WDIFF

draft-ietf-svrloc-protocol-15.txt --> draft-ietf-svrloc-protocol-16.txt

Internet Engineering Task Force John Veizades
INTERNET DRAFT @Home Network
8 January
13 March 1997 Erik Guttman
Sun Microsystems
Charles Perkins
IBM Research
Sun Microsystems
Scott Kaplan

Service Location Protocol
draft-ietf-svrloc-protocol-15.txt
draft-ietf-svrloc-protocol-16.txt

Status of This Memo

This draft document is a product of the Service Location Working
Group of the Internet Engineering Task Force (IETF); it will be
submitted to the RFC editor as a standards document. Please respond
with comments to the srvloc@tgv.com mailing list.

Distribution of this memo is unlimited.

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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Abstract

The Service Location Protocol provides a scalable framework for
the discovery and selection of network services. Using this
protocol, computers using the Internet no longer need so much static
configuration of network services for network based applications.
This is especially important as computers become more portable, and
users less tolerant or able to fulfill the demands of network system
administration.

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Contents

Status of This Memo i

Abstract i

1. Introduction 2 1

2. Terminology 2 1
2.1. Notation Conventions . . . . . . . . . . . . . . . . . . 4 3
2.2. Service Information and Predicate Representation . . . . 4 3
2.3. Specification Language . . . . . . . . . . . . . . . . . 4

3. Protocol Overview 5 4
3.1. Protocol Transactions . . . . . . . . . . . . . . . . . . 6 5
3.2. Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. The ``service:'' URL scheme . . . . . . . . . . 8 7
3.3. Standard Attribute Definitions . . . . . . . . . . . . . 8 7
3.4. Naming Authority . . . . . . . . . . . . . . . . . . . . 9 8
3.5. Interpretation of Service Location Replies . . . . . . . 9 8
3.6. Use of TCP, UDP and Multicast in Service Location . . . . 10 9
3.6.1. Multicast vs. Broadcast . . . . . . . . . . . . 10 9
3.6.2. Service-Specific Multicast Address . . . . . . . 11 10
3.7. Service Location Scaling, and Multicast Operating Modes . 11

4. Service Location General Message Format 13 12
4.1. Use of Transaction IDs (XIDs) . . . . . . . . . . . . . . 15 14
4.2. URL Entries . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Authentication Blocks . . . . . . . . . . . . . . . . . . 15
4.4. URL Entry Lifetime . . . . . . . . . . . . . . . . . . . 16 18

5. Service Request Message Format 16 18
5.1. Service Request Usage . . . . . . . . . . . . . . . . . . 18 20
5.2. Directory Agent Discovery Request . . . . . . . . . . . . 19 21
5.3. Explanation of Terms of Predicate Grammar . . . . . . . . 20 22
5.4. Service Request Predicate Grammar . . . . . . . . . . . . 22 24
5.5. String Matching for Requests . . . . . . . . . . . . . . 24 26

6. Service Reply Message Format 24 27

7. Service Type Request Message Format 25 28

8. Service Type Reply Message Format 27 30

9. Service Registration Message Format 28

10. Service Acknowledgement Message Format 31

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10. Service Acknowledgement Message Format 34

11. Service Deregister Message Format 32 36

12. Attribute Request Message Format 33 37

13. Attribute Reply Message Format 35 39

14. Directory Agent Advertisement Message Format 37 41

15. Directory Agents 38 42
15.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 38 42
15.2. Finding Directory Agents . . . . . . . . . . . . . . . . 38 42

16. Scope Discovery and Use 40

17. Language and Character Encoding Issues 41
17.1. Character Encoding and String Issues 44
16.1. Protected Scopes . . . . . . . . . . 42
17.1.1. Substitution of Character Escape Sequences . . . 43
17.2. Language Dialect . . . . . . . 45

17. Language and Character Encoding Issues 46
17.1. Character Encoding and String Issues . . . . . . . . . . 47
17.1.1. Substitution of Character Escape Sequences . . . 43
17.3. 48
17.2. Language-Independent Strings . . . . . . . . . . . . . . 44 48

18. Service Location Transactions 44 49
18.1. Service Location Connections . . . . . . . . . . . . . . 44 49
18.2. No Synchronous Assumption . . . . . . . . . . . . . . . . 45 50
18.3. Idempotency . . . . . . . . . . . . . . . . . . . . . . . 45 50

19. Security Considerations 45 50

20. String Formats used with Service Location Messages 46 52
20.1. Previous Responders' Address Specification . . . . . . . 47 52
20.2. Formal Definition of the ``service:'' Scheme . . . . . . 47 52
20.2.1. Service Type String . . . . . . . . . . . . . . . 48 53
20.3. Attribute Information . . . . . . . . . . . . . . . . . . 48 53
20.4. Address Specification in Service Location . . . . . . . . 49 54
20.5. Attribute Value encoding rules . . . . . . . . . . . . . 49 55

21. Implementation Protocol Requirements 50

22. Configurable Parameters and Default Values 54
22.1. 55
21.1. User Agent Requirements . . . . . . . . . . . . . . . . . 56
21.2. Service Agent: Use Predefined Directory Agent(s) Agent Requirements . . . . . . . 55
22.2. Time Out Intervals . . . . . . . . 57
21.3. Directory Agent Requirements . . . . . . . . . . . 56

23. Non-configurable . . . 59

22. Configurable Parameters 56

24. Acknowledgments 57 and Default Values 60
22.1. Service Agent: Use Predefined Directory Agent(s) . . . . 62
22.2. Time Out Intervals . . . . . . . . . . . . . . . . . . . 62

23. Non-configurable Parameters 63

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24. Acknowledgments 63

A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for
the representation of names of languages" 58

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B. SLP Certificates 66

C. Example of deploying SLP security using MD5 and RSA 68

D. Example of use of SLP Certificates by mobile nodes 68

E. Appendix: For Further Reading 59 69

1. Introduction

Traditionally, users find services by using the name of a network
host (a human readable text string) which is an alias for a network
address. The Service Location Protocol eliminates the need for
a user to know the name of a network host supporting a service.
Rather, the user names the service and supplies a set of attributes
which describe the service. The Service Location Protocol allows the
user to bind this description to the network address of the service.

Service Location provides a dynamic configuration mechanism for
applications in local area networks. It is not a global resolution
system for the entire Internet; rather it is intended to serve
enterprise networks with shared services. Applications are modeled
as clients that need to find servers attached to the enterprise
network at a possibly distant location. For cases where there are
many different clients and/or services available, the protocol
is adapted to make use of nearby Directory Agents that offer a
centralized repository for advertised services.

2. Terminology

User Agent (UA)
A process working on the user's behalf to acquire
service attributes and configuration. The User Agent
retrieves service information from the Service Agents or
Directory Agents.

Service Agent (SA)
A process working on the behalf of one or more services
to advertise service attributes and configuration.

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Service Information
A collection of attributes and configuration information
associated with a single service. The Service Agents
advertise service information for a collection of
service instances.

Service The service is a process or system providing a facility
to the network. The service itself is accessed using
a communication mechanism external to the the Service
Location Protocol.

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Directory Agent (DA)
A process which collects information from Service Agents
to provide a single repository of service information
in order to centralize it for efficient access by User
Agents. There can only be one DA present per given
host.

Service Type
Each type of service has a unique Service Type string.
The Service Type defines a template template, called a "service
scheme", including expected attributes, values and
protocol behavior.

Naming Authority
The agency or group which catalogues given Service Types
and Attributes. The default Naming Authority is IANA. IANA,
the Internet Assigned Numbers Authority.

Keyword
A string describing a characteristic of a service.

Attribute
A (class, value-list) pair of strings describing a
characteristic of a service. The value string may be
interpreted as a boolean, integer or opaque value if it
takes specific forms (see section 20.5).

Predicate
A boolean expression of attributes, relations and
logical operators. The predicate is used to find
services which satisfy particular requirements. See
section 5.3.

Alphanumeric
A character within the range 'a' to 'z', 'A' to 'Z', or
'0' to '9'.

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Scope A collection of services that make up a logical group.
See sections 16 3.7 and 3.7. 16.

Site Network
All the hosts accessible within the Agent's multicast
radius, which defaults to a value appropriate for
reaching all hosts within a site (see section 22). If
the site does not support multicast, the agent's site
network is restricted to a single subnet.

URL A Universal Resource Locator - see [6].

Address Specification
This is the network layer protocol dependent mechanism
for specifying an Agent. For Internet systems this is
part of a URL (Universal Resource Locator - see [7]).

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2.1. Notation Conventions

CAPS Strings which appear in all capital letters are protocol
literal. All string comparison is case insensitive,
however, (see section 5.5). Some strings are quoted in
this document to indicate they should be used literally.
Single characters inside apostrophes are included
literally.

<> Values set off in this manner are fully described in
section 20. In general, all definitions of items in
messages are described in section 20 or immediately
following their first use.

| |
\ \ Message layouts with this notation indicate a variable
| | length field.

2.2. Service Information and Predicate Representation

Service information is represented in a text format. The goal is
that the format be human readable and transmissible via email. The
location of network services is encoded as a Universal Resource
Locator (URL) which is also human readable and well defined. readable. Only the datagram headers are
encoded in a form which is not human readable. Strings used in the
Service Location Protocol are NOT null-terminated.

Predicates are expressed in a simple boolean notation using keywords,
attributes, and logical connectives, as described in Section 5.4.

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The logical connectives and subexpressions are presented in
prefix-order, so that the connective comes first and the expressions
it operates on follow afterwards.

2.3. Specification Language

In this document, several words are used to signify the requirements
of the specification. specification [8]. These words are often capitalized.

MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
specification.

MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.

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SHOULD This word, or the adjective "recommended", means
that, in some circumstances, valid reasons may exist
to ignore this item, but the full implications must
be understood and carefully weighed before choosing
a different course. Unexpected results may result
otherwise.

MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST
be prepared to interoperate with another implementation
which does include the option.

silently discard
The implementation discards the datagram without
further processing, and without indicating an error
to the sender. The implementation SHOULD provide the
capability of logging the error, including the contents
of the discarded datagram, and SHOULD record the event
in a statistics counter.

3. Protocol Overview

The basic operation in Service Location is that a client attempts
to discover the location of a Service. In smaller installations,
each service will be configured to respond individually to each
client. In larger installations, services will register their
services with one or more Directory Agents, and clients will
contact the Directory Agent to fulfill requests for Service Location
information. Clients may discover the whereabouts of a Directory

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Agent by preconfiguration, DHCP [2, 10], 12], or by issuing queries to the
Directory Agent Discovery multicast address.

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3.1. Protocol Transactions

The diagram below illustrates the relationships described below:

+---------------+ we want this info: +-----------+
| Application | - - - - - - - - - - - -> | Service |
+---------------+ +-----------+
/|\ | |
| +-------------+ |
| | |
\|/ \|/ \|/
+---------------+ +-----------+ +----------------+
| User Agent |<-------->| Service | | Service |
+---------------+ | Agent | | Agent which |
| +-----------+ | does not reply |
| | | to UA requests |
| \|/ +----------------+
| +-------------+ |
+------------------>| Directory |<----------+
| Agent |
+-------------+ ___________
/|\ / Many other\
+------------>| SA's |
\___________/

The following describes the operations a User Agent would employ
to find services on the site's network. The User Agent needs no
configuration to begin network interaction. The User Agent can
acquire information to construct predicates which describe the
services that match the user's needs. The User Agent may build on
the information received in earlier network requests to find the
Service Agents advertising service information.

A User Agent will operate two ways: If the User Agent has already
obtained the location of a Directory Agent, the User Agent will
unicast a request to it in order to resolve a particular request.
The Directory Agent will unicast a reply to the User Agent. The
User Agent will retry a request to a Directory Agent until it gets
a reply, so if the Directory Agent cannot service the request (say
it has no information) it must return an response with zero values,
possibly with an error code set.

If the User Agent does not have knowledge of a Directory Agent or if
there are no Directory Agents available on the site network, a second
mode of discovery is

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mode of discovery may be used. The User Agent multicasts a request
to the service-specific multicast address, to which the service it
wishes to locate will respond. All the Service Agents which are
listening to this multicast address will respond, provided they can

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satisfy the User Agent's request. A similar mechanism is used for
Directory Agent discovery; see section 5.2. Service Agents which
have no information for the User Agent MUST NOT respond.

When a User Agent wishes to obtain an enumeration of ALL services
which satisfy the query, a retransmission/convergence algorithm is
used. The User Agent resends the request, together with a list of
previous responders. Only those Service Agents which are not on
the list respond. Once there are no new responses to the request
the accumulation of responses is deemed complete. Depending on the
length of the request, around 60 previous responders may be listed
in a single datagram. If there are more responders than this, the
scaling mechanisms described in section 3.7 should be used.

While the multicast/convergence model may be important for
discovering services (such as Directory Agents) it is the exception
rather than the rule. Once a User Agent knows of the location of a
Directory Agent, it will use a unicast request/response transaction.

The Service Agent SHOULD listen for multicast requests on the
service-specific multicast address, and MUST register with an
available Directory Agent. This Directory Agent will resolve
requests from User Agents as described above. which are unicasted using TCP or UDP. This
means that a Directory Agent must first be discovered, using DHCP,
the DA Discovery Multicast address, the multicast mechanism described above.
above, or manual configuration. See section 5.2.

A Service Agent which does not respond to multicast requests will not
be useful in the absence of Directory Agents. Some Service Agents
may not include this functionality, if an especially light-weight lightweight
implementation is required.

If the service is to become unavailable, it should be deregistered
with the Directory Agent. The Directory Agent responds with an
acknowledgment to either a registration or deregistration. Service
Registrations include a Lifetime value lifetime, and will eventually expire.
Service Registrations need to be refreshed by the Service Agent
before their Lifetime runs out. If need be, Service Agents can
advertise signed URLs to prove that they are authorized to provide
the service.

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3.2. Schemes

The Service Location Protocol, designed as a way for clients to
access resources on the network, is a natural application for
Universal Resource Locators (URLs). It is intended that by re-using
URL specification and technology from the World Wide Web, clients and
servers will be more flexible and able to be written using already
existing code. Moreover, it is hoped that browsers will be written
to take advantage of the similarity in locator format, so that a

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client can dynamically formulate requests for services that are
resolved differently depending upon the circumstances. There is
the possibility for beneficial interaction between Directory Agents
and Web Browsers which we wish to facilitate by means of compatible
locator format.

3.2.1. The ``service:'' URL scheme

The service URL scheme is used by Service Location. It is used to
specify a Service Location. Many Service Types will be named by
including a scheme name after the ``service:'' scheme name. Service
Types are used by SAs to register and deregister Services with DAs.
It is also used by SAs and DAs to return Service Replies to UAs. The
formal definition of the ``service:'' URL scheme is in section 20.2.
The format of the information which follows the ``service:'' scheme
should as closely as possible follow the URL structure and semantics
as formalized by the IETF standardization process.

Well known Service Types are registered with the IANA and templates
are available as RFCs. Private Service Types may also be supported.

3.3. Standard Attribute Definitions

Service Types used with the Service Location Protocol must describe
the following:

Service Type string of the service
Service-specific multicast address, if used
Attributes and Keywords
Attribute Descriptions and interpretations

Service Types note not registered with IANA will use their own Naming
Authority string and, possibly, a service-specific multicast
address from the unassigned range. This is only an option for
a site-specific deployment, as there may be conflicts with this
multicast address somewhere, in some other site.

If a service-specific multicast address is not supplied by a
standards document registered with IANA, nor is a site specific
address being used, the Service Location General Multicast address
is the default. All Service Agents SHOULD listen to this address,
especially if they have not registered their service information
with any Directory Agent. string. The service-specific multicast address is
merely used for efficiency and is not strictly needed registration process for correct
operation.

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defined in [14].

Services which advertise a particular Service Type must support the
complete set of standardized attributes. They may support additional
attributes, beyond the standardized set. Unrecognized attributes
MUST be ignored by User Agents.

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Service Type names which begin with "x-" are guaranteed not to
conflict with any officially registered Service Type names. It
is suggested that this prefix be used for experimental or private
Service Type names. Similarly, attribute names which begin with "x-"
are guaranteed not to be used for any officially registered attribute
names.

A service of a given Service Type should accept the networking
protocol which is implied in its definition. If a Service Type
can accept multiple protocols, configuration information SHOULD
be included in the Service Type attribute information. This
configuration information will enable an application to use the
results of a Service Request and Attribute Request to directly
connect to a service.

See section 20.2.1 for the format of a Service Type String as used in
the Service Location Protocol.

3.4. Naming Authority

The Naming Authority of a service defines the meaning of the
Service Types and attributes registered with and provided by Service
Location. The Naming Authority itself is a string which uniquely
identifies an organization. If no string is provided IANA is the
default.

Naming Authorities may define IANA stands for the Internet Assigned Numbers Authority.

Naming Authorities may define Service Types which are experimental,
proprietary or for private use. The procedure to use is to create
a 'unique' Naming Authority string and then specify the Standard
Attribute Definitions as described above. This Naming Authority will
accompany registration and queries, as described in sections 9 5 and 5. 9.

3.5. Interpretation of Service Location Replies

Replies should be considered to be valid at the time of delivery.
The service may, however, fail or change between the time of the
reply and the moment an application seeks to make use of the service.
The application making use of Service Location MUST be prepared for
the possibility that the service information provided is either stale
or incomplete. In the case where the service information provided

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does not allow a User Agent to connect to a service as desired, the
Service Request and/or Attribute Request may be resubmitted.

Service specific configuration information (such as which protocol
to use) should be included as attribute information in Service

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Registrations. These configuration attributes will be used by
applications which interpret the Service Location Reply.

3.6. Use of TCP, UDP and Multicast in Service Location

The Service Location Protocol requires the implementation of
connectionless UDP
(connectionless) and a connection oriented TCP (connection oriented) transport protocols.
The latter is used for bulk transfer, only when necessary.
Connections are always initiated by an agent request or registration,
not by a replying Directory Agent. Service Agents and User Agents
use ephemeral ports for transmitting information to the service
location port, which is 427.

The Service Location discovery mechanisms use possibly internetwork-wide multicast. typically multicast
messages to as many enterprise networks as needed to establish
service availability. The protocol will operate in a broadcast
environment with limitations detailed in section 3.6.1.

3.6.1. Multicast vs. Broadcast

The Service Location Protocol was designed for use in networks
where DHCP is available, or multicast is supported at the network layer is supported; in some instances
multicast may not be supported.
layer. To support this protocol in networks
where multicast is not supported the following modifications are
made to support the protocol in an environment where when only network layer broadcast is supported.
supported, the following procedures may be followed.

3.6.1.1. Single Subnet

If a network is not connected to any other networks simple network
layer broadcasts will work in place of multicast.

Service Agents SHOULD and Directory Agents MUST listen for broadcast
Service Location request messages to the Service Location port. This
allows UAs which lack multicast capabilities to still make use of
Service Location on a single subnet.

3.6.1.2. Multiple Subnets

The Directory Agent provides a central clearing house of information
for User Agents. If the network is designed so that a Directory
Agent address is statically configured with each User Agent
and Service Agent, the Directory Agent will act as a bridge for

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information that resides on different subnets. The Directory Agent
address can be dynamically configured with Agents using DHCP or
staticly configured, but Agents will not DHCP. The
address can also be able to discover DAs on
non-bridged subnets. determined by static configuration.

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As dynamic discovery is not feasible in a broadcast environment with
multiple subnets and manual configuration is difficult, deploying
multiple
DAs in to serve enterprises with multiple subnets will require use of
multicast discovery with multiple hops (i.e., TTL > 1 in the IP
header).

3.6.2. Service-Specific Multicast Address

Each service type MAY have a unique multicast address which is
expected to be used for discovering services of that type. This
multicast address may be obtained from the naming authority (e.g.,
IANA).

This mechanism is used so that the number of datagrams any one
service agent receives is minimized. The Service Location General
Multicast Address may MAY be used to query for any service, though one
should
SHOULD use the service-specific multicast address if it exists.

If the site network does not support multicast then the query
should
SHOULD be broadcast to the Service Location port. If If, on the other
hand, the underlying hardware will not support the number of needed
multicast addresses the Service Location General Multicast Address may
MAY be used. Service Agents MUST listen on this multicast address
as well as the service-specific multicast addresses for the service
types they advertise.

3.7. Service Location Scaling, and

Service-Specific Multicast Operating Modes

In a very small network, with few nodes, no DA is required. A User
Agent can detect services Addresses are computed by multicasting requests. Service calculating a
string hash on the Service Type string. The Service Type string MUST
first be converted to an ASCII string from whatever character set it
is represented in, so the hash will have well-defined results.

The string hash function is modified from a code fragment attributed
to Chris Torek:

/*
* SLPhash returns a hash value in the range 0-1023 for a
* string of single-byte characters, of specified length.
*/
unsigned long SLPhash (const char *pc, unsigned int length) {
unsigned long h = 0;
while (length-- != 0) {
h *= 33;
h += *pc++;
}
return (0x3FF & h); /* round to a range of 0-1023 */
}

This value is added to the base range of Service Specific Discovery
Addresses, to be assigned by IANA. These will be 1024 contiguous
multicast addresses.

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3.7. Service Location Scaling, and Multicast Operating Modes

In a very small network, with few nodes, no DA is required. A user
agent can detect services by multicasting requests. Service Agents
will then reply to them. Further, Service Agents which respond to
user requests must be used to make service information available.
This does not scale to environments with many hosts and services.

When scaling Service Location systems to intermediate sized networks,
a central repository (Directory Agent) may be added to reduce the
number of Service Location messages transmitted in the network
infrastructure. Since the central repository can respond to
all Service and Attribute Requests, fewer Service and Attribute
Replies will be needed; for the same reason, there is no need to
differentiate between Directory Agents.

A site may also grow to such a size that it is not feasible to
maintain only one central repository of service information. In this

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case more Directory Agents are needed. The services (and service
agents) advertised by the several Directory Agents are collected
together into logical groupings called "Scopes".

All Service Registrations that have a Scope scope must be registered with
all DAs (within the appropriate multicast radius) of that Scope scope which
have been or are subsequently discovered. Service Registrations
which have no Scope scope are only registered with unscoped DAs. User
Agents make requests of DAs whose Scope scope they are configured to use.

Service Agents MUST register with unscoped DAs even if they are
configured to specifically register with DAs which have a specific
Scope
scope or set of Scopes. scopes. User Agents MAY query DAs without Scopes, scopes,
even if they are configured to use DAs with a certain Scope. scope. This
is because any DA with no Scope scope will have all the available service
information.

Scoped User Agents user agents SHOULD always use a DA which supports their
configured Scope scope when possible instead of an unscoped DA. This will
prevent the unscoped DAs from becoming overused and thus a scaling
problem.

It is possible to specially configure Service Agents to register
only with a specific set of DAs (see Section 22.1). In that case,
services may not be available to User Agents via all Directory
Agents, but some network administrators may deem this appropriate.

There are thus 3 distinct operating modes. The first requires no
administrative intervention. The second requires only that a DA be
run. The last requires that all DAs be configured to have Scope scope and

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that a coherent strategy of assigning Scopes scopes to services be followed.
Users must be instructed which Scopes scopes are appropriate for them to
use. This administrative effort will allow users and applications to
subsequently dynamically discover services without assistance.

The first mode (no DAs) is intended for a LAN. The second mode
(using a DA or DAs, but not using Scopes) scopes) scales well to a group
of interconnected LANs with a limited number of hosts. The third
mode (with DAs and Scopes) scopes) allows the SLP protocol to be used in an
internetworked campus environment.

If Scoped scoped DAs are used, they will not accept unscoped registrations
or requests. UAs which issue unscoped requests will discover only
unscoped services. They SHOULD use a Scope scope in their requests if
possible and SHOULD use a DA with their Scope scope in preference to an
unscoped DA. In a large campus environment it would be a bad idea to
have ANY unscoped DAs: They attract ALL registrations and will thus
present a scaling problem eventually.

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A subsequent protocol document will describe mechanisms for
supporting a service discovery protocol for the global Internet.

4. Service Location General Message Format

The following header is used in all of the message descriptions below
and is abbreviated by using "Service Location header =" followed by
the function being used.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Function | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|M| rsvd |
|O|M|U|A|F| rsvd| Dialect | Language Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Char Encoding | XID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version This protocol document defines version 1 of the Service
Location protocol.

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Function Service Location datagrams can be identified as to their
operation by the function field. The following are the
defined operations:

Message Type Abbreviation Function Value

Service Request SrvReq 1
Service Reply SrvRply 2
Service Registration SrvReg 3
Service Deregister SrvDereg 4
Service Acknowledge SrvAck 5
Attribute Request AttrRqst 6
Attribute Reply AttrRply 7
DA Advertisement DAAdvert 8
Service Type Request SrvTypeRqst 9
Service Type Reply SrvTypeRply 10

Length The number of bytes in the message, including the Service
Location Header.

O The 'Overflow' bit. See Section 18 for the use of this
field.

M The 'Monolingual' bit. Requests with this bit set
indicate the User Agent will only accept responses in
the language (see section 17) that is indicated by the
Service or Attribute Request.

U The 'URL Authentication Present' bit. See sections 4.2,
4.3, 9, and 11 for the use of this field.

A The 'Attribute Authentication Present' bit. See
sections 4.2, 4.3, and 13 for the use of this field.

F If the 'F' bit is set in a Service Acknowledgement, the
directory agent has registered the service as a new
entry, not as an updated entry.

rsvd MUST be zero.

Dialect Dialect tags are will be used in by future versions of the
Service Location messages Protocol to indicate a variant of
vocabulary used. This field is reserved and MUST be
set to 0 for compatibility with future versions of the
Service Location Protocol.

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Language Code
Strings within the remainder of the message which follows
are to be interpreted in the language encoded (see
section 17 and appendix A) in this field. This header field MUST be
set. See also section 17.

Character Encoding
The characters making up strings within the remainder of
the message may be encoded in any standardized encoding
(see section 17.1). This header field MUST be set.

Transaction Identifier (XID)
The XID (transaction ID) field allows the requester to
match replies to individual requests (see section 4.1).

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When URLs are registered, they have lengths and lifetimes. These
two values are associated with the URL for the duration of the
registration. The triplet (length, lifetime, URL)

Note that, whenever there is known as an Attribute Authentication
block, there will also be a
"URL-entry", and has the following format when used in Service
Replies and Service Registrations:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The URL conforms Authentication block.
Thus, it is an error to RFC 1738 [7]. If the scheme used in the URL does
not have a standardized representation, the minimal requirement is:

service:<srvtype>://<addr-spec>

The "SERVICE" string is the URL scheme of all Service Location
Information included in Service Registrations and Service Replies.
Each entry in the Reply will always have a <srvtype>. It may 'A' bit set without also
include an <addr-spec> except in
having the case of a reply to a Service
Type request (see section 7). 'U' bit set.

4.1. Use of Transaction IDs (XIDs)

Retransmission is used to ensure reliable transactions in the
Service Location Protocol. If a User Agent or Service Agent sends
a message and fails to receive an expected response, the message
will be sent again. Retransmission of the same Service Location
datagram should not contain an updated XID. It is quite possible the
original request reached the DA or SA, but reply failed to reach the
requester. Using the same XID allows the DA or SA to cache its reply
to the original request and then send it again, should a duplicate
request arrive. This cached information should only be held very
briefly (CONFIG_INTERVAL_0.) Any registration or deregistration at
a Directory Agent, or change of service information at a SA should
flush this cache so that the information returned to the client is
always valid.

The requester creates the XID from an initial random seed and
increments it by one for each request it makes. The XIDs will
eventually wrap back to zero and continue incrementing from there.

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Directory Agents use XID values in their DA Advertisements to
indicate their state (see section 15.2).

4.2. URL Entry Lifetime

The Lifetime field is set to the number of seconds the reply can be
cached by any agent. A value of 0 means the information must not
be cached. User Agents MAY cache service information, but if they
do, Entries

When URLs are registered, they must provide a way for applications to flush this cached
information have lifetimes and issue the request directly onto the network.

Services should lengths, and may
be registered with DAs authenticated. These values are associated with a Lifetime, the suggested
value being CONFIG_INTERVAL_1. The service must be reregistered
before this interval elapses, or the service advertisement will
no longer be available. Thus, services which vanish and fail to
deregister eventually become automatically deregistered.

5. Service Request Message Format

The Service Request is used to obtain URLs from a Directory Agent or URL for

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The format Location Protocol 13 March 1997

the duration of the Service Request registration. The association is known as follows: a
"URL-entry", and has the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReq) Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length Length of prev resp list string|<Previous Responders Addr Spec>| URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of predicate string | Service Request <predicate> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ Service Request <predicate>, contd. \
| | (if present) URL Authentication Block .....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Lifetime The <Previous Responders Addr Spec> length of time that the registration is described valid, in sections 7
the absence of later registrations or deregistration.

Length of URL
The length of the URL, measured in bytes and 20.1.

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After a User Agent restarts (say, after rebooting of a system,
loading of the network kernel), Service Requests should be delayed
for some random time uniformly distributed within a one second
interval centered about a configured delay value (by default,
CONFIG_INTERVAL_4). < 32768.

URL Authentication Block
(if present) A timestamped authenticator (section 4.3)

The Service Request allows the User Agent URL conforms to specify the Service Type
of RFC 1738 [6]. If the service and a Predicate in a specific language. The general
form of a Service Request is shown below:

<srvtype>[.<na>]/[<scope>]/[<where>]/

The punctuation 'U' bit is necessary even where set in the fields are omitted.

- The <srvtype> refers to
message header, the Service Type. For each type of
service available, there URL is followed by an URL Authentication
Block. If the scheme used in the URL does not have a unique Service type name string.
See section 20.2.1.

- The <na> is standardized
representation, the Naming Authority. This string determines minimal requirement is:

service:<srvtype>://<addr-spec>

"service" is the
semantic interpretation URL scheme of all Service Location Information
included in service registrations and service replies. Each URL
entry contains the attribute information service:<srvtype> scheme name. It may also
include an <addr-spec> except in the
<where> part case of the Service Request.

- The <scope> is a string reply to a Service
Type request (see section 7).

4.3. Authentication Blocks

Authentication blocks are used to restrict authenticate service registrations
and deregistrations. URLs are registered along with an URL
Authentication block to retain the range of authentication information in the query.
Scope is determined administratively, at
URL entry for subsequent use by User Agents who receive a given site. It is
not necessarily related to network topology (see Section 16).
Leaving this field out means that the request can be satisfied
only by unscoped service advertisements.

- The <where> string is the Where Clause of the request. It
contains query which specify which service instances the User
Agent is interested in. The query includes attributes, boolean
operators and relations. (See section 5.3.)

In the case of a multicast Service request, a list of previous
responders is sent. This list will prevent those in
Reply containing the list from
responding, to be sure that responses from other sources are not
drowned out. The request is multicast repeatedly (with a recommended
wait interval of CONFIG_INTERVAL_2) until there are no new responses,
or a certain time (CONFIG_INTERVAL_3) has elapsed. Different timing
values are applied to a URL entry. Service Request used for Directory Agent
Discovery, see Section 5.2.

In order for a request to succeed in matching attributes are registered information,
the following conditions must be met:

1. The result must have the same Service Type as the request.

2. It must
along with an Attribute Authentication block. Both authentication
blocks have the same Naming Authority. format illustrated below.

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3. It must have the same Scope. (If the <scope> of

If a service registration is accompanied by authentication which can
be validated by the request
was omitted, DA, the request will only match services which were
registered with no Scope. Note DA MUST validate any subsequent service
deregistrations, so that unauthorized entities cannot invalidate
such registered services. Likewise, if a Scoped request WILL match
all unscoped Services).

4. The conditions specified in service registration
is accompanied by an Attribute Authentication block which can be
validated by the Where Clause must match DA, the
attributes and keywords DA MUST validate any subsequent attribute
registrations, so that unauthorized entities cannot invalidate such
registered for the service.

5.1. Service Request Usage

The User Agent may form Service Requests using preconfigured
knowledge of a Service Type's attributes. It may also issue
Attribute Requests to obtain

To avoid replay attacks which use previously validated
deregistrations, the deregistration or attribute values for registration
message must contain a Service Type
before issuing Service Requests (see Section 12). Having obtained timestamp for use by the attributes DA. To avoid replay
attacks which describe use previously validated registrations to nullify a particular kind of service from an
Attribute Request, (or using configured knowledge of
valid deregistration, registrations must also contain a service's
attributes,) timestamp.

An authentication block has the User Agent can build a predicate that describes following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Structure Descriptor | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Structured Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Timestamp A 64-bit value formatted as specified by the
service needs of Network
Time Protocol (NTP) [17].

Block Structure Descriptor (BSD)
A value describing the user.

Service Requests may be sent directly to a Directory Agent. Suppose
a printer supporting structure of the lpr protocol Authenticator.
The only value currently defined is needed on 1, for
Object-Identifier.

Length The length of the 12th floor
which has UNRESTRICTED_ACCESS Authenticator

Structured Authenticator
An algorithm specification, and prints 12 pages per minute.
Suppose further that a Attribute Request indicates that there is a
printer on the 12th floor, a printer that prints 12 pages per minute, authentication data
produced by the algorithm.

The algorithms to use for encryption, decryption and a printer that offers UNRESTRICTED_ACCESS. To check whether they message
digest calculation are same printer, issue identified within the following request:

lpr//(& (PAGES PER MINUTE==12)
(UNRESTRICTED_ACCESS)
(LOCATION==12TH FLOOR))/

Suppose there is no such printer. The Directory Agent responds
with a Service Reply with 0 in the number of responses and no reply
values.

The User Agent then tries a less restrictive query to find authenticator itself.
When producing a printer,
using the 12th floor as "where" criteria.

lpr//(LOCATION==12TH FLOOR)/

In this case, there is now only one reply:

Returned URL: service:lpr://igore.wco.ftp.com:515/draft

The Address Specification for URL Authentication block, the printer is: igore.wco.ftp.com:515.
This is authentication
data produced by the location of algorithm identified within the printer. Files would be printed by Structured

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spooling to that port on that host. The word 'draft' refers to

Authenticator is a message digest D calculated over the
name following
ordered stream of data:

CHARACTER ENCODING OF URL (2 bytes in network byte order)
LIFETIME (2 bytes in network byte order)
LENGTH OF URL (2 bytes in network byte order)
URL (n bytes)
TIMESTAMP (8 bytes in SNTP format [17])

When producing a URL Authentication block, the print queue authentication
data produced by the lpr server supports.

In algorithm identified within the absence of Structured
Authenticator is a Directory Agent, message digest D calculated over the request above could
be multicast. following
ordered stream of data:

ATTRIBUTE CHARACTER ENCODING (2 bytes in network byte order)
LENGTH OF ATTRIBUTES (2 bytes in network byte order)
ATTRIBUTES (n bytes)
TIMESTAMP (8 bytes in SNTP format [17])

In this case it would be sent to the printer
Multicast Address and not to either case, D is then encrypted using the Directory Agent. Service Agents
that can satisfy key of the predicate will reply. Service Agents which
cannot support protected
scope to produce N bytes of authenticator info. The length field of
the character authenticator block is set to N. The N bytes of the request MUST return
CHARSET_NOT_UNDERSTOOD in encrypted
message digest D follow.

To verify an authenticator, the SrvRply. In all other circumstances,
Service Agents which cannot satisfy UA or DA computes the reply do not send any reply
at all.

The only way a User Agent can be sure there are no services which
match message digest
D from the query is by retrying same data as the request (CONFIG_INTERVAL_8). If
no response comes, SA did. The UA or DA then decrypts the User Agent gives up and assumes there are no
such printers.

Another form N
bytes of query is a simpler 'join' query. Its syntax has no
parentheses or logical operators. Each term authenticator data to obtain D'. If D is conjoined (AND-ed
together.) Rewriting the initial query provides an example:

lpr//PAGES PER MINUTE==12,
UNRESTRICTED_ACCESS,
LOCATION==12TH FLOOR/

5.2. Directory Agent Discovery Request

Normally a Service Request returns a Service Reply. The sole
exception to this same as D' the
URL has been authenticated.

There is a Service Request currently only one value defined for the Service Type
"directory-agent". This Service Request is answered with a DA
Advertisement.

Without configured knowledge of a Directory Agent (DA), a User Agent
or Service Agent Block Structure
Descriptor (BSD):

1 The Structured Authenticator uses a Service Request AlgorithmIdentifiers [10]
in ASN.1 [9] notation to discover a DA. (See
section 15.1 identify the algorithm and
encrypted data format.

BSD values 0 and 2-255 are reserved for mechanisms by which a client future standardized structure
descriptors. Other values may be configured to
have knowledge of a DA.) Such defined in a Service Request site-dependent
fashion, and are not intended be used for Directory
Agent Discovery includes a predicate of the form:

directory-agent///

This query is always sent to the Directory Agent Discovery multicast
address. The denote well-known structure
descriptor formats.

Every Service Type of a Location Protocol entity (User Agent, Service Agent,
or Directory Agent Agent) which is "directory-agent",
hence to be configured for use with protected
scopes MUST implement "md5WithRSAEncryption" [4] and be able to
associate it is with BSD value == 1 to verify the Service Type used in the request. No Scope is
included in the request, so all Directory Agents will reply. This is
the only request which omits a Scope which all Directory Agents MUST
respond to. Normally, a Directory Agent with a Scope ONLY responds
to requests with that Scope. No Naming Authority is included, so authentication.

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"IANA" is assumed. We want to reach all the available directory
agents. If the Scope were supplied, only DAs supporting that Scope
would reply.

DA Advertisement Replies may arrive from different sources, similar
in form to:

URL returned: service:directory-agent://slp-resolver.catch22.com
Scope returned: ACCOUNTING

4.4. URL returned: service:directory-agent://204.182.15.66
Scope returned: JANITORIAL SERVICES Entry Lifetime

The DA Advertisement format is defined in Section 14.

If the goal Lifetime field is merely set to discover any Directory Agent, the first
reply will do. If number of seconds the goal, however, is to discover all reachable
DAs, reply can be
cached by any agent. A value of 0 means the request information must not
be retransmitted after an interval (the
recommended time is CONFIG_INTERVAL_5). This retransmitted request
will include a list of DAs which have already responded. See
sections 7 and 20.1. Directory cached. User Agents which receive the request will
only respond MAY cache service information, but if they are not on this list. After there are no new
replies, all DAs are presumed to have been discovered.

If
do, they must provide a DA fails way for applications to respond after CONFIG_INTERVAL_6 seconds, flush this cached
information and issue the UA or
Service Agent should use a different DA. DA addresses may be cached
from previous discovery attempts, preconfigured, or by use of DHCP
(see section 15.2). If no such DA responds, DA discovery request directly onto the network.

Services should be
used to find registered with DAs with a new DA. Only after CONFIG_INTERVAL_7 seconds should
it Lifetime, the suggested
value being CONFIG_INTERVAL_1. The service must be assumed that reregistered
before this interval elapses, or the service advertisement will
no DA exists and multicast based Service Requests
should longer be used.

5.3. Explanation of Terms of Predicate Grammar

A predicate has a simple structure, available. Thus, services which depends on parentheses,
commas vanish and slashes fail to delimit the elements. Examples of proper usage
are given throughout this document.
deregister eventually become automatically deregistered.

5. Service Request Message Format

The terms used in the grammar
are as follows:

predicate:

Placed in a Service Request, this Request is interpreted by used to obtain URLs from a Service
Agent or Directory Agent to determine what information to
return.

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scope:

If this is absent in a Service Request, the request will match
only services registered without a Scope. If it is present,
only services registered under that Scope or are unscoped will
match the request.

where-clause:

This determines which services the request matches. An empty
where-clause will match all services.
Service Agents.

The request will be
limited to services which have format of the specified Service Type, so
the where-clause is not the sole factor in picking out which
services match the request.

where-list:

The where-list Request is a logical expression. It can be a single
expression, a disjunction as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReq) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of predicate string | Service Request <predicate> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ Service Request <predicate>, contd. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If a UA issues a request which will result in a reply which is
too large, the SA or DA will return an abbreviated response (in a conjunction. A single
expression must apply for
datagram the where-clause to match. A
disjunction matches if any expression in size of the OR list matches.
A conjunction matches only if all elements in site's MTU) which has the AND list
match.

Note that there is no logical negation operator: This is
because there 'Overflow' bit
flag set. The UA must then issue the request again using TCP.

The <Previous Responders Addr Spec> is no notion described in sections 7
and 20.1.

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After a User Agent restarts (say, after rebooting of returning "everything except"
what matches a given criteria.

A where-list can be nested and complex. For example, system,
loading of the
following requires that three subexpressions must all network kernel), Service Requests should be true:

(& (| <query-item> <query-item>)
<query-item>
(& <query-item> <query-item> <query-item>)
)

Notice that white space, tabs or carriage returns can be added
anywhere outside query-items. Each list has 2 or more items in
it, and lists can be nested. Services which fulfill delayed
for some random time uniformly distributed within a one second
interval centered about a configured delay value (by default,
CONFIG_INTERVAL_4).

The Service Request allows the entire
logical expression match User Agent to specify the where-clause.

'(' '|' <query-item> ')' Service Type
of the service and '(' '&' <query-item> ')' are
degenerate expressions but they should be tolerated. They a Predicate in a specific language. The general
form of a Service Request is shown below:

<srvtype>[.<na>]/[<scope>]/[<where>]/

The punctuation is necessary even where the fields are
equivalent omitted.

- The <srvtype> refers to <query-item>.

query-item:

A query item has the form:

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'(' <attr-tag> <comp-op> <attr-val> ')'

'(' <keyword> ')'

Examples Type. For each type of this would be:

(SOME ATTRIBUTE == SOME VALUE)
(RESERVED)
(QUEUE LENGTH <= 234)

query-join:

The query-join
service available, there is a comma delimited list unique Service type name string.
See section 20.2.1.

- The <na> is the Naming Authority. This string determines the
semantic interpretation of conditions which the service must satisfy attribute information in order to match the query. The
items are considered to be logically conjoined. Thus the
query-join:

ATTR1=VALUE1, KEYWORD1, KEYWORD2, ATTR2>=34

is equivalent to
<where> part of the where-list:

(& (ATTR1=VALUE1) (KEYWORD1) (KEYWORD2) (ATTR2>=34)) Service Request.

- The query-join cannot be mixed with a where-list. It <scope> is
provided as a convenient mechanism string used to provide a statement restrict the range of
necessary conditions without building the query.
Scope is determined administratively, at a logical expression.

5.4. Service Request Predicate Grammar

Service Requests can precisely describe given site. It is
not necessarily related to network topology (see Section 16).
Leaving this field out means that the services they need request can be satisfied
only by
including a Predicate unscoped service advertisements.

- The <where> string is the body Where Clause of the Request. This Predicate must
be constructed according to request. It
contains a query which allows the grammar below.

<predicate> ::= <srvtype>['.'<na>]'/'<scope>'/'<where>'/'

<srvtype> ::= string representing type selection of service. Only
'a' to 'z', 'A' to 'Z', '+' and '-' are allowed.

<na> ::= string representing those service
instances which the Naming Authority.
Only characters from 'a' to 'z', 'A' to 'z',
'+' and '-' are allowed. If this field is
omitted then "IANA" User Agent is assumed.

<scope> ::= string representing the directory agent scope.
'/', ',' (comma) and ':' are not allowed in

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this string. interested in. The scopes "LOCAL" query
includes attributes, boolean operators and "REMOTE"
are reserved.

<attr-tag> ::= class name of an attribute relations. (See
section 5.3.)

In the case of a given Service
Type. This tag cannot include the following
characters: '(', ')', ',', '=', '!', '>',
'<', '/', '*', except where escaped (see 17.1.)

<keyword> ::= multicast service request, a class name list of an attribute which will have
no values. previous
responders is sent. This string has the same limits
as list will prevent those in the <attr-tag>. In addition white space
internal list from
responding, to the keyword is illegal.

<where-any> ::=
That be sure that responses from other sources are not
drowned out. The request is NOTHING multicast repeatedly (with a recommended
wait interval of CONFIG_INTERVAL_2) until there are no new responses,
or white space.

<where-list> ::= '(' '&' <where-list> <query-list> ')' |
'(' '|' <where-list> <query-list> ')' |
'(' <keyword> ')'
'(' <attr-tag> <comp-op> <attr-val> ')'

<query-list> ::= <where-list> |
<where-list> <query-list>
<query-join> ::= <keyword> |
<join-item> |
<query-join> ',' <keyword> |
<query-join> ',' <join-item>

<join-item> ::= <attr-tag> <comp-op> <attr-val>

<comp-op> ::= "!=" | "==" | '<' | "<=" | '>' | ">="

<attr-val> ::= any string (see a certain time (CONFIG_INTERVAL_3) has elapsed. Different timing
values are applied to a Service Request used for Directory Agent
Discovery, see Section 20.5 5.2.

In order for the ways a request to succeed in which attr-vals are interpreted.)
Value strings may not contain '/', ','
'=', '<', '>', except where escaped (see 17.1.).

'(' and ')' may matching registered information,
the following conditions must be used in attribute values
for met:

1. The result must have the purpose of encoding a binary values.
Binary encodings (See 20.5) may
include same Service Type as the above reserved characters. request.

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5.5. String Matching for Requests

All strings are case insensitive,

2. It must have the same Naming Authority.

3. It must have the same scope. (If the scope of the request
was omitted, the request will only match services which were
registered with respect to string matching on
queries. All preceding or trailing blanks should not be considered
for no scope. Note that a match, but blanks internal to scoped request WILL match
all unscoped Services).

4. The conditions specified in the Where Clause must match the
attributes and keywords registered for the service.

5.1. Service Request Usage

The User Agent may form Service Requests using preconfigured
knowledge of a string are relevant.
For example, " Some String " matches "SOME STRING",
but not "some string".

String matching Service Type's attributes. It may only be performed over also issue
Attribute Requests to obtain the same character sets.
If attribute values for a request cannot be satisfied due to Service Type
before issuing Service Requests (see Section 13). Having obtained
the attributes which describe a lack particular kind of support for the
character set service from an
Attribute Request, or using configured knowledge of a service's
attributes, the request User Agent can build a CHARSET_NOT_UNDERSTOOD error is
returned.

String comparisons (using comparison operators such as '<' or
'>=') are done using lexical ordering in predicate that describes the character set
service needs of the
registration, not using any language specific rules. The ordering
is strictly by user.

Service Requests may be sent directly to a Directory Agent. Suppose
a printer supporting the character value, i.e. "0" < "A" lpr protocol is true when needed on the
character set is US-ASCII, since "0" 12th floor
which has the value of 48 UNRESTRICTED_ACCESS and "A" has
the value 65.

String matching prints 12 pages per minute.
Suppose further that a Attribute Request indicates that there is done after escape sequences have been substituted.
See sections 17, 5.3, 17.1.

6. Service Reply Message Format

The format of a
printer on the 12th floor, a printer that prints 12 pages per minute,
and a printer that offers UNRESTRICTED_ACCESS. To check whether they
are same printer, issue the following request:

lpr//(& (PAGES PER MINUTE==12)
(UNRESTRICTED_ACCESS)
(LOCATION==12th FLOOR))/

Suppose there is no such printer. The Directory Agent responds
with a Service Reply Message is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 with 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | in the number of replies returned |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry>-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
\ . \
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry>-N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Each Service Reply message is composed of a list of URL Entries. responses and no reply
values.

The Error Code may have one of User Agent then tries a less restrictive query to find a printer,
using the following values: 12th floor as "where" criteria.

lpr//(LOCATION==12th FLOOR)/

In this case, there is now only one reply:

Returned URL: service:lpr://igore.wco.ftp.com:515/draft

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0 Success

LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received
from a UA which is in a language

The Address Specification for which there is no
registered Service Information and the request arrived
with printer is: igore.wco.ftp.com:515,
containing the Monolingual bit set. See Section 17.

PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvReq is received
which cannot name of the host managing the requested printer.
Files would be parsed.

SCOPE_NOT_SUPPORTED
A DA which is configured to have a Scope will return this
error if it receives a request which is set printed by spooling to have a
Scope which it does not support. An SA will not return
this error, it will simply not reply that port on that host. The
word 'draft' refers to the multicast
request.

CHARSET_NOT_UNDERSTOOD
If the DA or SA receives a request or registration in a
character set which it does not support, it will return
this error.

Each <URL Entry> in the list has the form defined at the end name of
Section 4.

The URLs in the reply have no delimiters between them, other than the
length fields. The length fields indicate where print queue the strings end.

7. Service Type Request Message Format

The Service Type Request is used to determine all lpr server
supports.

In the types absence of
services supported on a network.

The Directory Agent, the request should above could be sent directly to a DA (though
multicast. In this case it may also would be sent to the Service Location General Specific
Multicast Address), in order Address for "service:printer" and not to find out all services available on the site network (which are
advertised by Directory
Agent. Service Agents and that can satisfy the predicate will reply.
Service Agents.) If no DA is
available, Agents which cannot support the character set of the request
MUST return CHARSET_NOT_UNDERSTOOD in the SrvRply. In all other
circumstances, Service Agents which cannot satisfy the reply do not
send any reply at all.

The only way a User Agent MAY issue more than one can be sure there are no services which
match the query is by retrying the request to insure
that all replies have been received. In each subsequent request, a (CONFIG_INTERVAL_8). If
no response comes, the User Agent included those Service Types that it gives up and assumes there are no
such printers.

Another form of query is aware of. When a simpler 'join' query. Its syntax has no
new replies arrive within CONFIG_INTERVAL_3 from
parentheses or logical operators. Each term is conjoined (AND-ed
together.) Rewriting the initial query provides an example:

lpr//PAGES PER MINUTE==12,
UNRESTRICTED_ACCESS,
LOCATION==12th FLOOR/

5.2. Directory Agent Discovery Request

Normally a request, Service Request returns a Service Reply. The sole
exception to this is a Service Request for the Service Type
"directory-agent". This Service Request is answered with a DA
Advertisement.

Without configured knowledge of a Directory Agent (DA), a User Agent can presume that it has acquired
or Service Agent uses a complete set Service Request to discover a DA. (See
section 15.1 for mechanisms by which a client may be configured to
have knowledge of available a DA.) Such a Service Types. Request used for Directory
Agent Discovery includes a predicate of the form:

directory-agent///

This query is always sent to the Directory Agent Discovery multicast
address. The Service Type of a Directory Agent is "directory-agent",
hence it is the Service Type used in the request. No scope is
included in the request, so all Directory Agents will reply. This is

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The format of

the only request which omits a Service Type Request is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of prev resp string |<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of naming authority | <Naming Authority String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Naming Authority String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Scope String | <Scope String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note that the <Previous Responders Addr Spec> is scope which all Directory Agents MUST
respond to. Normally, a comma delimited
list. (See section 20.1.) The 'length of prev responder list' field
indicates the length of the comma delimited list string. A previous
responder list Directory Agent with 3 elements takes this form:

<addr-spec>,<addr-spec>,<addr-spec>

The Naming Authority, if included, will limit the replies to Service
Type Requests a scope ONLY responds
to Service Types which have the specified Naming
Authority. If this field is omitted (i.e., the length field is
zero), the default requests with that scope. No Naming Authority ("IANA") is assumed. If the
length field is -1, service types from all naming authorities are
requested.

The Scope String Field, if included, will limit replies so
"IANA" is assumed. We want to Service
Types which have reach all the specified Scope or are unscoped. available directory
agents. If this field
is omitted, all Service Types (from the specified Naming scope were supplied, only DAs supporting that scope
would reply.

DA Advertisement Replies may arrive from different sources, similar
in form to:

URL returned: service:directory-agent://slp-resolver.catch22.com
Scope returned: ACCOUNTING

URL returned: service:directory-agent://204.182.15.66
Scope returned: JANITORIAL SERVICES

The DA Advertisement format is defined in Section 14.

If the goal is merely to discover any Directory Agent, the first
reply will do. If the goal, however, is to discover all reachable
DAs, the request must be retransmitted after an interval (the
recommended time is CONFIG_INTERVAL_5). This retransmitted request
will include a list of DAs which have already responded. See
sections 7 and 20.1. Directory Agents which receive the request will
only respond if they are not on this list. After there are no new
replies, all DAs are presumed to have been discovered.

If a DA fails to respond after CONFIG_INTERVAL_6 seconds, the UA or
Service Agent should use a different DA. DA addresses may be cached
from previous discovery attempts, preconfigured, or by use of DHCP
(see section 15.2). If no such DA responds, DA discovery should be
used to find a new DA. Only after CONFIG_INTERVAL_7 seconds should
it be assumed that no DA exists and multicast based Service Requests
should be used.

5.3. Explanation of Terms of Predicate Grammar

A predicate has a simple structure, which depends on parentheses,
commas and slashes to delimit the elements. Examples of proper usage
are given throughout this document. The terms used in the grammar
are as follows:

predicate:

Placed in a Service Request, this is interpreted by a Service
Agent or Directory Agent to determine what information to
return.

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scope:

If this is absent in a Service Request, the request will match
only services registered without a scope. If it is present,
only services registered under that scope or are unscoped will
match the request.

where-clause:

This determines which services the request matches. An empty
where-clause will match all services. The request will be
limited to services which have the specified Service Type, so
the where-clause is not the sole factor in picking out which
services match the request.

where-list:

The where-list is a logical expression. It can be a single
expression, a disjunction or a conjunction. A single
expression must apply for the where-clause to match. A
disjunction matches if any expression in the OR list matches.
A conjunction matches only if all elements in the AND list
match.

Note that there is no logical negation operator: This is
because there is no notion of returning "everything except"
what matches a given criteria.

A where-list can be nested and complex. For example, the
following requires that three subexpressions must all be true:

(& (| <query-item> <query-item>)
<query-item>
(& <query-item> <query-item> <query-item>)
)

Notice that white space, tabs or carriage returns can be added
anywhere outside query-items. Each list has 2 or more items in
it, and lists can be nested. Services which fulfill the entire
logical expression match the where-clause.

'(' '|' <query-item> ')' and '(' '&' <query-item> ')' are
degenerate expressions but they should be tolerated. They are
equivalent to <query-item>.

query-item:

A query item has the form:

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'(' <attr-tag> <comp-op> <attr-val> ')'

'(' <keyword> ')'

Examples of this would be:

(SOME ATTRIBUTE == SOME VALUE)
(RESERVED)
(QUEUE LENGTH <= 234)

query-join:

The query-join is a comma delimited list of conditions which
the service must satisfy in order to match the query. The
items are considered to be logically conjoined. Thus the
query-join:

ATTR1=VALUE1, KEYWORD1, KEYWORD2, ATTR2>=34

is equivalent to the where-list:

(& (ATTR1=VALUE1) (KEYWORD1) (KEYWORD2) (ATTR2>=34))

The query-join cannot be mixed with a where-list. It is
provided as a convenient mechanism to provide a statement of
necessary conditions without building a logical expression.

5.4. Service Request Predicate Grammar

Service Requests can precisely describe the services they need by
including a Predicate the body of the Request. This Predicate must
be constructed according to the grammar below.

<predicate> ::= <srvtype>['.'<na>]'/'<scope>'/'<where>'/'

<srvtype> ::= string representing type of service. Only
alphanumeric characters, '+', and '-' are allowed.

<na> ::= string representing the Naming Authority.
Only alphanumeric characters, '+',
and '-' are allowed. If this field is
omitted then "IANA" is assumed.

<scope> ::= string representing the directory agent scope.
'/', ',' (comma) and ':' are not allowed in

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this string. The scopes "LOCAL" and "REMOTE"
are reserved.

<attr-tag> ::= class name of an attribute of a given Service
Type. This tag cannot include the following
characters: '(', ')', ',', '=', '!', '>',
'<', '/', '*', except where escaped (see 17.1.)

<keyword> ::= a class name of an attribute which will have
no values. This string has the same limits
as the <attr-tag>, except that white space
internal to the keyword is illegal.

<where-any> ::=
That is NOTHING, or white space.

<where-list> ::= '(' '&' <where-list> <query-list> ')' |
'(' '|' <where-list> <query-list> ')' |
'(' <keyword> ')'
'(' <attr-tag> <comp-op> <attr-val> ')'

<query-list> ::= <where-list> |
<where-list> <query-list>
<query-join> ::= <keyword> |
<join-item> |
<query-join> ',' <keyword> |
<query-join> ',' <join-item>

<join-item> ::= <attr-tag> <comp-op> <attr-val>

<comp-op> ::= "!=" | "==" | '<' | "<=" | '>' | ">="

<attr-val> ::= any string (see Section 20.5 for the ways
in which attr-vals are interpreted.)
Value strings may not contain '/', ','
'=', '<', '>', or '*' except where escaped (see 17.1.).

'(' and ')' may be used in attribute values
for the purpose of encoding a binary values.
Binary encodings (See 20.5) may
include the above reserved characters.

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5.5. String Matching for Requests

All strings are case insensitive, with respect to string matching on
queries. All preceding or trailing blanks should not be considered
for a match, but blanks internal to a string are relevant.
For example, " Some String " matches "SOME STRING",
but not "some string".

String matching may only be performed over the same character sets.
If a request cannot be satisfied due to a lack of support for the
character set of the request a CHARSET_NOT_UNDERSTOOD error is
returned.

String comparisons (using comparison operators such as '<' or
'>=') are done using lexical ordering in the character set of the
registration, not using any language specific rules. The ordering
is strictly by the character value, i.e. "0" < "A" is true when the
character set is US-ASCII, since "0" has the value of 48 and "A" has
the value 65.

The special character '*' may precede or follow a string in order to
allow substring matching. If the '*' precedes a string, it matches
any attribute value which ends with the string. If the string ends
with a '*', it matches any attribute value which begins with the
string. Finally, if a string begins and ends with a '*', the string
will match any attribute value which contains the string.

Examples:

"bob*" matches "bob", "bobcat", and "bob and sue"
"*bob" matches "bob", "bigbob", and "sue and bob"
"*bob*" matches "bob", "bobcat", "bigbob", and "a bob I know"

String matching is done after escape sequences have been substituted.
See sections 17, 5.3, 17.1.

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6. Service Reply Message Format

The format of the Service Reply Message is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | URL Entry count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry 1> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
\ . \
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry N> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Each Service Reply message is composed of a list of URL Entries.

The Error Code may have one of the following values:

0 Success

LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received
from a UA which is in a language for which there is no
registered Service Information and the request arrived
with the Monolingual bit set. See Section 17.

PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvRply is received
which cannot be parsed or the declared string lengths
overrun the message.

SCOPE_NOT_SUPPORTED
A DA will return this error if it receives a request
which has a scope not supported by the DA. An SA will
not return this error; it will simply not reply to the
multicast request.

CHARSET_NOT_UNDERSTOOD
If the DA or SA receives a request or registration in a
character set which it does not support, it will return
this error.

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Each <URL Entry> in the list has the form defined in Section 4.2.
The URL entries in the reply have no delimiters between them, other
than the length fields. The URL length fields indicate where the
URL strings end. If the presence of an URL Authenticator block is
signalled by the 'U' bit, the length of the authenticator block
is determined by information within the block as discussed in
section 4.3. A User Agent MAY use the authentication block to
determine whether the Service Agent advertising the URL is, in fact,
authorized to offer the indicated service. If, in a list of URL
entries, some of the URLs indicate services which are in protected
scopes (see section 16.1) while other URLs in the list indicate
services which are not in protected scopes, the latter must still
have Authentication Blocks, but the length of the authentcitor is
shown as zero, and no authentication need be done.

7. Service Type Request Message Format

The Service Type Request is used to determine all the types of
services supported on a network.

The request should be sent directly to a DA (though it may also be
sent to the Service Location General Multicast Address), in order
to find out all services available on the site network (which are
advertised by Directory Agents and Service Agents.) If no DA is
available, a User Agent MAY issue more than one request to insure
that all replies have been received. In each subsequent request, a
User Agent includes those Service Types that it is aware of. When no
new replies arrive within CONFIG_INTERVAL_3 from a request, the User
Agent can presume that it has acquired a complete set of available
Service Types.

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The format of a Service Type Request is:

Note that the <Previous Responders Addr Spec> is a comma delimited
list. (See section 20.1.) The 'length of prev responder list' field
indicates the length of the comma delimited list string. A previous
responder list with 3 elements takes this form:

<addr-spec>,<addr-spec>,<addr-spec>

The Naming Authority, if included, will limit the replies to Service
Type Requests to Service Types which have the specified Naming
Authority. If this field is omitted (i.e., the length field is
zero), the default Naming Authority ("IANA") is assumed. If the
length field is -1, service types from all naming authorities are
requested.

The Scope String Field, if included, will limit replies to Service
Types which have the specified scope or are unscoped. If this field
is omitted, all Service Types (from the specified Naming Authority)
are returned. returned.

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8. Service Type Reply Message Format

The Service Type Reply has the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | number of service types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item 1> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item N> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of a Service Type Item is as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Service Type String | <Service Type String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code may have one of the following values:

0 Success

PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvTypeRqst is
received which cannot be parsed.

SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives a SrvTypeRqst which is set to have a
scope which it does not support. An SA will not return
this error, it will simply silently discard the multicast
request.

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CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvTypeRqst in a character set which
it does not support, it MUST use this error.

The service type's name is provided in the <Service Type String>. If
the service type has a naming authority other than "IANA" it should
be returned following the service type string and a "." character.
See section 20.2.1 for the formal definition of this field. User
Agents calculate Service Specific Multicast addresses based on a hash
of the Service Type (see Section 3.6.2). This multicast address may
then be used for issuing Service and Attribute Requests directly to
SAs.

The following are examples of Service Type Strings which might be
found in Service Type Replies:

service:lpr://
service:http://
service:nfs://

9. Service Registration Message Format

After a Service Agent has found a Directory Agent, it begins to
register its advertised services one at a time. A Service Agent
must wait for some random time uniformly distributed within the
range specified by CONFIG_INTERVAL_11 before registering again.
Registration is done using the Service Registration message
specifying all attributes for a service. If the service registration
in a protected scope 16.1, then the service MUST include both a URL
Authentication block and an Attribute Authentication block (see
section 4.3). In that case, the service agent MUST set both the 'U'
bit and the 'A' bit (see section 4).

A Directory Agent must acknowledge each service registration request.
If authentication blocks are included, the Directory Agent MUST
verify the authentication before registering the service. This
requires obtaining key information, either by preconfiguration,
maintenance of a security association with the service agent, or
acquiring the appropriate certificate.

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The format of a Service Registration is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL-Entry> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Attr List String | <attr-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) Attribute Authentication Block ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The <URL-Entry> is defined at the end of Section 4.2. The
<attr-list> is defined in Section 20.3. The Attribute Authentication
Block, which is only present if the 'A' bit is set in the message
header, is defined in section 4.3.

Service registration may use a connectionless protocol (e.g. UDP),
or a connection oriented protocol (e.g. TCP). If the registration
operation may contain more information than can be sent in one
datagram, the Service Agent MUST use a connection oriented protocol
to register itself with the DA. When a Service Agent registers the
same attribute class more than once for a service instance, the
Directory Agent overwrites the all the values associated with that
attribute class for that service instance. Separate registrations
must be made for each language that the service is to be advertised
in.

If a SA attempts to register a service with a DA and the registration
is larger than the site path MTU, then the DA will reply with a
SrvAck, with the error set to INVALID_REGISTRATION and the 'Overflow'
byte set.

An example of Service Registration information is:

Lifetime (seconds): 16-bit unsigned integer
URL (at least): service:<srvtype>://<addr-spec>
Attributes (if any): (ATTR1=VALUE),KEYWORD,(ATTR2 = VAL1, VAL2)

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In order to offer continuously advertised services, Service Agents
should start the reregistration process before the Lifetime they used
in the registration expires.

An example of a service registration (valid for 3 hours) is as
follows:

Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
Attributes: (SCOPE=DEVELOPMENT),
(PAPER COLOR=WHITE),
(PAPER SIZE=LETTER),
UNRESTRICTED_ACCESS,
(LANGUAGE=POSTSCRIPT, HPGCL),
(LOCATION=12 FLOOR)

The same registration could be done again, as shown below, in German;
however, note that "lpr", "service", and "SCOPE" are reserved terms
and will remain in the language they were originally registered
(English).

Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
Attributes: (SCOPE=ENTWICKLUNG),
(PAPIERFARBE=WEISS),
(PAPIERFORMAT=BRIEF),
UNBEGRENTZTER_ZUGANG,
(DRUECKERSPRACHE=POSTSCRIPT,HPGCL),
(STANDORT=11 ETAGE)

Scoped registrations must contain the SCOPE attribute. Unscoped
registrations must be registered with all unscoped Directory Agents.

Registrations of a previously registered service are considered
an update. If such an attribute registration is performed in a
protected scope (see section 16.1), a new Attribute Authentication
block must also be included, and the 'A' bit set in the registration
message header.

The new registration's attributes replace the previous
registration's, but do not effect attributes which were
included previously and are not present in the update.

For example, suppose service:x://a.org has been registered
with attributes A=1, B=2, C=3. If a new registration comes for
service:x://a.org with attributes C=30, D=40, then the attributes for
the service after the update are A=1, B=2, C=30, D=40.

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In the example above, the SCOPE is set to DEVELOPMENT (in English)
and ENTWICKLUNG (in German). Recall that all strings in a message
must be in one language, which is specified in the header. The
string SCOPE is *not* translated, as it is one of the reserved
strings in the Service Location Protocol (see section 17.2.)

The Directory Agent may return a server error in the acknowledgment.
This error is carried in the Error Codes field of the service
location message header. A Directory Agent MUST decline to register
a service if it is specified with an unsupported scope. In this case
a SCOPE_NOT_SUPPORTED error is returned in the SrvAck. A Directory
Agent MUST NOT accept Service Registrations which have an unsupported
scope unless it is an unscoped Directory Agent, in which case it MUST
accept all Service Registrations.

An unscoped Service Registration will match all requests. A request
which specifies a certain scope will therefore return services which
have that scope and services which are unscoped. It is strongly
suggested that one should use scopes in all registrations or none.
See Sections 16 and 3.7 for details.

When the URL entry accompanying a registration also contains an
authentication block (section 4.3), the DA MUST perform the indicated
authentication, and subsequently indicate the results in the Service
Acknowledgement message.

10. Service Acknowledgement Message Format

A Service Acknowledgement is sent as the result of a DA receiving
and processing a Service Registration or Service Deregistration. An
acknowledgment indicating success must have the error code set to
zero. Once a DA acknowledges a service registration it makes the
information available to clients.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvAck) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code may have one of the following values:

0 Success

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PROTOCOL_PARSE_ERROR
A DA returns this error when the SrvReg or SrvDereg is
received which cannot be parsed or the declared string
lengths overrun the message.

INVALID_REGISTRATION
A DA returns this error when a SrvReg or SrvDeReg is
invalid. For instance, an invalid URL, unknown or
malformed attributes, or deregistering an unregistered
service all cause this error to be reported.

SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives a SrvReq which is set to have a
scope which it does not support.

CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvReg or SrvDereg in a character
set which it does not support, it will return this error.

AUTHENTICATION_ABSENT
If DA has been configured to require an authentication
for any service registered in the requested scope, and
there are no authentication blocks in the registration,
the DA will return this error.

AUTHENTICATION_FAILED
If the registration contains an authentication block
which fails to match the correct result as calculated
(see section 4.3) over the URL or attribute data to be
authenticated, the DA will return this error.

If the Directory Agent accpets a Service Registration, and already
has an existing entry, it updates the existing entry with the new
lifetime information and possibly new attributes and new attribute
values. Otherwise, if the registration is acceptable (including all
necessary authentication checks) the Directory Agent creates a new
entry, and sets the 'F' bit in the Service Acknowledgement returned
to the Service Agent.

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11. Service Type Reply Deregister Message Format

The

When a service is no longer available for use, the Service Type Reply Agent must
deregister itself from Directory Agents that it has been registered
with. A service uses the following format: PDU to deregister itself.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRply) SrvDereg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | number length of service types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item>-1 \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ URL | . . . URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item>-N \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of a Service Type Item is as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length URL of Service Type String to Deregister, contd. \
| <Service Type String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type String>, continued \
| | (if present) authentication block .....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of addr spec <tag spec> string | <addr <tag spec> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <addr <tag spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Service Agent should retry this operation if there is no response
from the Directory Agent. The Directory Agent acknowledges this
operation with a Service Acknowledgment message. Once the Service
Agent receives an acknowledgment indicating success, it can assume
that the service is no longer advertised by the Directory Agent. The
Error Code in the Acknowledgment of the Service Deregistration may
have one of the same values as described in section 10.

The Service Deregister Information sent to the directory agent has
the following values:

0 Success

PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvTypeRqst form:

service:<srvtype>://<addr-spec>
Attribute tags (if any): ATTR1,KEYWORD,ATTR2

This will deregister the specified attributes from the service
information from the directory agent. If no attribute tags are
included, the entire service information is
received which cannot be parsed. deregistered in every
language and every scope it was registered in. To deregister the
printer from the preceding example, use:

service:lpr://igore.wco.ftp.com:515/draft

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SCOPE_NOT_SUPPORTED
A DA which is configured to have

If the service was originally registered with a Scope will return this
error if it receives URL entry containing
a SrvTypeRqst which is set to URL authentication block, then the Service Deregistration message
header MUST have a
Scope which it does not support. An SA will not return
this error, it will simply silently discard the multicast
request.

CHARSET_NOT_UNDERSTOOD
If 'U' bit set, and the DA receives a SrvTypeRqst URL entry is then followed
by the authentication block, with the authenticator calculated over
the URL data, the timestamp, and the length of the authenticator as
explained in a character set which
it does not support, it MUST section 4.3. In this error.

The service type's name calculation, the lifetime of the
URL data is provided in considered to be zero, no matter what the <Service Type String>.
See section 20.2.1 current value
for the formal definition remaining lifetime of this field. the registered URL.

12. Attribute Request Message Format

The
<addr spec> format Attribute Request is described in 20.4. This field provides
the service specific multicast address. If used to obtain attribute information. The
UA supplies a request and the service specific
multicast address appropriate attribute information is omitted,
returned.

If the General UA supplies only a Service Location Multicast
Address is assumed. User Agents may Type, then use this multicast address the reply includes
all attributes and all values for issuing that Service Type. The reply
includes only those attributes for which services exist and Attribute Requests directly to SAs.

Example Service Type Replies might be:

Multicast Address Service Type String

224.0.3.10 service:lpr://
224.0.3.24 service:http://
224.0.3.115 service:nfs://

NOTE: These multicast addresses are examples only,
advertised by the official
numbers DA or SA which received the Attribute Request.
Since different instances of a given service can, and very likely
will, have not yet been assigned.

9. different values for the attributes defined by the Service Registration Message Format

After
Type, the User Agent must form a union of all attributes returned by
all service Agents. The Attribute information will be used to form
Service Agent has found Requests.

If the UA supplies a Directory Agent, it begins URL, the reply will contain service information
corresponding to
register its advertised services one at that URL.

Attribute Requests include a time. A Service Agent
must wait for some random time uniformly distributed within 'select clause'. This may be used to
limit the amount of information returned. If the select clause is
empty, all information is returned. Otherwise, the UA supplies
a comma delimited list of attribute tags and keywords. If the
range specified by CONFIG_INTERVAL_11 before registering again.
Registration
attribute or keyword is done using defined for a service, it will be returned
in the Service Registration message
specifying Attribute Reply, along with all attributes registered values for a service. A Directory Agent must
acknowledge each service registration request. that
attribute. If the attribute selected has not been registered for
that URL or Service Type, the attribute or keyword information is
simply not returned.

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The format of a Service Registration is: Attribute Request message has the following form:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReg) AttrRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL-Entry> <Previous Responders Addr Spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length length of Attr List String URL | <attr-list> URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list>, Continued. URL, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The <attr-list>
is defined <Previous Responder Address List> functions exactly as introduced
in Section 20.3.

Service registration may use 7. See also Section 20.1.

The URL can take two forms: Either it is simply a connectionless protocol (e.g. UDP), Service
Type, such as "service:http:", or it can be a connection oriented protocol (e.g. TCP). If URL, such as
"service:lpr://igore.wco.ftp.com:515/draft". In the former case, all
attributes and the full range of values for each attribute for the
Service Type is returned. In the latter case, only the attributes
for the service whose URL is defined are returned.

The Scope String is provided so that Attribute Requests for Service
Types can be made so that only the registration
operation may contain more Attribute information than can pertaining
to a specific scope will be sent returned. This field is ignored in one
datagram, the Service Agent MUST use
case when a connection oriented protocol
to register itself with full URL is sent in the DA. When a Attribute Request. The rules for
encoding of the Scope String are given in Section 5.4.

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The select list takes the
same form:

<select-list> ::= <select-item> |
<select-item> ',' <select-list>

<select-item> ::= <keyword> | <attr-tag> | <partial-tag> '*'

<partial-tag> ::= the partial class name of an attribute
If followed by an '*', it matches all class more than once for a service instance, the
Directory Agent overwrites names
which begin with the partial tag. If preceded by a
'*' it matches all the values associated class names which end with that
attribute
partial tag. If both preceded and followed by '*'
it matches all class for that service instance. Separate registrations
must be made for each language that names which contain the service is to be advertised
in.
partial tag.

For definitions of <attr-tag> and <keyword> see 5.4.

An example of Service Registration information a select-list following the printer example is:

Lifetime (seconds): 16-bit unsigned integer
URL (at least): service:<srvtype>://<addr-spec>
Attributes (if any): (ATTR1=VALUE),KEYWORD,(ATTR2 = VAL1, VAL2)

In order

PAGES PER MINUTE, UNRESTRICTED_ACCESS, LOCATION

If sent to offer continuously advertised services, Service Agents
should start a Directory Agent, the number of previous responders is
zero and there are no Previous Responder Address Specification.
These fields are only used for repeated multicasting, exactly as for
the reregistration process before Service Request.

13. Attribute Reply Message Format

An Attribute Reply Message takes the Lifetime they used
in form:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | length of <attr-list> string |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code may have the registration expires. following values:

0 Success

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An example of

LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a service registration (valid for 3 hours) request is received
from a UA which is as
follows:

The same registration could be done again, as shown below, in German;
however, note that "lpr", "service", and "SCOPE" are reserved terms
and will remain in the a language they were originally for which there is no
registered
(English).

Registrations must contain an Attribute of SCOPE unless they are
unscoped Service Information and then they must be registered the request arrived
with all unscoped Directory
Agents.

Registrations of the Monolingual bit set. See Section 17.

PROTOCOL_PARSE_ERROR
A DA or SA returns this error when a AttrRqst is received
which cannot be parsed or the declared string lengths
overrun the message.

SCOPE_NOT_SUPPORTED
A DA which is configured to have a previously registered service are considered scope will return this
error if it receives an
update. The new registration's attributes replace the previous
registration's, but AttrRqst which is set to have
a scope which it does not support. SAs will silently
discard multicast AttrRqst messages for scopes they do
not effect attributes support.

CHARSET_NOT_UNDERSTOOD
If the DA receives an AttrRqst in a character set which were included
previously and are
it does not present in the update. For example:
service:x://a.org has been registered with attributes A=1, B=2, C=3.
A new registration comes for service:x://a.org with attributes C=30,
D=40. support, it will return this error. SAs will
silently discard multicast AttrRqst messages which arrive
using character sets they do not support.

The attributes for the service after <attr-list> (attribute list) has the update are A=1, B=2,
C=30, D=40.

In same form as the example above, attribute
list in a Service Registration, see Section 20.3 for a formal
definition of this field.

An Attribute Request for "lpr" might elicit the SCOPE following reply
(UNRESTRICTED_ACCESS is set to DEVELOPMENT (in English)
and ENTWICKLUNG (in German). Recall that all strings in a keyword):

(PAPER COLOR=WHITE,BLUE),
(PAPER SIZE=LEGAL,LETTER,ENVELOPE,TRACTOR FEED),
UNRESTRICTED_ACCESS,
(PAGES PER MINUTE=1,3,12),
(LOCATION=12th, NEAR ARUNA'S OFFICE),
(QUEUES=LEGAL,LETTER,ENVELOPE,LETTER HEAD)

If the message header has the 'A' bit set, the Attribute Reply will
have an Attribute Authentication block set. In this case, the
Attribute Authenticator must be in one language, which is specified in returned with the header. The
string SCOPE is *not* translated, as it is one entire list of the reserved
strings
attributes, exactly as it was registered by an SA in the Service Location Protocol (see section 17.3.)

The Directory Agent may return a server error in protected
scope. In this case, the acknowledgment.
This error is carried URL was registered in a protected scope
and the Error Codes field of UA included a URL but not a select clause. If the service
location message header. A Directory Agent MUST decline AttrRqst
specifies that only certain attributes are to register be returned, the DA
does not (typically cannot) compute a new Authenticator so it simply
returns the attributes without an authenticator block.

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a service if it is specified with an unsupported Scope. In this case
a SCOPE_NOT_SUPPORTED error is returned in the SrvAck.

A Directory
Agent MUST NOT accept Service Registrations UA which have an unsupported
Scope unless it is an unscoped Directory Agent, wishes to obtain authenticated attributes for a service in which case it MUST
accept all Service Registrations.

An unscoped Service Registration will match all requests. A request
which specifies
a certain protected scope will MUST therefore return services which
have that Scope and services which are unscoped. It is strongly
suggested that one should use Scopes in all registrations or none.
See Sections 16 must include a particular URL and 3.7 for details.

10. Service Acknowledgement no
select list with the AttrRqst.

14. Directory Agent Advertisement Message Format

A Service Acknowledgement is sent as the result of a DA receiving
and processing a Service Registration or Service Deregistration. An
acknowledgment indicating success must

Directory Agent Advertisement Messages have the error code set to
zero. Once a DA acknowledges a service registration it makes the
information available to clients. following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvAck) DAAdvert) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of <Scope-list> | <Scope-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope-list>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code is set when a DA Advertisement is returned as the
result of a Service Request. It will always be set to 0 in the case
of an unsolicited DA Advertisement. The Error Code may take the
values specified in Section 6.

The URL corresponds to the Directory Agent's location. The
<Scope-list> is a comma delimited list of scopes which the DA
supports, in the following format:

<Scope-list> ::= <Scope> | <Scope-list> ',' <Scope>
<Scope> ::= String representing a scope

See Section 5.4 for the lexical rules regarding <Scope>.

DA Advertisements sent in reply to a Directory Agent Discovery
Request has the same format as the unsolicited DA Advertisement, for
example:

URL: service:directory-agent://SLP-RESOLVER.CATCH22.COM
SCOPE List: ADMIN

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The Directory Agent can be reached at the Address Specification
returned, and supports the SCOPE called "ADMIN".

15. Directory Agents

15.1. Introduction

A Directory Agent acts on behalf of many Service Agents. It acquires
information from them and acts as a single point of contact to supply
that information to User Agents.

The queries that a User Agent multicasts to Service Agents (in an
environment without a Directory Agent) are the same as queries that
the User Agent might unicast to a Directory Agent. A User Agent may
cache information about the presence of alternate Directory Agents to
use in case a selected Directory Agent fails.

Aside from enhancing the scalability of the protocol (see
section 3.7), running multiple DAs provides robustness of operation.
The Error Code DAs may have replicated service information which remain
accessible even when one of the following values:

0 Success

PROTOCOL_PARSE_ERROR
A DA returns this error when DAs fail. Directory Agents, in the SrvReg or SrvDereg could
not be parsed.

INVALID_REGISTRATION
A DA returns
future, may use mechanisms outside of this error when protocol to coordinate
the maintenance of a SrvReg is invalid (it
parses badly distributed database of Service Location
information, and thus scale to enterprise networks or larger
administrative domains.

Each Service Agent must register with all DAs they are configured to
use. UAs may choose among DAs they are configured to use.

Locally, Directory Agent consistency is poorly formed guaranteed using mechanisms
in some way.)

SCOPE_NOT_SUPPORTED the protocol. There isn't any Directory to Directory Agent
protocol yet. Rather, passive detection of DAs by SAs ensures that
eventually service information will be registered consistently
between DAs. Invalid data will age out of the Directory Agents
leaving only transient stale registrations even in the case of a
failure of a Service Agent.

15.2. Finding Directory Agents

A DA which is User or Service Agent may be statically configured to have use a Scope will return this
error if it receives
particular DA. This is discouraged unless the application resides on
a SrvReq which network where any form of multicast or broadcast is set to have impossible.

Alternatively, a
Scope host which uses DHCP [2, 12] may use it does not support. to obtain a
Directory Agent's address. DHCP options 78 and 79 have been assigned
for this purpose [22].

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CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvReg or SrvDereg in a character
set which it does not support, it will return this error.

AUTHENTICATION_FAILED
If the DA uses IP Security Authentication and the
SA

The third way to discover DAs is dynamically. This is done by
sending out a SrvReg or SrvDereg message fails to be
authenticated, Directory Agent Discovery request (see Section 5.2).

Lastly, the DA will return this error.

11. Service Deregister Message Format agent may be informed passively as follows:

When a service is no longer available for use, the Service Agent must
deregister itself from Directory Agents that Agent first comes on-line it has been registered
with. A service uses the following PDU sends an unsolicited DA
Advertisement to deregister itself.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| the Service Location header (function = SrvDereg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of URL | URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL of Service to Deregister, contd. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length general multicast address. If
a DA supports a particular scope or set of <tag spec> string | <tag spec> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <tag spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Service Agent should retry this operation if there is no response
from scopes these are placed in
the Directory Agent. reply. The Directory Agent acknowledges class for this
operation with attribute is 'SCOPE'.

Every CONFIG_INTERVAL_9 a service acknowledgment. Once the Service Directory Agent
receives will send an acknowledgment indicating success, it can assume unsolicited
DA Advertisement. This will ensure that the
service is no longer advertised eventually it will be
discovered by the all applications which are concerned.

When a Directory Agent. The Error
Code in the Acknowledgment of the Service Deregistration may have the
same values Agent first comes up it begins with 0 as described in section 10.

The Service Deregister Information sent its XID,
and increments this by one each time it sends an unsolicited DA
Advertisement. When the counter wraps, it should go from 0xFFFF to
0x0100, not 0.

If the Directory Agent has stored all of the following form:

service:<srvtype>://<addr-spec>
Attribute tags (if any): ATTR1,KEYWORD,ATTR2

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This will deregister the specified attributes from the service
information from the Directory Agent. If no attribute tags are
included, the entire service information is deregistered in every
language and every Scope
a nonvolatile store, it was registered in. To deregister the
printer from should initially set the preceding example, use:

service:lpr://igore.wco.ftp.com:515/draft

12. Attribute Request Message Format

The Attribute Request is used XID to obtain attribute information. The
UA supplies a request and the appropriate attribute information 0x100, as it
is
returned. not coming up 'stateless.' If the UA supplies only a Service Type, and the reply includes
all attributes and all values for that it stores service registrations in
memory only, it will restart without any state. It should indicate
this by resetting its XID to 0.

All Service Type. The reply
includes only those attributes for Agents which services exist and are
advertised by receive the unsolicited DA Advertisement
should examine its XID. If the Directory Agent has never before
been heard from or SA which received if the Attribute Request.
Since different instances of a given service can, XID is less than it was previously and very likely
will, have different values for the attributes defined by
less than 256, the Service
Type, the User Agent must form a union of all attributes returned by
all should assume the DA does not have
its service Agents. The Attribute information will be used to form
Service Requests. registration, even if it once did. If this is the UA supplies a URL, case
and the reply will contain service information
corresponding to that URL.

Attribute Requests include a 'select clause'. This may be used to
limit DA has the amount of information returned. If proper scope, the select clause is
empty, SA should register all service
information is returned. Otherwise, with the UA supplies Directory Agent, after waiting a comma delimited list of attribute tags and keywords. If the
attribute random interval
CONFIG_INTERVAL_10.

When a Service Agent or keyword is defined for User Agent first comes on-line it must issue
a service, Directory Agent Discovery Request unless it will be returned is using static or DHCP
configuration, as described in 5.2.

A Service Agent registers information with ALL newly discovered
Directory Agents when either of the Attribute Reply, along above two events take place.
When scopes are being used, a Service Agent SHOULD choose a set of
scopes to be advertised in and need only register with all registered values for Directory
Agents that
attribute. If support the attribute selected has not ben scopes in which they wish to be registered.
Services MUST be registered for with DAs that
URL or Service Type, the attribute or keyword information is simply support their scope and
those which have no scope, unless specifically configured not returned. to do
so (see section 22.1.)

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The Attribute Request message has the following form:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of URL | URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <Scope> | <Scope> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length

Once a User Agent becomes aware of <select-list> | <select-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <select-list>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The <Previous Responder Address List> functions exactly as introduced
in Section 7. See also Section 20.1.

The URL can take two forms: Either it is simply a Service
Type, such as "service:http:", or Directory Agent it can be a URL, such as
"service:lpr://igore.wco.ftp.com:515/draft". In the former case, all
attributes and the full range of values for each attribute for the
Service Type is returned. will unicast
its queries there. In the latter case, only the attributes
for the service whose URL event that more than one Directory Agent
is defined detected, it will select one to communicate with. When scopes
are returned.

The Scope String is provided so that Attribute Requests supported, the User Agent will direct its queries to different
Directory Agents depending on which scopes are appropriate domains
for Service
Types can be made so that only the Attribute information pertaining query to a specific Scope will be returned. This field is ignored in the
case when a full URL is sent in the Attribute Request. answered in.

The rules for
encoding of protocol will cause all DAs (of the same scope) to eventually
obtain consistent information. Thus one DA should be as good as any
other for obtaining service information. There may be temporary
inconsistencies between DAs.

16. Scope String are given Discovery and Use

The scope mechanism in Section 5.4.

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scalability. The select list takes the form:

<select-list> ::= <select-item> |
<select-item> ',' <select-list>

<select-item> ::= <keyword> | <attr-tag> | <partial-tag> '*'

<partial-tag> ::= primary use of scopes is to provide the partial class name capability
to organize a site network along administrative lines. A set of
services can be assigned to a given department of an attribute
followed organization,
to a certain building or geographical area or for a certain purpose.
The users of the system can be presented with these organizational
elements as a top level selection, before services within this domain
are sought.

A site network that has grown beyond a size that can be reasonably
serviced by an '*' matches all class names
which begin with a few DAs can use the characters preceding scope mechanism. DAs have the '*'

For definitions
attribute class "SCOPE". The values for this attribute are a list
of <attr-tag> strings that represent the administrative areas for which this
Directory Agent is configured. The semantics and <keyword> see 5.4.

An example language of a select-list following the printer example is:

PAGES PER MINUTE, UNRESTRICTED_ACCESS, LOCATION

If sent
strings used to a Directory Agent, describe the number of previous responders is
zero and there are no Previous Responder Address Specification.
These fields scope are only used for repeated multicasting, exactly as for almost entirely the Service Request.

13. Attribute Reply Message Format

An Attribute Reply Message takes choice of
the form:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | length administrative entity of <attr-list> string |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code may have the following values:

0 Success

LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received

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from a UA particular domain in which these
scopes exist. The values of SCOPE should be configurable, so the
system administrator can set its value. The scopes "LOCAL" and
"REMOTE" are reserved and SHOULD NOT be used. Use of these reserved
values is to be defined in a language for which there is no
registered Service Information and future protocol document.

Services with the request arrived attribute SCOPE should only be registered with DAs
which support the Monolingual bit set. See Section 17.

PROTOCOL_PARSE_ERROR
A DA same scope or SA returns this error when the AttrRqst could not
be parsed.

SCOPE_NOT_SUPPORTED
A DA DAs which is configured to have no scope.

Directory Agents advertise their available scopes. A Service Agent
may then choose a Scope will return this
error if it receives an AttrRqst scope in which is set to have
a Scope which it does not support. SAs will silently
discard multicast AttrRqst messages for Scopes they do
not support.

CHARSET_NOT_UNDERSTOOD
If the DA receives an AttrRqst register, and SHOULD register
with all Directory Agents in a character set that scope, as well as all DAs which
it does not support, it will return
have no scope. Failure to be comprehensive in registration according
to this error. SAs rule will
silently discard multicast AttrRqst messages which arrive
using character sets they do mean that the service advertisement may not support.

The <attr-list> (attribute list) be
available to all User Agents.

A Directory Agent which has the same form as the attribute
list in a Service Registration, see Section 20.3 for a formal
definition of this field.

An Attribute Request for "lpr" might elicit scope will return advertisements
in response to Directory Agent Discovery requests with the following reply
(UNRESTRICTED_ACCESS is a keyword):

(PAPER COLOR=WHITE,BLUE),
(PAPER SIZE=LEGAL,LETTER,ENVELOPE,TRACTOR FEED),
UNRESTRICTED_ACCESS,
(PAGES PER MINUTE=1,3,12),
(LOCATION=12TH, NEAR ARUNA'S OFFICE), scope

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14. Directory Agent Advertisement Message Format

Directory Agent Advertisement Messages have

information included. Note that the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = DAAdvert) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of <Scope-list> | <Scope-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope-list>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Error Code is set when a DA Advertisement "service:directory-agent" scheme
is returned as registered with the
result of a Service Request. It will always be set to 0 in IANA naming authority (which is automatically
selected by leaving the case
of an unsolicited DA Advertisement. Naming Authority field empty.)

The Error Code may take query:

directory-agent/MATH DEPT//

Could receive the
values specified in Section 6. following DA Advertisement:

Returned URL: service:directory-agent://diragent.blah.edu
Returned SCOPE: MATH DEPT

The URL corresponds to the same Directory Agent's location. The
<Scope-list> is a comma delimited list of Scopes which the DA
supports, in the following format:

<Scope-list> ::= <Scope> | <Scope-list> ',' <Scope>
<Scope> ::= String representing Agent if it had no scope value would reply:

Returned URL: service:directory-agent://diragent.void.com
Returned SCOPE:

If a Directory Agent supported more than one scope

See Section 5.4 for the lexical rules regarding <Scope>. it would reply as:

Returned URL: service:directory-agent://srv.domain.org
Returned SCOPE: MATH DEPT,ENGLISH DEPT,CS DEPT

A DA Advertisements sent in which has no scope will reply to a any Directory Agent Discovery
Request has the same format as the unsolicited DA Advertisement, for
example:

URL: service:directory-agent://SLP-RESOLVER.CATCH22.COM
SCOPE List: ADMIN

The Directory Agent can be reached at the Address Specification
returned, and supports the SCOPE called "ADMIN".

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15. Directory Agents

15.1. Introduction

A Directory Agent acts on behalf of many Service Agents. It acquires
information from them and acts as
Request.

Being a single point member of contact to supply
that information to User Agents.

The queries that a User Agent multicasts to Service Agents (in scope means that an
environment without a agent SHOULD use those
Directory Agent) are the same as queries Agents that
the User Agent might unicast to a Directory Agent. A support its scope. User Agent may
cache information about the presence of alternate Directory Agents send all
requests to
use in case a selected Directory Agent fails.

Aside from enhancing the scalability of the protocol (see
section 3.7), running multiple DAs provides robustness of operation.
The DAs have replicated service information which remain accessible
even when one of the DAs fail. Directory Agents, in the future, may
use mechanisms outside of this protocol to coordinate support the maintenance
of a distributed database of Service Location information, and thus
scale to enterprise networks or larger administrative domains.

Each Service Agent must register with all DAs they are configured to
use. UAs may choose among DAs they indicated scope. Services are configured to use.

Locally, Directory Agent consistency is guaranteed using mechanisms
in
registered with the protocol. There isn't any Directory DA(s) in their scope. For a UA to Directory Agent
protocol yet. Rather, passive detection of DAs by SAs ensures that
eventually find a service information will be
that is registered consistently
between DAs. Invalid data will age out of the Directory Agents
leaving only transient stale registrations even in the case of a
failure of particular scope it must send requests to a
DA which supports the indicated scope. There is no limitation on
scope membership built into the protocol; that is to say, a Service Agent.

15.2. Finding Directory Agents

A User
Agent or Service Agent may be statically configured to use a
particular DA. This member of more than one scope.
Membership is discouraged open to all, unless the application resides on
a network where any form of multicast or broadcast some external authorization
mechanism is impossible.

Alternatively, a host which uses DHCP [2, 10] may use it added to obtain a
Directory Agent's address. A DHCP option will be assigned limit access.

16.1. Protected Scopes

Scope membership MAY also define the security access and
authorization for this
purpose. It has not yet been, at services in the time this document was written.

The third way to discover DAs is dynamically. This occurs actively
by sending out scope; such scopes are called
protected scopes. If a Directory User Agent Discovery wishes to be sure that Service
Agents are authorized to provide the service they advertise, then
the User Agent should request (see Section 5.2).

Lastly, services from a protected scope which
has been configured to have the necessary authentication mechanism
and keys distributed to the Service Agents within the scope. A
directory agent may be informed passively as follows: distributing URLs for services in a protected scope

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When a Directory Agent first comes on-line it sends an unsolicited DA
Advertisement

will reject any registrations or deregistrations for service agents
which cannot provide cryptographically strong authentication to prove
their authorization to provide the Service Location general multicast address. If services.

For instance, if a DA supports campus registrar wishes to find a particular Scope or set of Scopes these are placed working printer
to produce student grade information for mailing, the registrar would
require the printing user agent to transmit the printable output
only to those printing Service Agents which have been registered in
the reply. The class for this attribute is 'SCOPE'.

Every CONFIG_INTERVAL_9 a Directory Agent will send an unsolicited
DA Advertisement again. This will ensure appropriate protected scope. Notice that eventually it will be
discovered by all applications which are concerned.

When a Directory Agent first comes up it begins each service agent
is, under normal circumstances, validated two times: once when
registering with 0 as its XID, the directory agent, and increments this by one each time it sends an unsolicited DA
Advertisement. When once when the counter wraps, it should go from 0xFFFF to
0x0100, not 0.

If user agent
validates the Directory Agent has stored all of URL received with the service information in
a nonvolatile store, it should initially set Service Reply. This protects
against the XID to 0x100, possibilities of malicious Directory Agents as it
is not coming up 'stateless.' If it stores service registrations in
memory only, it will restart without any state. It should indicate
this by resetting its XID to 0.

All well as
malicious Service Agents.

Note that services in protected scopes provide separate
authentication for their URL entry, and for their attributes. This
follows naturally from the needs of the protocol operation. User
Agents which specify a service type and attributes needed for service
in that service type will not receive attribute information from the unsolicited DA Advertisement
should examine its XID. If
directory agent; they will only receive the Directory Agent has never before
been heard from or if appropriate URL entries.
Only the XID is less than it was previously and
less than 256, information returned needs to be authenticated.

User agents which receive attribute information for a particular
URL (see section 12), on the Service Agent should assume other hand, need to authenticate the DA does not have
its service registration, even if it once did. If
attributes when they are returned (see section 13). In this case,
there may be much more data to authenticate, but this operation
is also performed much less often, usually only while the case user is
browsing the available network resources.

17. Language and Character Encoding Issues

All Service Registrations declare the DA has language in which the proper Scope, strings
in the SA should register all service
information with attributes are written by specifying the Directory Agent, after waiting appropriate
code in the message header. For each language the Service advertises
a random interval
CONFIG_INTERVAL_10.

When separate registration takes place. Each of these registrations
uses the same URL to indicate that they refer to the same service.

If a Service Agent or User Agent first comes on-line it must issue
a Directory Agent Discovery Request unless it is using static or DHCP
configuration, as described fully deregistered (the URL is given in 5.2.

A the Service Agent registers information with ALL newly discovered
Directory Agents when either of
Deregister request, without any attribute information) then the above two events take place.
When Scopes are being used, a
Service Agent SHOULD choose a set of
Scopes needs to be advertised in and need deregistered only register with Directory
Agents that support once. This will effectively
deregister the Scopes service in which they wish to be registered.
Services MUST be registered with DAs that support their Scope and
those which have no Scope, unless specifically configured not to do
so (see section 22.1.)

Once a User Agent becomes aware of a Directory Agent all languages it will unicast
its queries there. In has been registered in.

If, on the event that more than one Directory Agent other hand, attribute information is detected, it will select one to communicate with. When Scopes
are supported, included in the User Agent will direct its queries to different
Directory Agents depending on
Service Deregistration request, a separate Service Deregistration
of selected attributes must be undertaken in each language in which Scopes are appropriate domains
for the query
service information has been provided to be answered in. the DA by a Service Agent.

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The protocol will cause all DAs (of

Service Registrations in different languages are mutually
unintelligible. They share no information except for their service
type and URL with which they were registered. No attempt is made
to match queries with "language independence." Instead, queries are
handled using string matching against registrations in the same Scope) to eventually
obtain consistent information. Thus one DA should be as good
language as any
other the query.

Service Types which are standardized will have definitions for obtaining service information. There
all attributes and value strings. Official translations to other
languages of the attribute tags and values may be temporary
inconsistencies between DAs.

16. Scope Discovery created and Use

The Scope mechanism in the Service Location Protocol enhances its
scalability. The primary use
submitted as part of Scopes the standard; this is to provide not feasible for all
languages. For those languages which are not defined as part of the capability
to organize
Service Type, a site network along administrative lines. A set best effort translation of
services can be assigned to a given department the standard definitions
of an organization,
to a certain building or geographical area or for the Service type's attribute strings MAY be used.

All Service Requests specify a certain purpose. requested language in the message
header. The users of Directory Agent or Service Agent will respond in the system can be presented with these organizational
elements
same language as the request, if it has a top level selection, before services within registration in the same
language as the request. If this domain
are sought.

A site network that has grown beyond language is not supported, and the
Monolingual bit is not specified, a size that reply can be reasonably
serviced by a few DAs can use sent in the Scope mechanism. DAs have default
language (which is English.) If the
attribute class "SCOPE". The values for this attribute are a list
of strings that represent 'monolingual bit' flag in the administrative areas for which this
Directory Agent
header is an authority. The semantics set and the requested language of is not supported, a SrvRply
is returned with the error field set to LANGUAGE_NOT_SUPPORTED.

If a query is in a supported language on a SA or DA, but has a
different dialect than the
strings used to describe available service information, the Scope are almost entirely query
MUST be serviced on a best-effort basis. If possible, the choice of query
should be matched against the administrative entity same dialect. If that is not possible,
it MAY be matched against any dialect of the particular domain same language.

17.1. Character Encoding and String Issues

Values for character encoding can be found in which these
Scopes exist. The IANA's database
http://www.isi.edu/in-notes/iana/assignments/character-sets
and have the values of SCOPE should be configurable, so referred by the
system administrator can set its MIBEnum value.

The scopes "LOCAL" and
"REMOTE" are reserved and SHOULD NOT be used. Use encoding will determine the interpretation of these reserved
values all character data
which follows the Service Location Protocol header. There is no way
to mix ASCII and UNICODE, for example. All responses must be defined in a future protocol document.

Services with the attribute SCOPE should only be registered with DAs
which support
character set of the same Scope request, or DAs which have no Scope.

Directory Agents advertise their available Scopes. A Service Agent
may then choose use US-ASCII. If a scope in which to register, and SHOULD register
with all Directory Agents in that Scope, as well as all DAs which
have no Scope. Failure request is sent
to be comprehensive in a DA or SA or a registration according is sent to this rule will mean that the service advertisement may not be
discoverable by all User Agents.

A Directory Agent which has a Scope will send replies to Directory
Agent Discovery requests with the scope information included. Note
that the directory-agent Service Type is registered with the IANA
naming authority (which DA, which is automatically selected by leaving unable to
manipulate or store the
Naming Authority field empty.) character set of the incoming message, the
request will fail. The query:

directory-agent/MATH DEPT// SA or DA returns a CHARSET_NOT_UNDERSTOOD
error in a SrvAck message in this case. Requests using US-ASCII will
never fail for this reason, since all SAs and DAs must be able to
accept this character set.

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Could receive the following DA Advertisement:

Returned URL: service:directory-agent://diragent.blah.edu
Returned SCOPE: MATH DEPT

The same Directory Agent if it had no Scope value would reply:

Returned URL: service:directory-agent://diragent.void.com
Returned SCOPE:

If a Directory Agent supported more than one Scope it would reply as:

Returned URL: service:directory-agent://srv.domain.org
Returned SCOPE: MATH DEPT,ENGLISH DEPT,CS DEPT

A DA which has no Scope will reply to any Directory Agent Discovery
Request.

Being a member

Certain characters are illegal in certain contexts of a Scope means that an agent SHOULD use those
Directory Agents that support its Scope. User Agents send all
requests to DAs which support the indicated Scope. Services are
registered with protocol.
Since the DA(s) in their Scope. For a UA to find a service
that protocol is registered largely character string based, in a particular Scope it some
contexts characters are used as protocol delimiters. In these cases
the delimiting characters must send requests to a
DA not be used as 'data text.'

17.1.1. Substitution of Character Escape Sequences

The Service Location Protocol has an 'escape mechanism' which supports the indicated Scope. There
is no limitation on
Scope membership built into consistent with HTTP 2.0 [5] and SGML [16]. If the protocol; that character
sequence "&#" is to say, a User
Agent followed by one or Service Agent may be a member of more than one Scope.
Membership is open to all, unless some external authorization
mechanism is added to limit access.

17. Language and Character Encoding Issues

All Service Registrations declare digits, followed by
a semicolon ';' the language entire sequence is interpreted as a single
character. The digits are interpreted as a decimal value in which the strings
in
character set of the service attributes are written request, as specified by specifying the appropriate
code in the message header. For each language the Service advertises Thus, in
US-ASCII &#44; would be interpreted as a separate registration takes place. Each comma. Substitution of
these registrations
uses the same URL to indicate that they refer escape strings must be done in all <attr-list> and strings
present in SrvReq and AttrRqst messages. Only numerical character
references are accepted, not 'Entity References,' as defined in HTML.
These escape values should only be used to the same service.

If provide a Service is fully deregistered (the URL is given mechanism for
including reserved characters in the Service
Deregister request, without any attribute information) then tag and value strings.

The interpretation of these escape values is different than in
HTML in one respect: In HTML the
Service needs escape values are considered to
be deregistered only once. This will effectively
deregister in the service ISO Latin-1 character set. In Service Location they
are interpreted in all languages it has been registered in.

If, on the other hand, attribute information is character set defined in the header of the
message.

This escape mechanism allows characters like commas to be included in
attribute tags and values, which would otherwise be illegal as the
Service Deregistration request,
comma is a separate Service Deregistration protocol delimiter.

Attribute tags and values of selected attributes must different languages are considered to be undertaken
mutually unintelligible. A query in each one language in which SHOULD use service
information has been provided registered in that language.

17.2. Language-Independent Strings

Some strings, such as Service Type names, have standard definitions.
These strings should be considered as tokens and not as words in a
language to be translated.

Reserved String Section xDefinition
--------------- ------- --------------------------------------
SCOPE 3, 15 Used to limit the matching of requests.
SERVICE 6, 9 The URL scheme of all Service Location
information registered with a DA by or
returned from a Service Agent. Request.

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Service Registrations

<srvtype> 20.2.1 Used in different languages are mutually
unintelligible. They share no information except for their all service
type and URL with which they were registered. No attempt is made
to match queries with "language independence." Instead, queries are
handled using string matching against registrations in the same
language as the query.

Service Types which are standardized will have definitions for
all attributes and value strings. Official translations to other
languages of the attribute tags
and values may be created replies.
domain names 20.4 A fully qualified domain name, used
in registrations and
submitted as part of the standard; this is not feasible for all
languages. For those languages which are not defined as part of the
Service Type, a best effort translation of the standard definitions
of the replies.
IANA 3.3 The default naming authority.
LOCAL 16 Reserved.
REMOTE 16 Reserved.
TRUE 20.5 Boolean true.
FALSE 20.5 Boolean false.

18. Service type's attribute strings MAY be used.

All Location Transactions

18.1. Service Requests specify Location Connections

When a requested language in the message
header. The Directory Agent Service Location Request or Attribute Request results in a
UDP reply from a Service or Directory Agent that will respond in the
same language as the request, if it has overflow a registration in
datagram, the same
language as User Agent can open a connection to the request. If this language is not supported, Agent and
reissue the
Monolingual bit is not specified, a request over the connection. The reply can will be sent in the default
language (which is English.) If the 'monolingual bit' flag in the
header is set and the requested language is not supported, a SrvRply
is returned
with the error field overflow bit set to LANGUAGE_NOT_SUPPORTED.

If a query is in a supported language on a SA or DA, but has a
different dialect than the available service information, the query
MUST be serviced on a best-effort basis. If possible, the query
should be matched against the same dialect. If that is not possible,
it MAY be matched against any dialect of the same language.

17.1. Character Encoding and String Issues

Values for character encoding can be found in the Internet Assigned
Numbers Authority's (IANA) database
(http://www.isi.edu/in-notes/iana/assignments/character-sets), and
have the values referred by the MIBEnum value. (see section 4). The encoding reply will determine the interpretation of all character contain as
much data
which follows the Service Location Protocol header. There as will fit into a single datagram. If no MTU information
is no way
to mix ASCII and UNICODE, available for example. All responses must be in the
character set of route, assume that the MTU is 1400; this value
is configurable (see section 22).

When a request results in overflowed data that cannot be correctly
parsed (say, because of duplicate or use US-ASCII. If dropped IP datagrams), a request is sent
User Agent that wishes to reliably obtain the overflowed data must
establish a DA or SA TCP connection with the Directory Agent or a registration Service Agent
with the data. When the request is sent to again with a DA, which is unable to
manipulate or store new XID, the character set of
reply is returned over the incoming message, connection.

When registration data exceeds one datagram in length, the
request will fail. The SA or DA returns Service
Registration should be made by establishing a CHARSET_NOT_UNDERSTOOD
error in connection with a SrvAck message in this case.
Directory Agent and sending the registration over the connection
stream.

Directory Agents and Service Agents must respond to connection
requests; services whose registration data can overflow a datagram
must be able to use TCP to send the registration. User Agents
should be able to make Service and Attribute Requests using US-ASCII will
never TCP. If
they fail for this reason, since all SAs and DAs to implement this, they must be able to
accept this character set. interpret partial
replies and/or reissue requests with more selective criteria to
reduce the size of the replies.

A connection initiated by an Agent may be used for a single
transaction. It may also be used for multiple transactions. Since
there are length fields in the message headers, the Agents may send

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Certain characters are illegal in certain contexts of the protocol.
Since

multiple requests along a connection and read the protocol return stream for
acknowledgments and replies.

The initiating agent is largely character string based, in some
contexts characters are used as protocol delimiters. In these cases responsible for closing the delimiter characters must not be used as 'data text.'

17.1.1. Substitution of Character Escape Sequences TCP connection.
The Service Location Protocol has DA should wait at least CONFIG_INTERVAL_12 before closing an 'escape mechanism' which
is consistent with HTTP 2.0 [6] idle
connection. DAs and SGML [13]. If the character
sequence "&#" is followed by one or more digits, followed by
a semicolon ';' the entire sequence is interpreted as a single
character. The digits are interpreted as a decimal value in the
character set of the request, as specified by SAs SHOULD eventually close idle connections
to ensure robust operation, even when the header. Thus, in
US-ASCII &#44; would be interpreted as agent which opened a comma. Substitution of
these escape strings must be done in all <attr-list> and strings
present in SrvReq and AttrRqst messages. Only numerical character
references are accepted, not 'Entity References,' as defined in HTML.
These escape values should only be used
connection neglects to provide close it.

18.2. No Synchronous Assumption

There is no requirement that one transaction complete before a mechanism for
including reserved characters in attribute tag
given host begins another. An agent may have multiple outstanding
transactions, initiated either using UDP or TCP.

18.3. Idempotency

All Service Location actions are idempotent. Of course registration
and value strings.

The interpretation deregistration will change the state of a DA, but repeating these escape values is different than in
HTML in one respect: In HTML
actions with the escape values are considered to
be in same XID will have exactly the ISO Latin-1 character set. In Service Location they
are interpreted in same effect each
time. Repeating a registration with a new XID has the character set defined in effect of
extending the header lifetime of the
message.

This escape mechanism allows characters like commas to be included in
attribute tags and values, which would otherwise be illegal registration.

19. Security Considerations

The Service Location Protocol provides for authentication of Service
Agents as part of the
comma is a protocol delimiter.

Attribute tags scope mechanism, and values consequently, integrity
of different languages are considered to be
mutually unintelligible. A query in one language SHOULD use service
information registered in that language.

17.2. Language Dialect

Dialect tags are used in the data received as part of such registrations. Service
Location messages does not provide confidentiality. Because the objective
of this protocol is to advertise services to indicate a
variant community of vocabulary used. If one service users,
confidentiality might not generally be needed when this protocol is registered
used in more
than one dialect, a DA or SA SHOULD return the one with the same
dialect tag as non-sensitive environments. Specialized schemes might be
able to provide confidentiality, if needed in the query, but MAY choose future. Sites
requiring confidentiality should implement the IP Encapsulating
Security Payload (ESP) [3] to return any registered
service that matches provide confidentiality for Service
Location messages.

Using unprotected scopes, an adversary might easily use this protocol
to advertise services on servers controlled by the criteria.

Dialects (unlike languages) are assumed adversary and
thereby gain access to be mutually intelligible,
but may have variations in spelling. Since string matching is used, users' private information. Further, an
adversary using this protocol will find it is advantageous in some cases much easier to register engage in
selective denial of service information attacks. Sites that are in
multiple dialects. potentially
hostile environments (e.g. are directly connected to the Internet)
should consider the advantages of distributing keys associated with

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Dialect tags will be assigned as enumerated values to correspond

protected scopes prior to deploying the official dialects registered with the IANA. There are as of this
writing no enumerated dialect values; they will be created as needed.

17.3. Language-Independent Strings

Some strings, such as sensitive directory agents or
service agents.

Service Type names, have standard definitions.
These strings should be considered as tokens and not Location is useful as words in a
language to bootstrap protocol. It may be translated.

Reserved String Section Definition
--------------- ------- --------------------------------------
SCOPE 3, 15 Used to limit the matching of requests.
SERVICE 9, 6 The URL scheme of all Service Location
information registered with a DA or
returned from a Service Request.
<srvtype> 20.2.1 Used in all service registrations
and replies.
domain names 20.4 A fully qualified domain name, used
in registrations and replies.
IANA 3.3 The default naming authority.
LOCAL 16 Reserved.
REMOTE 16 Reserved.
TRUE 20.5 Boolean true.
FALSE 20.5 Boolean false.

18. Service Location Transactions

18.1. Service Location Connections

When environments in which no preconfiguration is possible. In such
situations, a certain amount of "blind faith" is required: Without
any prior configuration it is impossible to use any of the security
mechanisms described above. Service Location Request or Attribute Request results in a
UDP reply from a Service or Directory Agent that will overflow a
datagram, the User Agent can open a connection to make use of
the Agent and
reissue mechanisms provided by the request over Security Area of the connection. The reply will IETF for key
distribution as they become available. At this point it would only
be returned possible to gain the benefits associated with the overflow bit set (see section 4). The reply will contain as
much data as will fit into a single datagram. If no MTU use of protected
scopes if some cryptographic information
is available for the route, assume that can be preconfigured with
the MTU is 1400; end systems before they use Service Location. For User Agents,
this value
is configurable (see section 22).

When a request results in overflowed data that cannot could be correctly
parsed (say, because as simple as supplying the public key of duplicate or dropped IP datagrams), a
User Agent that wishes to reliably obtain Certificate
Authority. See Appendix B.

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20. String Formats used with Service Location Messages

The following section supplies formal definitions for fields and
protocol elements introduced in the overflowed data must
establish sections indicated.

Protocol Element Defined in Used in
----------------------------------- ------------ ------------
<Previous Responders' Addr Spec> 20.1 SrvReq
Service Request <predicate> 5.4 SrvReq
URL 20.2 SrvReg,
SrvDereg,
SrvRply
<attr-list> 20.3 SrvReg,
SrvRply,
AttrRply
<Service Registration Information> 9 SrvReg
<Service Deregister Information> 11 SrvDereg
<Service Type String> 20.2.1 AttrRqst

20.1. Previous Responders' Address Specification

The previous responders' Address Specification is specified as

i.e., a TCP connection list separated by commas with no intervening white space.
The Address Specification is the address of the Directory Agent
or Service Agent
with which supplied the data. previous response. The request
format for Address Specifications in Service Location is sent again defined
in section 20.4. The comma delimiter is required between each
<addr-spec>. The use of dotted decimal IP address notation should
only be used in environments which have no Domain Name Service.

Example:

RESOLVO.NEATO.ORG,128.127.203.63

20.2. Formal Definition of the ``service:'' Scheme

A URL with a new XID. The reply ``service:'' scheme is returned over used in the connection stream. SrvReg, SrvDereg,
SrvRply and AttrRqst messages in Service Location. URLs are defined
in RFC 1738 [6]. A URL with the ``service:'' scheme must contain at
least:

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When registration data exceeds one datagram in length, the Service
Registration should be made by establishing a connection with a
Directory Agent and sending the registration over

<url> ::= service:<srvtype>://<addr-spec>

where:

service the connection
stream.

Directory Agents and URL scheme for Service Agents must respond Location, to connection
requests; services whose registration data can overflow return
Replies.

<srvtype> a datagram
must string; Service Types may be able to use TCP to send standardized
by developing a specification for the registration. User Agents
should be able to make Service "service
type"-specific part and Attribute Requests using TCP. If
they fail to implement this, they must be able to interpret partial
replies and/or reissue requests with more selective criteria to
reduce registering it with IANA.
See sections 20.2.1 and 3.3.

<addr-spec> the size service access point of the replies.

A connection initiated by an Agent service. It is the
network address or domain name where the service can
be accessed. See section 20.4.

The ``service:'' scheme may be used followed by any legal URL. The
'minimal' service URL provides a service type and an access point for
a single
transaction. It may also be particular service. The protocol used for multiple transactions. Since
there are length fields in to access the message headers, service at
the Agents given service access <addr-spec> may send
multiple requests along a connection and read be implicit in the return stream for
acknowledgments and replies.

The initiating agent Service
Type name. If this is responsible for closing not the TCP connection.
The DA should wait at least CONFIG_INTERVAL_12 before closing an idle
connection. DAs case, the Service Type MUST be defined
in such a way that attribute information will include all necessary
configuration and SAs SHOULD eventually close idle connections protocol information. A User Agent MUST therefore
be able to ensure robust operation, even when the agent which opened use either a
connection neglects ``service:'' URL alone or a ``service:''
URL in conjunction with service attributes to close it.

18.2. No Synchronous Assumption

There make use of a service.

20.2.1. Service Type String

The Service Type is no requirement that one transaction complete before a
given host begins another. An agent string describing the type of service. These
strings may have multiple outstanding
transactions, initiated either using UDP or TCP.

18.3. Idempotency

All Service Location actions are idempotent. Of course registration only be comprised of alphanumeric characters, '+', and deregistration will change
'-'. Upper case is considered equivalent to lower case in Service
Type names.

If the state of Service Type name is followed by a DA, but repeating these
actions will have exactly '.' and a string (which
has the same effect each time.

19. Security Considerations

The Service Location Protocol does not provide authentication,
integrity or confidentiality. Because limitations) the objective of this
protocol 'suffix' is considered to advertise services to a community be the Naming
Authority of users,
confidentiality might not generally the service. If the Naming Authority is omitted, IANA
is assumed to be needed when this protocol the Naming Authority.

Service Types developed for in-house or experimental use may have any
name and attribute semantics provided that they do not conflict with
the standardized Service Types.

20.3. Attribute Information

The <attr-list> is
used returned in non-sensitive environments. Authentication and integrity the Attribute Reply if the Attribute
Request does not result in an empty result.

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are functionally equivalent in the context of this protocol.
Authentication is generally needed with this protocol.

<attr-list> ::= <attribute> | <attribute>, <attr-list>
<attribute> ::= (<attr-tag>=<attr-val-list>) | <keyword>
<attr-val-list> ::= <attr-val> | <attr-val>, <attr-val-list>

An adversary can easily use this protocol to advertise services on
servers controlled by the adversary and thereby gain access to users'
private information. Further, an adversary using this protocol
will find it much easier <attr-list> must be scanned prior to engage in selective denial evaluation for all
occurrences of service
attacks. Sites that are the string "&#" followed by one or more digit followed
by ';'. See Section 17.1.1.

A keyword has only an <attr-tag>, and no values.

A comma cannot appear in potentially hostile environments (e.g.
are directly connected to an <attr-val>, as the Internet) should consider comma is used as the security
risks
multiple value delimiter. Examples of deploying this protocol prior to deploying it. an <attr-list> are:

(SCOPE=ADMINISTRATION)
(COLOR=RED, WHITE, BLUE)
(DELAY=10 MINS),BUSY,(LATEST BUILD=10-5-95),(PRIORITY=L,M,H)

The security risks third example has three attributes in this protocol the list. Color can be significantly reduced or
eliminated by using take
on the IP Authentication Header [5, 3] with all
Service Location messages. It is recommended that sites use values red, white and blue. There are several other examples
of replies throughout the
IP Authentication header with all document.

20.4. Address Specification in Service Location messages. For
the security considerations listed above, it is recommended that
all nodes which implement

The address specification used in Service Location also implement the IP
Authentication Header.

Sites requiring confidentiality should implement the is:

<addr-spec> ::= [<user>:<password>@]<host>[:<port>]

<host> ::= Fully qualified domain name |
dotted decimal IP Encapsulating
Security Payload (ESP) [4] to provide confidentiality for Service
Location messages.

Service Location is useful as a bootstrap protocol. It may be
used in environments in which address notation

When no preconfiguration is possible. In
such situations, a certain amount of "blind faith" Domain Name Server is required:
Without any prior configuration available, SAs and DAs must use dotted
decimal conventions for IP addresses. Otherwise, it is impossible preferable to
use any a fully qualified domain name wherever possible as renumbering of the
security mechanisms described above. Service Location
host addresses will make
use of the mechanisms provided by the Security Area of IP addresses invalid over time.

Generally, just the IETF host domain name (or address) is returned.
When there is a non-standard port for
key distribution as they become available. At this point it would
only be possible to deploy the IP Authentication Header if some
certificate information can protocol, that should
be preconfigured with the end systems
before they returned as well. Some applications may make use Service Location.

20. String Formats used with Service Location Messages

The following section supplies formal definitions for fields and
protocol elements introduced in of the sections indicated.

Protocol Element Defined
<user>:<password>@ syntax, but its use is not encouraged in Used this
context until mechanisms are established to maintain confidentiality.

Address specification in
----------------------------------- ------------ ------------
<Previous Responders' Addr Spec> 20.1 SrvReq Service Request <predicate> 5.4 SrvReq Location is consistent with standard
URL 20.2 SrvReg,
SrvDereg,
SrvRply
<attr-list> 20.3 SrvReg, format [6].

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SrvRply,
AttrRply
<Service Registration Information> 9 SrvReg
<Service Deregister Information> 11 SrvDereg
<Service Type String> 20.2.1 AttrRqst

20.1. Previous Responders' Address Specification

The previous responders' Address Specification is specified as

i.e., a list separated

20.5. Attribute Value encoding rules

Attribute values, and attribute tags are CASE INSENSITIVE for
purposes of lexical comparison.

Attribute values are strings containing any characters with the
exception of '(', ')', '=', '>', '<', '/', '*', and ',' (the comma)
except in the case described below where opaque values are encoded.
These characters may be included using the character value escape
mechanism described in section 17.1.1.

While an attribute can take any value, there are three types
of values which differentiate themselves from general strings:
Booleans, Integers and terminated by commas with no intervening
white space. The Address Specification Opaque values.

- Boolean values are either "TRUE" or "FALSE". This is the address case
regardless of the
Directory Agent or Service Agent which supplied the previous
response. The format for Address Specifications language (i.e. in Service Location French or Telugu, Boolean
TRUE is defined "TRUE", as well as in section 20.4. The comma delimiter is required between
each <addr-spec>. English.) Boolean attributes can
take only one value.

- Integer values are expressed as a sequence of numbers. The use
range of dotted decimal IP address notation
should only be used in environments which have no Domain Name
Service.

Example:

RESOLVO.NEATO.ORG,128.127.203.63

20.2. Formal Definition allowable values for integers is "-2147483648" to
"2147483647". No other form of the ``service:'' Scheme

A URL with a ``service:'' scheme numeric representation is used in the SrvReg, SrvDereg,
SrvRply and AttrRqst messages in Service Location. URLs
interpreted as such except integers. For example, hexadecimal
numbers such as "0x342" are defined not interpreted as integers, but as
strings.

- Opaque values (i.e. binary values) are expressed in RFC 1738 [7]. A URL with radix-64
notation. The syntax is:

<opaque-val> ::= (<len>:<radix-64-data>)
<len> ::= number of bytes of the ``service:'' scheme must contain at
least:

<url> original data
<radix-64-data> ::= service:<srvtype>://<addr-spec>

where:

service radix-64 encoding of the URL scheme for Service Location, to return
Replies.

<srvtype> original data

<len> is a string; Service Types may be standardized 16-bit binary number. Radix-64 encodes every 3 bytes
of binary data into 4 bytes of ASCII data which is in the range
of characters which are fully printable and transferable by developing mail.
For a specification for formal definition of the "service
type"-specific part and registering it with IANA.
See sections 20.2.1 Radix-64 format see RFC 1521 [7],
MIME Part One, Section 5.2 Base64 Content Transfer Encoding, page
21.

21. Protocol Requirements

In this section are listed various protocol requirements for User
Agents, Service Agents, and 3.3. Directory Agents.

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<addr-spec> the service access point of the service. It is the
network address or domain name where the service can
be accessed. See section 20.4.

The ``service:'' scheme may be followed by any legal URL. The
'minimal' service URL provides

21.1. User Agent Requirements

A User Agent MAY:

- Provide a service type and an access point way for
a particular service. The protocol used to access the service at
the given service access <addr-spec> may be implicit in the Service
Type name. If this is not the case, application to configure the Service Type MUST be defined
in such a way default DA, so
that attribute information will include all necessary
configuration and protocol information. A User Agent MUST therefore it can be able used without needing to use either a ``service:'' URL alone or a ``service:''
URL in conjunction with service attributes find it each initially.

- Be able to make use of a service.

20.2.1. Service Type String

The Service Type is a string describing request the type of service. These
strings may only be comprised address of 'a' through 'z', '+' and '-'. Upper
case is considered equivalent a DA from DHCP, if configured
to do so.

- Ignore any unauthenticated Service Reply.

- Be able to lower case issue requests in Service Type names.

If any language or character set
provided that it can switch to the Service Type name is followed by a '.' default language and a string (which
has the same limitations) character
set if the 'suffix' is considered to request can not be serviced by DAs and SAs at the Naming
Authority
site.

- Require an authentication block in any URL entry returned as
part of a Service Request, before making use of the advertised
service. If

A User Agent SHOULD:

- Try to contact DHCP to obtain the Naming Authority is omitted, IANA
is assumed address of a DA.

- Use a scope in all requests, if possible.

- Issue requests to be scoped DAs if the Naming Authority.

Service Types developed for in-house or experimental use may have any
name and attribute semantics provided that they do not conflict UA has been configured with a
scope.

- Listen on the standardized Service Types. The Service Type's Service specific Location General Multicast Address used should taken from address for
unsolicited DA Advertisements. This will increase the range set of experimental
multicast addresses reserved by the IANA.

20.3. Attribute Information

The <attr-list> is returned in the Attribute Reply if the Attribute
Request does not result in an empty result.

<attr-list> ::= <attribute> | <attribute>, <attr-list>
<attribute> ::= (<attr-tag>=<attr-val-list>) | <keyword>
<attr-val-list> ::= <attr-val> | <attr-val>, <attr-val-list>

An <attr-list> must be scanned prior
Directory Agents available to evaluation it for all
occurrences making requests. See
Section 15.2.

- Be able to be configured to require an authentication block in
any received URL entry advertised as belonging to a protected
scope, before making use of the string "&#" followed by one or more digit followed
by ';'. See Section 17.1 and specifically Section 17.1.1. service.

If the UA does not listen for DA Advertisements, new DAs will not
be passively detected. A keyword UA which does not have a configured DA
and has only an <attr-tag>, not yet discovered one and no values. is not listening for unsolicited
DA Advertisements will remain ignorant of DAs. It may then do a DA
discovery before each query performed or it may simply use multicast
queries to Service Agents.

A User Agent MUST:

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A comma cannot appear in an <attr-val>, as the comma is used as the
multiple value delimiter. Examples of an <attr-list> are:

(SCOPE=ADMINISTRATION)
(COLOR=RED, WHITE, BLUE)
(DELAY=10 MINS),BUSY,(LATEST BUILD=10-5-95),(PRIORITY=L,M,H)

The third example has three attributes in the list. Color can take
on the values red, white and blue. There are several other examples
of replies throughout the document.

20.4. Address Specification in Service Location

The address specification used in Service Location is:

<addr-spec> ::= [<user>:<password>@]<host>[:<port>]

<host> ::= Fully qualified domain name |
dotted decimal IP address notation

It is preferable to use a fully qualified domain name wherever
possible as renumbering of host addresses will make IP addresses
invalid over time.

When no Domain Name Server is available SAs and DAs must use dotted
decimal conventions for IP addresses.

Generally, just the host domain name (or address) is sufficient

- Be able to
return. When there is a non-standard port for the protocol, that unicast requests and receive replies from a DA.
Transactions should be returned as well. Some applications may make use made reliable by using retransmission
of the
<user>:<password>@ syntax, but its use is request if the reply does not encouraged in this
context until mechanisms are established arrive within a timeout
interval.

- Be able to maintain confidentiality.

Address specification in detect DAs using a Directory Agent Discovery request
issued when the UA starts up.

- Be able to send requests to a multicast address. Service Location is consistent with standard
URL format [7].

20.5. Attribute Value encoding rules

Attribute values, and attribute tags
Specific Multicast addresses are CASE INSENSITIVE for
purposes computed based on a hash of lexical comparison.

Attribute values can have the
Service Type. See Section 3.6.2.

- Be able to handle numerous replies after a multicast request.
The implementation may be configurable so it will either return
the first reply, all replies until a timeout or keep trying till
the results converge.

- Ignore any string with unauthenticated Service Reply or Attribute Reply when
an appropriate IPSec Security Association for that Reply exists.

- Whenever it obtains its IP address from DHCP in the exception of '(',
')', '=', '>', '<', '/' first place,
also attempt to obtain scope information, and ',' (the comma) except the address of a
DA, from DHCP.

- Use the IP Authentication Header or IP Encapsulating Payload in
all Service Location messages, whenever an appropriate IPSec
Security Association exists.

- Be able to issue requests using the case
described below where opaque values are encoded. These characters
may US-ASCII character set.

- If configured to use a protected scope, be able to use
"md5WithRSAEncryption" [4] to verify the signed data.

21.2. Service Agent Requirements

A Service Agent MAY be included using able to:

- Get the address of a local Directory Agent by way of DHCP.

- Accept requests in non-US-ASCII character value escape mechanism described encodings. This is
encouraged, especially for UNICODE [1] and UTF-8 [25] encodings.

- Register services with a DA in section 17.1. non-US-ASCII character encodings.
This is encouraged, especially for UNICODE [1] and UTF-8 [25]
encodings.

A Service Agent SHOULD be able to:

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While an attribute can take any value, there are three types
of values which differentiate themselves from general strings:
Booleans, Integers and Opaque values.

- Boolean values are either "TRUE" or "FALSE". This Listen to the service-specific multicast address of the service
it is advertising. The incoming requests should be filtered:
If the case
regardless Address Specification of the language (i.e. in French or Telugu, Boolean
TRUE SA is "TRUE", as well as in English.) Boolean attributes can
take only one value.

- Integer values are expressed as the Previous
Responders Address Specification list, the SA SHOULD NOT respond.
Otherwise, a sequence of numbers. The range
of allowable values, response to the multicast query SHOULD be unicast to
the UA which sent the request.

- Listen for this 32 bit quantity, is "-2147483648" and respond to broadcast requests and TCP connection
requests, to the Service Location port.

- Be configurable to calculate authentication blocks and thereby
be enabled to register in protected scopes. This requires that
the service agent be configured to possess the necessary keys to
calculate the authenticator.

A Service Agent MUST be able to:

- Listen to the Service Location General Multicast address for
queries (e.g., Service Type Requests). If the query can be
replied to "2147483647". Note: No other form of numeric representation by the Service Agent, the Service Agent MUST do
so. It MUST check first to make sure it is interpreted as such, save integers. For example, hexadecimal
numbers such as "0x342" are not interpreted as integers, but as
strings.

- Opaque values (i.e. binary values) are expressed in radix-64
notation. The syntax is:

<opaque-val> ::= (<len>:<radix-64-data>)
<len> ::= number of bytes of on the original data
<radix-64-data> ::= radix-64 encoding list of
'previous responders.'

- Listen to the original data

Radix-64 encodes every 3 bytes of binary data into 4 bytes of
ASCII data which Service Location General Multicast address for
unsolicited DA Advertisements. If one is in detected, and the range of characters DA
has the right scope, (or has no scope), all services which are fully
printable and transferable by mail. For
currently being advertised MUST be registered with the DA (unless
configured to only use a formal definition of single DA (see section 22.1), or the Radix-64 format see RFC 1521 [8], MIME Part One, Section 5.2
Base64 Content Transfer Encoding, page 21.

21. Implementation Requirements

A User Agent MAY:
DA has already been detected, subject to certain rules (see
section 15.2)).

- Provide a way for Whenever it obtains its IP address from DHCP in the application first place,
also attempt to configure obtain scope information, and the default address of a
DA, so
that it can be used without needing from DHCP.

- Unicast registrations and deregistrations to find it each initially. a DA. Transactions
should be made reliable by using retransmission of the request if
the reply does not arrive within a timeout interval.

- Be able to detect DAs using a Directory Agent Discovery request
issued when the address of a DA from DHCP, if SA starts up (unless configured to do so. only use a
single DA, see section 22.1.)

- Ignore any unauthenticated Service Reply Use the IP Authentication Header or Attribute Reply.

- Be able to issue requests IP Encapsulating Payload in any language or character set
provided that it can switch to the default language and character
set if the request can not be serviced by DAs and SAs at the
site.

A User Agent SHOULD:
all Service Location messages, whenever an appropriate IPSec
Security Association exists.

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- Use a Scope in all requests, if possible.

- Issue requests Be able to scoped DAs if the UA has been configured register service information with a
scope. DA using US-ASCII
character encoding. It must also be able to reply to requests
from UAs which use US-ASCII character encoding.

- Listen on the Service Location General Multicast address for
unsolicited Reregister with a DA Advertisements. This will increase before the set Lifetime of Directory Agents available registered service
information elapses.

- If configured to it use a protected scope, be able to use
"md5WithRSAEncryption" [4] to produce the signed data.

21.3. Directory Agent Requirements

A Directory Agent MAY:

- Accept registrations and requests in non-US-ASCII character
encodings. This is encouraged, especially for making replies. See
Section 15.2.

If UNICODE [1] and
UTF-8 [25] encodings.

A Directory Agent SHOULD:

- Be able to configure certain scopes as protected scopes, so that
registrations within those scopes require the calculation of
cryptographically strong authenticators. This requires that the UA does not listen fro
DA Advertisements, new DAs will not be passively detected. A UA which does not have a configured DA
and has not yet discovered one and is not listening able to possess the keys needed for unsolicited the authentication,
or that the DA Advertisements will remain ignorant of DAs. It may then do be able to acquire a DA discovery certificate generated by a
trusted Certificate Authority [24], before each query performed or it may simply use
multicasted queries to completing Service Agents.
Registrations for protected scopes.

A User Directory Agent MUST:

- Be MUST be able to:

- Send an unsolicited DA Advertisements to unicast requests the Service Location
General Multicast address on startup and receive replies from a DA.
Transactions should be made reliable repeat it periodically.
This reply has an XID which is incremented by using retransmission
of one each time. If
the request if DA starts with state, it initializes the reply does not arrive within a timeout
interval.

- Be able XID to detect DAs using 0x0100. If
it starts up stateless, it initializes the XID to 0x0000.

- Ignore any unauthenticated Service Registration or Service
Deregistration from an entity with which it maintains a security
association.

- Listen on the Directory Agent Discovery request
issued when Multicast Address for
Directory Agent Discovery requests. Filter these requests if the UA starts up.
Previous Responder Address Specification list includes the DA's
Address Specification.

- Be able to send Listen for broadcast requests to a multicast address. If the
multicast address is not known, Service Location port.

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- Listen on the UA must be able to use TCP and UDP Service Location Ports for unicast
requests, registrations and deregistrations and service them.

- Provide a
Service Type query way in which scope information can be used to obtain the multicast address for configure
the
Service Type of Directory Agent.

- Expire registrations when the request. service registration's lifetime
expires.

- Be able to handle numerous replies after When a multicast request.
The implementation may be configurable so Directory Agent has been configured with a scope, it will either return
the first reply, MUST
refuse all requests and registrations which do not have this
scope. The DA replies until with a timeout or keep trying till
the results converge. SCOPE_NOT_SUPPORTED error. There
is one exception: All DAs MUST respond to DA discovery requests
which have no scope.

- When a Directory Agent has been configured without a scope, it
MUST accept ALL registrations and requests.

- Ignore any unauthenticated Service Reply or Attribute Reply Location messages when an
appropriate IPSec Security Association exists for that Reply exists. request.

- Use the IP Authentication Header or and IP Encapsulating Security Payload
in Service Location messages whenever an appropriate IPSec
Security Association exists.

- Accept requests and registrations in US-ASCII.

- If configured with a protected scope, be able to authenticate (at
least by using "md5WithRSAEncryption" [4]) Service Registrations
advertising services purporting to belong to such configured
protected scopes.

22. Configurable Parameters and Default Values

There are several configuration parameters for Service Location.
Default values are chosen to allow protocol operation without the
need for selection of these configuration parameters, but other
values may be selected by the site administrator. The configurable
parameters will allow an implementation of Service Location to be
more useful in a variety of scenarios.

Multicast vs. Broadcast
All Service Location entities must use multicast by
default. The ability to use broadcast messages must be
configurable for UAs and SAs. Broadcast messages are to
be used in environments where not all Service Location messages, whenever an appropriate IPSec
Security Association exists.

- Be able to issue requests using the US-ASCII character set.

A Service Agent MAY be able to:

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- Get the address of a local Directory Agent by way of DHCP.

- Accept

entities have hardware or software which supports
multicast.

Multicast Radius
Multicast requests should be sent to all subnets in non-US-ASCII character encodings. This is
encouraged, especially a
site. The default multicast radius for UNICODE [1] and UTF-8 [17] encodings.

- Register services with a DA in non-US-ASCII character encodings.
This site is encouraged, especially for UNICODE [1] and UTF-8 [17]
encodings.

A Service Agent SHOULD 32.
This value must be able to:

- Listen to the service-specific multicast address of the service
it is advertising. configurable. The incoming requests should be filtered:
If the Address Specification of the SA is in the Previous
Responders Address Specification list, the SA SHOULD NOT respond.
Otherwise, a response to value for the
site's multicast query SHOULD TTL may be unicast to
the UA obtained from DHCP using an
option which sent the request.

- Listen for and respond to broadcast requests and TCP connection
requests, to the Service Location port.

- Listen to the Service Location General Multicast address for
queries (e.g., Service Type Requests). If the query can be
replied to by the Service Agent, the Service Agent must do
so. It MUST check first to make sure it is not on the list of
'previous responders.'

A Service currently unassigned.

Directory Agent Address
The Directory Agent MUST be able to:

- Listen to the Service Location General Multicast address discovery mechanism must be
configurable. There are three possibilities for
unsolicited DA Advertisements. If one is detected, this
configuration: A default address, no default address
and the use of DHCP to locate a DA
has the right Scope, (or has no Scope), all services which are
currently being advertised MUST as described in
section 15.2. The default value should be registered use of DHCP,
with "no default address" used if DHCP does not respond.
In this case the DA (unless
configured to only use UA or SA must do a single DA (see section 22.1), Directory Agent
Discovery query.

Directory Agent Scope Assignment
The scope or the scopes of a DA has already been detected, subject must be configurable. The
default value for a DA is to certain rules (see
section 15.2)).

- Unicast registrations and deregistrations have no scope if not
otherwise configured.

Path MTU
The default path MTU is assumed to a DA. Transactions
should be made reliable by using retransmission 1400. This value
may be too large for the infrastructure of some sites.
For this reason this value MUST be configurable for all
SAs and DAs.

Keys for Protected Scopes

If the request if local administration designates certain scopes
as "protected scopes", the reply does not arrive within a timeout interval.

- Be agents making use of those
scopes have to be able to detect DAs using acquire keys to authenticate
data sent by services along with their advertised
URLs for services within the protected scope. For
instance, service agents would use a Directory Agent Discovery request
issued when private key to
produce authentication data. By default, service agents
use "md5WithRSAEncryption" [4] to produce the SA starts up (unless configured signed
data, to only use a
single DA, see section 22.1.)

- Use be be included with service registrations
and deregistrations (see appendix B, 4.3). This
authentication data could be verified by user agents and
directory agents that possess the IP Authentication Header or IP Encapsulating Payload in
all Service Location messages, whenever an appropriate IPSec
Security Association exists. corresponding public
key.

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- Be able

22.1. Service Agent: Use Predefined Directory Agent(s)

A Service Agent's default configuration is to register service information with a do passive and active
DA using US-ASCII
character encoding. It must also be able to reply discovery and to requests
from UAs which use US-ASCII character encoding.

- Reregister register with a DA before the Lifetime of registered service
information elapses. all DAs which are properly scoped.

A Directory Agent MAY:

- Ignore any unauthenticated Service Registration or Service
Deregistration.

- Accept registrations and requests in non-US-ASCII character
encodings. This is encouraged, especially for UNICODE [1] and
UTF-8 [17] encodings.

A Directory Agent MUST SHOULD be able to:

- Send an unsolicited DA Advertisements configurable to the Service Location
General Multicast address on startup and repeat it periodically. allow a special mode of
operation: They will use only preconfigured DAs. This reply has an XID which is incremented by one each time. If
the DA starts with state, it initializes the XID to 0x0100. If
it starts up stateless, it initializes the XID to 0x0000.

- Listen on the Directory Agent Discovery Multicast Address for
Directory agent discovery requests. Filter these requests if means they
will *NOT* actively or passively detect DAs.

If a Service Agent is configured this way, knowledge of the
Previous Responder Address Specification list includes DA must
come through another channel, either static configuration or by the DA's
Address Specification.

- Listen for broadcast requests to
use of DHCP.

The availability of the Service Location port.

- Listen on information will not be consistent
between DAs. The mechanisms which achieve eventual consistency
between DAs are ignored by the TCP and UDP Service Location Ports for unicast
requests, registrations and deregistrations and SA, so their service them.

- Provide a way in which Scope information can will
not be used to configure
the Directory Agent.

- Age out distributed. This leaves the services which have been registered so that when SA open to failure if the service registration's Lifetime expires, DA
they are configured to use fails.

22.2. Time Out Intervals

These values should be configurable in case the service
advertisement is withdrawn.

- When a Directory Agent site deploying
Service Location has been configured with a Scope, it MUST
refuse all requests and registrations which do not have this
Scope. The DA special requirements (such as very slow links.)

Interval name Section Default Value Meaning
----------------- ------- ------------- -----------------------
CONFIG_INTERVAL_0 4.1 1 minute Cache replies with by XID.
CONFIG_INTERVAL_1 4.4 10800 seconds registration Lifetime,
(ie. 3 hours)after which ad expires
CONFIG_INTERVAL_2 5 each second, Retry multicast query
backing off until no new values
gradually arrive.
CONFIG_INTERVAL_3 5 15 seconds Max time to wait for a SCOPE_NOT_SUPPORTED error. There
is one exception: All DAs MUST respond
complete multicast query
response (all values.)
CONFIG_INTERVAL_4 9 3 seconds Wait to register on
reboot.
CONFIG_INTERVAL_5 5.2 3 seconds Retransmit DA discovery,
try it 3 times.
CONFIG_INTERVAL_6 5.2 5 seconds Give up on requests sent
to a DA.
CONFIG_INTERVAL_7 5.2 15 seconds Give up on DA discovery
CONFIG_INTERVAL_8 5.1 15 seconds Give up on requests
which have no Scope.
sent to SAs.
CONFIG_INTERVAL_9 15.2 3 hours DA Heartbeat, so that SAs
passively detect new DAs.
CONFIG_INTERVAL_10 15.2 1-3 seconds Wait to register services

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- When a Directory Agent has been configured without a Scope, it
MUST accept ALL registrations and requests.

- Ignore any unauthenticated Service Location messages when an
appropriate IPSec Security Association exists for that request.

- Use the IP Authentication and IP Encapsulating Security Payload
in Service Location messages whenever an appropriate IPSec
Security Association exists.

- Accept requests and registrations in US-ASCII.

NOTE: Service Agents and User Agents use ephemeral ports for
transmitting information

on passive DA discovery.
CONFIG_INTERVAL_11 9 1-3 seconds Wait to the service location port.

22. Configurable Parameters register services
on active DA discovery.
CONFIG_INTERVAL_12 18.1 5 minutes DAs and Default Values

There are several configuration parameters for Service Location.
The protocol will work fine if only default values are used. Due SAs close idle
connections.

A note on CONFIG_INTERVAL_9: While it might seem advantageous to the nature
have frequent heartbeats, this poses a significant risk of the protocol, it may be deployed in many different
environments. The configuration options parameters will allow an
implementation generating
a lot of Service Location overhead traffic. This value should be kept high to prevent
routine protocol operations from using any significant bandwidth.

23. Non-configurable Parameters

IP Port number for unicast requests to be useful in a variety of
different scenarios. Directory Agents:

UDP and TCP Port Number: 427

Multicast vs. Broadcast
All Addresses

Service Location entities must use General Multicast Address: 224.0.1.22
Directory Agent Discovery Multicast Address: 224.0.1.35

A range of 1024 contiguous multicast by
default. The ability to addresses for use broadcast messages must
be configurable. Broadcast messages are to be used in
environments where not all as Service Location entities have
hardware or software which supports multicast.

Multicast Radius
Specific Discovery Multicast requests should Addresses will be sent assigned by IANA.

Error Codes:

No Error 0
LANGUAGE_NOT_SUPPORTED 1
PROTOCOL_PARSE_ERROR 2
INVALID_REGISTRATION 3
SCOPE_NOT_SUPPORTED 4
CHARSET_NOT_UNDERSTOOD 5
AUTHENTICATION_ABSENT 6
AUTHENTICATION_FAILED 7

24. Acknowledgments

This protocol owes some of the original ideas to all subnets other service
location protocols found in a
site. The default multicast radius many other networking protocols. Leo
McLaughlin and Mike Ritter (Metricom) provided much input into early
version of this document. Thanks also to Steve Deering (Xerox) for a site is 32.
This value must be configurable. The value
providing his insight into distributed multicast protocols. Harry
Harjono and Charlie Perkins supplied the basis for the
site's multicast TTL may be obtained from DHCP. The DHCP
option has not yet been assigned.

Directory Agent Address
The Directory Agent address discovery mechanism must be
configurable. There are three possibilities URL based

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wire protocol in their Resource Discovery Protocol. Thanks also to
Peerlogic, Inc. for supporting this work.

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A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for this
configuration: A default address, no default address
and the use
representation of DHCP to locate a DA as described in
section 15.2. names of languages"

Two-letter lower-case symbols are used. The default value should Registration Authority
for ISO 639 [15] is Infoterm, Osterreiches Normungsinstitut (ON),
Postfach 130, A-1021 Vienna, Austria. Contains additions from ISO
639/RA Newsletter No.1/1989

aa Afar ga Irish mg Malagasy
ab Abkhazian gd Scots Gaelic mi Maori
af Afrikaans gl Galician mk Macedonian
am Amharic gn Guarani ml Malayalam
ar Arabic gu Gujarati mn Mongolian
as Assamese mo Moldavian
ay Aymara ha Hausa mr Marathi
az Azerbaijani he Hebrew ms Malay
hi Hindi mt Maltese
ba Bashkir hr Croatian my Burmese
be "no default
address." In this case the UA Byelorussian hu Hungarian
bg Bulgarian hy Armenian na Nauru
bh Bihari ne Nepali
bi Bislama ia Interlingua nl Dutch
bn Bengali; Bangla in Indonesian no Norwegian
bo Tibetan ie Interlingue
br Breton ik Inupiak oc Occitan
is Icelandic om (Afan) Oromo
ca Catalan it Italian or SA must do a Directory
Agent Discovery query. Oriya
co Corsican ja Japanese
cs Czech jw Javanese pa Punjabi
cy Welsh pl Polish
ka Georgian ps Pashto, Pushto
da Danish kk Kazakh pt Portuguese
de German kl Greenlandic
dz Bhutani km Cambodian qu Quechua
rw Kinyarwanda
el Greek kn Kannada rm Rhaeto-Romance
en English ko Korean rn Kirundi
eo Esperanto ks Kashmiri ro Romanian
es Spanish ku Kurdish ru Russian
et Estonian ky Kirghiz
eu Basque
la Latin
fa Persian ln Lingala
fi Finnish lo Laothian
fj Fiji lt Lithuanian
fo Faeroese lv Latvian, Lettish
fr French
fy Frisian

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Directory Agent Scope Assignment
The Scope or Scopes of a DA must be configurable. The
default value for a DA is to have no Scope if not
otherwise configured.

Default Path MTU
The default path MTU is assumed

sa Sanskrit ta Tamil ug Uigar
sd Sindhi te Telugu uk Ukrainian
sg Sangro tg Tajik ur Urdu
sh Serbo-Croatian th Thai uz Uzbek
si Singhalese ti Tigrinya
sk Slovak tk Turkmen vi Vietnamese
sl Slovenian tl Tagalog vo Volapuk
sm Samoan tn Setswana
sn Shona to be 1400. This value Tonga wo Wolof
so Somali tr Turkish
sq Albanian ts Tsonga xh Xhosa
sr Serbian tt Tatar
ss Siswati tw Twi yi Yiddish
st Sesotho yo Yoruba
su Sundanese
sv Swedish za Zhuang
sw Swahili zh Chinese
zu Zulu

B. SLP Certificates

Certificates may be too large for the infrastructure of some sites.
For this reason this value MUST be configurable for all
SAs and DAs.

If a UA issues a request which will result used in a
reply which is too large, the SA or DA will return an
abbreviated response (in a datagram the size of the
site's MTU) which has the 'Overflow' bit flag set.
The UA must then issue the request again using a tcp
connection.

Similarly, if a SA attempts SLP in order to register a service with a
DA and the registration is larger than the site path MTU
the DA will reply with a SrvAck, with distribute the error set to
INVALID_REGISTRATION and public
keys of trusted protected scopes. Assuming public keys, this
appendix discusses the use of such certificates in the 'Overflow' byte set.

22.1. Service Agent: Use Predefined Directory Agent(s)

A Service Agent's default configuration
Location Protocol.

Possession of the private key of a protected scope is equivalent
to do passive being a trusted SA. The trustworthiness of the protected scope
depends upon all of these private keys being held by trusted
hosts, and active
DA discovery used only for legitimate service registrations and
deregistrations.

With access to register with all the proper Certificate Authority (CA), DAs and UAs
do not need (in advance) hold public keys which are properly scoped.

A Service Agent SHOULD be configurable correspond to allow a special mode of
operation: these
protected scopes. They will use only preconfigured DAs. This means they
will *NOT* actively or passively detect DAs.

If a Service Agent is configured this way, knowledge of the DA must
come through another channel, either static configuration or by do require the
use of DHCP.

The availability public key of the Service information will not be consistent
between DAs. CA. The mechanisms which achieve eventual consistency
between DAs are ignored by the SA, so their service information will
not be distributed. CA
produces certificates using its unique private key. This leaves private key
is not shared with any other system, and must remain secure. The
certificates declare that a given protected scope has a given public
key, as well as the SA open to failure if expiration date of the DA
they are configured to use fails. certificate.

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22.2. Time Out Intervals

These values should be configurable

The ASCII (mail-safe) string format for the certificate is the
following list of tag and value pairs:

"certificate-alg=" 1*ASN1CHAR CRLF
"scope-charset=" 1*DIGIT CRLF
"scope=" 1*RADIX-64-CHAR CRLF
"timestamp=" 16HEXDIGIT CRLF
"public-key=" 1*RADIX-64-CHAR CRLF
"cert-digest=" 1*RADIX-64-CHAR CRLF

ASN1CHAR = DIGIT | '.'
HEXDIGIT = DIGIT | 'a'..'f' | 'A'..'F'
RADIX-64-CHAR = DIGIT | 'a'..'z' | 'A'..'Z' | '+' | '/' | '='

The radix-64 notation is described in RFC 1521 [7]. Spaces are
ignored in case the site deploying
Service Location has special requirements (such as very slow links.)

Interval name Section Default Value Meaning
----------------- ------- ------------- -----------------------
CONFIG_INTERVAL_0 4.1 1 minute Cache replies by XID.
CONFIG_INTERVAL_1 4.2 10800 seconds registration Lifetime,
(ie. 3 hours)after which ad expires
CONFIG_INTERVAL_2 5 each second, Retry multicast query
backing off until no new values
gradually arrive.
CONFIG_INTERVAL_3 5 15 seconds Max time to wait for a
complete multicast query
response (all values.)
CONFIG_INTERVAL_4 9 3 seconds Wait to register on
reboot.
CONFIG_INTERVAL_5 5.2 3 seconds Retransmit DA discovery,
try it 3 times.
CONFIG_INTERVAL_6 5.2 5 seconds Give up on requests sent computation of the binary value corresponding to a DA.
CONFIG_INTERVAL_7 5.2 15 seconds Give up on DA discovery
CONFIG_INTERVAL_8 5.1 15 seconds Give
Radix-64 string. If the value for scope, public-key or cert-digest
is greater than 72 characters, the Radix-64 notation may be broken
up on requests
sent to SAs.
CONFIG_INTERVAL_9 15.2 3 hours DA Heartbeat, so that SAs
will
passively detect new DAs.
CONFIG_INTERVAL_10 15.2 1-3 seconds Wait to register services
on passive DA discovery.
CONFIG_INTERVAL_11 9 1-3 seconds Wait to register services
on active DA discovery.
CONFIG_INTERVAL_12 18.1 5 minutes DAs and SAs close idle
connections.

A note on CONFIG_INTERVAL_9: While it might seem advantageous to
have frequent heartbeats, this poses a significant risk separate lines. The continuation lines must be preceded
by one or more spaces. Only the tags listed above may start in the
first column of generating the certificate string. This removes ambiguity in
parsing the Radix-64 values (since the tags consist of legal Radix-64
values.)

The certificate-alg is the ASN.1 string for the Object Identifier
value of the algorithm used to produce the "cert-digest". The
scope-charset is a lot decimal representation of overhead traffic. This the MIBEnum value should be kept high to prevent
routine protocol operations from using any significant bandwidth.

23. Non-configurable Parameters

IP Port number for unicast requests to Directory Agents:

UDP and TCP Port Number: 427

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the character set in which the scope is represented.

The radix-64 encoding of the scope string will allow the ASCII
rendering of a scope string any character set.

The 8 January 1997

Multicast Addresses

Service Location General Multicast Address: 224.0.1.22
Directory Agent Discovery Multicast Address: 224.0.1.35

Further service-specific multicast address byte NTP format timestamp is represented as 16 hex digits.
This timestamp is the time at which the certificate will be assigned expire.

The format for
specific types of service through the IANA.

Error Codes:

No Error 0
LANGUAGE_NOT_SUPPORTED 1
PROTOCOL_PARSE_ERROR 2
INVALID_REGISTRATION 3
SCOPE_NOT_SUPPORTED 4
CHARSET_NOT_UNDERSTOOD 5
AUHENTICATION_FAILED 6

24. Acknowledgments

This protocol owes some public key will depend on the type of cryptosystem
used, which is identified by the original ideas certificate-alg. When the CA
generated the certificate holding the public key being obtained,
it used the message digest algorithm identified by certificate-alg
to other service
location protocols found in many other networking protocols. Leo
McLaughlin and Mike Ritter (Metricom) provided much input into early
version calculate a digest D on the string encoding of the certificate,
excepting the cert-digest. The CA then encrypted this document. Thanks also value using
the CA's private key to Steve Deering (Xerox) for
providing his insight into distributed multicast protocols. Harry
Harjono and Charlie Perkins supplied produce the basis for cert-digest, which is included in
the URL based
wire protocol certificate.

The CA generates the certificate off-line. The mechanism to
distibute certificates is not specified in the Service Location
Protocol, but may be in their Resource Discovery Protocol. Thanks also the future. The CA specifies the algorithms
to
Peerlogic, Inc. use for supporting this work. message digest and public key decryption. The DA or SA

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A. Appendix: Technical contents

need only obtain the certificate, have a preconfigured public key for
the CA and support the algorithm specified in the certificate-alg in
order to obtain certified new public keys for protected scopes.

The DA or UA may confirm the certificate by calculating the message
digest D, using the message digest algorithm identified by the
certificate-alg. The input to the message digest algorithm is the
string encoding of ISO 639:1988 (E/F): "Code the certificate, excepting the cert-digest.
The cert-digest is decrypted using the CA's public key to produce
D'. If D is the same as D', the certificate is legitimate. The
public-key for the protected scope may be used until the expiration
date indicated by the certificate timestamp.

The certificate may be distributed along untrusted channels, such as
email or through file transfer, as it must be verified anyhow. The
CA's public key must be delivered using a trusted channel.

C. Example of deploying SLP security using MD5 and RSA

In our site, we have a protected scope "CONTROLLED". We generate a
private key - public key pair for the scope, using RSA. The private
key is maintained on a secret key ring by all SAs in the protected
scope. The public key is available to all DAs which support the
protected scope and to all UAs which will use it.

In order to register or deregister a URL, the data required to be
authenticated (as described in section 4.3) is digestified using
MD5 [23] to create a digital signature, then encrypted by RSA with
the protected scope's private key. The output of RSA is used in the
authenticator data field of the authenticator block.

The DA or UA discovers the appropriate method for verifying the
authentication by looking inside the authentication block. Suppose
that the "md5WithRSAEncryption" [4] algorithm has to be used
to verify the signed data. The DA or UA calculates the
representation of names message
digest of languages"

Two-letter lower-case symbols are used. the URL Entry by using md5, exactly as the SA did. The Registration Authority
authenticator block is decrypted using the public key for ISO 639 [12] the
"CONTROLLED" scope, which is Infoterm, Osterreiches Normungsinstitut (ON),
Postfach 130, A-1021 Vienna, Austria. Contains additions from ISO
639/RA Newsletter No.1/1989

aa Afar ga Irish mg Malagasy
ab Abkhazian gd Scots Gaelic mi Maori
af Afrikaans gl Galician mk Macedonian
am Amharic gn Guarani ml Malayalam
ar Arabic gu Gujarati mn Mongolian
as Assamese mo Moldavian
ay Aymara ha Hausa mr Marathi
az Azerbaijani he Hebrew ms Malay
hi Hindi mt Maltese
ba Bashkir hr Croatian my Burmese
be Byelorussian hu Hungarian
bg Bulgarian hy Armenian na Nauru
bh Bihari ne Nepali
bi Bislama ia Interlingua nl Dutch
bn Bengali; Bangla stored in Indonesian no Norwegian
bo Tibetan ie Interlingue
br Breton ik Inupiak oc Occitan
is Icelandic om (Afan) Oromo
ca Catalan it Italian the public key ring of the UA
or Oriya
co Corsican ja Japanese
cs Czech jw Javanese pa Punjabi
cy Welsh pl Polish
ka Georgian ps Pashto, Pushto
da Danish kk Kazakh pt Portuguese
de German kl Greenlandic
dz Bhutani km Cambodian qu Quechua
rw Kinyarwanda
el Greek kn Kannada rm Rhaeto-Romance
en English ko Korean rn Kirundi
eo Esperanto ks Kashmiri ro Romanian
es Spanish ku Kurdish ru Russian
et Estonian ky Kirghiz
eu Basque
la Latin
fa Persian ln Lingala
fi Finnish lo Laothian
fj Fiji lt Lithuanian
fo Faeroese lv Latvian, Lettish
fr French
fy Frisian DA under the name "CONTROLLED". If the digest calculated by the
UA or DA matches that of the SA, the URL Entry has been validated.

D. Example of use of SLP Certificates by mobile nodes

Say a mobile node needs to make use of protected scopes. The mobile
node is first preconfigured by adding a single public key to its
public key ring: We will call it the CA-Key. This key will be used

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to Tonga wo Wolof
so Somali tr Turkish
sq Albanian ts Tsonga xh Xhosa
sr Serbian tt Tatar
ss Siswati tw Twi yi Yiddish
st Sesotho yo Yoruba
su Sundanese
sv Swedish za Zhuang
sw Swahili zh Chinese
zu Zulu

B. obtain SLP certificates in the format described in Appendix B.
The corresponding private key will be used by the CA to create the
certificates in the necessary format.

The CA might be operated by a system administrator using a computer
which is not connected to any networks. The certificate's duration
will depend on the policy of the site. The duration, scope, and
public key for the protected scope, are used as input to 'md5sum'.
This sum is then encrypted with RSA using the CA's private key. The
radix 64 encoding of this is added to the mail-safe string based
certificate encoding defined in Appendix B.

The certificate, say for the protected scope "CONTROLLED" could be
made available to the mobile node. For example, it might be on a
web page. The mobile node could then process the certificate in
order to obtain the public key for the CONTROLLED scope. There is
still no reason to *trust* this key is really the one to use (as in
Appendix C). To trust it, calculate the md5 checksum of the ascii
encoded certificate, excluding the cert-digest. Next, decrypt the
cert-digest using the CA's public key and RSA. If the cert-digest
matches the output of MD5, the certificate may be trusted (until it
expires).

The mobile node requires only one key (CA-key) in order to obtain
others dynamically and make use of protected scopes. Notice that we
do not define any method for access control by arbitrary UAs to SAs
in protected scopes. This could be done fairly easily by limiting
access to the public key certificates.

E. Appendix: For Further Reading

Three related resource discovery protocols are NBP and ZIP
which are part of the AppleTalk protocol family [11], [13], the Legato
Resource Administration Platform [18], [26], and the Xerox Clearinghouse
system [16]. [21]. Domain names and representation of addresses are
used extensively in the Service Location Protocol. The references
for these are RFCs 1034 and 1035 [14, 15]. An example [18, 19]. Example of service a discovery
protocol for a specific service is routers include Router Discovery [9]. [11] and Neighbor
Discovery [20].

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References

[1] Unicode Technical Report #4. The unicode standard, version 1.1
(volumes 1 and 2). Technical Report (ISBN 0-201-56788-1) and
(ISBN 0-201-60845-6), Unicode Consortium, 1994.

[2] S. Alexander and R. Droms. DHCP Options and BOOTP Vendor
Extensions. RFC 1533, October 1993.

[3] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.

[4] R. Atkinson. IP Encapsulating Security Payload. RFC 1827,
August 1995.

[5] R. Atkinson. Security Architecture

[4] D. Balenson. Privacy Enhancement for the Internet Protocol. Electronic
Mail: Part III: Algorithms, Modes, and Identifiers. RFC 1825, August 1995.

[6] 1423,
February 1993.

[5] T. Berners-Lee and D. Connolly. Hypertext Markup Language -
2.0. RFC 1866, November 1995.

[7]

[6] T. Berners-Lee, L. Masinter, and M. McCahill. Uniform Resource
Locators (URL). RFC 1738, December 1994.

[8]

[7] N. Borenstein and N. Freed. MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies. RFC 1521, September
1993.

[8] Scott Bradner. Key words for use in RFCs to Indicate
Requirement Levels, January 1997. draft-bradner-key-words-03.txt
(work in progress).

[9] CCITT. Specification of the Abstract Syntax Notation One
(ASN.1). Recommendation X.208, 1988.

[10] CCITT. The Directory Authentication Framework. Recommendation
X.509, 1988.

[11] Stephen E. Deering, editor. ICMP Router Discovery Messages.
RFC 1256, September 1991.

[10]

[12] Ralph Droms. Dynamic Host Configuration Protocol. RFC 1541,
October 1993.

[11]

[13] S. Gursharan, R. Andrews, and A. Oppenheimer. Inside AppleTalk.
Addison-Wesley, 1990.

[12]

[14] E. Guttman. The service: URL scheme, November 1996.
draft-ietf-svrloc-service-scheme-00.txt (work in progress).

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[15] Geneva ISO. Code for the representation of names of languages.
ISO 639:1988 (E/F), 1988.

[13] Geneva

[16] ISO 8879. 8879, Geneva. Information Processing -- Text and Office
Systems - Standard Generalized Markup Language (SGML).
<URL:http://www.iso.ch/cate/d16387.html>, 1986.

[14]

[17] D. Mills. Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI. RFC 2030, October 1996.

[18] P. Mockapetris. Domain Names - Concepts and Facilities. RFC
1034, November 1987.

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Internet Draft Service Location Protocol 8 January 1997

[15]

[19] P. Mockapetris. DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION. RFC 1035, November 1987.

[16]

[20] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP version 6 (IPv6). RFC 1970, August 1996.

[21] D. Oppen and Y. Dalal. The clearinghouse: A decentralized
agent for locating named objects in a distributed environment.
Technical Report Tech. Rep. OPD-78103, Xerox Office Products
Division, 1981.

[17]

[22] C. Perkins. DHCP Options for Service Location Protocol, August
1996. draft-ietf-dhc-slp-00.txt (work in progress).

[23] Ronald L. Rivest. The MD5 Message-Digest Algorithm. RFC 1321,
April 1992.

[24] Bruce Schneier. Applied Cryptography: Protocols, Algorithms,
and Source Code in C. John Wiley, New York, NY, USA, 1994.

[25] X/Open Preliminary Specification. File System Safe UCS
Transformation Format (FSS_UTF). Technical Report Document
Number: P316, X/Open Company Ltd., 1994.

[18]

[26] Legato Systems. The Legato Resource Administration Platform.
Legato Systems, 1991.

Authors' Addresses

Questions about this memo can be directed to:

Veizades,Guttman,Perkins,Kaplan Expires 13 September 1997 [Page 71]

Internet Draft Service Location Protocol 13 March 1997

John Veizades Erik Guttman
@Home Network Sun Microsystems
385 Ravendale Dr. Gaisbergstr. 6
Mountain View, CA 94043 69115 Heidelberg Germany

Phone: +1 415 944 7332 Phone: +49 6221 601649 +1 415 336 6697
Fax: +1 415 944 8500 Fax: +49 6221 161019

Email: veizades@home.com Email: Erik.Guttman@eng.sun.com

Charles E. Perkins Scott Kaplan
IBM Corporation
P.O. Box 704
Sun Microsystems
2550 Garcia Avenue 346 Fair Oaks St.
Yorktown Heights NY 10598
Mountain View, CA 94043 San Francisco, CA 94110

Phone: +1 914 784 7350 415 336 7153 Phone: +1 415 285 4526
Fax: +1 914 784 6205 415 336 0670

EMail: perk@watson.ibm.com cperkins@Corp.sun.com Email: scott@catch22.com

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