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Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method.
O. Levin, A. Johnston. May 2006.

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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:;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" <> ;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: <;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: Alan Johnston Avaya St. Louis, MO 63124 EMail: 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. 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 in BCP 78 and BCP 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 at 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 Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Standards Track [Page 6]


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