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Network Working Group                                           O. LevinRequest for Comments: 4508                         Microsoft CorporationCategory: Standards Track                                    A. Johnston                                                                   Avaya                                                                May 2006Conveying Feature Tags with theSession Initiation Protocol (SIP) REFER MethodStatus 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 inRFC 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 inRFC 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   ofRFC 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 Contents1. Introduction ....................................................22. Terminology .....................................................23. Definitions .....................................................34. Examples ........................................................34.1. isfocus Feature Tag Usage ..................................34.2. Voice and Video Feature Tags Usage .........................34.3. Example with URI parameters and multiple feature tags ......35. Security Considerations .........................................46. Acknowledgements ................................................47. Normative References ............................................4                            Standards Track                     [Page 1]

RFC 4508              Feature Tags with SIP REFER               May 20061.  Introduction   This document extends the SIP [2] REFER method defined inRFC 3515   [3] to be used with feature parameters defined inRFC 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 inRFC 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 20063.  Definitions   The Refer-To BNF fromRFC 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 inSection 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.  Examples4.1.  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;isfocus4.2.  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;video4.3.  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;audio5.  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 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).                            Standards Track                     [Page 6]