rfc2165.txt
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Network Working Group J. Veizades
Request for Comments: 2165 @Home Network
Category: Standards Track E. Guttman
C. Perkins
Sun Microsystems
S. Kaplan
June 1997
Service Location Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
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.
Table of Contents
1. Introduction 3
2. Terminology 3
2.1. Notation Conventions . . . . . . . . . . . . . . . . . . 5
2.2. Service Information and Predicate Representation . . . . 5
2.3. Specification Language . . . . . . . . . . . . . . . . . 6
3. Protocol Overview 6
3.1. Protocol Transactions . . . . . . . . . . . . . . . . . . 7
3.2. Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. The "service:" URL scheme . . . . . . . . . . . . 9
3.3. Standard Attribute Definitions . . . . . . . . . . . . . 9
3.4. Naming Authority . . . . . . . . . . . . . . . . . . . . 10
3.5. Interpretation of Service Location Replies . . . . . . . 10
3.6. Use of TCP, UDP and Multicast in Service Location . . . . 10
3.6.1. Multicast vs. Broadcast . . . . . . . . . . . . 11
3.6.2. Service-Specific Multicast Address . . . . . . . 11
3.7. Service Location Scaling, and Multicast Operating Modes . 12
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RFC 2165 Service Location Protocol June 1997
4. Service Location General Message Format 14
4.1. Use of Transaction IDs (XIDs) . . . . . . . . . . . . . . 15
4.2. URL Entries . . . . . . . . . . . . . . . . . . . . . . . 16
4.3. Authentication Blocks . . . . . . . . . . . . . . . . . . 17
4.4. URL Entry Lifetime . . . . . . . . . . . . . . . . . . . 19
5. Service Request Message Format 19
5.1. Service Request Usage . . . . . . . . . . . . . . . . . . 22
5.2. Directory Agent Discovery Request . . . . . . . . . . . . 23
5.3. Explanation of Terms of Predicate Grammar . . . . . . . . 24
5.4. Service Request Predicate Grammar . . . . . . . . . . . . 26
5.5. String Matching for Requests . . . . . . . . . . . . . . 27
6. Service Reply Message Format 28
7. Service Type Request Message Format 29
8. Service Type Reply Message Format 31
9. Service Registration Message Format 32
10. Service Acknowledgement Message Format 35
11. Service Deregister Message Format 37
12. Attribute Request Message Format 38
13. Attribute Reply Message Format 40
14. Directory Agent Advertisement Message Format 42
15. Directory Agents 43
15.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 43
15.2. Finding Directory Agents . . . . . . . . . . . . . . . . 43
16. Scope Discovery and Use 45
16.1. Protected Scopes . . . . . . . . . . . . . . . . . . . . 46
17. Language and Character Encoding Issues 47
17.1. Character Encoding and String Issues . . . . . . . . . . 48
17.1.1. Substitution of Character Escape Sequences . . . 49
17.2. Language-Independent Strings . . . . . . . . . . . . . . 49
18. Service Location Transactions 50
18.1. Service Location Connections . . . . . . . . . . . . . . 50
18.2. No Synchronous Assumption . . . . . . . . . . . . . . . . 51
18.3. Idempotency . . . . . . . . . . . . . . . . . . . . . . . 51
19. Security Considerations 51
20. String Formats used with Service Location Messages 52
20.1. Previous Responders' Address Specification . . . . . . . 53
20.2. Formal Definition of the "service:" Scheme . . . . . . . 53
20.2.1. Service Type String . . . . . . . . . . . . . . . 54
20.3. Attribute Information . . . . . . . . . . . . . . . . . . 54
20.4. Address Specification in Service Location . . . . . . . . 55
20.5. Attribute Value encoding rules . . . . . . . . . . . . . 55
21. Protocol Requirements 56
21.1. User Agent Requirements . . . . . . . . . . . . . . . . . 56
21.2. Service Agent Requirements . . . . . . . . . . . . . . . 58
21.3. Directory Agent Requirements . . . . . . . . . . . . . . 59
22. Configurable Parameters and Default Values 61
22.1. Service Agent: Use Predefined Directory Agent(s) . . . . 62
22.2. Time Out Intervals . . . . . . . . . . . . . . . . . . . 63
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RFC 2165 Service Location Protocol June 1997
23. Non-configurable Parameters 63
24. Acknowledgments 64
A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for
the representation of names of languages" 65
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 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.
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.
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RFC 2165 Service Location Protocol June 1997
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.
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, 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,
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
Scope A collection of services that make up a logical group.
See sections 3.7 and 16.
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RFC 2165 Service Location Protocol June 1997
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.
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 human 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.
The logical connectives and subexpressions are presented in prefix-
order, so that the connective comes first and the expressions it
operates on follow afterwards.
Veizades, et. al. Standards Track [Page 5]
RFC 2165 Service Location Protocol June 1997
2.3. Specification Language
In this document, several words are used to signify the requirements
of the 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.
SHOULD This word, or the adjective "recommended", means
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