Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method
RFC 4508

Document Type RFC - Proposed Standard (May 2006; No errata)
Updated by RFC 8217
Authors Alan Johnston  , Orit Levin 
Last updated 2015-10-14
Replaces draft-olson-sipping-refer-extensions
Stream Internet Engineering Task Force (IETF)
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IESG IESG state RFC 4508 (Proposed Standard)
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Responsible AD Allison Mankin
Send notices to,
Network Working Group                                           O. Levin
Request for Comments: 4508                         Microsoft Corporation
Category: Standards Track                                    A. Johnston
                                                                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).


   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

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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

   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

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   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

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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:


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" <>

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.

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RFC 4508              Feature Tags with SIP REFER               May 2006

   Refer-To: <;transport=tcp>

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

   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

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.

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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

   Phone: 425-722-2225

   Alan Johnston
   St. Louis, MO 63124


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RFC 4508              Feature Tags with SIP REFER               May 2006

Full Copyright Statement

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