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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AI ELI5
On this page
§1Why Feature Tags Help a REFER
RFC 4508 extends the SIP REFER method from RFC 3515 so that it can carry the feature parameters defined in RFC 3840. Feature tags are how a user agent (UA) tells another UA about its capabilities and features.
Feature tag information can already be shared through REGISTER requests and responses, OPTIONS responses, or the remote target (Contact) URI inside a dialog. The RFC says this information is especially useful before a session is established. If a UA knows the remote UA supports both video and audio, it can offer video in the SDP when it calls.
The new mechanism lets the party sending the REFER pass along what it knows about the party to be contacted, by putting feature tags in the Refer-To header field.
Before you call someone, it helps to know what kind of phone they have. This lets a person who knows pass that along when asking you to make the call.
§2Three Roles in a REFER
To keep the discussion clear, the RFC defines three terms.
- REFER-Issuer: the UA issuing the REFER request.
- REFER-Recipient: the UA receiving the REFER request.
- REFER-Target: the UA designated in the
Refer-ToURI.
The Issuer describes the Target to the Recipient. The Recipient is then expected to send the induced request to the Target.
There is the person who asks, the person who is asked, and the person who gets called. The asker tells the helper about the person to be called.
§3The Extended Refer-To Header
RFC 3515 defines Refer-To as a name-address or address-spec followed by any number of generic parameters. RFC 4508 replaces the generic parameter with a refer-param, which is either a generic-param or a feature-param. The feature-param is the one defined in Section 9 of RFC 3840.
One syntax detail matters. If any URI parameters are present, the entire URI must be enclosed in < and >. Without the angle brackets, every parameter after the URI is a header parameter, not a URI parameter.
The address line now has room for little labels about the person at that address. If the address itself has extras, you have to wrap it in brackets so the labels do not get mixed up with it.
§4Examples in the RFC
The RFC gives three examples.
- The
isfocustag tells the Recipient that the Target is a conference focus, so sending an INVITE will bring the Recipient into the conference. - The
audioandvideotags tell the Recipient that the Target supports both, so a video and audio session can be established by sending an INVITE. - A voicemail server is described with
actor="msg-taker",automataandaudio. Thetransport=tcpURI parameter sits inside the angle brackets so it is not read as a header parameter.
You can say the person is a conference host, that they have a video phone, or that they are an answering machine. Each is a small tag stuck onto the address.
Refer-To: <sip:alice-vm@example.com;transport=tcp>
;actor="msg-taker";automata;audio§5Security Considerations
Feature tags can reveal sensitive information about a user or UA. RFC 3840 cautions against giving such information to another party, and once it is given out, any use may be made of it, including relaying it to a third party as this specification does.
A REFER-Issuer MUST NOT create or guess feature tags. A tag in a REFER SHOULD be discovered in an authenticated and secure way, such as an OPTIONS response or the remote target URI in a dialog, directly from the REFER-Target. It is RECOMMENDED that the Issuer include all feature tags from the Target's most recent Contact header field.
The Recipient cannot authenticate a tag from the REFER itself, so it MUST treat the information as a hint. If its logic or the user's action depends on the feature, it can verify it, for example by sending an OPTIONS query to the Target over a secure, mutually authenticated and integrity-protected connection.
Treat what you are told as a hint, not a promise. If it really matters, ask the person directly in a safe way.