Network Working Group                                         E. Guttman
Request for Comments: 3059                              Sun Microsystems
Category: Standards Track                                  February 2001


       Attribute List Extension for the 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.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   The Service Location Protocol, Version 2 (SLPv2) provides a mechanism
   for a service to be discovered in a single exchange of messages.
   This exchange of messages does not presently include any of the
   service's attributes.  This document specifies a SLPv2 extension
   which allows a User Agent (UA) to request a service's attributes be
   included as an extension to Service Reply messages.  This will
   eliminate the need for multiple round trip messages for a UA to
   acquire all service information.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
       1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
       1.2. Notation Conventions  . . . . . . . . . . . . . . . . . . 3
   2. Attribute List Extension  . . . . . . . . . . . . . . . . . . . 3
   3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   4. Internationalization Considerations . . . . . . . . . . . . . . 4
   5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 4
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 6

1. Introduction

   The Service Location Protocol, Version 2 [3] provides a mechanism for
   a service to be discovered in a single exchange of messages.  The UA
   sends a Service Request message and the DA or SA (as appropriate)
   sends a Service Reply message.

   It is clearly advantageous to be able to obtain all service
   information at once.  The Service Location Protocol separates
   messages which obtain different classes of information.  This
   extension enables an optimization to the basic exchange of messages,
   which currently does not include service attributes in Service Reply
   messages.

   This document specifies a SLPv2 extension which allows a UA to
   request that a service's attributes be included in Service Reply
   messages.  This will eliminate the need for multiple round trip
   messages for a UA to acquire all service information.

   If the DA or SA does not support the Attrlist extension, it will
   simply return a Service Reply (without the extension).  Support of
   this extension is OPTIONAL.  Existing implementations will ignore the
   Attrlist extension since it has been assigned a identifying number
   from the range which indicates that the receiver MUST ignore the
   extension if it is not recognized.  See RFC 2608 [3].

   If the UA receives a Service Reply message without an Attrlist
   Extension it must assume the SA or DA does not support the extension.
   In this case, the UA must send an Attribute Request for each URL it
   obtains in the Service Reply message in order to obtain the
   attributes for these services.

1.1. Terminology

   User Agent (UA)
         A process working on the user's behalf to establish contact
         with some service.  The UA 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 the services.

   Directory Agent (DA)
         A process which collects service advertisements.  There can
         only be one DA present per given host.

1.2. Notation Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].

2. Attribute List Extension

   The format of the Attribute List Extension 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Extension ID = 0x0002    |     Next Extension Offset     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Offset, contd.|      Service URL Length       |  Service URL  /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Attribute List Length     |         Attribute List        /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |# of AttrAuths |(if present) Attribute Authentication Blocks.../
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Extension ID is 0x0002.

   The Next Extension Offset value indicates the position of the next
   extension as offset from the beginning of the SLP message.  If the
   next extension offset value is 0, there are no more extensions in the
   message.

   A UA sends an Attribute List Extension with a Service Request.  The
   Service URL Length and Attribute List Length are set to 0 and the
   Service URL and Attribute List fields omitted in this case.  The UA
   thereby requests that the SA or DA include an Attribute List
   Extension in its Service Reply by including such an 'empty' Attribute
   List Extension in the Service Request.

   A SA or DA which supports the Attribute List Extension returns one
   Attribute List extension for every URL Entry in the Service Reply
   message.  The order of the Attribute List Extensions SHOULD be the
   same as the URL Entries in the Service Reply.

   The Service URL [4] identifies the corresponding URL Entry.

   The Attribute List field is the entire attribute list of the service.
   These attributes must be in the same language as that indicated in
   the Service Request message.

   If the Service Request message includes a SLP SPI string, then the
   attribute list extension MUST include an authentication block.  If
   the SA or DA does not support or is unable to return an
   authentication block for the SLP SPI included in the Service Request,
   then the SA or DA MUST NOT return an Attribute List Extension.  The
   format of the authentication block(s) is exactly the same as would be
   included in an Attribute Reply or Service Registration message.

3. IANA Considerations

   IANA has assigned an extension ID number of 0x0002 for the Attribute
   List Extension.

4. Internationalization Considerations

   The Service Location Protocol, version 2 has mechanisms for allowing
   attributes to be transmitted with explicit language tagging [6].  The
   same mechanisms are used for this protocol extension.

5. Security Considerations

   The Service Location Protocol, version 2 has mechanisms for allowing
   authenticators to be returned with attribute lists so that UAs are
   able to verify a digital signature over the attributes they obtain.
   This same mechanism is used for this protocol extension.  The
   Attribute List Extension used in conjunction with SLPv2 is no less
   secure than SLPv2 without the extension.

6. Acknowledgments

   The author benefited from preliminary conversations about this
   extension with Charlie Perkins.

References

   [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
       9, RFC 2026, October 1996.

   [2] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

   [3] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
       Location Protocol, Version 2", RFC 2608, June 1999.

   [4] Guttman, E., Perkins, C. and J. Kempf, "Service Templates and
       service: Schemes", RFC 2609, June 1999.

   [5] Narten, T and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [6] Alvestrand, H., "Tags for the Identification of Languages", BCP
       47, RFC 3066, January 2001.

Author's Address

   Erik Guttman
   Sun Microsystems
   Eichhoelzelstr. 7
   74915 Waibstadt
   Germany

   Phone:    +49 6227 356 202
   EMail:    Erik.Guttman@sun.com

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