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 RFC2608 [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 RFC2119 [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, RFC2026, October 1996.

[2] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[3] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2", RFC2608, June 1999.

[4] Guttman, E., Perkins, C. and J. Kempf, "Service Templates and
service: Schemes", RFC2609, June 1999.

[5] Narten, T and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC2434, October 1998.

[6] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC3066, 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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