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Versions: (draft-olson-sipping-refer-extensions) 00 01 RFC 4508

SIP Working Group                                               O. Levin
Internet-Draft                                     Microsoft Corporation
Updates: 3515 (if approved)                                  A. Johnston
Expires: January 6, 2006                                             MCI
                                                            July 5, 2005


  Conveying Feature Tags with Session Initiation Protocol REFER Method
                 draft-ietf-sip-refer-feature-param-00

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document extends the SIP REFER method, defined in RFC 3515, to
   convey feature parameters defined in RFC 3840.

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this



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   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 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 SIP User Agent (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 (Uniform Resource Identifier),
   such as a 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 its session description.  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)

   is extended to:

   Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
              *(SEMI refer-param)
   refer-param = generic-param / feature-param



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   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@vphone.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

   This document requires no actions by IANA.  Note that this document
   does not define any elements in the SIP Header Parameter Registry
   [5], since it incorporates media feature parameters instead of SIP
   header parameters.






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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.

   As a result, it is NOT RECOMMENDED that all feature tag information
   be passed using the mechanism described in this specification.
   Instead, only feature tags that directly relate to a requested
   operation should be used.  For example, the "isfocus" feature tag has
   clear operation semantics and utility.  However, the "mobility" or
   "class" feature tags have no obvious use in a REFER scenario and
   should not be included unless their application is defined in the
   future.

   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.

   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.

7.  Acknowledgements

   The authors would like to thank Jonathan Rosenberg for providing
   helpful guidance to this work.

8.  References

8.1  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.




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   [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.

8.2  Informative References

   [5]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
        Header Field Parameter Registry for the Session Initiation
        Protocol (SIP)", BCP 98, RFC 3968, December 2004.


Authors' Addresses

   Orit Levin
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: 425-722-2225
   Email: oritl@microsoft.com


   Alan Johnston
   MCI
   100 South 4th Street
   St. Louis, MO  63102

   Email: alan.johnston@mci.com



















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