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rfc:rfc4508

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

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).
                          Standards Track                     [Page 6]
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