SIP REFER feature tags: Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method
How the sender of a SIP REFER can tell the recipient what the referred-to party can do.
Levin, O. and A. Johnston, "Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method", RFC 4508, DOI 10.17487/RFC4508, May 2006, <https://www.rfc-editor.org/info/rfc4508>.
@misc{rfc4508,
series = {Request for Comments},
number = 4508,
howpublished = {RFC 4508},
publisher = {RFC Editor},
doi = {10.17487/RFC4508},
url = {https://www.rfc-editor.org/info/rfc4508},
author = {O. Levin and A. Johnston},
title = {{Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method}},
pagetotal = 6,
year = 2006,
month = may,
} RFC text
Contents
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
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
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.
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.
[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
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ISO Plain Language
On this page
§1Introduction
This document extends the SIP REFER method defined in RFC 3515 to be used with feature parameters defined in RFC 3840.
Feature tags are used by a user agent to convey to another user agent information about capabilities and features. This information can be shared by a user agent using a number of mechanisms, including REGISTER requests and responses and OPTIONS responses. It can also be shared in the context of a dialog by inclusion with a remote target URI, that is, a Contact URI.
Feature tag information can be very useful to another user agent, and it is especially useful prior to the establishment of a session. For example, if a user agent knows, through an OPTIONS query, that the remote user agent supports both video and audio, the calling user agent might call offering video in the SDP. Another example is when a user agent knows that a remote user agent is acting as a focus and hosting a conference. In this case, the user agent 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's capabilities and functionality to the REFER-Recipient, by including feature tags in the Refer-To header field in a REFER request.
Before this change, the phone that receives the referral knows only an address. With it, the address can also say whether the far end handles video, hosts a conference, and so on.
§2Terminology
The document introduces three terms to simplify discussions of the REFER method and its extensions. The REFER-Issuer is the user agent issuing the REFER request. The REFER-Recipient is the user agent receiving the REFER request. The REFER-Target is the user agent designated in the Refer-To URI.
Three roles: the one asking for the referral, the one that receives the request, and the one that ends up being called.
- REFER-IssuerThe user agent that issues the REFER request.
- REFER-RecipientThe user agent that receives the REFER request.
- REFER-TargetThe user agent designated in the Refer-To URI.
§3The Refer-To Header Extension
The Refer-To grammar from RFC 3515 is extended. The original form is Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) *(SEMI generic-param), and it becomes Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) *(SEMI refer-param), where refer-param = generic-param / feature-param and feature-param is defined in section 9 of RFC 3840.
The document notes that if any URI parameters are present, the entire URI must be enclosed in angle brackets. If they are not present, all parameters after the URI are header parameters and not URI parameters.
The header already accepted a list of parameters. This change says one of those parameters may be a feature tag, and warns that a URI parameter and a header parameter look the same unless the URI is put in angle brackets.
Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
*(SEMI generic-param)
becomes
Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
*(SEMI refer-param)
refer-param = generic-param / feature-param§4.1The isfocus Feature Tag
The example shows how the isfocus feature tag can be used by the 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.
The header is written as Refer-To: sip:conf44@example.com;isfocus.
One tag says the far end runs a conference. The receiving phone can then tell the user that picking up will join a conference rather than start a one-to-one call.
Refer-To: sip:conf44@example.com;isfocus
Refer-To: "Alice's Videophone" <sip:alice@videophone.example.com>
;audio;video
Refer-To: <sip:alice-vm@example.com;transport=tcp>
;actor="msg-taker";automata;audio§4.2Voice, Video and URI Parameters
The second example 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. The header is written as Refer-To: "Alice's Videophone" <sip:alice@videophone.example.com> ;audio;video.
The third example shows how the REFER-Issuer can tell the REFER-Recipient that the REFER-Target is a voicemail server. The transport URI parameter is enclosed within the angle brackets so that it is not interpreted as a header parameter. The header is written as Refer-To: <sip:alice-vm@example.com;transport=tcp> ;actor="msg-taker";automata;audio.
Several tags can be given at once, and a tag that belongs to the address rather than to the header has to be kept inside the angle brackets so the two are not confused.
§5Security Considerations
Feature tags can provide sensitive information about a user or a user agent. 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. 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, so the REFER-Recipient MUST treat the information as a hint. If the REFER-Recipient's application logic or the user's action depends on the presence of the expressed feature, the feature tag can be verified. For example, the REFER-Recipient can directly send an OPTIONS query to the REFER-Target over a secure connection, such as a mutually authenticated and integrity-protected one. This protects the REFER-Recipient against the sending of incorrect or malicious feature tags.
The tags are only a hint. Whoever sent them could have made them up, so the receiving phone must check them itself if it is going to act on them.