Network Working Group O. Levin
Request for Comments: 4508 Microsoft Corporation
Category: Standards Track A. Johnston
Avaya
May 2006
Conveying Feature Tags with the
Session Initiation Protocol (SIP) REFER Method
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 (2006).
Abstract
The SIP "Caller Preferences" extension defined in RFC 3840 provides a
mechanism that allows a SIP request to convey information relating to
the originator's capabilities and preferences for handling of that
request. The SIP REFER method defined in RFC 3515 provides a
mechanism that allows one party to induce another to initiate a SIP
request. This document extends the REFER method to use the mechanism
of RFC 3840. By doing so, the originator of a REFER may inform the
recipient as to the characteristics of the target that the induced
request is expected to reach.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Definitions .....................................................3
4. Examples ........................................................3
4.1. isfocus Feature Tag Usage ..................................3
4.2. Voice and Video Feature Tags Usage .........................3
4.3. Example with URI parameters and multiple feature tags ......3
5. Security Considerations .........................................4
6. Acknowledgements ................................................4
7. Normative References ............................................4
Standards Track [Page 1]RFC 4508 Feature Tags with SIP REFER May 2006
1. Introduction
This document extends the SIP [2] 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 REGISTER requests and
responses and OPTIONS responses. This information can also be 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.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1].
To simplify discussions of the REFER method and its extensions, three
new terms are 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
Standards Track [Page 2]RFC 4508 Feature Tags with SIP REFER May 2006
3. Definitions
The Refer-To BNF from RFC 3515:
Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
*(SEMI generic-param)
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 the "<" and ">" are not 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, that 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.
Standards Track [Page 3]RFC 4508 Feature Tags with SIP REFER May 2006
Refer-To: <sip:alice-vm@example.com;transport=tcp>
;actor="msg-taker";automata;audio
5. 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 be 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 cannot be authenticated or
certified directly from the REFER request. As such, the REFER-
Recipient MUST treat the information as a 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 the sending of incorrect or malicious feature
tags.
6. Acknowledgements
The authors would like to thank Jonathan Rosenberg for providing
helpful guidance to this work.
7. 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.
Standards Track [Page 4]RFC 4508 Feature Tags with SIP REFER May 2006
[4] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User
Agent Capabilities in the Session Initiation Protocol (SIP)",
RFC 3840, August 2004.
Authors' Addresses
Orit Levin
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: 425-722-2225
EMail: oritl@microsoft.com
Alan Johnston
Avaya
St. Louis, MO 63124
EMail: ajohnston@ipstation.com
Standards Track [Page 5]RFC 4508 Feature Tags with SIP REFER May 2006
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Standards Track [Page 6]
