVITE sip:callee@gateway.leftprivatespace.com SIP/2.0
Contact: <sip:caller@u1.leftprivatespace.com>
P1 uses its location service and sends the following to U2:
INVITE sip:callee@rightprivatespace.com SIP/2.0
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Contact: <sip:caller@u1.leftprivatespace.com>
Record-Route: <sip:gateway.rightprivatespace.com;lr>
U2 sends this 200 (OK) back to PI:
SIP/2.0 200 OK
Contact: <sip:callee@u2.rightprivatespace.com>
Record-Route: <sip:gateway.rightprivatespace.com;lr>
P1 rewrites its Record-Route header parameter to provide a value that
U1 will find useful, and sends the following to U1:
SIP/2.0 200 OK
Contact: <sip:callee@u2.rightprivatespace.com>
Record-Route: <sip:gateway.leftprivatespace.com;lr>
Later, U1 sends the following BYE request to P1:
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BYE sip:callee@u2.rightprivatespace.com SIP/2.0
Route: <sip:gateway.leftprivatespace.com;lr>
which P1 forwards to U2 as
BYE sip:callee@u2.rightprivatespace.com SIP/2.0
17 Transactions
SIP is a transactional protocol: interactions between components take
place in a series of independent message exchanges. Specifically, a
SIP transaction consists of a single request and any responses to
that request, which include zero or more provisional responses and
one or more final responses. In the case of a transaction where the
request was an INVITE (known as an INVITE transaction), the
transaction also includes the ACK only if the final response was not
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a 2xx response. If the response was a 2xx, the ACK is not considered
part of the transaction.
The reason for this separation is rooted in the importance
of delivering all 200 (OK) responses to an INVITE to the
UAC. To deliver them all to the UAC, the UAS alone takes
responsibility for retransmitting them (see Section
13.3.1.4), and the UAC alone takes responsibility for
acknowledging them with ACK (see Section 13.2.2.4). Since
this ACK is retransmitted only by the UAC, it is
effectively considered its own transaction.
Transactions have a client side and a server side. The client side is
known as a client transaction and the server side as a server
transaction. The client transaction sends the request, and the server
transaction sends the response. The client and server transactions
are logical functions that are embedded in any number of elements.
Specifically, they exist within user agents and stateful proxy
servers. Consider the example in Section 4. In this example, the UAC
executes the client transaction, and its outbound proxy executes the
server transaction. The outbound proxy also executes a client
transaction, which sends the request to a server transaction in the
inbound proxy. That proxy also executes a client transaction, which
in turn sends the request to a server transaction in the UAS. This is
shown in Figure 4.
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+---------+ +---------+ +---------+ +---------+
| +-+|Request |+-+ +-+|Request |+-+ +-+|Request |+-+ |
| |C||------->||S| |C||------->||S| |C||------->||S| |
| |l|| ||e| |l|| ||e| |l|| ||e| |
| |i|| ||r| |i|| ||r| |i|| ||r| |
| |e|| ||v| |e|| ||v| |e|| ||v| |
| |n|| ||e| |n|| ||e| |n|| ||e| |
| |t|| ||r| |t|| ||r| |t|| ||r| |
| | || || | | || || | | || || | |
| |T|| ||T| |T|| ||T| |T|| ||T| |
| |r|| ||r| |r|| ||r| |r|| ||r| |
| |a|| ||a| |a|| ||a| |a|| ||a| |
| |n|| ||n| |n|| ||n| |n|| ||n| |
| |s||Response||s| |s||Response||s| |s||Response||s| |
| +-+|<-------|+-+ +-+|<-------|+-+ +-+|<-------|+-+ |
+---------+ +---------+ +---------+ +---------+
UAC Outbound Inbound UAS
Proxy Proxy
Figure 4: Transaction relationships
A stateless proxy does not contain a client or server transaction.
The transaction exists between the UA or stateful proxy on one side,
and the UA or stateful proxy on the other side. As far as SIP
transactions are concerned, stateless proxies are effectively
transparent. The purpose of the client transaction is to receive a
request from the element in which the client is embedded (call this
element the "Transaction User" or TU; it can be a UA or a stateful
proxy), and reliably deliver the request to a server transaction. The
client transaction is also responsible for receiving responses and
delivering them to the TU, filtering out any response retransmissions
or disallowed responses (such as a response to ACK). Additionally, in
the case of an INVITE request, the client transaction is responsible
for generating the ACK request for any final response excepting a 2xx
response.
Similarly, the purpose of the server transaction is to receive
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requests from the transport layer and deliver them to the TU. The
server transaction filters any request retransmissions from the
network. The server transaction accepts responses from the TU and
delivers them to the transport layer for transmission over the
network. In the case of an INVITE transaction, it absorbs the ACK
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+---------+ +---------+ +---------+ +---------+
| +-+|Request |+-+ +-+|Request |+-+ +-+|Request |+-+ |
| |C||------->||S| |C||------->||S| |C||------->||S| |
| |l|| ||e| |l|| ||e| |l|| ||e| |
| |i|| ||r| |i|| ||r| |i|| ||r| |
| |e|| ||v| |e|| ||v| |e|| ||v| |
| |n|| ||e| |n|| ||e| |n|| ||e| |
| |t|| ||r| |t|| ||r| |t|| ||r| |
| | || || | | || || | | || || | |
| |T|| ||T| |T|| ||T| |T|| ||T| |
| |r|| ||r| |r|| ||r| |r|| ||r| |
| |a|| ||a| |a|| ||a| |a|| ||a| |
| |n|| ||n| |n|| ||n| |n|| ||n| |
| |s||Response||s| |s||Response||s| |s||Response||s| |
| +-+|<-------|+-+ +-+|<-------|+-+ +-+|<-------|+-+ |
+---------+ +---------+ +---------+ +---------+
UAC Outbound Inbound UAS
Proxy Proxy
Figure 4: Transaction relationships
request for any final response excepting a 2xx response.
The 2xx response and its ACK receive special treatment. This response
is retransmitted only by a UAS, and its ACK generated only by the
UAC. This end-to-end treatment is needed so that a caller knows the
entire set of users that have accepted the call. Because of this
special handling, retransmissions of the 2xx response are handled by
the UA core, not the transaction layer. Similarly, generation of the
ACK for the 2xx is handled by the UA core. Each proxy along the path
merely forwards each 2xx response to INVITE and its corresponding
ACK.
17.1 Client Transaction
The client transaction provides its functionality through the
maintenance of a state machine.
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The TU communicates with the client transaction through a simple
interface. When the TU wishes to initiate a new transaction, it
creates a client transaction and passes it the SIP request to send
and an IP address, port, and transport to which to send it. The
client transaction begins execution of its state machine. Valid
responses are passed up to the TU from the client transaction.
There are two types of client transaction state machines, depending
on the method of the request passed by the TU. One handles client
transactions for INVITE requests. This type of machine is referred to
as an INVITE client transaction. Another type handles client
transactions for all requests except INVITE and ACK. This is referred
to as a non-INVITE client transaction. There is no client transaction
for ACK. If the TU wishes to send an ACK, it passes one directly to
the transport layer for transmission.
The INVITE transaction is different from those of other methods
because of its extended duration. Normally, human input is required
in order to respond to an INVITE. The long delays expected for
sending a response argue for a three-way handshake. On the other
hand, requests of other methods are expected to complete rapidly.
Because of the non-INVITE transaction's reliance on a two-way
handshake, TUs SHOULD respond immediately to non-INVITE requests.
17.1.1 INVITE Client Transaction
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17.1.1.1 Overview of INVITE Transaction
The INVITE transaction consists of a three-way handshake. The client
transaction sends an INVITE, the server transaction sends responses,
and the client transaction sends an ACK. For unreliable transports
(such as UDP), the client transaction retransmits requests at an
interval that starts at T1 seconds and doubles after every
retransmission. T1 is an estimate of the round-trip time (RTT), and
it defaults to 500 ms. Nearly all of the transaction timers described
here scale with T1, and changing T1 adjusts their values. The request
is not retransmitted over reliable transports. After receiving a 1xx
response, any retransmissions cease altogether, and the client waits
for further responses. The server transaction can send additional 1xx
responses, which are not transmitted reliably by the server
transaction. Eventually, the server transaction decides to send a
final response. For unreliable transports, that response is
retransmitted periodically, and for reliable transports, it is sent
once. For each final response that is received at the client
transaction, the client transaction sends an ACK, the purpose of
which is to quench retransmissions of the response.
17.1.1.2 Formal Description
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The state machine for the INVITE client transaction is shown in
Figure 5. The initial state, "calling", MUST be entered when the TU
initiates a new client transaction with an INVITE request. The client
transaction MUST pass the request to the transport layer for
transmission (see Section 18). If an unreliable transport is being
used, the client transaction MUST start timer A with a value of T1.
If a reliable transport is being used, the client transaction SHOULD
NOT start timer A (Timer A controls request retransmissions). For any
transport, the client transaction MUST start timer B with a value of
64*T1 seconds (Timer B controls transaction timeouts).
When timer A fires, the client transaction MUST retransmit the
request by passing it to the transport layer, and MUST reset the
timer with a value of 2*T1. The formal definition of retransmit r transmit
within the context of the transaction layer is to take the message
previously sent to the transport layer and pass it to the transport
layer once more.
When timer A fires 2*T1 seconds later, the request MUST be
retransmitted again (assuming the client transaction is still in this
state). This process MUST continue so that the request is
retransmitted with intervals that double after each transmission.
These retransmissions SHOULD only be done while the client
transaction is in the "calling" state.
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The default value for T1 is 500 ms. T1 is an estimate of the RTT
between the client and server transactions. Elements MAY (though it
is NOT RECOMMENDED) use smaller values of T1 within closed, private
networks that do not permit general Internet connection. T1 MAY be
chosen larger, and this is RECOMMENDED if it is known in advance
(such as on high latency access links) that the RTT is larger.
Whatever the value of T1, the exponential backoffs on retransmissions
described in this section MUST be used.
If the client transaction is still in the "calling" state when timer
B fires, the client transaction SHOULD inform the TU that a timeout
has occurred. The client transaction MUST NOT generate an ACK. The
value of 64*T1 is equal to the amount of time required to send seven
requests in the case of an unreliable transport.
If the client transaction receives a provisional response while in
the "Calling" state, it transitions to the "proceeding" state. In the
"proceeding" state, the client transaction SHOULD NOT retransmit the
request any longer. Furthermore, the provisional response MUST be
passed to the TU. Any further provisional responses MUST be passed up
to the TU while in the "proceeding" state.
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When in either the "Calling" or "Proceeding" states, reception of a
response with status code from 300-699 MUST cause the client
transaction to transition to "Completed". The client transaction MUST
pass the received response up to the TU, and the client transaction
MUST generate an ACK request, even if the transport is reliable
(guidelines for constructing the ACK from the response are given in
Section 17.1.1.3) and then pass the ACK to the transport layer for
transmission. The ACK MUST be sent to the same address, port, and
transport to which the original request was sent. The client
transaction SHOULD start timer D when it enters the "Completed"
state, with a value of at least 32 seconds for unreliable transports,
and a value of zero seconds for reliable transports. Timer D reflects
the amount of time that the server transaction can remain in the
"Completed" state when unreliable transports are used. This is equal
to Timer H in the INVITE server transaction, whose default is 64*T1.
However, the client transaction does not know the value of T1 in use
by the server transaction, so an absolute minimum of 32s is used
instead of basing Timer D on T1.
Any retransmissions of the final response that are received while in
the "Completed" state MUST cause the ACK to be re-passed to the
transport layer for retransmission, but the newly received response
MUST NOT be passed up to the TU. A retransmission of the response is
defined as any response which would match the same client transaction
based on the rules of Section 17.1.3.
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|INVITE from TU
Timer A fires |INVITE sent
Reset A, V Timer B fires
INVITE sent +-----------+ or Transport Err.
+---------| |---------------+inform TU
| | Calling | |
+-------->| |-------------->|
+-----------+ 2xx |
| | 2xx to TU |
| |1xx |
300-699 +---------------+ |1xx to TU |
ACK sent | | |
resp. to TU | 1xx V |
| 1xx to TU -----------+ |
| +---------| | |
| | |Proceeding |-------------->|
| +-------->| | 2xx |
| +-----------+ 2xx to TU |
| 300-699 | |
| ACK sent, | |
| resp. to TU| |
| | | NOTE:
| 300-699 V |
| ACK sent +-----------+Transport Err. | transitions
| +---------| |Inform TU | labeled with
| | | Completed |-------------->| the event
| +-------->| | | over the action
| +-----------+ | to take
| ^ | |
| | | Timer D fires |
+--------------+ | - |
| |
V |
+-----------+ |
| | |
| Terminated|<--------------+
| |
+-----------+
Figure 5: INVITE client transaction
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If timer D fires while the client transaction is in the "Completed"
state, the client transaction MUST move to the terminated state, and
it MUST inform the TU of the timeout.
When in either the "Calling" or "Proceeding" states, reception of a
2xx response MUST cause the client transaction to enter the
"Terminated" state, and the response MUST be passed up to the TU. The
handling of this response depends on whether the TU is a proxy core
or a UAC core. A UAC core will handle generation of the ACK for this
response, while a proxy core will always forward the 200 (OK)
upstream. The differing treatment of 200 (OK) between proxy and UAC
is the reason that handling of it does not take place in the
transaction layer.
The client transaction MUST be destroyed the instant it enters the
"Terminated" state. This is actually necessary to guarantee correct
operation. The reason is that 2xx responses to an INVITE are treated
differently; each one is forwarded by proxies, and the ACK handling
in a UAC is different. Thus, each 2xx needs to be passed to a proxy
core (so that it can be forwarded) and to a UAC core (so it can be
acknowledged). No transaction layer processing takes place. Whenever
a response is received by the transport, if the transport layer finds
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|INVITE from TU
Timer A fires |INVITE sent
Reset A, V Timer B fires
INVITE sent +-----------+ or Transport Err.
+---------| |---------------+inform TU
| | Calling | |
+-------->| |-------------->|
+-----------+ 2xx |
| | 2xx to TU |
| |1xx |
300-699 +---------------+ |1xx to TU |
ACK sent | | |
resp. to TU | 1xx V |
| 1xx
no matching client transaction (using the rules of Section 17.1.3),
the response is passed directly to TU -----------+ |
| +---------| | |
| | |Proceeding |-------------->|
| +-------->| | 2xx |
| +-----------+ the core. Since the matching
client transaction is destroyed by the first 2xx, subsequent 2xx will
find no match and therefore be passed to TU |
| 300-699 | |
| the core.
17.1.1.3 Construction of the ACK sent, | |
| resp. to TU| |
| | | NOTE:
| 300-699 V |
| Request
This section specifies the construction of ACK sent +-----------+Transport Err. | transitions
| +---------| |Inform TU | labeled with
| | | Completed |-------------->| the event
| +-------->| | | over the action
| +-----------+ | to take
| ^ | |
| | | Timer D fires |
+--------------+ | - |
| |
V |
+-----------+ |
| | |
| Terminated|<--------------+
| |
+-----------+
Figure 5: INVITE client transaction
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no matching client transaction (using the rules of Section 17.1.3),
the response is passed directly to the core. Since the matching
client transaction is destroyed by the first 2xx, subsequent 2xx will
find no match and therefore be passed to the core.
17.1.1.3 Construction of the ACK Request
This section specifies the construction of ACK requests requests sent within
the client transaction. A UAC core that generates an ACK for 2xx MUST
instead follow the rules described in Section 13.
The ACK request constructed by the client transaction MUST contain
values for the Call-ID, From, and Request-URI that are equal to the
values of those header fields in the request passed to the transport
by the client transaction (call this the "original request"). The To
header field in the ACK MUST equal the To header field in the
response being acknowledged, and therefore will usually differ from
the To header field in the original request by the addition of the
tag parameter. The ACK MUST contain a single Via header field, and
this MUST be equal to the top Via header field of the original
request. The CSeq header field in the ACK MUST contain the same value
for the sequence number as was present in the original request, but
the method parameter MUST be equal to "ACK".
If the INVITE request whose response is being acknowledged had Route
header fields, those header fields MUST appear in the ACK. This is
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to ensure that the ACK can be routed properly through any downstream
stateless proxies.
Although any request MAY contain a body, a body in an ACK is special
since the request cannot be rejected if the body is not understood.
Therefore, placement of bodies in ACK for non-2xx is NOT RECOMMENDED,
but if done, the body types are restricted to any that appeared in
the INVITE, assuming that the response to the INVITE was not 415. If
it was, the body in the ACK MAY be any type listed in the Accept
header field in the 415.
For example, consider the following request:
INVITE sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.com>;tag=88sja8x
Max-Forwards: 70
Call-ID: 987asjd97y7atg
CSeq: 986759 INVITE
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The ACK request for a non-2xx final response to this request would
look like this:
ACK sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
To: Bob <sip:bob@biloxi.com>;tag=99sa0xk
From: Alice <sip:alice@atlanta.com>;tag=88sja8x
Max-Forwards: 70
Call-ID: 987asjd97y7atg
CSeq: 986759 ACK
17.1.2 Non-INVITE Client Transaction
17.1.2.1 Overview of the non-INVITE Transaction
Non-INVITE transactions do not make use of ACK. They are simple
request-response interactions. For unreliable transports, requests
are retransmitted at an interval which starts at T1 and doubles until
it hits T2. If a provisional response is received, retransmissions
continue for unreliable transports, but at an interval of T2. The
server transaction retransmits the last response it sent, which can
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be a provisional or final response, only when a retransmission of the
request is received. This is why request retransmissions need to
continue even after a provisional response, they are to ensure
reliable delivery of the final response.
Unlike an INVITE transaction, a non-INVITE transaction has no special
handling for the 2xx response. The result is that only a single 2xx
response to a non-INVITE is ever delivered to a UAC.
17.1.2.2 Formal Description
The state machine for the non-INVITE client transaction is shown in
Figure 6. It is very similar to the state machine for INVITE.
The "Trying" state is entered when the TU initiates a new client
transaction with a request. When entering this state, the client
transaction SHOULD set timer F to fire in 64*T1 seconds. The request
MUST be passed to the transport layer for transmission. If an
unreliable transport is in use, the client transaction MUST set timer
E to fire in T1 seconds. If timer E fires while still in this state,
the timer is reset, but this time with a value of MIN(2*T1, T2). When
the timer fires again, it is reset to a MIN(4*T1, T2). This process
continues so that retransmissions occur with an exponentially
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increasing interval that caps at T2. The default value of T2 is 4s,
and it represents the amount of time a non-INVITE server transaction
will take to respond to a request, if it does not respond
immediately. For the default values of T1 and T2, this results in
intervals of 500 ms, 1 s, 2 s, 4 s, 4 s, 4 s, etc.
If Timer F fires while the client transaction is still in the
"Trying" state, the client transaction SHOULD inform the TU about the
timeout, and then it SHOULD enter the "Terminated" state. If a
provisional response is received while in the "Trying" state, the
response MUST be passed to the TU, and then the client transaction
SHOULD move to the "Proceeding" state. If a final response (status
codes 200-699) is received while in the "Trying" state, the response
MUST be passed to the TU, and the client transaction MUST transition
to the "Completed" state.
If Timer E fires while in the "Proceeding" state, the request MUST be
passed to the transport layer for retransmission, and Timer E MUST be
reset with a value of T2 seconds. If timer F fires while in the
"Proceeding" state, the TU MUST be informed of a timeout, and the
client transaction MUST transition to the terminated state. If a
final response (status codes 200-699) is received while in the
"Proceeding" state, the response MUST be passed to the TU, and the
client transaction MUST transition to the "Completed" state.
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Once the client transaction enters the "Completed" state, it MUST set
Timer K to fire in T4 seconds for unreliable transports, and zero
seconds for reliable transports. The "Completed" state exists to
buffer any additional response retransmissions that may be received
(which is why the client transaction remains there only for
unreliable transports). T4 represents the amount of time the network
will take to clear messages between client and server transactions.
The default value of T4 is 5s. A response is a retransmission when it
matches the same transaction, using the rules specified in Section
17.1.3. If Timer K fires while in this state, the client transaction
MUST transition to the "Terminated" state.
Once the transaction is in the terminated state, it MUST be
destroyed.
17.1.3 Matching Responses to Client Transactions
When the transport layer in the client receives a response, it has to
determine which client transaction will handle the response, so that
the processing of Sections 17.1.1 and 17.1.2 can take place. The
branch parameter in the top Via header field is used for this
purpose. A response matches a client transaction under two
conditions:
1. If the response has the same value of the branch parameter
in the top Via header field as the branch parameter in the
top Via header field of the request that created the
transaction.
2. If the method parameter in the CSeq header field matches
the method of the request that created the transaction. The
method is needed since a CANCEL request constitutes a
different transaction, but shares the same value of the
branch parameter.
A response that matches a transaction matched by a previous response
is considered a retransmission of that response.
If a request is sent via multicast, it is possible that it will
generate multiple responses from different servers. These responses
will all have the same branch parameter in the topmost Via, but vary
in the To tag. The first response received, based on the rules above,
will be used, and others will be viewed as retransmissions. That is
not an error; multicast SIP provides only a rudimentary "single-hop-
discovery-like" service that is limited to processing a single
response. See Section 18.1.1 for details.
17.1.4 Handling Transport Errors
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|Request from TU
|send request
Timer E V
send request +-----------+
+---------| |-------------------+
| | Trying | Timer F |
+-------->| | or Transport Err.|
+-----------+ inform TU |
200-699 | | |
resp. to TU | |1xx |
+---------------+ |resp. to TU |
| | |
| Timer E V Timer F |
| send req +-----------+ or Transport Err. |
| +---------| | inform TU |
| | |Proceeding |------------------>|
| +-------->| |-----+ |
| +-----------+ |1xx |
| | ^ |resp to TU |
| 200-699 | +--------+ |
| resp. to TU | |
| | |
| V |
| +-----------+ |
| | | |
| | Completed | |
| | | |
| +-----------+ |
| ^ | |
| | | Timer K |
+--------------+ | - |
| |
V |
NOTE: +-----------+ |
| | |
transitions | Terminated|<------------------+
labeled with | |
the event +-----------+
over the action
to take
Figure 6: non-INVITE client transaction
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1. If the response has the same value of
When the branch parameter
in client transaction sends a request to the top Via header field as transport layer to
be sent, the branch parameter in following procedures are followed if the
top Via header field of transport layer
indicates a failure.
The client transaction SHOULD inform the request TU that created a transport failure
has occurred, and the
transaction.
2. If client transaction SHOULD transition directly
to the method parameter in "Terminated" state. The TU will handle the CSeq header field matches failover mechanisms
described in [4].
17.2 Server Transaction
The server transaction is responsible for the method delivery of requests to
the request that TU and the reliable transmission of responses. It accomplishes
this through a state machine. Server transactions are created by the transaction. The
method is needed since a CANCEL request constitutes a
different transaction, but shares the same value of the
branch parameter.
A response that matches a transaction matched by a previous response
is considered a retransmission of that response.
If a request is sent via multicast, it is possible that it will
generate multiple responses from different servers. These responses
will all have the same branch parameter in the topmost Via, but vary
in the To tag. The first response received, based on the rules above,
will be used, and others will be viewed as retransmissions. That is
not an error; multicast SIP provides only a rudimentary "single-hop-
discovery-like" service that is limited to processing a single
response. See Section 18.1.1 for details.
17.1.4 Handling Transport Errors
When the client transaction sends a request to the transport layer to
be sent, the following procedures are followed if the transport layer
indicates a failure.
The client transaction SHOULD inform the TU that a transport failure
has occurred, and the client transaction SHOULD transition directly
to the "Terminated" state. The TU will handle the failover
mechanisms described in [4].
17.2 Server Transaction
The server transaction is responsible for the delivery of requests to
the TU and the reliable transmission of responses. It accomplishes
this through a state machine. Server transactions are created by the
core when
core when a request is received, and transaction handling is desired
for that request (this is not always the case).
As with the client transactions, the state machine depends on whether
the received request is an INVITE request.
17.2.1 INVITE Server Transaction
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The state diagram for the INVITE server transaction is shown in
Figure 7.
When a server transaction is constructed with a request, it enters
the "Proceeding" state. The server transaction MUST generate a 100
(Trying) response unless it knows that the TU will generate a
provisional or final response within 200 ms, in which case it MAY
generate a 100 (Trying) response. This provisional response is needed
to quench request retransmissions rapidly in order to avoid network
congestion. The 100 (Trying) response is constructed according to the
procedures in Section 8.2.6, except that the insertion of tags in the
To header field of the response (when none was present in the
request) is downgraded from MAY to SHOULD NOT. The request MUST be
passed to the TU.
The TU passes any number of provisional responses to the server
transaction. So long as the server transaction is in the "Proceeding"
state, each of these MUST be passed to the transport layer for
transmission. They are not sent reliably by the transaction layer
(they are not retransmitted by it) and do not cause a change in the
state of the server transaction. If a request retransmission is
received while in the "Proceeding" state, the most recent provisional
response that was received from the TU MUST be passed to the
transport layer for retransmission. A request is a retransmission if
it matches the same server transaction based on the rules of Section
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17.2.3.
If, while in the "Proceeding" state, the TU passes a 2xx response to
the server transaction, the server transaction MUST pass this
response to the transport layer for transmission. It is not
retransmitted by the server transaction; retransmissions of 2xx
responses are handled by the TU. The server transaction MUST then
transition to the "Terminated" state.
While in the "Proceeding" state, if the TU passes a response with
status code from 300 to 699 to the server transaction, the response
MUST be passed to the transport layer for transmission, and the state
machine MUST enter the "Completed" state. For unreliable transports,
timer G is set to fire in T1 seconds, and is not set to fire for
reliable transports.
This is a change from RFC 2543, where responses were always
retransmitted, even over reliable transports.
When the "Completed" state is entered, timer H MUST be set to fire in
64*T1 seconds for all transports. Timer H determines when the server
transaction abandons retransmitting the response. Its value is chosen
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|INVITE
|pass INV to TU
INVITE V send 100 if TU won't in 200ms
send response+-----------+
+--------| |--------+101-199 from TU
| | Proceeding| |send response
+------->| |<-------+
| | Transport Err.
| | Inform TU
| |--------------->+
+-----------+ |
300-699 from TU | |2xx from TU |
send response | |send response |
| +------------------>+
| |
INVITE V Timer G fires |
send response+-----------+ send response |
+--------| |--------+ |
| | Completed | | |
+------->| |<-------+ |
+-----------+ |
| | |
ACK | | |
- | +------------------>+
| Timer H fires |
V or Transport Err.|
+-----------+ Inform TU |
| | |
| Confirmed | |
| | |
+-----------+ |
| |
|Timer I fires |
|- |
| |
V |
+-----------+ |
| | |
| Terminated|<---------------+
| |
+-----------+
Figure 7: INVITE server transaction
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to equal Timer B, the amount of time a client transaction will
continue to retry sending a request. If timer G fires, the response
is passed to the transport layer once more for retransmission, and
timer G is set to fire in MIN(2*T1, T2) seconds. From then on, when
timer G fires, the response is passed to the transport again for
transmission, and timer G is reset with a value that doubles, unless
that value exceeds T2, in which case it is reset with the value of
T2. This is identical to the retransmit behavior for requests in the
"Trying" state of the non-INVITE client transaction. Furthermore,
while in the "Completed" state, if a request retransmission is
received, the server SHOULD pass the response to the transport for
retransmission.
If an ACK is received while the server transaction is in the
"Completed" state, the server transaction MUST transition to the
"Confirmed" state. As Timer G is ignored in this state, any
retransmissions of the response will cease.
If timer H fires while in the "Completed" state, it implies that the
ACK was never received. In this case, the server transaction MUST
transition to the "Terminated" state, and MUST indicate to the TU
that a transaction failure has occurred.
The purpose of the "Confirmed" state is to absorb any additional ACK
messages that arrive, triggered from retransmissions of the final
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|INVITE
|pass INV to TU
INVITE V send 100 if TU won't in 200ms
send response+-----------+
+--------| |--------+101-199 from TU
| | Proceeding| |send response
+------->| |<-------+
| | Transport Err.
| | Inform TU
| |--------------->+
+-----------+ |
300-699 from TU | |2xx from TU |
send response | |send response |
| +------------------>+
| |
INVITE V Timer G fires |
send response+-----------+ send response |
+--------| |--------+ |
| | Completed | | |
+------->| |<-------+ |
+-----------+ |
| | |
ACK | | |
- | +------------------>+
| Timer H fires |
V or Transport Err.|
+-----------+ Inform TU |
| | |
| Confirmed | |
| | |
+-----------+ |
| |
|Timer I fires |
|- |
| |
V |
+-----------+ |
| | |
| Terminated|<---------------+
| |
+-----------+
Figure 7: INVITE server transaction
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response. When this state is entered, timer I is set to fire in T4
seconds for unreliable transports, and zero seconds for reliable
transports. Once timer I fires, the server MUST transition to the
"Terminated" state.
Once the transaction is in the "Terminated" state, it MUST be
destroyed. As with client transactions, this is needed to ensure
reliability of the 2xx responses to INVITE.
17.2.2 Non-INVITE Server Transaction
The state machine for the non-INVITE server transaction is shown in
Figure 8.
The state machine is initialized in the "Trying" state and is passed
a request other than INVITE or ACK when initialized. This request is
passed up to the TU. Once in the "Trying" state, any further request
retransmissions are discarded. A request is a retransmission if it
matches the same server transaction, using the rules specified in
Section 17.2.3.
While in the "Trying" state, if the TU passes a provisional response
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to the server transaction, the server transaction MUST enter the
"Proceeding" state. The response MUST be passed to the transport
layer for transmission. Any further provisional responses that are
received from the TU while in the "Proceeding" state MUST be passed
to the transport layer for transmission. If a retransmission of the
request is received while in the "Proceeding" state, the most
recently sent provisional response MUST be passed to the transport
layer for retransmission. If the TU passes a final response (status
codes 200-699) to the server while in the "Proceeding" state, the
transaction MUST enter the "Completed" state, and the response MUST
be passed to the transport layer for transmission.
When the server transaction enters the "Completed" state, it MUST set
Timer J to fire in 64*T1 seconds for unreliable transports, and zero
seconds for reliable transports. While in the "Completed" state, the
server transaction MUST pass the final response to the transport
layer for retransmission whenever a retransmission of the request is
received. Any other final responses passed by the TU to the server
transaction MUST be discarded while in the "Completed" state. The
server transaction remains in this state until Timer J fires, at
which point it MUST transition to the "Terminated" state.
The server transaction MUST be destroyed the instant it enters the
"Terminated" state.
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17.2.3 Matching Requests to Server Transactions
When a request is received from the network by the server, it has to
be matched to an existing transaction. This is accomplished in the
following manner.
The branch parameter in the topmost Via header field of the request
is examined. If it is present and begins with the magic cookie
"z9hG4bK", the request was generated by a client transaction
compliant to this specification. Therefore, the branch parameter will
be unique across all transactions sent by that client. The request
matches a transaction if the branch parameter in the request is equal
to the one in the top Via header field of the request that created
the transaction, the sent-by value in the top Via of the request is
equal to the one in the request that created the transaction, and in
the case of a CANCEL request, the method of the request that created
the transaction was also CANCEL. This matching rule applies to both
INVITE and non-INVITE transactions alike.
The sent-by value is used as part of the matching process
because there could be accidental or malicious duplication
of branch parameters from different clients.
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|Request received
|pass to TU
V
+-----------+
| |
| Trying |-------------+
| | |
+-----------+ |200-699 from TU
| |send response
|1xx from TU |
|send response |
| |
Request V 1xx from TU |
send response+-----------+send response|
+--------| |--------+ |
| | Proceeding| | |
+------->| |<-------+ |
+<--------------| | |
|Trnsprt Err +-----------+ |
|Inform TU | |
| | |
| |200-699 from TU |
| |send response |
| Request V |
| send response+-----------+ |
| +--------| | |
| | | Completed |<------------+
| +------->| |
+<--------------| |
|Trnsprt Err +-----------+
|Inform TU |
| |Timer J fires
| |-
| |
| V
| +-----------+
| | |
+-------------->| Terminated|
| |
+-----------+
Figure 8: non-INVITE server transaction
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If the branch parameter in the top Via header field is not present,
or does not contain the magic cookie, the following procedures are
used. These exist to handle backwards compatibility with RFC 2543
compliant implementations.
The INVITE request matches a transaction if the Request-URI, To tag,
From tag, Call-ID, CSeq, and top Via header field match those of the
INVITE request which created the transaction. In this case, the
INVITE is a retransmission of the original one that created the
transaction. The ACK request matches a transaction if the Request-
URI, From tag, Call-ID, CSeq number (not the method), and top Via
header field match those of the INVITE request which created the
transaction, and the To tag of the ACK matches the To tag of the
response sent by the server transaction. Matching is done based on
the matching rules defined for each of those header fields. The usage Inclusion
of the tag in the To header field in the ACK matching process helps
disambiguate ACK for 2xx from ACK for other responses at a proxy,
which may have forwarded both responses (which (This can occur in unusual conditions).
conditions. Specifically, when a proxy forked a request, and then
crashes, the responses may be delivered to another proxy, which might
end up forwarding multiple responses upstream). An ACK request that
matches an INVITE transaction matched by a previous ACK is considered
a retransmission of that previous ACK.
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|Request received
|pass to TU
V
+-----------+
| |
| Trying |-------------+
| | |
+-----------+ |200-699 from TU
| |send response
|1xx from TU |
|send response |
| |
Request V 1xx from TU |
send response+-----------+send response|
+--------| |--------+ |
| | Proceeding| | |
+------->| |<-------+ |
+<--------------| | |
|Trnsprt Err +-----------+ |
|Inform TU | |
| | |
| |200-699 from TU |
| |send response |
| Request V |
| send response+-----------+ |
| +--------| | |
| | | Completed |<------------+
| +------->| |
+<--------------| |
|Trnsprt Err +-----------+
|Inform TU |
| |Timer J fires
| |-
| |
| V
| +-----------+
| | |
+-------------->| Terminated|
| |
+-----------+
Figure 8: non-INVITE server transaction
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For all other request methods, a request is matched to a transaction
if the Request-URI, To tag, From tag, Call-ID Cseq (including the
method), and top Via header field match those of the request that
created the transaction. Matching is done based on the matching rules
defined for each of those header fields. When a non-INVITE request
matches an existing transaction, it is a retransmission of the
request that created that transaction.
Because the matching rules include the Request-URI, the server cannot
match a response to a transaction. When the TU passes a response to
the server transaction, it must pass it to the specific server
transaction for which the response is targeted.
17.2.4 Handling Transport Errors
When the server transaction sends a response to the transport layer
to be sent, the following procedures are followed if the transport
layer indicates a failure.
First, the procedures in [4] are followed, which attempt to deliver
the response to a backup. If those should all fail, based on the
definition of failure in [4], the server transaction SHOULD inform
the TU that a failure has occurred, and SHOULD transition to the
terminated state.
18 Transport
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The transport layer is responsible for the actual transmission of
requests and responses over network transports. This includes
determination of the connection to use for a request or response in
the case of connection-oriented transports.
The transport layer is responsible for managing persistent
connections for transport protocols like TCP and SCTP, or TLS over
those, including ones opened to the transport layer. This includes
connections opened by the client or server transports, so that
connections are shared between client and server transport functions.
These connections are indexed by the tuple formed from the address,
port, and transport protocol at the far end of the connection. When a
connection is opened by the transport layer, this index is set to the
destination IP, port and transport. When the connection is accepted
by the transport layer, this index is set to the source IP address,
port number, and transport. Note that, because the source port is
often ephemeral, but it cannot be known whether it is ephemeral or
selected through procedures in [4], connections accepted by the
transport layer will frequently not be reused. The result is that two
proxies in a "peering" relationship using a connection-oriented
transport frequently will have two connections in use, one for
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transactions initiated in each direction.
It is RECOMMENDED that connections be kept open for some
implementation-defined duration after the last message was sent or
received over that connection. This duration SHOULD at least equal
the longest amount of time the element would need in order to bring a
transaction from instantiation to the terminated state. This is to
make it likely that transactions complete over the same connection on
which they are initiated (for example, request, response, and in the
case of INVITE, ACK for non-2xx responses). This usually means at
least 64*T1 (see Section 17.1.1.1 for a definition of T1). However,
it could be larger in an element that has a TU using a large value
for timer C (bullet 11 of Section 16.6), for example.
All SIP elements MUST implement UDP and TCP. SIP elements MAY
implement other protocols.
Making TCP mandatory for the UA is a substantial change
from RFC 2543. It has arisen out of the need to handle
larger messages, which MUST use TCP, as discussed below.
Thus, even if an element never sends large messages, it may
receive one and needs to be able to handle them.
18.1 Clients
18.1.1 Sending Requests
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The client side of the transport layer is responsible for sending the
request and receiving responses. The user of the transport layer
passes the client transport the request, an IP address, port,
transport, and possibly TTL for multicast destinations.
If a request is within 200 bytes of the path MTU, or if it is larger
than 1300 bytes and the path MTU is unknown, the request MUST be sent
using TCP. This prevents fragmentation of messages over UDP and
provides congestion control for larger messages. However,
implementations MUST be able to handle messages up to the maximum
datagram packet size. For UDP, this size is 65,535 bytes, including
IP and UDP headers.
The 200 byte "buffer" between the message size and the MTU
accommodates the fact that the response in SIP can be
larger than the request. This happens due to the addition
of Record-Route header field values to the responses to
INVITE, for example. With the extra buffer, the response
can be about 170 bytes larger than the request, and still
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not be fragmented on IPv4 (about 30 bytes is consumed by
IP/UDP, assuming no IPSec). 1300 is chosen when path MTU is
not known, based on the assumption of a 1500 byte Ethernet
MTU.
If an element sends a request over TCP because of these message size
constraints, and that request would have otherwise been sent over
UDP, if the attempt to establish the connection generates either an
ICMP Protocol Not Supported, or results in a TCP reset, the element
SHOULD retry the request, using UDP. This is only to provide
backwards compatibility with RFC 2543 compliant implementations that
do not support UDP. It is anticipated that this behavior will be
deprecated in a future revision of this specification.
A client that sends a request to a multicast address MUST add the
"maddr" parameter to its Via header field value containing the
destination multicast address, and for IPv4, SHOULD add the "ttl"
parameter with a value of 1. Usage of IPv6 multicast is not defined
in this specification, and will be a subject of future
standardization when the need arises.
These rules result in a purposeful limitation of multicast in SIP.
Its primary function is to provide an "single-hop-discovery-like"
service, delivering a request to a group of homogeneous servers,
where it is only required to process the response from any one of
them. This functionality is most useful for registrations. In fact,
based on the transaction processing rules in Section 17.1.3, the
client transaction will accept the first response, and view any
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others as retransmissions because they all contain the same Via
branch identifier.
Before a request is sent, the client transport MUST insert a value of
the "sent-by" field into the Via header field. This field contains an
IP address or host name, and port. The usage of an FQDN is
RECOMMENDED. This field is used for sending responses under certain
conditions, described below. If the port is absent, the default value
depends on the transport. It is 5060 for UDP, TCP and SCTP, 5061 for
TLS.
For reliable transports, the response is normally sent on the
connection on which the request was received. Therefore, the client
transport MUST be prepared to receive the response on the same
connection used to send the request. Under error conditions, the
server may attempt to open a new connection to send the response. To
handle this case, the transport layer MUST also be prepared to
receive an incoming connection on the source IP address from which
the request was sent and port number in the "sent-by" field. It also
MUST be prepared to receive incoming connections on any address and
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port that would be selected by a server based on the procedures
described in Section 5 of [4].
For unreliable unicast transports, the client transport MUST be
prepared to receive responses on the source IP address from which the
request is sent (as responses are sent back to the source address)
and the port number in the "sent-by" field. Furthermore, as with
reliable transports, in certain cases the response will be sent
elsewhere. The client MUST be prepared to receive responses on any
address and port that would be selected by a server based on the
procedures described in Section 5 of [4].
For multicast, the client transport MUST be prepared to receive
responses on the same multicast group and port to which the request
is sent (that is, it needs to be a member of the multicast group it
sent the request to.)
If a request is destined to an IP address, port, and transport to
which an existing connection is open, it is RECOMMENDED that this
connection be used to send the request, but another connection MAY be
opened and used.
If a request is sent using multicast, it is sent to the group
address, port, and TTL provided by the transport user. If a request
is sent using unicast unreliable transports, it is sent to the IP
address and port provided by the transport user.
18.1.2 Receiving Responses
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When a response is received, the client transport examines the top
Via header field value. If the value of the "sent-by" parameter in
that header field value does not correspond to a value that the
client transport is configured to insert into requests, the response
MUST be silently discarded.
If there are any client transactions in existence, the client
transport uses the matching procedures of Section 17.1.3 to attempt
to match the response to an existing transaction. If there is a
match, the response MUST be passed to that transaction. Otherwise,
the response MUST be passed to the core (whether it be stateless
proxy, stateful proxy, or UA) for further processing. Handling of
these "stray" responses is dependent on the core (a proxy will
forward them, while a UA will discard, for example).
18.2 Servers
18.2.1 Receiving Requests
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A server SHOULD be prepared to received requests on any IP address,
port and transport combination that can be the result of a DNS lookup
on a SIP or SIPS URI [4] that is handed out for the purposes of
communicating with that server. In this context, "handing out"
includes placing a URI in a Contact header field in a REGISTER
request or a any redirect response, or in a Record-Route header field
in a request or response. A URI can also be "handed out" by placing
it on a web page or business card. It is also RECOMMENDED that a
server listen for requests on the default SIP ports (5060 for TCP and
UDP, 5061 for TLS over TCP) on all public interfaces. The typical
exception would be private networks, or when multiple server
instances are running on the same host. For any port and interface
that a server listens on for UDP, it MUST listen on that same port
and interface for TCP. This is because a message may need to be sent
using TCP, rather than UDP, if it is too large. As a result, the
converse is not true. A server need not listen for UDP on a
particular address and port just because it is listening on that same
address and port for TCP. There may, of course, be other reasons why
a server needs to listen for UDP on a particular address and port.
When the server transport receives a request over any transport, it
MUST examine the value of the "sent-by" parameter in the top Via
header field value. If the host portion of the "sent-by" parameter
contains a domain name, or if it contains an IP address that differs
from the packet source address, the server MUST add a "received"
parameter to that Via header field value. This parameter MUST contain
the source address from which the packet was received. This is to
assist the server transport layer in sending the response, since it
must be sent to the source IP address from which the request came.
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Consider a request received by the server transport which looks like,
in part:
INVITE sip:bob@Biloxi.com SIP/2.0
Via: SIP/2.0/UDP bobspc.biloxi.com:5060
The request is received with a source IP address of 192.0.2.4. Before
passing the request up, the transport adds a "received" parameter, so
that the request would look like, in part:
INVITE sip:bob@Biloxi.com SIP/2.0
Via: SIP/2.0/UDP bobspc.biloxi.com:5060;received=192.0.2.4
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Next, the server transport attempts to match the request to a server
transaction. It does so using the matching rules described in Section
17.2.3. If a matching server transaction is found, the request is
passed to that transaction for processing. If no match is found, the
request is passed to the core, which may decide to construct a new
server transaction for that request. Note that when a UAS core sends
a 2xx response to INVITE, the server transaction is destroyed. This
means that when the ACK arrives, there will be no matching server
transaction, and based on this rule, the ACK is passed to the UAS
core, where it is processed.
18.2.2 Sending Responses
The server transport uses the value of the top Via header field in
order to determine where to send a response. It MUST follow the
following process:
o If the "sent-protocol" is a reliable transport protocol such
as TCP or SCTP, or TLS over those, the response MUST be sent
using the existing connection to the source of the original
request that created the transaction, if that connection is
still open. This requires the server transport to maintain an
association between server transactions and transport
connections. If that connection is no longer open, the server
SHOULD open a connection to the IP address in the "received"
parameter, if present, using the port in the "sent-by" value,
or the default port for that transport, if no port is
specified. If that connection attempt fails, the server SHOULD
use the procedures in [4] for servers in order to
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the IP address and port to open the connection and send the
response to.
o Otherwise, if the Via header field value contains a "maddr"
parameter, the response MUST be forwarded to the address
listed there, using the port indicated in "sent-by", or port
5060 if none is present. If the address is a multicast
address, the response SHOULD be sent using the TTL indicated
in the "ttl" parameter, or with a TTL of 1 if that parameter
is not present.
o Otherwise (for unreliable unicast transports), if the top Via
has a "received" parameter, the response MUST be sent to the
address in the "received" parameter, using the port indicated
in the "sent-by" value, or using port 5060 if none is
specified explicitly. If this fails, for example, elicits an
ICMP "port unreachable" response, the procedures of Section 5
of [4] SHOULD be used to determine where to send the response.
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o Otherwise, if it is not receiver-tagged, the response MUST be
sent to the address indicated by the "sent-by" value, using
the procedures in Section 5 of [4].
18.3 Framing
In the case of message-oriented transports (such as UDP), if the
message has a Content-Length header field, the message body is
assumed to contain that many bytes. If there are additional bytes in
the transport packet beyond the end of the body, they MUST be
discarded. If the transport packet ends before the end of the message
body, this is considered an error. If the message is a response, it
MUST be discarded. If the message is a request, the element SHOULD
generate a 400 (Bad Request) response. If the message has no
Content-Length header field, the message body is assumed to end at
the end of the transport packet.
In the case of stream-oriented transports such as TCP, the Content-
Length header field indicates the size of the body. The Content-
Length header field MUST be used with stream oriented transports.
18.4 Error Handling
Error handling is independent of whether the message was a request or
response.
If the transport user asks for a message to be sent over an
unreliable transport, and the result is an ICMP error, the behavior
depends on the type of ICMP error. Host, network, port or protocol
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unreachable errors, or parameter problem errors SHOULD cause the
transport layer to inform the transport user of a failure in sending.
Source quench and TTL exceeded ICMP errors SHOULD be ignored.
If the transport user asks for a request to be sent over a reliable
transport, and the result is a connection failure, the transport
layer SHOULD inform the transport user of a failure in sending.
19 Common Message Components
There are certain components of SIP messages that appear in various
places within SIP messages (and sometimes, outside of them) that
merit separate discussion.
19.1 SIP and SIPS Uniform Resource Indicators
A SIP or SIPS URI identifies a communications resource. Like all
URIs, SIP and SIPS URIs may be placed in web pages, email messages,
or printed literature. They contain sufficient information to
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initiate and maintain a communication session with the resource.
Examples of communications resources include the following:
o a user of an online service
o an appearance on a multi-line phone
o a mailbox on a messaging system
o a PSTN number at a gateway service
o a group (such as "sales" or "helpdesk") in an organization
A SIPS URI specifies that the resource be contacted securely. This
means, in particular, that TLS is to be used between the UAC and the
domain that owns the URI. From there, secure communications are used
to reach the user, where the specific security mechanism depends on
the policy of the domain. Any resource described by a SIP URI can be
"upgraded" to a SIPS URI by just changing the scheme, if it is
desired to communicate with that resource securely.
19.1.1 SIP and SIPS URI Components
The "sip:" and "sips:" schemes follow the guidelines in RFC 2396 [5].
They use a form similar to the mailto URL, allowing the specification
of SIP request-header fields and the SIP message-body. This makes it
possible to specify the subject, media type, or urgency of sessions
initiated by using a URI on a web page or in an email message. The
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formal syntax for a SIP or SIPS URI is presented in Section 25. Its
general form, in the case of a SIP URI, is
sip:user:password@host:port;uri-parameters?headers
The format for a SIPS URI is the same, except that the scheme is
"sips" instead of sip. These tokens, and some of the tokens in their
expansions, have the following meanings:
user: The identifier of a particular resource at the host being
addressed. The term "host" in this context frequently
refers to a domain. The "userinfo" of a URI consists of
this user field, the password field, and the @ sign
following them. The userinfo part of a URI is optional and
MAY be absent when the destination host does not have a
notion of users or when the host itself is the resource
being identified. If the @ sign is present in a SIP or SIPS
URI, the user field MUST NOT be empty.
If the host being addressed can process telephone numbers,
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for instance, an Internet telephony gateway, a telephone-
subscriber field defined in RFC 2806 [9] MAY be used to
populate the user field. There are special escaping rules
for encoding telephone-subscriber fields in SIP and SIPS
URIs described in Section 19.1.2.
password: A password associated with the user. While the SIP
and SIPS URI syntax allows this field to be present, its
use is NOT RECOMMENDED, because the passing of
authentication information in clear text (such as URIs) has
proven to be a security risk in almost every case where it
has been used. For instance, transporting a PIN number in
this field exposes the PIN.
Note that the password field is just an extension of user
portion. Implementations not wishing to give special
significance to the password portion of the field MAY
simply treat "user:password" as a single string.
host: The host providing the SIP resource. The host part
contains either a fully-qualified domain name or numeric
IPv4 or IPv6 address. Using the fully-qualified domain name
form is RECOMMENDED whenever possible.
port: The port number where the request is to be sent.
URI parameters: Parameters affecting a request constructed from
the URI.
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URI parameters are added after the hostport component and
are separated by semi-colons.
URI parameters take the form:
parameter-name "=" parameter-value
Even though an arbitrary number of URI parameters may be
included in a URI, any given parameter-name MUST NOT appear
more than once.
This extensible mechanism includes the transport, maddr,
ttl, user, method and lr parameters.
The transport parameter determines the transport mechanism
to be used for sending SIP messages, as specified in [4].
SIP can use any network transport protocol. Parameter names
are defined for UDP (RFC 768 [14]), TCP (RFC 761 [15]), and
SCTP (RFC 2960 [16]). For a SIPS URI, the transport
parameter MUST indicate a reliable transport.
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The maddr parameter indicates the server address to be
contacted for this user, overriding any address derived
from the host field. When an maddr parameter is present,
the port and transport components of the URI apply to the
address indicated in the maddr parameter value. [4]
describes the proper interpretation of the transport,
maddr, and hostport in order to obtain the destination
address, port, and transport for sending a request.
The maddr field has been used as a simple form of
loose source routing. It allows a URI to specify a
proxy that must be traversed en-route to the
destination. Continuing to use the maddr parameter
this way is strongly discouraged (the mechanisms that
enable it are deprecated). Implementations should
instead use the Route mechanism described in this
document, establishing a pre-existing route set if
necessary (see Section 8.1.1.1). This provides a full
URI to describe the node to be traversed.
The ttl parameter determines the time-to-live value of the
UDP multicast packet and MUST only be used if maddr is a
multicast address and the transport protocol is UDP. For
example, to specify to call alice@atlanta.com using
multicast to 239.255.255.1 with a ttl of 15, the following
URI would be used:
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sip:alice@atlanta.com;maddr=239.255.255.1;ttl=15
The set of valid telephone-subscriber strings is a subset
of valid user strings. The user URI parameter exists to
distinguish telephone numbers from user names that happen
to look like telephone numbers. If the user string
contains a telephone number formatted as a telephone-
subscriber, the user parameter value "phone" SHOULD be
present. Even without this parameter, recipients of SIP and
SIPS URIs MAY interpret the pre-@ part as a telephone
number if local restrictions on the name space for user
name allow it.
The method of the SIP request constructed from the URI can
be specified with the method parameter.
The lr parameter, when present, indicates that the element
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responsible for this resource implements the routing
mechanisms specified in this document. This parameter will
be used in the URIs proxies place into Record-Route header
field values, and may appear in the URIs in a pre-existing
route set.
This parameter is used to achieve backwards
compatibility with systems implementing the strict-
routing mechanisms of RFC 2543 and the rfc2543bis
drafts up to bis-05. An element preparing to send a
request based on a URI not containing this parameter
can assume the receiving element implements strict-
routing and reformat the message to preserve the
information in the Request-URI.
Since the uri-parameter mechanism is extensible, SIP
elements MUST silently ignore any uri-parameters that they
do not understand.
Headers: Header fields to be included in a request constructed
from the URI.
Headers fields in the SIP request can be specified with the
"?" mechanism within a URI. The header names and values are
encoded in ampersand separated hname = hvalue pairs. The
special hname "body" indicates that the associated hvalue
is the message-body of the SIP request.
Table 1 summarizes the use of SIP and SIPS URI components based on
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the context in which the URI appears. The external column describes
URIs appearing anywhere outside of a SIP message, for instance on a
web page or business card. Entries marked "m" are mandatory, those
marked "o" are optional, and those marked "-" are not allowed.
Elements processing URIs SHOULD ignore any disallowed components if
they are present. The second column indicates the default value of an
optional element if it is not present. "--" indicates that the
element is either not optional, or has no default value.
URIs in Contact header fields have different restrictions depending
on the context in which the header field appears. One set applies to
messages that establish and maintain dialogs (INVITE and its 200 (OK)
response). The other applies to registration and redirection messages
(REGISTER, its 200 (OK) response, and 3xx class responses to any
method).
19.1.2 Character Escaping Requirements
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dialog
reg./redir. Contact/
default Req.-URI To From Contact R-R/Route external
user -- o o o o o o
password -- o o o o o o
host -- m m m m m m
port (1) o - - o o o
user-param ip o o o o o o
method INVITE - - - - - o
maddr-param -- o - - o o o
ttl-param 1 o - - o - o
transp.-param (2) o - - o o o
lr-param -- o - - - o o
other-param -- o o o o o o
headers -- - - - o - o
(1): The default port value is transport and scheme dependent. The
default is 5060 for sip: using UDP, TCP, or SCTP. The default is
5061 for sip: using TLS over TCP and sips: over TCP.
(2): The default transport is scheme dependent. For sip:, it is UDP.
For sips:, it is TCP.
Table 1: Use and default values of URI components for SIP header
field values, Request-URI and references
19.1.2 Character Escaping Requirements
SIP follows the requirements and guidelines of RFC 2396 [5] when
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defining the set of characters that must be escaped in a SIP URI, and
uses its ""%" HEX HEX" mechanism for escaping. From RFC 2396 [5]:
The set of characters actually reserved within any given
URI component is defined by that component. In general, a
character is reserved if the semantics of the URI changes
if the character is replaced with its escaped US-ASCII
encoding. [5]. Excluded US-ASCII characters (RFC 2396
[5]), such as space and control characters and characters
used as URI delimiters, also MUST be escaped. URIs MUST NOT
contain unescaped space and control characters.
For each component, the set of valid BNF expansions defines exactly
which characters may appear unescaped. All other characters MUST be
escaped.
For example, "@" is not in the set of characters in the user
component, so the user "j@s0n" must have at least the @ sign encoded,
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as in "j%40s0n".
Expanding the hname and hvalue tokens in Section 25 show that all URI
reserved characters in header field names and values MUST be escaped.
The telephone-subscriber subset of the user component has special
escaping considerations. The set of characters not reserved in the
RFC 2806 [9] description of telephone-subscriber contains a number of
characters in various syntax elements that need to be escaped when
used in SIP URIs. Any characters occurring in a telephone-subscriber
that do not appear in an expansion of the BNF for the user rule MUST
be escaped.
Note that character escaping is not allowed in the host component of
a SIP or SIPS URI (the % character is not valid in its expansion).
This is likely to change in the future as requirements for
Internationalized Domain Names are finalized. Current implementations
MUST NOT attempt to improve robustness by treating received escaped
characters in the host component as literally equivalent to their
unescaped counterpart. The behavior required to meet the
requirements of IDN may be significantly different.
19.1.3 Example SIP and SIPS URIs
sip:alice@atlanta.com
sip:alice:secretword@atlanta.com;transport=tcp
sips:alice@atlanta.com?subject=project%20x&priority=urgent
sip:+1-212-555-1212:1234@gateway.com;user=phone
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sips:1212@gateway.com
sip:alice@192.0.2.4
sip:atlanta.com;method=REGISTER?to=alice%40atlanta.com
sip:alice;day=tuesday@atlanta.com
The last sample URI above has a user field value of
"alice;day=tuesday". The escaping rules defined above allow a
semicolon to appear unescaped in this field. For the purposes of this
protocol, the field is opaque. The structure of that value is only
useful to the SIP element responsible for the resource.
19.1.4 URI Comparison
Some operations in this specification require determining whether two
SIP or SIPS URIs are equivalent. In this specification, registrars
need to compare bindings in Contact URIs in REGISTER requests (see
Section 10.3.) SIP and SIPS URIs are compared for equality according
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to the following rules:
o A SIP and SIPS URI are never equivalent.
o Comparison of the userinfo of SIP and SIPS URIs is case-
sensitive. This includes userinfo containing passwords or
formatted as telephone-subscribers. Comparison of all other
components of the URI is case-insensitive unless explicitly
defined otherwise.
o The ordering of parameters and header fields is not
significant in comparing SIP and SIPS URIs.
o Characters other than those in the "reserved" and "unsafe"
sets (see RFC 2396 [5]) are equivalent to their ""%" HEX HEX"
encoding.
o An IP address that is the result of a DNS lookup of a host
name does not match that host name.
o For two URIs to be equal, the user, password, host, and port
components must match.
A URI omitting the user component will not match a URI that
includes one. A URI omitting the password component will not
match a URI that includes one.
A URI omitting any component with a default value will not
match a URI explicitly containing that component with its
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default value. For instance, a URI omitting the optional port
component will not match a URI explicitly declaring port 5060.
The same is true for the transport-parameter, ttl-parameter,
user-parameter, and method components.
Defining sip:user@host to not be equivalent to
sip:user@host:5060 is a change from RFC 2543. When
deriving addresses from URIs, equivalent addresses are
expected from equivalent URIs. The URI
sip:user@host:5060 will always resolve to port 5060.
The URI sip:user@host may resolve to other ports
through the DNS SRV mechanisms detailed in [4].
o URI uri-parameter components are compared as follows
- Any uri-parameter appearing in both URIs must match.
- A user, ttl, or method uri-parameter appearing in only one
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URI never matches, even if it contains the default value.
- A URI that includes an maddr parameter will not match a URI
that contains no maddr parameter.
- All other uri-parameters appearing in only one URI are
ignored when comparing the URIs.
o URI header components are never ignored. Any present header
component MUST be present in both URIs and match for the URIs
to match. The matching rules are defined for each header field
in Section 20.
The URIs within each of the following sets are equivalent:
sip:%61lice@atlanta.com;transport=TCP
sip:alice@AtLanTa.CoM;Transport=tcp
sip:carol@chicago.com
sip:carol@chicago.com;newparam=5
sip:carol@chicago.com;security=on
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sip:biloxi.com;transport=tcp;method=REGISTER?to=sip:bob%40biloxi.com
sip:biloxi.com;method=REGISTER;transport=tcp?to=sip:bob%40biloxi.com
sip:alice@atlanta.com?subject=project%20x&priority=urgent
sip:alice@atlanta.com?priority=urgent&subject=project%20x
The URIs within each of the following sets are not equivalent:
SIP:ALICE@AtLanTa.CoM;Transport=udp (different usernames)
sip:alice@AtLanTa.CoM;Transport=UDP
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sip:bob@biloxi.com (can resolve to different ports)
sip:bob@biloxi.com:5060
sip:bob@biloxi.com (can resolve to different transports)
sip:bob@biloxi.com;transport=udp
sip:bob@biloxi.com (can resolve to different port and transports)
sip:bob@biloxi.com:6000;transport=tcp
sip:carol@chicago.com (different header component)
sip:carol@chicago.com?Subject=next%20meeting
sip:bob@phone21.boxesbybob.com (even though that's what
sip:bob@192.0.2.4 phone21.boxesbybob.com resolves to)
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Note that equality is not transitive:
o sip:carol@chicago.com and sip:carol@chicago.com;security=on
are equivalent
o sip:carol@chicago.com and sip:carol@chicago.com;security=off
are equivalent
o sip:carol@chicago.com;security=on and
sip:carol@chicago.com;security=off are not equivalent
19.1.5 Forming Requests from a URI
An implementation needs to take care when forming requests directly
from a URI. URIs from business cards, web pages, and even from
sources inside the protocol such as registered contacts may contain
inappropriate header fields or body parts.
An implementation MUST include any provided transport, maddr, ttl, or
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user parameter in the Request-URI of the formed request. If the URI
contains a method parameter, its value MUST be used as the method of
the request. The method parameter MUST NOT be placed in the Request-
URI. Unknown URI parameters MUST be placed in the message's Request-
URI.
An implementation SHOULD treat the presence of any headers or body
parts in the URI as a desire to include them in the message, and
choose to honor the request on a per-component basis.
An implementation SHOULD NOT honor these obviously dangerous header
fields: From, Call-ID, CSeq, Via, and Record-Route.
An implementation SHOULD NOT honor any requested Route header field
values in order to not be used as an unwitting agent in malicious
attacks.
An implementation SHOULD NOT honor requests to include header fields
that may cause it to falsely advertise its location or capabilities.
These include: Accept, Accept-Encoding, Accept-Language, Allow,
Contact (in its dialog usage), Organization, Supported, and User-
Agent.
An implementation SHOULD verify the accuracy of any requested
descriptive header fields, including: Content-Disposition, Content-
Encoding, Content-Language, Content-Length, Content-Type, Date,
Mime-Version, and Timestamp.
If the request formed from constructing a message from a given URI is
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not a valid SIP request, the URI is invalid. An implementation MUST
NOT proceed with transmitting the request. It should instead pursue
the course of action due an invalid URI in the context it occurs.
The constructed request can be invalid in many ways. These
include, but are not limited to, syntax error in header
fields, invalid combinations of URI parameters, or an
incorrect description of the message body.
Sending a request formed from a given URI may require capabilities
unavailable to the implementation. The URI might indicate use of an
unimplemented transport or extension, for example. An implementation
SHOULD refuse to send these requests rather than modifying them to
match their capabilities. An implementation MUST NOT send a request
requiring an extension that it does not support.
For example, such a request can be formed through the
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presence of a Require header parameter or a method URI
parameter with an unknown or explicitly unsupported value.
19.1.6 Relating SIP URIs and tel URLs
When a tel URL (RFC 2806 [9]) is converted to a SIP or SIPS URI, the
entire telephone-subscriber portion of the tel URL, including any
parameters, is placed into the userinfo part of the SIP or SIPS URI.
Thus, tel:+358-555-1234567;postd=pp22 becomes
sip:+358-555-1234567;postd=pp22@foo.com;user=phone
or
sips:+358-555-1234567;postd=pp22@foo.com;user=phone
not
sip:+358-555-1234567@foo.com;postd=pp22;user=phone
or
sips:+358-555-1234567@foo.com;postd=pp22;user=phone
In general, equivalent "tel" URLs converted to SIP or SIPS URIs in
this fashion may not produce equivalent SIP or SIPS URIs. The
userinfo of SIP and SIPS URIs are compared as a case-sensitive
string. Variance in case-insensitive portions of tel URLs and
reordering of tel URL parameters does not affect tel URL equivalence,
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but does affect the equivalence of SIP URIs formed from them.
For example,
tel:+358-555-1234567;postd=pp22
tel:+358-555-1234567;POSTD=PP22
are equivalent, while
sip:+358-555-1234567;postd=pp22@foo.com;user=phone
sip:+358-555-1234567;POSTD=PP22@foo.com;user=phone
are not.
Likewise,
tel:+358-555-1234567;postd=pp22;isub=1411
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tel:+358-555-1234567;isub=1411;postd=pp22
are equivalent, while
sip:+358-555-1234567;postd=pp22;isub=1411@foo.com;user=phone
sip:+358-555-1234567;isub=1411;postd=pp22@foo.com;user=phone
are not.
To mitigate this problem, elements constructing telephone-subscriber
fields to place in the userinfo part of a SIP or SIPS URI SHOULD fold
any case-insensitive portion of telephone-subscriber to lower case,
and order the telephone-subscriber parameters lexically by parameter
name. (All components of a tel URL except for future-extension
parameters are defined to be compared case-insensitive.)
Following this suggestion, both
tel:+358-555-1234567;postd=pp22
tel:+358-555-1234567;POSTD=PP22
become
sip:+358-555-1234567;postd=pp22@foo.com;user=phone
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and both
tel:+358-555-1234567;postd=pp22;isub=1411
tel:+358-555-1234567;isub=1411;postd=pp22
become
sip:+358-555-1234567;isub=1411;postd=pp22;user=phone
19.2 Option Tags
Option tags are unique identifiers used to designate new options
(extensions) in SIP. These tags are used in Require (Section 20.32),
Proxy-Require (Section 20.29), Supported (Section 20.37) and
Unsupported (Section 20.40) header fields. Note that these options
appear as parameters in those header fields in an option-tag = token
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form (see Section 25 for the definition of token).
Option tags are defined in standards track RFCs. This is a change
from past practice, and is instituted to ensure continuing multi-
vendor interoperability (see discussion in Section 20.32 and Section
20.37). An IANA registry of option tags is used to ensure easy
reference.
19.3 Tags
The "tag" parameter is used in the To and From header fields of SIP
messages. It serves as a general mechanism to identify a dialog,
which is the combination of the Call-ID along with two tags, one from
each participant in the dialog. When a UA sends a request outside of
a dialog, it contains a From tag only, providing "half" of the dialog
ID. The dialog is completed from the response(s), each of which
contributes the second half in the To header field. The forking of
SIP requests means that multiple dialogs can be established from a
single request. This also explains the need for the two-sided dialog
identifier; without a contribution from the recipients, the
originator could not disambiguate the multiple dialogs established
from a single request.
When a tag is generated by a UA for insertion into a request or
response, it MUST be globally unique and cryptographically random
with at least 32 bits of randomness. A property of this selection
requirement is that a UA will place a different tag into the From
header of an INVITE as it would place into the To header of the
response to the same INVITE. This is needed in order for a UA to
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invite itself to a session, a common case for "hairpinning" of calls
in PSTN gateways. Similarly, two INVITEs for different calls will
have different From tags, and two responses for different calls will
have different To tags.
Besides the requirement for global uniqueness, the algorithm for
generating a tag is implementation-specific. Tags are helpful in
fault tolerant systems, where a dialog is to be recovered on an
alternate server after a failure. A UAS can select the tag in such a
way that a backup can recognize a request as part of a dialog on the
failed server, and therefore determine that it should attempt to
recover the dialog and any other state associated with it.
20 Header Fields
The general syntax for header fields is covered in Section 7.3. This
section lists the full set of header fields along with notes on
syntax, meaning, and usage. Throughout this section, we use [HX.Y]
to refer to Section X.Y of the current HTTP/1.1 specification RFC
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2616 [8]. Examples of each header field are given.
Information about header fields in relation to methods and proxy
processing is summarized in Tables 2 and 3.
The "where" column describes the request and response types in which
the header field can be used. Values in this column are:
R: header field may only appear in requests;
r: header field may only appear in responses;
2xx, 4xx, etc.: A numerical value or range indicates response
codes with which the header field can be used;
c: header field is copied from the request to the response.
An empty entry in the "where" column indicates that the header
field may be present in all requests and responses.
The "proxy" column describes the operations a proxy may perform on a
header field:
a: A proxy can add or concatenate the header field if not
present.
m: A proxy can modify an existing header field value.
d: A proxy can delete a header field value.
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r: A proxy must be able to read the header field, and thus this
header field cannot be encrypted.
The next six columns relate to the presence of a header field in a
method:
c: Conditional; requirements on the header field depend on the
context of the message.
m: The header field is mandatory.
m*: The header field SHOULD be sent, but clients/servers need to
be prepared to receive messages without that header field.
o: The header field is optional.
t: The header field SHOULD be sent, but clients/servers need to
be prepared to receive messages without that header field.
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If a stream-based protocol (such as TCP) is used as a
transport, then the header field MUST be sent.
*: The header field is required if the message body is not
empty. See sections 20.14, 20.15 and 7.4 for details.
-: The header field is not applicable.
"Optional" means that a UA MAY include the header field in a request
or response, and a UA MAY ignore the header field if present in the
request or response (The exception to this rule is the Require header
field discussed in 20.32). A "mandatory" header field MUST be present
in a request, and MUST be understood by the UAS receiving the
request. A mandatory response header field MUST be present in the
response, and the header field MUST be understood by the UAC
processing the response. "Not applicable" means that the header field
MUST NOT be present in a request. If one is placed in a request by
mistake, it MUST be ignored by the UAS receiving the request.
Similarly, a header field labeled "not applicable" for a response
means that the UAS MUST NOT place the header field in the response,
and the UAC MUST ignore the header field in the response.
A UA SHOULD ignore extension header parameters that are not
understood.
A compact form of some common header field names is also defined for
use when overall message size is an issue.
The Contact, From, and To header fields contain a URI. If the URI
contains a comma, question mark or semicolon, the URI MUST be
enclosed in angle brackets (< and >). Any URI parameters are
contained within these brackets. If the URI is not enclosed in angle
brackets, any semicolon-delimited parameters are header-parameters,
not URI parameters.
20.1 Accept
The Accept header field follows the syntax defined in [H14.1]. The
semantics are also identical, with the exception that if no Accept
header field is present, the server SHOULD assume a default value of
application/sdp
An empty Accept header field means that no formats are acceptable.
Example:
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Header field where proxy ACK BYE CAN INV OPT REG
___________________________________________________________
Accept R - o - o m* o
Accept 2xx - - - o m* o
Accept 415 - c - c c c
Accept-Encoding R - o - o o o
Accept-Encoding 2xx - - - o m* o
Accept-Encoding 415 - c - c c c
Accept-Language R - o - o o o
Accept-Language 2xx - - - o m* o
Accept-Language 415 - c - c c c
Alert-Info R ar - - - o - -
Alert-Info 180 ar - - - o - -
Allow R - o - o o o
Allow 2xx - o - m* m* o
Allow r - o - o o o
Allow 405 - m - m m m
Authentication-Info 2xx - o - o o o
Authorization R o o o o o o
Call-ID c r m m m m m m
Call-Info ar - - - o o o
Contact R o - - m o o
Contact 1xx - - - o - -
Contact 2xx - - - m o o
Contact 3xx d - o - o o o
Contact 485 - o - o o o
Content-Disposition o o - o o o
Content-Encoding o o - o o o
Content-Language o o - o o o
Content-Length ar t t t t t t
Content-Type * * - * * *
CSeq c r m m m m m m
Date a o o o o o o
Error-Info 300-699 a - o o o o o
Expires - - - o - o
From c r m m m m m m
In-Reply-To R - - - o - -
Max-Forwards R amr m m m m m m
Min-Expires 423 - - - - - m
MIME-Version o o - o o o
Organization ar - - - o o o
Table 2: Summary of header fields, A--O
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Header field where proxy ACK BYE CAN INV OPT REG
___________________________________________________________________
Priority R ar - - - o - -
Proxy-Authenticate 407 ar - m - m m m
Proxy-Authenticate 401 ar - o o o o o
Proxy-Authorization R dr o o - o o o
Proxy-Require R ar - o - o o o
Record-Route R ar o o o o o -
Record-Route 2xx,18x mr - o o o o -
Reply-To - - - o - -
Require ar - c - c c c
Retry-After 404,413,480,486 - o o o o o
500,503 - o o o o o
600,603 - o o o o o
Route R adr c c c c c c
Server r - o o o o o
Subject R - - - o - -
Supported R - o o m* o o
Supported 2xx - o o m* m* o
Timestamp o o o o o o
To c(1) r m m m m m m
Unsupported 420 - m - m m m
User-Agent o o o o o o
Via R amr m m m m m m
Via rc dr m m m m m m
Warning r - o o o o o
WWW-Authenticate 401 ar - m - m m m
WWW-Authenticate 407 ar - o - o o o
Table 3: Summary of header fields, P--Z; (1): copied with possible
addition of tag
The Contact, From, and To header fields contain a URI. If the URI
contains a comma, question mark or semicolon, the URI MUST be
enclosed in angle brackets (< and >). Any URI parameters are
contained within these brackets. If the URI is not enclosed in angle
brackets, any semicolon-delimited parameters are header-parameters,
not URI parameters.
20.1 Accept
The Accept header field follows the syntax defined in [H14.1]. The
semantics are also identical, with the exception that if no Accept
header field is present, the server SHOULD assume a default value of
application/sdp
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An empty Accept header field means that no formats are acceptable.
Example:
Accept: application/sdp;level=1, application/x-private, text/html
20.2 Accept-Encoding
The Accept-Encoding header field is similar to Accept, but restricts
the content-codings [H3.5] that are acceptable in the response. See
[H14.3]. The syntax of this header field is defined in [H14.3]. The
semantics in SIP are identical to those defined in [H14.3].
An empty Accept-Encoding header field is permissible, even though the
syntax in [H14.3] does not provide for it. It is equivalent to
Accept-Encoding: identity, that is, only the identity encoding,
meaning no encoding, is permissible.
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If no Accept-Encoding header field is present, the server SHOULD
assume a default value of identity.
This differs slightly from the HTTP definition, which indicates that
when not present, any encoding can be used, but the identity encoding
is preferred.
Example:
Accept-Encoding: gzip
20.3 Accept-Language
The Accept-Language header field is used in requests to indicate the
preferred languages for reason phrases, session descriptions, or
status responses carried as message bodies in the response. If no
Accept-Language header field is present, the server SHOULD assume all
languages are acceptable to the client.
The Accept-Language header field follows the syntax defined in
[H14.4]. The rules for ordering the languages based on the "q"
parameter apply to SIP as well.
Example:
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Accept-Language: da, en-gb;q=0.8, en;q=0.7
20.4 Alert-Info
When present in an INVITE request, the Alert-Info header field
specifies an alternative ring tone to the UAS. When present in a 180
(Ringing) response, the Alert-Info header field specifies an
alternative ringback tone to the UAC. A typical usage is for a proxy
to insert this header field to provide a distinctive ring feature.
The Alert-Info header field can introduce security risks. These risks
and the ways to handle them are discussed in Section 20.9, which
discusses the Call-Info header field since the risks are identical.
In addition, a user SHOULD be able to disable this feature
selectively.
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This helps prevent disruptions that could result from the
use of this header field by untrusted elements.
Example:
Alert-Info: <http://www.example.com/sounds/moo.wav>
20.5 Allow
The Allow header field lists the set of methods supported by the UA
generating the message.
All methods, including ACK and CANCEL, understood by the UA MUST be
included in the list of methods in the Allow header field, when
present. The absence of an Allow header field MUST NOT be interpreted
to mean that the UA sending the message supports no methods. Rather,
it implies that the UA is not providing any information on what
methods it supports.
Supplying an Allow header field in responses to methods other than
OPTIONS reduces the number of messages needed.
Example:
Allow: INVITE, ACK, OPTIONS, CANCEL, BYE
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20.6 Authentication-Info
The Authentication-Info header field provides for mutual
authentication with HTTP Digest. A UAS MAY include this header field
in a 2xx response to a request that was successfully authenticated
using digest based on the Authorization header field.
Syntax and semantics follow those specified in RFC 2617 [17].
Example:
Authentication-Info: nextnonce="47364c23432d2e131a5fb210812c"
20.7 Authorization
The Authorization header field contains authentication credentials of
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a UA. Section 22.2 overviews the use of the Authorization header
field, and Section 22.4 describes the syntax and semantics when used
with HTTP authentication.
This header field, along with Proxy-Authorization, breaks the general
rules about multiple header field values. Although not a comma-
separated list, this header field name may be present multiple times,
and MUST NOT be combined into a single header line using the usual
rules described in Section 7.3.
In the example below, there are no quotes around the Digest
parameter:
Authorization: Digest username="Alice", realm="atlanta.com",
nonce="84a4cc6f3082121f32b42a2187831a9e",
response="7587245234b3434cc3412213e5f113a5432"
20.8 Call-ID
The Call-ID header field uniquely identifies a particular invitation
or all registrations of a particular client. A single multimedia
conference can give rise to several calls with different Call-IDs,
for example, if a user invites a single individual several times to
the same (long-running) conference. Call-IDs are case-sensitive and
are simply compared byte-by-byte.
The compact form of the Call-ID header field is i.
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Examples:
Call-ID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6@biloxi.com
i:f81d4fae-7dec-11d0-a765-00a0c91e6bf6@192.0.2.4
20.9 Call-Info
The Call-Info header field provides additional information about the
caller or callee, depending on whether it is found in a request or
response. The purpose of the URI is described by the "purpose"
parameter. The "icon" parameter designates an image suitable as an
iconic representation of the caller or callee. The "info" parameter
describes the caller or callee in general, for example, through a web
page. The "card" parameter provides a business card, for example, in
vCard [35] [36] or LDIF [36] [37] formats. Additional tokens can be registered
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using IANA and the procedures in Section 27.
Use of the Call-Info header field can pose a security risk. If a
callee fetches the URIs provided by a malicious caller, the callee
may be at risk for displaying inappropriate or offensive content,
dangerous or illegal content, and so on. Therefore, it is RECOMMENDED
that a UA only render the information in the Call-Info header field
if it can verify the authenticity of the element that originated the
header field and trusts that element. This need not be the peer UA; a
proxy can insert this header field into requests.
Example:
Call-Info: <http://wwww.example.com/alice/photo.jpg> ;purpose=icon,
<http://www.example.com/alice/> ;purpose=info
20.10 Contact
A Contact header field value provides a URI whose meaning depends on
the type of request or response it is in.
A Contact header field value can contain a display name, a URI with
URI parameters, and header parameters.
This document defines the Contact parameters "q" and "expires". These
parameters are only used when the Contact is present in a REGISTER
request or response, or in a 3xx response. Additional parameters may
be defined in other specifications.
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When the header field value contains a display name, the URI
including all URI parameters is enclosed in "<" and ">". If no "<"
and ">" are present, all parameters after the URI are header
parameters, not URI parameters. The display name can be tokens, or a
quoted string, if a larger character set is desired.
Even if the "display-name" is empty, the "name-addr" form MUST be
used if the