SIP Working Group O. Levin
Internet-Draft Microsoft Corporation
Expires: July 17, 2006 A. Johnston
Tello Corporation
January 13, 2006
Conveying Feature Tags with Session Initiation Protocol REFER Method
draft-ietf-sip-refer-feature-param-01
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Copyright (C) The Internet Society (2006).
Abstract
This document extends the REFER method, defined in RFC 3515, to
convey feature parameters defined in RFC 3840.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. isfocus Feature Tag Usage . . . . . . . . . . . . . . . . . 4
4.2. Voice and Video Feature Tags Usage . . . . . . . . . . . . 4
4.3. Example with URI parameters and multiple feature tags . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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1. Terminology
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 [1].
To simplify discussions of the REFER method and its extensions, three
new terms are being used throughout the document:
o REFER-Issuer: the UA issuing the REFER request
o REFER-Recipient: the UA receiving the REFER request
o REFER-Target: the UA designated in the Refer-To URI
2. Introduction
This document extends REFER method defined in RFC 3515 [3] to be used
with feature parameters defined in RFC 3840 [4].
Feature tags are used by a UA to convey to another UA information
about capabilities and features. This information can be shared by a
UA using a number of mechanisms including registration requests,
OPTIONS responses, or shared in the context of a dialog by inclusion
with a remote target URI (Contact URI).
Feature tag information can be very useful to another UA. It is
especially useful prior to the establishment of a session. For
example, if a UA knows (through an OPTIONS query, for example) that
the remote UA supports both video and audio, the calling UA might
call offering video in the SDP. Another example is when a UA knows
that a remote UA is acting as a focus and hosting a conference. In
this case, the UA might first subscribe to the conference URI and
find out details about the conference prior to sending an INVITE to
join.
This extension to the REFER method provides a mechanism by which the
REFER-Issuer can provide this useful information about the REFER-
Target capabilities and functionality to the REFER-Recipient by
including feature tags in the Refer-To header field in a REFER
request.
3. Definitions
The Refer-To BNF from RFC 3515:
Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
* (SEMI generic-param)
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is extended to:
Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
* (SEMI refer-param)
refer-param = generic-param / feature-param
where feature-param is defined in Section 9 of RFC 3840 [4].
Note that if any URI parameters are present, the entire URI must be
enclosed in "<" and ">". If no "<" and ">" are present, all
parameters after the URI are header parameters, not URI parameters.
4. Examples
4.1. isfocus Feature Tag Usage
The example below shows how the "isfocus" feature tag can be used by
REFER-Issuer to tell the REFER-Recipient that the REFER-Target is a
conference focus and, consequently, sending an INVITE will bring the
REFER-Recipient into the conference:
Refer-To: sip:conf44@example.com;isfocus
4.2. Voice and Video Feature Tags Usage
The example below shows how a REFER-Issuer can tell the REFER-
Recipient that the REFER-Target supports audio and video and,
consequently, that a video and audio session can be established by
sending an INVITE to the REFER-Target:
Refer-To: "Alice's Videophone" <sip:alice@videophone.example.com>
;audio;video
4.3. Example with URI parameters and multiple feature tags
The example below shows how the REFER-Issuer can tell the REFER-
Recipient that the REFER-Target is a voicemail server. Note that the
transport URI parameter is enclosed within the "<" and ">" so
that it is not interpreted as a header parameter.
Refer-To: <sip:alice-vm@example.com;transport=tcp>
;actor="msg-taker";automata;audio
5. IANA Considerations
None.
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6. Security Considerations
Feature tags can provide sensitive information about a user or a UA.
As such, RFC 3840 cautions against providing sensitive information to
another party. Once this information is given out, any use may be
made of it, including relaying to a third party as in this
specification.
A REFER-Issuer MUST NOT create or guess feature tags - instead a
feature tag included in a REFER SHOULD have been discovered in an
authenticated and secure method (such as an OPTIONS response or from
a remote target URI in a dialog) directly from the REFER-Target.
It is RECOMMENDED that the REFER-Issuer includes in the Refer-To
header field all feature tags that were listed in the most recent
Contact header field of the REFER-Target.
A feature tag provided by a REFER-Issuer can not be authenticated or
certified directly from the REFER request. As such, the REFER-
Recipient MUST treat the information as hint. If the REFER-Recipient
application logic or user's action depends on the presence of the
expressed feature, the feature tag can be verified. For example, in
order to do so, the REFER-Recipient can directly send an OPTIONS
query to the REFER-Target over a secure (e.g. mutually authenticated
and integrity protected) connection. This protects the REFER-
Recipient against incorrect or malicious feature tags being sent.
7. Acknowledgements
The authors would like to thank Jonathan Rosenberg for providing
helpful guidance to this work.
8. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[3] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[4] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User
Agent Capabilities in the Session Initiation Protocol (SIP)",
RFC 3840, August 2004.
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Authors' Addresses
Orit Levin
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: 425-722-2225
Email: oritl@microsoft.com
Alan Johnston
Tello Corporation
999 Baker Way, Suite 250
San Mateo, CA 94404
Email: ajohnston@tello.com
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