SIP O. Levin
Internet-Draft Microsoft Corporation
Expires: April 17, 2005 October 17, 2004
Suppression of REFER Implicit Subscription
draft-ietf-sip-refer-with-norefersub-00
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This specification defines the way to suppress an implicit
subscription with REFER method in SIP.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Definition . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Preventing Forking of REFER Requests . . . . . . . . . . . . 7
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1 Normative References . . . . . . . . . . . . . . . . . . . 12
10.2 Informational References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . 13
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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
To simplify discussions of the REFER method and its extensions, three
new terms are being 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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2. Introduction
The REFER specification specifies that every REFER creates an
implicit subscription between the REFER-Issuer and the
REFER-Recipient. This document defines a new option tag,
"norefersub", which specifies that an implicit subscription for event
package refer should not be created as a result of accepting this
REFER request.
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3. Motivation
The REFER specification mandates that every REFER creates an implicit
subscription between the REFER-Issuer and the REFER-Recipient. This
subscription results in at least one NOTIFY being sent from the
REFER-Recipient to the REFER-Issuer. The REFER-Recipient may choose
to cancel the implicit subscription with this NOTIFY. The
REFER-Issuer may choose to cancel this implicit subscription with an
explicit SUBSCRIBE (Expires: 0) after receipt of the initial NOTIFY
or by sending a 481 response to this initial NOTIFY request.
One purpose of requiring the implicit subscription and initial NOTIFY
is to allow for the situation where the REFER request gets forked and
the REFER-Issuer needs a way to see the multiple dialogs that may be
established as a result of the forked REFER. This is the same
approach used to handle forking of SUBSCRIBE [4] requests. Where the
REFER-Issuer explicitly specifies that forking not occur, the
requirement that an implicit subscription be established is
unnecessary.
Another purpose of the NOTIFY is to inform the REFER-Issuer of the
progress of the SIP transaction that results from the REFER at the
REFER-Recipient. In the case where the REFER-Issuer is already aware
of the progress of the requested operation, such as when the
REFER-Issuer has an explicit subscription to the dialog event package
at the REFER-Recipient, the implicit subscription and resultant
NOTIFY traffic related to the REFER can create an unnecessary network
overhead.
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4. Definition
This document defines a new option tag, "norefersub", which specifies
that an implicit subscription for event package refer should not be
created as a result of accepting this REFER request.
The "norefersub" option tag MUST be used by the REFER-Issuer only
when the REFER-Issuer can be certain that the REFER request will not
be forked.
The REFER-Issuer can place the "norefersub" option tag either in the
Require header or in the Supported header of the REFER request,
subject to application requirements.
If the REFER-Issuer inserts the option tag in the Supported header
but the REFER-Recipient doesn't grant the suggestion, an implicit
subscription is created as in default case.
If the REFER-Issuer inserts the option tag in the Require header but
the REFER-Recipient is not willing to grant the request, the REFER
request is rejected.
If the REFER-Recipient is willing to grant the "norefersub" behavior
for the issued REFER request, it MUST insert a Supported: norefersub
header in the 2xx response to the REFER-Issuer. In this case no
dialog is created.
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5. Preventing Forking of REFER Requests
The REFER specification allows for the possibility of forking a REFER
request which is sent outside of an existing dialog. The
REFER-Issuer can ensure that REFER doesn't get forked by sending
REFER to a REFER-Recipient which has GRUU properties according to
definitions of [5].
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6. Example
An example of REFER which suppresses the implicit subscription is
shown below:
REFER sip:pc-b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1a
To: <sip:pc-b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: <sip:c@tradewind.com;method=INVITE>
Require: norefersub
Accept-Contact: *;audio;require
Contact: sip:a@issuer.tradewind.com
Content-Type: message/sipfrag
Content-Id: <1239103912039@issuer.tradewind.com>
Content-Length: ...
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7. IANA Considerations
This document defines a new option tag, "norefersub", which specifies
that no implicit subscription should be created as a result of
accepting the REFER request. This option tag is only meaningful for
the REFER request defined in RFC3515.
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8. Security Considerations
This extension doesn't introduce new security threads beyond those
identified and addressed in the core SIP specifications.
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9. Acknowledgements
The SIP community would like to thank Sriram Parameswar for his ideas
being originally presented in draft-parameswar-sipping-norefersub-00
and incorporated in this document.
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10. References
10.1 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] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
10.2 Informational References
[5] Rosenberg, J., "Obtaining and Using Globally Routable User Agent
(UA) URIs (GRUU) in the Session Initiation Protocol (SIP)",
draft-ietf-sip-gruu-02 (work in progress), July 2004.
Author's Address
Orit Levin
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1-425-722-2225
EMail: oritl@microsoft.com
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Internet Society.
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