rfc4975
Network Working Group B. Campbell, Ed.
Request for Comments: 4975 Estacado Systems
Category: Standards Track R. Mahy, Ed.
Plantronics
C. Jennings, Ed.
Cisco Systems, Inc.
September 2007
The Message Session Relay Protocol (MSRP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes the Message Session Relay Protocol, a
protocol for transmitting a series of related instant messages in the
context of a session. Message sessions are treated like any other
media stream when set up via a rendezvous or session creation
protocol such as the Session Initiation Protocol.
Campbell, et al. Standards Track [Page 1]
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Table of Contents
1. Introduction ....................................................4
2. Conventions .....................................................5
3. Applicability of MSRP ...........................................5
4. Protocol Overview ...............................................6
5. Key Concepts ....................................................9
5.1. MSRP Framing and Message Chunking ..........................9
5.2. MSRP Addressing ...........................................10
5.3. MSRP Transaction and Report Model .........................11
5.4. MSRP Connection Model .....................................12
6. MSRP URIs ......................................................14
6.1. MSRP URI Comparison .......................................15
6.2. Resolving MSRP Host Device ................................16
7. Method-Specific Behavior .......................................17
7.1. Constructing Requests .....................................17
7.1.1. Sending SEND Requests ..............................18
7.1.2. Sending REPORT Requests ............................21
7.1.3. Generating Success Reports .........................22
7.1.4. Generating Failure Reports .........................23
7.2. Constructing Responses ....................................24
7.3. Receiving Requests ........................................25
7.3.1. Receiving SEND Requests ............................25
7.3.2. Receiving REPORT Requests ..........................27
8. Using MSRP with SIP and SDP ....................................27
8.1. SDP Connection and Media-Lines ............................28
8.2. URI Negotiations ..........................................29
8.3. Path Attributes with Multiple URIs ........................30
8.4. Updated SDP Offers ........................................31
8.5. Connection Negotiation ....................................31
8.6. Content Type Negotiation ..................................32
8.7. Example SDP Exchange ......................................34
8.8. MSRP User Experience with SIP .............................35
8.9. SDP Direction Attribute and MSRP ..........................35
9. Formal Syntax ..................................................36
10. Response Code Descriptions ....................................38
10.1. 200 ......................................................38
10.2. 400 ......................................................38
10.3. 403 ......................................................38
10.4. 408 ......................................................39
10.5. 413 ......................................................39
10.6. 415 ......................................................39
10.7. 423 ......................................................39
10.8. 481 ......................................................39
10.9. 501 ......................................................39
10.10. 506 .....................................................40
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11. Examples ......................................................40
11.1. Basic IM Session .........................................40
11.2. Message with XHTML Content ...............................42
11.3. Chunked Message ..........................................43
11.4. Chunked Message with Message/CPIM Payload ................43
11.5. System Message ...........................................44
11.6. Positive Report ..........................................44
11.7. Forked IM ................................................45
12. Extensibility .................................................48
13. CPIM Compatibility ............................................48
14. Security Considerations .......................................49
14.1. Secrecy of the MSRP URI ..................................50
14.2. Transport Level Protection ...............................50
14.3. S/MIME ...................................................51
14.4. Using TLS in Peer-to-Peer Mode ...........................52
14.5. Other Security Concerns ..................................53
15. IANA Considerations ...........................................55
15.1. MSRP Method Names ........................................55
15.2. MSRP Header Fields .......................................55
15.3. MSRP Status Codes ........................................56
15.4. MSRP Port ................................................56
15.5. URI Schema ...............................................56
15.5.1. MSRP Scheme .......................................56
15.5.2. MSRPS Scheme ......................................57
15.6. SDP Transport Protocol ...................................57
15.7. SDP Attribute Names ......................................58
15.7.1. Accept Types ......................................58
15.7.2. Wrapped Types .....................................58
15.7.3. Max Size ..........................................58
15.7.4. Path ..............................................58
16. Contributors and Acknowledgments ..............................59
17. References ....................................................59
17.1. Normative References .....................................59
17.2. Informative References ...................................60
Campbell, et al. Standards Track [Page 3]
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1. Introduction
A series of related instant messages between two or more parties can
be viewed as part of a "message session", that is, a conversational
exchange of messages with a definite beginning and end. This is in
contrast to individual messages each sent independently. Messaging
schemes that track only individual messages can be described as
"page-mode" messaging, whereas messaging that is part of a "session"
with a definite start and end is called "session-mode" messaging.
Page-mode messaging is enabled in SIP via the SIP [4] MESSAGE method
[22]. Session-mode messaging has a number of benefits over page-mode
messaging, however, such as explicit rendezvous, tighter integration
with other media-types, direct client-to-client operation, and
brokered privacy and security.
This document defines a session-oriented instant message transport
protocol called the Message Session Relay Protocol (MSRP), whose
sessions can be negotiated with an offer or answer [3] using the
Session Description Protocol (SDP) [2]. The exchange is carried by
some signaling protocol, such as SIP [4]. This allows a
communication user agent to offer a messaging session as one of the
possible media-types in a session. For instance, Alice may want to
communicate with Bob. Alice doesn't know at the moment whether Bob
has his phone or his IM client handy, but she's willing to use
either. She sends an invitation to a session to the address of
record she has for Bob, sip:bob@example.com. Her invitation offers
both voice and an IM session. The SIP services at example.com
forward the invitation to Bob at his currently registered clients.
Bob accepts the invitation at his IM client, and they begin a
threaded chat conversation.
When a user uses an Instant Messaging (IM) URL, RFC 3861 [32] defines
how DNS can be used to map this to a particular protocol to establish
the session such as SIP. SIP can use an offer/answer model to
transport the MSRP URIs for the media in SDP. This document defines
how the offer/answer exchange works to establish MSRP connections and
how messages are sent across the MSRP, but it does not deal with the
issues of mapping an IM URL to a session establishment protocol.
This session model allows message sessions to be integrated into
advanced communications applications with little to no additional
protocol development. For example, during the above chat session,
Bob decides Alice really needs to be talking to Carol. Bob can
transfer [21] Alice to Carol, introducing them into their own
messaging session. Messaging sessions can then be easily integrated
into call-center and dispatch environments using third-party call
control [20] and conferencing [19] applications.
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This document specifies MSRP behavior only for peer-to-peer sessions,
that is, sessions crossing only a single hop. MSRP relay devices
[23] (referred to herein as "relays") are specified in a separate
document. An endpoint that implements this specification, but not
the relay specification, will be unable to introduce relays into the
message path, but will still be able to interoperate with peers that
do use relays.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].
This document consistently refers to a "message" as a complete unit
of MIME or text content. In some cases, a message is split and
delivered in more than one MSRP request. Each of these portions of
the complete message is called a "chunk".
3. Applicability of MSRP
MSRP is not designed for use as a standalone protocol. MSRP MUST be
used only in the context of a rendezvous mechanism meeting the
following requirements:
o The rendezvous mechanism MUST provide both MSRP URIs associated
with an MSRP session to each of the participating endpoints. The
rendezvous mechanism MUST implement mechanisms to protect the
confidentiality of these URIs -- they MUST NOT be made available
to an untrusted third party or be easily discoverable.
o The rendezvous mechanism MUST provide mechanisms for the
negotiation of any supported MSRP extensions that are not
backwards compatible.
o The rendezvous mechanism MUST be able to natively transport im:
URIs or automatically translate im: URIs [27] into the addressing
identifiers of the rendezvous protocol.
To use a rendezvous mechanism with MSRP, an RFC MUST be prepared that
describes how it exchanges MSRP URIs and meets these requirements
listed here. This document provides such a description for the use
of MSRP in the context of SIP and SDP.
SIP meets these requirements for a rendezvous mechanism. The MSRP
URIs are exchanged using SDP in an offer/answer exchange via SIP.
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The exchanged SDP can also be used to negotiate MSRP extensions.
This SDP can be secured using any of the mechanisms available in SIP,
including using the sips mechanism to ensure transport security
across intermediaries and Secure/Multipurpose Internet Mail
Extensions (S/MIME) for end-to-end protection of the SDP body. SIP
can carry arbitrary URIs (including im: URIs) in the Request-URI, and
procedures are available to map im: URIs to sip: or sips: URIs. It
is expected that initial deployments of MSRP will use SIP as its
rendezvous mechanism.
4. Protocol Overview
MSRP is a text-based, connection-oriented protocol for exchanging
arbitrary (binary) MIME [8] content, especially instant messages.
This section is a non-normative overview of how MSRP works and how it
is used with SIP.
MSRP sessions are typically arranged using SIP the same way a session
of audio or video media is set up. One SIP user agent (Alice) sends
the other (Bob) a SIP invitation containing an offered session-
description that includes a session of MSRP. The receiving SIP user
agent can accept the invitation and include an answer session-
description that acknowledges the choice of media. Alice's session
description contains an MSRP URI that describes where she is willing
to receive MSRP requests from Bob, and vice versa. (Note: Some lines
in the examples are removed for clarity and brevity.)
Alice sends to Bob:
INVITE sip:bob@biloxi.example.com SIP/2.0
To: <sip:bob@biloxi.example.com>
From: <sip:alice@atlanta.example.com>;tag=786
Call-ID: 3413an89KU
Content-Type: application/sdp
c=IN IP4 atlanta.example.com
m=message 7654 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://atlanta.example.com:7654/jshA7weztas;tcp
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Bob sends to Alice:
SIP/2.0 200 OK
To: <sip:bob@biloxi.example.com>;tag=087js
From: <sip:alice@atlanta.example.com>;tag=786
Call-ID: 3413an89KU
Content-Type: application/sdp
c=IN IP4 biloxi.example.com
m=message 12763 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
Alice sends to Bob:
ACK sip:bob@biloxi SIP/2.0
To: <sip:bob@biloxi.example.com>;tag=087js
From: <sip:alice@atlanta.example.com>;tag=786
Call-ID: 3413an89KU
Figure 1: Session Setup
MSRP defines two request types, or methods. SEND requests are used
to deliver a complete message or a chunk (a portion of a complete
message), while REPORT requests report on the status of a previously
sent message, or a range of bytes inside a message. When Alice
receives Bob's answer, she checks to see if she has an existing
connection to Bob. If not, she opens a new connection to Bob using
the URI he provided in the SDP. Alice then delivers a SEND request
to Bob with her initial message, and Bob replies indicating that
Alice's request was received successfully.
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MSRP a786hjs2 SEND
To-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: 87652491
Byte-Range: 1-25/25
Content-Type: text/plain
Hey Bob, are you there?
-------a786hjs2$
MSRP a786hjs2 200 OK
To-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
-------a786hjs2$
Figure 2: Example MSRP Exchange
Alice's request begins with the MSRP start line, which contains a
transaction identifier that is also used for request framing. Next
she includes the path of URIs to the destination in the To-Path
header field, and her own URI in the From-Path header field. In this
typical case, there is just one "hop", so there is only one URI in
each path header field. She also includes a message ID, which she
can use to correlate status reports with the original message. Next
she puts the actual content. Finally, she closes the request with an
end-line of seven hyphens, the transaction identifier, and a "$" to
indicate that this request contains the end of a complete message.
If Alice wants to deliver a very large message, she can split the
message into chunks and deliver each chunk in a separate SEND
request. The message ID corresponds to the whole message, so the
receiver can also use it to reassemble the message and tell which
chunks belong with which message. Chunking is described in more
detail in Section 5.1. The Byte-Range header field identifies the
portion of the message carried in this chunk and the total size of
the message.
Alice can also specify what type of reporting she would like in
response to her request. If Alice requests positive acknowledgments,
Bob sends a REPORT request to Alice confirming the delivery of her
complete message. This is especially useful if Alice sent a series
of SEND requests containing chunks of a single message. More on
requesting types of reports and errors is described in Section 5.3.
Alice and Bob choose their MSRP URIs in such a way that it is
difficult to guess the exact URI. Alice and Bob can reject requests
to URIs they are not expecting to service and can correlate the
specific URI with the probable sender. Alice and Bob can also use
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TLS [1] to provide channel security over this hop. To receive MSRP
requests over a TLS protected connection, Alice or Bob could
advertise URIs with the "msrps" scheme instead of "msrp".
MSRP is designed with the expectation that MSRP can carry URIs for
nodes on the far side of relays. For this reason, a URI with the
"msrps" scheme makes no assertion about the security properties of
other hops, just the next hop. The user agent knows the URI for each
hop, so it can verify that each URI has the desired security
properties.
MSRP URIs are discussed in more detail in Section 6.
An adjacent pair of busy MSRP nodes (for example, two relays) can
easily have several sessions, and exchange traffic for several
simultaneous users. The nodes can use existing connections to carry
new traffic with the same destination host, port, transport protocol,
and scheme. MSRP nodes can keep track of how many sessions are using
a particular connection and close these connections when no sessions
have used them for some period of time. Connection management is
discussed in more detail in Section 5.4.
5. Key Concepts
5.1. MSRP Framing and Message Chunking
Messages sent using MSRP can be very large and can be delivered in
several SEND requests, where each SEND request contains one chunk of
the overall message. Long chunks may be interrupted in mid-
transmission to ensure fairness across shared transport connections.
To support this, MSRP uses a boundary-based framing mechanism. The
start line of an MSRP request contains a unique identifier that is
also used to indicate the end of the request. Included at the end of
the end-line, there is a flag that indicates whether this is the last
chunk of data for this message or whether the message will be
continued in a subsequent chunk. There is also a Byte-Range header
field in the request that indicates the overall position of this
chunk inside the complete message.
For example, the following snippet of two SEND requests demonstrates
a message that contains the text "abcdEFGH" being sent as two chunks.
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MSRP dkei38sd SEND
Message-ID: 4564dpWd
Byte-Range: 1-*/8
Content-Type: text/plain
abcd
-------dkei38sd+
MSRP dkei38ia SEND
Message-ID: 4564dpWd
Byte-Range: 5-8/8
Content-Type: text/plain
EFGH
-------dkei38ia$
Figure 3: Breaking a Message into Chunks
This chunking mechanism allows a sender to interrupt a chunk part of
the way through sending it. The ability to interrupt messages allows
multiple sessions to share a TCP connection, and for large messages
to be sent efficiently while not blocking other messages that share
the same connection, or even the same MSRP session. Any chunk that
is larger than 2048 octets MUST be interruptible. While MSRP would
be simpler to implement if each MSRP session used its own TCP
connection, there are compelling reasons to conserve connections.
For example, the TCP peer may be a relay device that connects to many
other peers. Such a device will scale better if each peer does not
create a large number of connections. (Note that in the above
example, the initial chunk was interruptible for the sake of example,
even though its size is well below the limit for which
interruptibility would be required.)
The chunking mechanism only applies to the SEND method, as it is the
only method used to transfer message content.
5.2. MSRP Addressing
MSRP entities are addressed using URIs. The MSRP URI schemes are
defined in Section 6. The syntax of the To-Path and From-Path header
fields each allows for a list of URIs. This was done to allow the
protocol to work with relays, which are defined in a separate
document, to provide a complete path to the end recipient. When two
MSRP nodes communicate directly, they need only one URI in the To-
Path list and one URI in the From-Path list.
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5.3. MSRP Transaction and Report Model
A sender sends MSRP requests to a receiver. The receiver MUST
quickly accept or reject the request. If the receiver initially
accepted the request, it still may then do things that take
significant time to succeed or fail. For example, if the receiver is
an MSRP to Extensible Messaging and Presence Protocol (XMPP) [30]
gateway, it may forward the message over XMPP. The XMPP side may
later indicate that the request did not work. At this point, the
MSRP receiver may need to indicate that the request did not succeed.
There are two important concepts here: first, the hop-by-hop delivery
of the request may succeed or fail; second, the end result of the
request may or may not be successfully processed. The first type of
status is referred to as "transaction status" and may be returned in
response to a request. The second type of status is referred to as
"delivery status" and may be returned in a REPORT transaction.
The original sender of a request can indicate if they wish to receive
reports for requests that fail, and can independently indicate if
they wish to receive reports for requests that succeed. A receiver
only sends a success REPORT if it knows that the request was
successfully delivered, and the sender requested a success report. A
receiver only sends a failure REPORT if the request failed to be
delivered and the sender requested failure reports.
This document describes the behavior of MSRP endpoints. MSRP
relays will introduce additional conditions that indicate a
failure REPORT should be sent, such as the failure to receive a
positive response from the next hop.
Two header fields control the sender's desire to receive reports.
The Success-Report header field can have a value of "yes" or "no" and
the Failure-Report header field can have a value of "yes", "no", or
"partial".
The combinations of reporting are needed to meet the various
scenarios of currently deployed IM systems. Success-Report might be
"no" in many public systems to reduce load, but might be "yes" in
certain enterprise systems, such as systems used for securities
trading. A Failure-Report value of "no" is useful for sending system
messages such as "the system is going down in 5 minutes" without
causing a response explosion to the sender. A Failure-Report of
"yes" is used by many systems that wish to notify the user if the
message failed. A Failure-Report of "partial" is a way to report
errors other than timeouts. Timeout error reporting requires the
sending hop to run a timer and the receiving hop to send an
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acknowledgment to stop the timer. Some systems don't want the
overhead of doing this. "Partial" allows them to choose not to do
so, but still allows error responses to be sent in many cases.
The term "partial" denotes that the hop-by-hop acknowledgment
mechanism that would be required with a Failure-Report value of
"yes" is not invoked. Thus, each device uses only "part" of the
set of error detection tools available to them. This allows a
compromise between no reporting of failures at all, and reporting
every possible failure. For example, with "partial", a sending
device does not have to keep transaction state around waiting for
a positive acknowledgment. But it still allows devices to report
other types of errors. The receiving device could still report a
policy violation such as an unacceptable content-type, or an ICMP
error trying to connect to a downstream device.
5.4. MSRP Connection Model
When an MSRP endpoint wishes to send a request to a peer identified
by an MSRP URI, it first needs a transport connection, with the
appropriate security properties, to the host specified in the URI.
If the sender already has such a connection, that is, one associated
with the same host, port, and URI scheme, then it SHOULD reuse that
connection.
When a new MSRP session is created, the initiating endpoint MUST act
as the "active" endpoint, meaning that it is responsible for opening
the transport connection to the answerer, if a new connection is
required. However, this requirement MAY be weakened if standardized
mechanisms for negotiating the connection direction become available
and are implemented by both parties to the connection.
Likewise, the active endpoint MUST immediately issue a SEND request.
This initial SEND request MAY have a body if the sender has content
to send, or it MAY have no body at all.
The first SEND request serves to bind a connection to an MSRP
session from the perspective of the passive endpoint. If the
connection is not authenticated with TLS, and the active endpoint
did not send an immediate request, the passive endpoint would have
no way to determine who had connected, and would not be able to
safely send any requests towards the active party until after the
active party sends its first request.
When an element needs to form a new connection, it looks at the URI
to decide on the type of connection (TLS, TCP, etc.) then connects to
the host indicated by the URI, following the URI resolution rules in
Section 6.2. Connections using the "msrps" scheme MUST use TLS. The
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SubjectAltName in the received certificate MUST match the hostname
part of the URI and the certificate MUST be valid according to RFC
3280 [16], including having a date that is valid and being signed by
an acceptable certification authority. At this point, the device
that initiated the connection can assume that this connection is with
the correct host.
The rules on certificate name matching and CA signing MAY be relaxed
when using TLS peer-to-peer. In this case, a mechanism to ensure
that the peer used a correct certificate MUST be used. See Section
14.4 for details.
If the connection used mutual TLS authentication, and the TLS client
presented a valid certificate, then the element accepting the
connection can verify the identity of the connecting device by
comparing the hostname part of the target URI in the SDP provided by
the peer device against the SubjectAltName in the client certificate.
When mutual TLS authentication is not used, the listening device MUST
wait until it receives a request on the connection, at which time it
infers the identity of the connecting device from the associated
session description.
When the first request arrives, its To-Path header field should
contain a URI that the listening element provided in the SDP for a
session. The element that accepted the connection looks up the URI
in the received request, and determines which session it matches. If
a match exists, the node MUST assume that the host that formed the
connection is the host to which this URI was given. If no match
exists, the node MUST reject the request with a 481 response. The
node MUST also check to make sure the session is not already in use
on another connection. If the session is already in use, it MUST
reject the request with a 506 response.
If it were legal to have multiple connections associated with the
same session, a security problem would exist. If the initial SEND
request is not protected, an eavesdropper might learn the URI, and
use it to insert messages into the session via a different
connection.
If a connection fails for any reason, then an MSRP endpoint MUST
consider any sessions associated with the connection as also having
failed. When either endpoint notices such a failure, it MAY attempt
to re-create any such sessions. If it chooses to do so, it MUST use
a new SDP exchange, for example, in a SIP re-INVITE. If a
replacement session is successfully created, endpoints MAY attempt to
resend any content for which delivery on the original session could
not be confirmed. If it does this, the Message-ID values for the
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resent messages MUST match those used in the initial attempts. If
the receiving endpoint receives more than one message with the same
Message-ID, it SHOULD assume that the messages are duplicates. The
specific action that an endpoint takes when it receives a duplicate
message is a matter of local policy, except that it SHOULD NOT
present the duplicate messages to the user without warning of the
duplication. Note that acknowledgments as needed based on the
Failure-Report and Success-Report settings are still necessary even
for requests containing duplicate content.
When endpoints create a new session in this fashion, the chunks for a
given logical message MAY be split across the sessions. However,
endpoints SHOULD NOT split chunks between sessions under non-failure
circumstances.
If an endpoint attempts to re-create a failed session in this manner,
it MUST NOT assume that the MSRP URIs in the SDP will be the same as
the old ones.
A connection SHOULD NOT be closed while there are sessions associated
with it.
6. MSRP URIs
URIs using the "msrp" and "msrps" schemes are used to identify a
session of instant messages at a particular MSRP device, as well as
to identify an MSRP relay in general. This document describes the
former usage; the latter usage is described in the MSRP relay
specification [23]. MSRP URIs that identify sessions are ephemeral;
an MSRP device will use a different MSRP URI for each distinct
session. An MSRP URI that identifies a session has no meaning
outside the scope of that session.
An MSRP URI follows a subset of the URI syntax in Appendix A of RFC
3986 [10], with a scheme of "msrp" or "msrps". The syntax is
described in Section 9.
MSRP URIs are primarily expected to be generated and exchanged
between systems, and are not intended for "human consumption".
Therefore, they are encoded entirely in US-ASCII.
The constructions for "authority", "userinfo", and "unreserved" are
detailed in RFC 3986 [10]. URIs designating MSRP over TCP MUST
include the "tcp" transport parameter.
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Since this document only specifies MSRP over TCP, all MSRP URIs
herein use the "tcp" transport parameter. Documents that provide
bindings on other transports should define respective parameters
for those transports.
The MSRP URI authority field identifies a participant in a particular
MSRP session. If the authority field contains a numeric IP address,
it MUST also contain a port. The session-id part identifies a
particular session of the participant. The absence of the session-id
part indicates a reference to an MSRP host device, but does not refer
to a particular session at that device. A particular value of
session-id is only meaningful in the context of the associated
authority; thus, the authority component can be thought of as
identifying the "authority" governing a namespace for the session-id.
A scheme of "msrps" indicates that the underlying connection MUST be
protected with TLS.
MSRP has an IANA-registered recommended port defined in Section 15.4.
This value is not a default, as the URI negotiation process described
herein will always include explicit port numbers. However, the URIs
SHOULD be configured so that the recommended port is used whenever
appropriate. This makes life easier for network administrators who
need to manage firewall policy for MSRP.
The authority component will typically not contain a userinfo
component, but MAY do so to indicate a user account for which the
session is valid. Note that this is not the same thing as
identifying the session itself. A userinfo part MUST NOT contain
password information.
The following is an example of a typical MSRP URI:
msrp://host.example.com:8493/asfd34;tcp
6.1. MSRP URI Comparison
In the context of the MSRP protocol, MSRP URI comparisons MUST be
performed according to the following rules:
1. The scheme MUST match. Scheme comparison is case insensitive.
2. If the authority component contains an explicit IP address and/or
port, these are compared for address and port equivalence.
Percent-encoding normalization [10] applies; that is, if any
percent-encoded nonreserved characters exist in the authority
component, they must be decoded prior to comparison. Userinfo
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parts are not considered for URI comparison. Otherwise, the
authority component is compared as a case-insensitive character
string.
3. If the port exists explicitly in either URI, then it MUST match
exactly. A URI with an explicit port is never equivalent to
another with no port specified.
4. The session-id part is compared as case sensitive. A URI without
a session-id part is never equivalent to one that includes one.
5. URIs with different "transport" parameters never match. Two URIs
that are identical except for transport are not equivalent. The
transport parameter is case insensitive.
Path normalization [10] is not relevant for MSRP URIs.
6.2. Resolving MSRP Host Device
An MSRP host device is identified by the authority component of an
MSRP URI.
If the authority component contains a numeric IP address and port,
they MUST be used as listed.
If the authority component contains a host name and a port, the
connecting device MUST determine a host address by doing an A or AAAA
DNS query and use the port as listed.
If a connection attempt fails, the device SHOULD attempt to connect
to the addresses returned in any additional A or AAAA records, in the
order the records were presented.
This process assumes that the connection port is always known
prior to resolution. This is always true for the MSRP URI uses
described in this document, that is, URIs exchanged in the SDP
offer and answer. The introduction of relays creates situations
where this is not the case. For example, when a user configures
her client to use a relay, it is desirable that the relay's MSRP
URI is easy to remember and communicate to humans. Often this
type of MSRP will omit the port number. Therefore, the relay
specification [23] describes additional steps to resolve the port
number.
MSRP devices MAY use other methods for discovering other such
devices, when appropriate. For example, MSRP endpoints may use other
mechanisms to discover relays, which are beyond the scope of this
document.
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7. Method-Specific Behavior
7.1. Constructing Requests
To form a new request, the sender creates a transaction identifier
and uses this and the method name to create an MSRP request start
line. The transaction identifier MUST NOT collide with that of other
transactions that exist at the same time. Therefore, it MUST contain
at least 64 bits of randomness.
Next, the sender places the target path in a To-Path header field,
and the sender's URI in a From-Path header field. If multiple URIs
are present in the To-Path, the leftmost is the first URI visited;
the rightmost URI is the last URI visited. The processing then
becomes method specific. Additional method-specific header fields
are added as described in the following sections.
After any method-specific header fields are added, processing
continues to handle a body, if present. If the request has a body,
it MUST contain a Content-Type header field. It may contain other
MIME-specific header fields. The Content-Type header field MUST be
the last field in the message header section. The body MUST be
separated from the header fields with an extra CRLF.
Non-SEND requests are not intended to carry message content, and are
therefore not interruptible. Non-SEND request bodies MUST NOT be
larger than 10240 octets.
Although this document does not discuss any particular usage of
bodies in non-SEND requests, they may be useful in the future for
carrying security or identity information, information about a
message in progress, etc. The 10K size limit was chosen to be
large enough for most of such applications, but small enough to
avoid the fairness issues caused by sending arbitrarily large
content in non-interruptible method bodies.
A request with no body MUST NOT include a Content-Type or any other
MIME-specific header fields. A request without a body MUST contain
an end-line after the final header field. No extra CRLF will be
present between the header section and the end-line.
Requests with no bodies are useful when a client wishes to send
"traffic", but does not wish to send content to be rendered to the
peer user. For example, the active endpoint sends a SEND request
immediately upon establishing a connection. If it has nothing to
say at the moment, it can send a request with no body. Bodiless
requests may also be used in certain applications to keep Network
Address Translation (NAT) bindings alive, etc.
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Bodiless requests are distinct from requests with empty bodies. A
request with an empty body will have a Content-Type header field
value and will generally be rendered to the recipient according to
the rules for that type.
The end-line that terminates the request MUST be composed of seven
"-" (minus sign) characters, the transaction ID as used in the start
line, and a flag character. If a body is present, the end-line MUST
be preceded by a CRLF that is not part of the body. If the chunk
represents the data that forms the end of the complete message, the
flag value MUST be a "$". If the sender is aborting an incomplete
message, and intends to send no further chunks in that message, the
flag MUST be a "#". Otherwise, the flag MUST be a "+".
If the request contains a body, the sender MUST ensure that the end-
line (seven hyphens, the transaction identifier, and a continuation
flag) is not present in the body. If the end-line is present in the
body, the sender MUST choose a new transaction identifier that is not
present in the body, and add a CRLF if needed, and the end-line,
including the "$", "#", or "+" character.
Some implementations may choose to scan for the closing sequence as
they send the body, and if it is encountered, simply interrupt the
chunk at that point and start a new transaction with a different
transaction identifier to carry the rest of the body. Other
implementations may choose to scan the data and ensure that the body
does not contain the transaction identifier before they start sending
the transaction.
Once a request is ready for delivery, the sender follows the
connection management (Section 5.4) rules to forward the request over
an existing open connection or create a new connection.
7.1.1. Sending SEND Requests
When an endpoint has a message to deliver, it first generates a new
Message-ID. The value MUST be highly unlikely to be repeated by
another endpoint instance, or by the same instance in the future. If
necessary, the endpoint breaks the message into chunks. It then
generates a SEND request for each chunk, following the procedures for
constructing requests (Section 7.1).
The Message-ID header field provides a unique message identifier
that refers to a particular version of a particular message. The
term "Message" in this context refers to a unit of content that
the sender wishes to convey to the recipient. While such a
message may be broken into chunks, the Message-ID refers to the
entire message, not a chunk of the message.
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The uniqueness of the message identifier is ensured by the host
that generates it. This message identifier is intended to be
machine readable and not necessarily meaningful to humans. A
message identifier pertains to exactly one version of a particular
message; subsequent revisions to the message each receive new
message identifiers. Endpoints can ensure sufficient uniqueness
in any number of ways, the selection of which is an implementation
choice. For example, an endpoint could concatenate an instance
identifier such as a MAC address, its idea of the number of
seconds since the epoch, a process ID, and a monotonically
increasing 16-bit integer, all base-64 encoded. Alternately, an
endpoint without an on-board clock could simply use a 64-bit
random number.
Each chunk of a message MUST contain a Message-ID header field
containing the Message-ID. If the sender wishes non-default status
reporting, it MUST insert a Failure-Report and/or Success-Report
header field with an appropriate value. All chunks of the same
message MUST use the same Failure-Report and Success-Report values in
their SEND requests.
If success reports are requested, i.e., the value of the Success-
Report header field is "yes", the sending device MAY wish to run a
timer of some value that makes sense for its application and take
action if a success report is not received in this time. There is no
universal value for this timer. For many IM applications, it may be
2 minutes while for some trading systems it may be under a second.
Regardless of whether such a timer is used, if the success report has
not been received by the time the session is ended, the device SHOULD
inform the user.
If the value of "Failure-Report" is set to "yes", then the sender of
the request runs a timer. If a 200 response to the transaction is
not received within 30 seconds from the time the last byte of the
transaction is sent, or submitted to the operating system for
sending, the element MUST inform the user that the request probably
failed. If the value is set to "partial", then the element sending
the transaction does not have to run a timer, but MUST inform the
user if it receives a non-recoverable error response to the
transaction. Regardless of the Failure-Report value, there is no
requirement to wait for a response prior to sending the next request.
The treatment of timers for success reports and failure reports is
intentionally inconsistent. An explicit timeout value makes sense
for failure reports since such reports will usually refer to a
message "chunk" that is acknowledged on a hop-by-hop basis. This
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is not the case for success reports, which are end-to-end and may
refer to the entire message content, which can be arbitrarily
large.
If no Success-Report header field is present in a SEND request, it
MUST be treated the same as a Success-Report header field with a
value of "no". If no Failure-Report header field is present, it MUST
be treated the same as a Failure-Report header field with a value of
"yes". If an MSRP endpoint receives a REPORT for a Message-ID it
does not recognize, it SHOULD silently ignore the REPORT.
The Byte-Range header field value contains a starting value (range-
start) followed by a "-", an ending value (range-end) followed by a
"/", and finally the total length. The first octet in the message
has a position of one, rather than a zero.
The first chunk of the message SHOULD, and all subsequent chunks
MUST, include a Byte-Range header field. The range-start field MUST
indicate the position of the first byte in the body in the overall
message (for the first chunk this field will have a value of one).
The range-end field SHOULD indicate the position of the last byte in
the body, if known. It MUST take the value of "*" if the position is
unknown, or if the request needs to be interruptible. The total
field SHOULD contain the total size of the message, if known. The
total field MAY contain a "*" if the total size of the message is not
known in advance. The sender MUST send all chunks in Byte-Range
order. (However, the receiver cannot assume that the requests will
be delivered in order, as intervening relays may have changed the
order.)
There are some circumstances where an endpoint may choose to send an
empty SEND request. For the sake of consistency, a Byte-Range header
field referring to nonexistent or zero-length content MUST still have
a range-start value of 1. For example, "1-0/0".
To ensure fairness over a connection, senders MUST NOT send chunks
with a body larger than 2048 octets unless they are prepared to
interrupt them (meaning that any chunk with a body of greater than
2048 octets will have a "*" character in the range-end field). A
sender can use one of the following two strategies to satisfy this
requirement. The sender is STRONGLY RECOMMENDED to send messages
larger than 2048 octets using as few chunks as possible, interrupting
chunks (at least 2048 octets long) only when other traffic is waiting
to use the same connection. Alternatively, the sender MAY simply
send chunks in 2048-octet increments until the final chunk. Note
that the former strategy results in markedly more efficient use of
the connection. All MSRP nodes MUST be able to receive chunks of any
size from zero octets to the maximum number of octets they can
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receive for a complete message. Senders SHOULD NOT break messages
into chunks smaller than 2048 octets, except for the final chunk of a
complete message.
A SEND request is interrupted while a body is in the process of being
written to the connection by simply noting how much of the message
has already been written to the connection, then writing out the end-
line to end the chunk. It can then be resumed in a another chunk
with the same Message-ID and a Byte-Range header field range start
field containing the position of the first byte after the
interruption occurred.
SEND requests larger than 2048 octets MUST be interrupted if the
sender needs to send pending responses or REPORT requests. If
multiple SEND requests from different sessions are concurrently being
sent over the same connection, the device SHOULD implement some
scheme to alternate between them such that each concurrent request
gets a chance to send some fair portion of data at regular intervals
suitable to the application.
The sender MUST NOT assume that a message is received by the peer
with the same chunk allocation with which it was sent. An
intervening relay could possibly break SEND requests into smaller
chunks, or aggregate multiple chunks into larger ones.
The default disposition of messages is to be rendered to the user.
If the sender wants a different disposition, it MAY insert a Content-
Disposition [9] header field. Values MAY include any from RFC 2183
[9] or the IANA registry it defines. Since MSRP can carry unencoded
binary payloads, transfer encoding is always "binary", and transfer-
encoding parameters MUST NOT be present.
7.1.2. Sending REPORT Requests
REPORT requests are similar to SEND requests, except that report
requests MUST NOT include Success-Report or Failure-Report header
fields, and MUST contain a Status header field. REPORT requests MUST
contain the Message-ID header field from the original SEND request.
If an MSRP element receives a REPORT for a Message-ID it does not
recognize, it SHOULD silently ignore the REPORT.
An MSRP endpoint MUST be able to generate success REPORT requests.
REPORT requests will normally not include a body, as the REPORT
request header fields can carry sufficient information in most cases.
However, REPORT requests MAY include a body containing additional
information about the status of the associated SEND request. Such a
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body is informational only, and the sender of the REPORT request
SHOULD NOT assume that the recipient pays any attention to the body.
REPORT requests are not interruptible.
Success-Report and Failure-Report header fields MUST NOT be present
in REPORT requests. MSRP nodes MUST NOT send REPORT requests in
response to REPORT requests. MSRP nodes MUST NOT send MSRP responses
to REPORT requests.
Endpoints SHOULD NOT send REPORT requests if they have reason to
believe the request will not be delivered. For example, they SHOULD
NOT send a REPORT request for a session that is no longer valid.
7.1.3. Generating Success Reports
When an endpoint receives a message in one or more chunks that
contain a Success-Report value of "yes", it MUST send a success
report or reports covering all bytes that are received successfully.
The success reports are sent in the form of REPORT requests,
following the normal procedures (Section 7.1), with a few additional
requirements.
The receiver MAY wait until it receives the last chunk of a message,
and send a success report that covers the complete message.
Alternately, it MAY generate incremental success REPORTs as the
chunks are received. These can be sent periodically and cover all
the bytes that have been received so far, or they can be sent after a
chunk arrives and cover just the part from that chunk.
It is helpful to think of a success REPORT as reporting on a
particular range of bytes, rather than on a particular chunk sent
by a client. The sending client cannot depend on the Byte-Range
header field in a given success report matching that of a
particular SEND request. For example, an intervening MSRP relay
may break chunks into smaller chunks, or aggregate multiple chunks
into larger ones. A side effect of this is, even if no relay is
used, the receiving client may report on byte ranges that do not
exactly match those in the original chunks sent by the sender. It
can wait until all bytes in a message are received and report on
the whole, it can report as it receives each chunk, or it can
report on any other received range. Reporting on ranges smaller
than the entire message contents allows certain improved user
experiences for the sender. For example, a sending client could
display incremental status information showing which ranges of
bytes have been acknowledged by the receiver. However, the choice
on whether to report incrementally is entirely up to the receiving
client. There is no mechanism for the sender to assert its desire
to receive incremental reports or not. Since the presence of a
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relay can cause the receiver to see a very different chunk
allocation than the sender, such a mechanism would be of
questionable value.
When generating a REPORT request, the endpoint inserts a To-Path
header field containing the From-Path value from the original
request, and a From-Path header field containing the URI identifying
itself in the session. The endpoint then inserts a Status header
field with a namespace of "000", a status-code of "200", and an
implementation-defined comment phrase. It also inserts a Message-ID
header field containing the value from the original request.
The namespace field denotes the context of the status-code field.
The namespace value of "000" means the status-code should be
interpreted in the same way as the matching MSRP transaction
response code. If a future specification uses the status-code
field for some other purpose, it MUST define a new namespace field
value.
The endpoint MUST NOT send a success report for a SEND request that
either contained no Success-Report header field or contained such a
field with a value of "no". That is, if no Success-Report header
field is present, it is treated identically to one with a value of
"no".
7.1.4. Generating Failure Reports
If an MSRP endpoint receives a SEND request that it cannot process
for some reason, and the Failure-Report header field either was not
present in the original request or had a value of "yes", it SHOULD
simply include the appropriate error code in the transaction
response. However, there may be situations where the error cannot be
determined quickly, such as when the endpoint is a gateway that waits
for a downstream network to indicate an error. In this situation, it
MAY send a 200 OK response to the request, and then send a failure
REPORT request when the error is detected.
If the endpoint receives a SEND request with a Failure-Report header
field value of "no", then it MUST NOT send a failure REPORT request,
and MUST NOT send a transaction response. If the value is "partial",
it MUST NOT send a 200 transaction response to the request, but
SHOULD send an appropriate non-200 class response if a failure
occurs.
As stated above, if no Failure-Report header field is present, it
MUST be treated the same as a Failure-Report header field with a
value of "yes".
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Construction of failure REPORT requests is identical to that for
success REPORT requests, except the Status header field code field
MUST contain the appropriate error code. Any error response code
defined in this specification MAY also be used in failure reports.
If a failure REPORT request is sent in response to a SEND request
that contained a chunk, it MUST include a Byte-Range header field
indicating the actual range being reported on. It can take the
range-start and total values from the original SEND request, but MUST
calculate the range-end field from the actual body data.
This section only describes failure report generation behavior for
MSRP endpoints. Relay behavior is beyond the scope of this
document, and will be considered in a separate document [23]. We
expect failure reports to be more commonly generated by relays
than by endpoints.
7.2. Constructing Responses
If an MSRP endpoint receives a request that either contains a
Failure-Report header field value of "yes" or does not contain a
Failure-Report header field at all, it MUST immediately generate a
response. Likewise, if an MSRP endpoint receives a request that
contains a Failure-Report header field value of "partial", and the
receiver is unable to process the request, it SHOULD immediately
generate a response.
To construct the response, the endpoint first creates the response
start line, inserting the appropriate response code and optionally a
comment. The transaction identifier in the response start line MUST
match the transaction identifier from the original request.
The endpoint then inserts an appropriate To-Path header field. If
the request triggering the response was a SEND request, the To-Path
header field is formed by copying the first (leftmost) URI in the
From-Path header field of the request. (Responses to SEND requests
are returned only to the previous hop.) For responses to all other
request methods, the To-Path header field contains the full path back
to the original sender. This full path is generated by copying the
list of URIs from the From-Path of the original request into the To-
Path of the response. (Legal REPORT requests do not request
responses, so this specification doesn't exercise the behavior
described above; however, we expect that extensions for gateways and
relays will need such behavior.)
Finally, the endpoint inserts a From-Path header field containing the
URI that identifies it in the context of the session, followed by the
end-line after the last header field. Since a response is never
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chunked, the continuation flag in the end-line will always contain a
dollar sign ("$"). The response MUST be transmitted back on the same
connection on which the original request arrived.
7.3. Receiving Requests
The receiving endpoint MUST first check the URI in the To-Path to
make sure the request belongs to an existing session. When the
request is received, the To-Path will have exactly one URI, which
MUST map to an existing session that is associated with the
connection on which the request arrived. If this is not true, then
the receiver MUST generate a 481 error and ignore the request. Note
that if the Failure-Report header field had a value of "no", then no
error report would be sent.
Further request processing by the receiver is method specific.
7.3.1. Receiving SEND Requests
When the receiving endpoint receives a SEND request, it first
determines if it contains a complete message or a chunk from a larger
message. If the request contains no Byte-Range header field, or
contains one with a range-start value of "1", and the closing line
continuation flag has a value of "$", then the request contained the
entire message. Otherwise, the receiver looks at the Message-ID
value to associate chunks together into the original message. The
receiver forms a virtual buffer to receive the message, keeping track
of which bytes have been received and which are missing. The
receiver takes the data from the request and places it in the
appropriate place in the buffer. The receiver SHOULD determine the
actual length of each chunk by inspecting the payload itself; it is
possible the body is shorter than the range-end field indicates.
This can occur if the sender interrupted a SEND request unexpectedly.
It is worth noting that the chunk that has a termination character of
"$" defines the total length of the message.
It is technically illegal for the sender to prematurely interrupt
a request that had anything other than "*" in the last-byte
position of the Byte-Range header field. But having the receiver
calculate a chunk length based on actual content adds resilience
in the face of sender errors. Since this should never happen with
compliant senders, this only has a "SHOULD" strength.
Receivers MUST not assume that the chunks will be delivered in order
or that they will receive all the chunks with "+" flags before they
receive the chunk with the "$" flag. In certain cases of connection
failure, it is possible for information to be duplicated. If chunk
data is received that overlaps already received data for the same
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message, the last chunk received SHOULD take precedence (even though
this may not have been the last chunk transmitted). For example, if
bytes 1 to 100 were received and a chunk arrives that contains bytes
50 to 150, this second chunk will overwrite bytes 50 to 100 of the
data that had already been received. Although other schemes work,
this is the easiest for the receiver and results in consistent
behavior between clients.
There are situations in which the receiver may not be able to give
precedence to the last chunk received when chunks overlap. For
example, the recipient might incrementally render chunks as they
arrive. If a new chunk arrives that overlaps with a previously
rendered chunk, it would be too late to "take back" any
conflicting data from the first chunk. Therefore, the requirement
to give precedence to the most recent chunk is specified at a
"SHOULD" strength. This requirement is not intended to disallow
applications where this behavior does not make sense.
The seven "-" in the end-line are used so that the receiver can
search for the value "----", 32 bits at a time to find the probable
location of the end-line. This allows most processors to locate the
boundaries and copy the memory at the same rate that a normal memory
copy could be done. This approach results in a system that is as
fast as framing based on specifying the body length in the header
fields of the request, but also allows for the interruption of
messages.
What is done with the body is outside the scope of MSRP and largely
determined by the MIME Content-Type and Content-Disposition. The
body MAY be rendered after the whole message is received or partially
rendered as it is being received.
If the SEND request contained a Content-Type header field indicating
an unsupported media-type, and the Failure-Report value is not "no",
the receiver MUST generate a response with a status code of 415. All
MSRP endpoints MUST be able to receive the multipart/mixed [15] and
multipart/alternative [15] media-types.
If the Success-Report header field was set to "yes", the receiver
must construct and send one or more success reports, as described in
Section 7.1.3.
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7.3.2. Receiving REPORT Requests
When an endpoint receives a REPORT request, it correlates the report
to the original SEND request using the Message-ID and the Byte-Range,
if present. If it requested success reports, then it SHOULD keep
enough state about each outstanding sent message so that it can
correlate REPORT requests to the original messages.
An endpoint that receives a REPORT request containing a Status header
field with a namespace field of "000" MUST interpret the report in
exactly the same way it would interpret an MSRP transaction response
with a response code matching the status-code field.
It is possible to receive a failure report or a failure transaction
response for a chunk that is currently being delivered. In this
case, the entire message corresponding to that chunk SHOULD be
aborted, by including the "#" character in the continuation field of
the end-line.
It is possible that an endpoint will receive a REPORT request on a
session that is no longer valid. The endpoint's behavior if this
happens is a matter of local policy. The endpoint is not required to
take any steps to facilitate such late delivery; i.e., it is not
expected to keep a connection active in case late REPORTs might
arrive.
When an endpoint that sent a SEND request receives a failure REPORT
indicating that a particular byte range was not received, it MUST
treat the session as failed. If it wishes to recover, it MUST first
re-negotiate the URIs at the signaling level then resend that range
of bytes of the message on the resulting new session.
MSRP nodes MUST NOT send MSRP REPORT requests in response to other
REPORT requests.
8. Using MSRP with SIP and SDP
MSRP sessions will typically be initiated using the Session
Description Protocol (SDP) [2] via the SIP offer/answer mechanism
[3].
This document defines a handful of new SDP parameters to set up MSRP
sessions. These are detailed below and in the IANA Considerations
section.
An MSRP media-line (that is, a media-line proposing MSRP) in the
session description is accompanied by a mandatory "path" attribute.
This attribute contains a space-separated list of URIs to be visited
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to contact the user agent advertising this session description. If
more than one URI is present, the leftmost URI is the first URI to be
visited to reach the target resource. (The path list can contain
multiple URIs to allow for the deployment of gateways or relays in
the future.) MSRP implementations that can accept incoming
connections without the need for relays will typically only provide a
single URI here.
An MSRP media line is also accompanied by an "accept-types"
attribute, and optionally an "accept-wrapped-types" attribute. These
attributes are used to specify the media-types that are acceptable to
the endpoint.
8.1. SDP Connection and Media-Lines
An SDP connection-line takes the following format:
c=<network type> <address type> <connection address>
Figure 4: Standard SDP Connection Line
The network type and address type fields are used as normal for SDP.
The connection address field MUST be set to the IP address or fully
qualified domain name from the MSRP URI identifying the endpoint in
its path attribute.
The general format of an SDP media-line is:
m=<media> <port> <protocol> <format list>
Figure 5: Standard SDP Media Line
An offered or accepted media-line for MSRP over TCP MUST include a
protocol field value of "TCP/MSRP", or "TCP/TLS/MSRP" for TLS. The
media field value MUST be "message". The format list field MUST be
set to "*".
The port field value MUST match the port value used in the endpoint's
MSRP URI in the path attribute, except that, as described in [3], a
user agent that wishes to accept an offer, but not a specific media-
line, MUST set the port number of that media-line to zero (0) in the
response. Since MSRP allows multiple sessions to share the same TCP
connection, multiple m-lines in a single SDP document may share the
same port field value; MSRP devices MUST NOT assume any particular
relationship between m-lines on the sole basis that they have
matching port field values.
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MSRP devices do not use the c-line address field, or the m-line
port and format list fields to determine where to connect.
Rather, they use the attributes defined in this specification.
The connection information is copied to the c-line and m-line for
purposes of backwards compatibility with conventional SDP usages.
While MSRP could theoretically carry any media-type, "message" is
appropriate.
8.2. URI Negotiations
Each endpoint in an MSRP session is identified by a URI. These URIs
are negotiated in the SDP exchange. Each SDP offer or answer that
proposes MSRP MUST contain a "path" attribute containing one or more
MSRP URIs. The path attribute is used in an SDP a-line, and has the
following syntax:
path = path-label ":" path-list
path-label = "path"
path-list= MSRP-URI *(SP MSRP-URI)
Figure 6: Path Attribute
where MSRP-URI is an "msrp" or "msrps" URI as defined in Section 6.
MSRP URIs included in an SDP offer or answer MUST include explicit
port numbers.
An MSRP device uses the URI to determine a host address, port,
transport, and protection level when connecting, and to identify the
target when sending requests and responses.
The offerer and answerer each selects a URI to represent itself and
sends that URI to its peer in the SDP document. Each peer stores the
path value received from the other peer and uses that value as the
target for requests inside the resulting session. If the path
attribute received from the peer contains more than one URI, then the
target URI is the rightmost, while the leftmost entry represents the
adjacent hop. If only one entry is present, then it is both the peer
and adjacent hop URI. The target path is the entire path attribute
value received from the peer.
The following example shows an SDP offer with a session URI of
"msrp://alice.example.com:7394/2s93i9ek2a;tcp"
Campbell, et al. Standards Track [Page 29]
RFC 4975 MSRP September 2007
v=0
o=alice 2890844526 2890844527 IN IP4 alice.example.com
s= -
c=IN IP4 alice.example.com
t=0 0
m=message 7394 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://alice.example.com:7394/2s93i9ek2a;tcp
Figure 7: Example SDP with Path Attribute
The rightmost URI in the path attribute MUST identify the endpoint
that generated the SDP document, or some other location where that
endpoint wishes to receive requests associated with the session. It
MUST be assigned for this particular session, and MUST NOT duplicate
any URI in use for any other session in which the endpoint is
currently participating. It SHOULD be hard to guess, and protected
from eavesdroppers. This is discussed in more detail in Section 14.
8.3. Path Attributes with Multiple URIs
As mentioned previously, this document describes MSRP for peer-to-
peer scenarios, that is, when no relays are used. The use of relays
is described in a separate document [23]. In order to allow an MSRP
device that only implements the core specification to interoperate
with devices that use relays, this document must include a few
assumptions about how relays work.
An endpoint that uses one or more relays will indicate that by
putting a URI for each device in the relay chain into the SDP path
attribute. The final entry will point to the endpoint itself. The
other entries will indicate each proposed relay, in order. The first
entry will point to the first relay in the chain from the perspective
of the peer, that is, the relay to which the peer device, or a relay
operating on its behalf, should connect.
Endpoints that do not wish to insert a relay, including those that do
not support relays at all, will put exactly one URI into the path
attribute. This URI represents both the endpoint for the session and
the connection point.
Even though endpoints that implement only this specification will
never introduce a relay, they need to be able to interoperate with
other endpoints that do use relays. Therefore, they MUST be prepared
to receive more than one URI in the SDP path attribute. When an
endpoint receives more than one URI in a path attribute, only the
Campbell, et al. Standards Track [Page 30]
RFC 4975 MSRP September 2007
first entry is relevant for purposes of resolving the address and
port, and establishing the network connection, as it describes the
first adjacent hop.
If an endpoint puts more than one URI in a path attribute, the final
URI in the path attribute (the peer URI) identifies the session, and
MUST not duplicate the URI of any other session in which the endpoint
is currently participating. Uniqueness requirements for other
entries in the path attribute are out of scope for this document.
8.4. Updated SDP Offers
MSRP endpoints may sometimes need to send additional SDP exchanges
for an existing session. They may need to send periodic exchanges
with no change to refresh state in the network, for example, SIP
session timers or the SIP UPDATE [24] request. They may need to
change some other stream in a session without affecting the MSRP
stream, or they may need to change an MSRP stream without affecting
some other stream.
Either peer may initiate an updated exchange at any time. The
endpoint that sends the new offer assumes the role of offerer for all
purposes. The answerer MUST respond with a path attribute that
represents a valid path to itself at the time of the updated
exchange. This new path may be the same as its previous path, but
may be different. The new offerer MUST NOT assume that the peer will
answer with the same path it used previously.
If either party wishes to send an SDP document that changes nothing
at all, then it MUST use the same o-line as in the previous exchange.
8.5. Connection Negotiation
Previous versions of this document included a mechanism to negotiate
the direction for any required TCP connection. The mechanism was
loosely based on the Connection-Oriented Media (COMEDIA) [26] work
done by the MMUSIC working group. The primary motivation was to
allow MSRP sessions to succeed in situations where the offerer could
not accept connections but the answerer could. For example, the
offerer might be behind a NAT, while the answerer might have a
globally routable address.
The SIMPLE working group chose to remove that mechanism from MSRP, as
it added a great deal of complexity to connection management.
Instead, MSRP now specifies a default connection direction. The
party that sent the original offer is responsible for connecting to
its peer.
Campbell, et al. Standards Track [Page 31]
RFC 4975 MSRP September 2007
8.6. Content Type Negotiation
An SDP media-line proposing MSRP MUST be accompanied by an accept-
types attribute.
An entry of "*" in the accept-types attribute indicates that the
sender may attempt to send content with media-types that have not
been explicitly listed. Likewise, an entry with an explicit type and
a "*" character as the subtype indicates that the sender may attempt
to send content with any subtype of that type. If the receiver
receives an MSRP request and is able to process the media-type, it
does so. If not, it will respond with a 415 response. Note that all
explicit entries SHOULD be considered preferred over any non-listed
types. This feature is needed as, otherwise, the list of formats for
rich IM devices may be prohibitively large.
This specification requires the support of certain data formats.
Mandatory formats MUST be signaled like any other, either explicitly
or by the use of a "*".
The accept-types attribute may include container types, that is, MIME
formats that contain other types internally. If compound types are
used, the types listed in the accept-types attribute may be used as
the root payload or may be wrapped in a listed container type. Any
container types MUST also be listed in the accept-types attribute.
Occasionally, an endpoint will need to specify a MIME media-type that
can only be used if wrapped inside a listed container type.
Endpoints MAY specify media-types that are only allowed when wrapped
inside compound types using the "accept-wrapped-types" attribute in
an SDP a-line.
The semantics for accept-wrapped-types are identical to those of the
accept-types attribute, with the exception that the specified types
may only be used when wrapped inside container types listed in the
accept-types attribute. Only types listed in the accept-types
attribute may be used as the "root" type for the entire body. Since
any type listed in accept-types may be both used as a root body and
wrapped in other bodies, format entries from accept-types SHOULD NOT
be repeated in this attribute.
This approach does not allow for specifying distinct lists of
acceptable wrapped types for different types of containers. If an
endpoint understands a media-type in the context of one wrapper, it
is assumed to understand it in the context of any other acceptable
wrappers, subject to any constraints defined by the wrapper types
themselves.
Campbell, et al. Standards Track [Page 32]
RFC 4975 MSRP September 2007
The approach of specifying types that are only allowed inside of
containers separately from the primary payload types allows an
endpoint to force the use of certain wrappers. For example, a
Common Presence and Instant Messaging (CPIM) [12] gateway device
may require all messages to be wrapped inside message/cpim bodies,
but may allow several content types inside the wrapper. If the
gateway were to specify the wrapped types in the accept-types
attribute, its peer might attempt to use those types without the
wrapper.
If the recipient of an offer does not understand any of the payload
types indicated in the offered SDP, it SHOULD indicate that using the
appropriate mechanism of the rendezvous protocol. For example, in
SIP, it SHOULD return a SIP 488 response.
An MSRP endpoint MUST NOT send content of a type not signaled by the
peer in either an accept-types or an accept-wrapped-types attribute.
Furthermore, it MUST NOT send a top-level (i.e., not wrapped) MIME
document of a type not signaled in the accept-types attribute. In
either case, the signaling could be explicit, or implicit through the
use of the "*" character.
An endpoint MAY indicate the maximum size message it wishes to
receive using the max-size a-line attribute. Max-size refers to the
complete message in octets, not the size of any one chunk. Senders
SHOULD NOT exceed the max-size limit for any message sent in the
resulting session. However, the receiver should consider max-size
value as a hint.
Media format entries may include parameters. The interpretation of
such parameters varies between media-types. For the purposes of
media-type negotiation, a format-entry with one or more parameters is
assumed to match the same format-entry with no parameters.
Campbell, et al. Standards Track [Page 33]
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The formal syntax for these attributes is as follows:
accept-types = accept-types-label ":" format-list
accept-types-label = "accept-types"
accept-wrapped-types = wrapped-types-label ":" format-list
wrapped-types-label = "accept-wrapped-types"
format-list = format-entry *( SP format-entry)
format-entry = ( ( (type "/" subtype)
/ (type "/" "*") )
*( ";" type-param ) )
/ ("*")
type = token
subtype = token
type-param = parm-attribute "=" parm-value
parm-attribute = token
parm-value = token / quoted-string
max-size = max-size-label ":" max-size-value
max-size-label = "max-size"
max-size-value = 1*(DIGIT) ; max size in octets
Figure 8: Attribute Syntax
8.7. Example SDP Exchange
Endpoint A wishes to invite Endpoint B to an MSRP session. A offers
the following session description:
v=0
o=usera 2890844526 2890844527 IN IP4 alice.example.com
s= -
c=IN IP4 alice.example.com
t=0 0
m=message 7394 TCP/MSRP *
a=accept-types:message/cpim text/plain text/html
a=path:msrp://alice.example.com:7394/2s93i93idj;tcp
Figure 9: SDP from Endpoint A
Campbell, et al. Standards Track [Page 34]
RFC 4975 MSRP September 2007
B responds with its own URI:
v=0
o=userb 2890844530 2890844532 IN IP4 bob.example.com
s= -
c=IN IP4 bob.example.com
t=0 0
m=message 8493 TCP/MSRP *
a=accept-types:message/cpim text/plain
a=path:msrp://bob.example.com:8493/si438dsaodes;tcp
Figure 10: SDP from Endpoint B
8.8. MSRP User Experience with SIP
In typical SIP applications, when an endpoint receives an INVITE
request, it alerts the user, and waits for user input before
responding. This is analogous to the typical telephone user
experience, where the callee "answers" the call.
In contrast, the typical user experience for instant messaging
applications is that the initial received message is immediately
displayed to the user, without waiting for the user to "join" the
conversation. Therefore, the principle of least surprise would
suggest that MSRP endpoints using SIP signaling SHOULD allow a mode
where the endpoint quietly accepts the session and begins displaying
messages.
This guideline may not make sense for all situations, such as for
mixed-media applications, where both MSRP and audio sessions are
offered in the same INVITE. In general, good application design
should take precedence.
SIP INVITE requests may be forked by a SIP proxy, resulting in more
than one endpoint receiving the same INVITE. SIP early media [29]
techniques can be used to establish a preliminary session with each
endpoint so the initial message(s) are displayed on each endpoint,
and canceling the INVITE transaction for any endpoints that do not
send MSRP traffic after some period of time, so that they cease
receiving MSRP traffic from the inviter.
8.9. SDP Direction Attribute and MSRP
SDP defines a number of attributes that modify the direction of media
flows. These are the "sendonly", "recvonly", "inactive", and
"sendrecv" attributes.
Campbell, et al. Standards Track [Page 35]
RFC 4975 MSRP September 2007
If a "sendonly" or "recvonly" attribute modifies an MSRP media
description line, the attribute indicates the direction of MSRP SEND
requests that contain regular message payloads. Unless otherwise
specified, these attributes do not affect the direction of other
types of requests, such as REPORT. SEND requests that contain some
kind of control or reporting protocol rather than regular message
payload (e.g., Instant Message Delivery Notification (IMDN) reports)
should be generated according to the protocol rules as if no
direction attribute were present.
9. Formal Syntax
MSRP is a text protocol that uses the UTF-8 [14] transformation
format.
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 4234 [6].
msrp-req-or-resp = msrp-request / msrp-response
msrp-request = req-start headers [content-stuff] end-line
msrp-response = resp-start headers end-line
req-start = pMSRP SP transact-id SP method CRLF
resp-start = pMSRP SP transact-id SP status-code [SP comment] CRLF
comment = utf8text
pMSRP = %x4D.53.52.50 ; MSRP in caps
transact-id = ident
method = mSEND / mREPORT / other-method
mSEND = %x53.45.4e.44 ; SEND in caps
mREPORT = %x52.45.50.4f.52.54; REPORT in caps
other-method = 1*UPALPHA
status-code = 3DIGIT ; any code defined in this document
; or an extension document
MSRP-URI = msrp-scheme "://" authority
["/" session-id] ";" transport *( ";" URI-parameter)
; authority as defined in RFC3986
msrp-scheme = "msrp" / "msrps"
session-id = 1*( unreserved / "+" / "=" / "/" )
; unreserved as defined in RFC3986
transport = "tcp" / 1*ALPHANUM
URI-parameter = token ["=" token]
headers = To-Path CRLF From-Path CRLF 1*( header CRLF )
Campbell, et al. Standards Track [Page 36]
RFC 4975 MSRP September 2007
header = Message-ID
/ Success-Report
/ Failure-Report
/ Byte-Range
/ Status
/ ext-header
To-Path = "To-Path:" SP MSRP-URI *( SP MSRP-URI )
From-Path = "From-Path:" SP MSRP-URI *( SP MSRP-URI )
Message-ID = "Message-ID:" SP ident
Success-Report = "Success-Report:" SP ("yes" / "no" )
Failure-Report = "Failure-Report:" SP ("yes" / "no" / "partial" )
Byte-Range = "Byte-Range:" SP range-start "-" range-end "/" total
range-start = 1*DIGIT
range-end = 1*DIGIT / "*"
total = 1*DIGIT / "*"
Status = "Status:" SP namespace SP status-code [SP comment]
namespace = 3(DIGIT); "000" for all codes defined in this document.
ident = ALPHANUM 3*31ident-char
ident-char = ALPHANUM / "." / "-" / "+" / "%" / "="
content-stuff = *(Other-Mime-header CRLF)
Content-Type 2CRLF data CRLF
Content-Type = "Content-Type:" SP media-type
media-type = type "/" subtype *( ";" gen-param )
type = token
subtype = token
gen-param = pname [ "=" pval ]
pname = token
pval = token / quoted-string
token = 1*(%x21 / %x23-27 / %x2A-2B / %x2D-2E
/ %x30-39 / %x41-5A / %x5E-7E)
; token is compared case-insensitive
quoted-string = DQUOTE *(qdtext / qd-esc) DQUOTE
qdtext = SP / HTAB / %x21 / %x23-5B / %x5D-7E
/ UTF8-NONASCII
qd-esc = (BACKSLASH BACKSLASH) / (BACKSLASH DQUOTE)
BACKSLASH = "\"
UPALPHA = %x41-5A
ALPHANUM = ALPHA / DIGIT
Campbell, et al. Standards Track [Page 37]
RFC 4975 MSRP September 2007
Other-Mime-header = (Content-ID
/ Content-Description
/ Content-Disposition
/ mime-extension-field)
; Content-ID, and Content-Description are defined in RFC2045.
; Content-Disposition is defined in RFC2183
; MIME-extension-field indicates additional MIME extension
; header fields as described in RFC2045
data = *OCTET
end-line = "-------" transact-id continuation-flag CRLF
continuation-flag = "+" / "$" / "#"
ext-header = hname ":" SP hval CRLF
hname = ALPHA *token
hval = utf8text
utf8text = *(HTAB / %x20-7E / UTF8-NONASCII)
UTF8-NONASCII = %xC0-DF 1UTF8-CONT
/ %xE0-EF 2UTF8-CONT
/ %xF0-F7 3UTF8-CONT
/ %xF8-Fb 4UTF8-CONT
/ %xFC-FD 5UTF8-CONT
UTF8-CONT = %x80-BF
Figure 11: MSRP ABNF
10. Response Code Descriptions
This section summarizes the semantics of various response codes that
may be used in MSRP transaction responses. These codes may also be
used in the Status header field in REPORT requests.
10.1. 200
The 200 response code indicates a successful transaction.
10.2. 400
A 400 response indicates that a request was unintelligible. The
sender may retry the request after correcting the error.
10.3. 403
A 403 response indicates that the attempted action is not allowed.
The sender should not try the request again.
Campbell, et al. Standards Track [Page 38]
RFC 4975 MSRP September 2007
10.4. 408
A 408 response indicates that a downstream transaction did not
complete in the allotted time. It is never sent by any elements
described in this specification. However, 408 is used in the MSRP
relay extension; therefore, MSRP endpoints may receive it. An
endpoint MUST treat a 408 response in the same manner as it would
treat a local timeout.
10.5. 413
A 413 response indicates that the receiver wishes the sender to stop
sending the particular message. Typically, a 413 is sent in response
to a chunk of an undesired message.
If a message sender receives a 413 in a response, or in a REPORT
request, it MUST NOT send any further chunks in the message, that is,
any further chunks with the same Message-ID value. If the sender
receives the 413 while in the process of sending a chunk, and the
chunk is interruptible, the sender MUST interrupt it.
10.6. 415
A 415 response indicates that the SEND request contained a media type
that is not understood by the receiver. The sender should not send
any further messages with the same content-type for the duration of
the session.
10.7. 423
A 423 response indicates that one of the requested parameters is out
of bounds. It is used by the relay extensions to this document.
10.8. 481
A 481 response indicates that the indicated session does not exist.
The sender should terminate the session.
10.9. 501
A 501 response indicates that the recipient does not understand the
request method.
The 501 response code exists to allow some degree of method
extensibility. It is not intended as a license to ignore methods
defined in this document; rather, it is a mechanism to report lack
of support of extension methods.
Campbell, et al. Standards Track [Page 39]
RFC 4975 MSRP September 2007
10.10. 506
A 506 response indicates that a request arrived on a session that is
already bound to another network connection. The sender should cease
sending messages for that session on this connection.
11. Examples
11.1. Basic IM Session
This section shows an example flow for the most common scenario. The
example assumes SIP is used to transport the SDP exchange. Details
of the SIP messages and SIP proxy infrastructure are omitted for the
sake of brevity. In the example, assume that the offerer is
sip:alice@example.com and the answerer is sip:bob@example.com.
Alice Bob
| |
| |
|(1) (SIP) INVITE |
|----------------------->|
|(2) (SIP) 200 OK |
|<-----------------------|
|(3) (SIP) ACK |
|----------------------->|
|(4) (MSRP) SEND |
|----------------------->|
|(5) (MSRP) 200 OK |
|<-----------------------|
|(6) (MSRP) SEND |
|<-----------------------|
|(7) (MSRP) 200 OK |
|----------------------->|
|(8) (SIP) BYE |
|----------------------->|
|(9) (SIP) 200 OK |
|<-----------------------|
| |
| |
Figure 12: Basic IM Session Example
Campbell, et al. Standards Track [Page 40]
RFC 4975 MSRP September 2007
1. Alice constructs a local URI of
msrp://alicepc.example.com:7777/iau39soe2843z;tcp .
Alice->Bob (SIP): INVITE sip:bob@example.com
v=0
o=alice 2890844557 2890844559 IN IP4 alicepc.example.com
s= -
c=IN IP4 alicepc.example.com
t=0 0
m=message 7777 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://alicepc.example.com:7777/iau39soe2843z;tcp
2. Bob listens on port 8888, and sends the following response:
Bob->Alice (SIP): 200 OK
v=0
o=bob 2890844612 2890844616 IN IP4 bob.example.com
s= -
c=IN IP4 bob.example.com
t=0 0
m=message 8888 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://bob.example.com:8888/9di4eae923wzd;tcp
3. Alice->Bob (SIP): ACK sip:bob@example.com
4. (Alice opens connection to Bob.) Alice->Bob (MSRP):
MSRP d93kswow SEND
To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-16/16
Content-Type: text/plain
Hi, I'm Alice!
-------d93kswow$
5. Bob->Alice (MSRP):
MSRP d93kswow 200 OK
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
-------d93kswow$
Campbell, et al. Standards Track [Page 41]
RFC 4975 MSRP September 2007
6. Bob->Alice (MSRP):
MSRP dkei38sd SEND
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
Message-ID: 456s9wlk3
Byte-Range: 1-21/21
Content-Type: text/plain
Hi, Alice! I'm Bob!
-------dkei38sd$
7. Alice->Bob (MSRP):
MSRP dkei38sd 200 OK
To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
-------dkei38sd$
8. Alice->Bob (SIP): BYE sip:bob@example.com
Alice invalidates local session state.
9. Bob invalidates local state for the session.
Bob->Alice (SIP): 200 OK
11.2. Message with XHTML Content
MSRP dsdfoe38sd SEND
To-Path: msrp://alice.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
Message-ID: 456so39s
Byte-Range: 1-374/374
Content-Type: application/xhtml+xml
Campbell, et al. Standards Track [Page 42]
RFC 4975 MSRP September 2007
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"_http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd_">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>FY2005 Results</title>
</head>
<body>
<p>See the results at <a
href="http://example.org/">example.org</a>.</p>
</body>
</html>
-------dsdfoe38sd$
Figure 13: Example Message with XHTML
11.3. Chunked Message
For an example of a chunked message, see the example in Section 5.1.
11.4. Chunked Message with Message/CPIM Payload
This example shows a chunked message containing a CPIM message that
wraps a text/plain payload. It is worth noting that MSRP considers
the complete CPIM message before chunking the message; thus, the CPIM
headers are included in only the first chunk. The MSRP Content-Type
and Byte-Range headers, present in both chunks, refer to the whole
CPIM message.
MSRP d93kswow SEND
To-Path: msrp://bobpc.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7654/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-137/148
Content-Type: message/cpim
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>
DateTime: 2006-05-15T15:02:31-03:00
Content-Type: text/plain
ABCD
-------d93kswow+
Figure 14: First Chunk
Campbell, et al. Standards Track [Page 43]
RFC 4975 MSRP September 2007
Alice sends the second and last chunk.
MSRP op2nc9a SEND
To-Path: msrp://bobpc.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7654/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 138-148/148
Content-Type: message/cpim
1234567890
-------op2nc9a$
Figure 15: Second Chunk
11.5. System Message
Sysadmin->Alice (MSRP):
MSRP d93kswow SEND
To-Path: msrp://alicepc.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://example.com:7777/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-38/38
Failure-Report: no
Success-Report: no
Content-Type: text/plain
This conference will end in 5 minutes
-------d93kswow$
11.6. Positive Report
Alice->Bob (MSRP):
MSRP d93kswow SEND
To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-106/106
Success-Report: yes
Failure-Report: no
Content-Type: text/html
Campbell, et al. Standards Track [Page 44]
RFC 4975 MSRP September 2007
<html><body>
<p>Here is that important link...
<a href="http://www.example.com/foobar">foobar</a>
</p>
</body></html>
-------d93kswow$
Figure 16: Initial SEND Request
Bob->Alice (MSRP):
MSRP dkei38sd REPORT
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-106/106
Status: 000 200 OK
-------dkei38sd$
Figure 17: Success Report
11.7. Forked IM
Traditional IM systems generally do a poor job of handling multiple
simultaneous IM clients online for the same person. While some do a
better job than many existing systems, handling of multiple clients
is fairly crude. This becomes a much more significant issue when
always-on mobile devices are available, but it is desirable to use
them only if another IM client is not available.
Using SIP makes rendezvous decisions explicit, deterministic, and
very flexible. In contrast, "page-mode" IM systems use implicit
implementation-specific decisions that IM clients cannot influence.
With SIP session-mode messaging, rendezvous decisions can be under
control of the client in a predictable, interoperable way for any
host that implements callee capabilities [31]. As a result,
rendezvous policy is managed consistently for each address of record.
The following example shows Juliet with several IM clients where she
can be reached. Each of these has a unique SIP contact and MSRP
session. The example takes advantage of SIP's capability to "fork"
an invitation to several contacts in parallel, in sequence, or in
combination. Juliet has registered from her chamber, the balcony,
her PDA, and as a last resort, you can leave a message with her
nurse. Juliet's contacts are listed below. The q-values express
relative preference (q=1.0 is the highest preference).
Campbell, et al. Standards Track [Page 45]
RFC 4975 MSRP September 2007
When Romeo opens his IM program, he selects Juliet and types the
message "art thou hither?" (instead of "you there?"). His client
sends a SIP invitation to sip:juliet@thecapulets.example.com. The
proxy there tries first the balcony and the chamber simultaneously.
A client is running on each of those systems, both of which set up
early sessions of MSRP with Romeo's client. The client automatically
sends the message over MSRP to the two MSRP URIs involved. After a
delay of a several seconds with no reply or activity from Juliet, the
proxy cancels the invitation at her first two contacts, and forwards
the invitation on to Juliet's PDA. Since her father is talking to
her about her wedding, she selects "Do Not Disturb" on her PDA, which
sends a "Busy Here" response. The proxy then tries the nurse, who
answers and tells Romeo what is going on.
Campbell, et al. Standards Track [Page 46]
RFC 4975 MSRP September 2007
Romeo Juliet's Juliet/ Juliet/ Juliet/ Nurse
Proxy balcony chamber PDA
| | | | | |
|--INVITE--->| | | | |
| |--INVITE--->| | | |
| |<----180----| | | |
|<----180----| | | | |
|---PRACK---------------->| | | |
|<----200-----------------| | | |
|<===Early MSRP Session==>| art thou hither? | |
| | | | | |
| |--INVITE---------------->| | |
| |<----180-----------------| | |
|<----180----| | | | |
|---PRACK----------------------------->| | |
|<----200------------------------------| | |
|<========Early MSRP Session==========>| art thou hither? |
| | | | | |
| | | | | |
| | .... Time Passes .... | | |
| | | | | |
| | | | | |
| |--CANCEL--->| | | |
| |<---200-----| | | |
| |<---487-----| | | |
| |----ACK---->| | | |
| |--CANCEL---------------->| | |
| |<---200------------------| | |
| |<---487------------------| | |
| |----ACK----------------->| | |
| |--INVITE---------------------------->| romeo wants
| | | | | to IM w/ you
| |<---486 Busy Here--------------------| |
| |----ACK----------------------------->| |
| | | | | |
| |--INVITE---------------------------------------->|
| |<---200 OK---------------------------------------|
|<--200 OK---| | | | |
|---ACK------------------------------------------------------->|
|<================MSRP Session================================>|
| | | | | |
| Hi Romeo, Juliet is |
| with her father now |
| can I take a message?|
| |
| Tell her to go to confession tomorrow.... |
Figure 18: Forking Example
Campbell, et al. Standards Track [Page 47]
RFC 4975 MSRP September 2007
12. Extensibility
MSRP was designed to be only minimally extensible. New MSRP methods,
header fields, and status codes can be defined in standards-track
RFCs. MSRP does not contain a version number or any negotiation
mechanism to require or discover new features. If an extension is
specified in the future that requires negotiation, the specification
will need to describe how the extension is to be negotiated in the
encapsulating signaling protocol. If a non-interoperable update or
extension occurs in the future, it will be treated as a new protocol,
and MUST describe how its use will be signaled.
In order to allow extension header fields without breaking
interoperability, if an MSRP device receives a request or response
containing a header field that it does not understand, it MUST ignore
the header field and process the request or response as if t
