Internet DRAFT - draft-abhishek-refer-to-event-parameter
Network Working Group Abhishek Gupta
Internet-Draft Independent Consultant
Intended status: Standards Track June 20, 2015
Expires: Dec 19, 2015
Conveying Event Tag with the Session Initiation Protocol REFER Method
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This document extends the REFER method, defined in RFC 3515, to
convey feature parameters defined in RFC 3840.
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 .
To simplify discussions of the REFER method and its extensions, three
new terms are used throughout the document:
- REFER-Issuer: the UA issuing the REFER request
- REFER-Recipient: the UA receiving the REFER request
- REFER-Target: the UA designated in the Refer-To URI
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 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 OPTIONS responses.
Feature tag information can be very useful to another UA. It is
especially useful prior to the establishment of a subscription,
triggered only through REFER method. This is valid only for the request
containing parameter "method=SUBSCRIBE" in Refer-To header. While
sending this request if there is no mechanism to indicate as to which
event is to be subscribed, the REFER-Recipient may send SUBSCRIBE with
an undesired event. For example, if a REFER-Issuer knows (through an
OPTIONS query, for example) that the remote REFER-Target supports
events like dialog the REFER-Recipient might subscribe with dialog
or any other event as specified.
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
1. The example below shows how the REFER-Issuer tells the REFER-
Recipient that the REFER-Target supports dialog event package.
2. In case the REFER-Recipient doesn't support the desired event in
the Refer-To header, a 489 Bad Event will be responded to REFER
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 event tags - instead an
event tag included in a REFER SHOULD have been discovered in an
authenticated and secure method (such as an OPTIONS response)
directly from the REFER-Target.
It is RECOMMENDED that the REFER-Issuer includes in the Refer-To
header field the desired feature tag that were listed in the most
recent Allow-Events header of the REFER-Target.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 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.
 Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
 Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User
Agent Capabilities in the Session Initiation Protocol (SIP)",
RFC 3840, August 2004.
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