[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 RFC 4488

SIP                                                             O. Levin
Internet-Draft                                     Microsoft Corporation
Expires: July 17, 2006                                  January 13, 2006


    Suppression of Session Initiation Protocol REFER Method Implicit
                              Subscription
                draft-ietf-sip-refer-with-norefersub-04

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 17, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification defines a way to suppress an implicit subscription
   with the Session Initiation Protocol (SIP) REFER method.  A new SIP
   option tag "norefersub" is defined to indicate support for this
   extension.  A new SIP header field "Refer-Sub" is defined to request
   the usage of this extension.






Levin                     Expires July 17, 2006                 [Page 1]

Internet-Draft       SIP REFER without Subscription         January 2006


Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Preventing Forking of REFER Requests . . . . . . . . . . . . .  5
   6.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     10.1.  Normative References  . . . . . . . . . . . . . . . . . .  8
     10.2.  Informational References  . . . . . . . . . . . . . . . .  8
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10



































Levin                     Expires July 17, 2006                 [Page 2]

Internet-Draft       SIP REFER without Subscription         January 2006


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, the
   three terms below 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


2.  Introduction

   The REFER specification [3] specifies that every REFER creates an
   implicit subscription between the REFER-Issuer and the REFER-
   Recipient.

   This document defines a new SIP header field: "Refer-Sub" meaningful
   within a REFER transaction only.  This header field, when set to
   "false", specifies that a REFER-Issuer requests that the REFER-
   Recipient doesn't establish an implicit subscription and the
   resultant dialog.

   This document defines a new option tag: "norefersub".  This tag, when
   included in the Supported header field, indicates that a User Agent
   (UA) is capable of accepting a REFER request without creating an
   implicit subscription when acting as a REFER-Recipient.


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.

   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



Levin                     Expires July 17, 2006                 [Page 3]

Internet-Draft       SIP REFER without Subscription         January 2006


   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.


4.  Definitions

   This document defines a new SIP header field: "Refer-Sub".  This
   header field is meaningful and MAY be used with a REFER request and
   the corresponding 2XX response only.  This header field set to
   "false" specifies that a REFER-Issuer requests that the REFER-
   Recipient doesn't establish an implicit subscription and the
   resultant dialog.  Note that when using this extension, the REFER
   remains a target refresh request (as in the default case - when the
   extension is not used).

   This document adds the following entry to Table 2 of [2].  The
   additions to this table are also provided for extension methods at
   the time of publication of this document.  This is provided as a
   courtesy to the reader and is not normative in any way:

   Header field        where    proxy ACK  BYE  CAN  INV  OPT  REG  MSG

   Refer-Sub           R, 2xx          -    -    -    -    -    -    -

   Header field        where    SUB  NOT  REF  INF  UPD  PRA  PUB

   Refer-Sub           R, 2xx    -    -    o    -    -    -    -


   The Refer-Sub header field MAY be encrypted as part of end-to-end
   encryption.

   The syntax of the header field follows the BNF defined below:

    Refer-Sub       = "Refer-Sub" HCOLON refer-sub-value *(SEMI exten)
    refer-sub-value = "true" / "false"
    exten           = generic-param

   where the syntax of generic-param is defined in [2].




Levin                     Expires July 17, 2006                 [Page 4]

Internet-Draft       SIP REFER without Subscription         January 2006


   The "Refer-Sub" header field set to "false" MAY be used by the REFER-
   Issuer only when the REFER-Issuer can be certain that the REFER
   request will not be forked.

   If the REFER-Recipient supports the extension and is willing to
   process the REFER transaction without establishing an implicit
   subscription, it MUST insert the "Refer-Sub" header field set to
   "false" in the 2xx response to the REFER-Issuer.  In this case no
   implicit subscription is created.  Consequently, no new dialog is
   created if this REFER was issued outside any existing dialog.

   If the REFER-Issuer inserts the "Refer-Sub" header field set to
   "false", but the REFER-Recipient doesn't grant the suggestion (i.e.
   either does not include the "Refer-Sub" header field or includes the
   "Refer-Sub" header field set to "true" in the 2xx response), an
   implicit subscription is created as in default case.

   This document also defines a new option tag, "norefersub".  This tag,
   when included in the Supported header field, specifies that a User
   Agent (UA) is capable of accepting a REFER request without creating
   an implicit subscription when acting as a REFER-Recipient.

   The REFER-Issuer can know the capabilities of the REFER-Recipient
   from the presence of the option tags in the Supported header field of
   the dialog initiating request or response.  Another way of learning
   the capabilities would be by using presence, such as defined in [6].
   However, if the capabilities of the REFER-Recipient are not known,
   using the "norefersub" tag with the Require header field is NOT
   RECOMMENDED.  This is due to the fact that in the event the REFER-
   Recipient doesn't support the extension, in order to fallback to the
   normal REFER, the REFER-Issuer will need to issue a new REFER
   transaction thus resulting in additional round-trips.

   As described in Section 8.2.2.3 in [2], a REFER-Recipient will reject
   a REFER request containing a Require: norefersub header field with a
   420 (Bad Extension) response unless it supports this extension.  Note
   that Require: norefersub can be present with a Refer-Sub: false
   header field.


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.  In addition, a
   proxy may fork an unknown method type.  Should forking occur, the
   sender of the REFER with "Refer-Sub" will not be aware as only a
   single 2xx response will be forwarded by the forking proxy.  As a
   result, the responsibility is on the issuer of the REFER with "Refer-



Levin                     Expires July 17, 2006                 [Page 5]

Internet-Draft       SIP REFER without Subscription         January 2006


   Sub" to ensure that no forking will result.

   If a REFER request to a given Request-URI might fork, the REFER-
   Issuer SHOULD NOT include the "Refer-Sub" header field.  The REFER-
   Issuer SHOULD use standardized mechanisms for ensuring the REFER
   request does not fork.  In absence of any other mechanism, the
   Request-URI of the REFER request SHOULD have GRUU properties
   according to the definitions of [5] as those properties ensure the
   request will not fork.


6.  Example

   An example of REFER which suppresses the implicit subscription is
   shown below.  Note that the conventions used in the SIP Torture Test
   Messages [7] document are reused, specifically the <allOneLine> tag.

   REFER sip:pc-b@example.com SIP/2.0
   Via: SIP/2.0/TCP issuer.example.com;branch=z9hG4bK-a-1
   From: <sip:a@example.com>;tag=1a
   <allOneLine>
   To: sip:b@example.com;opaque=urn:uuid:f8
   1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
   </allOneLine>
   Call-ID: 1@issuer.example.com
   CSeq: 234234 REFER
   Max-Forwards: 70
   Refer-To: <sip:c@example.com;method=INVITE>
   Refer-Sub: false
   Supported: norefersub
   Contact: sip:a@issuer.example.com
   Content-Length: 0


7.  IANA Considerations

   This document registers a new SIP header field "Refer-Sub".  This
   header field is only meaningful for the REFER request defined in RFC
   3515 [3] and the corresponding response.  The following information
   to be added to the header field sub-registry under
   http://www.iana.org/assignments/sip-parameters:
   o  Header Name: Refer-Sub
   o  Compact Form: None
   o  Reference: [Substitute with this RFC number]

   This document also registers a new SIP option tag, "norefersub".  The
   required information for this registration, as specified in RFC 3261
   [2], is:



Levin                     Expires July 17, 2006                 [Page 6]

Internet-Draft       SIP REFER without Subscription         January 2006


   o  Name: norefersub
   o  Description: This option tag specifies a User Agent ability of
      accepting a REFER request without establishing an implicit
      subscription (compared to the default case defined in RFC 3515
      [3]).


8.  Security Considerations

   The purpose of this SIP extension is to modify the expected behavior
   of the REFER-Recipient.  The change in behavior is for the REFER-
   Recipient to not establish a dialog and to not send NOTIFY messages
   back to the REFER-Issuer.  As such, a malicious inclusion of a
   "Refer-Sub" header field set to "false" reduces the processing and
   state requirements on the recipient.  As a result, its use in a
   denial of service attack seems limited.

   Should an intermediary maliciously insert a "Refer-Sub" header field
   set to "false", two possibilities may occur.  If the REFER-Recipient
   does not support the extension, the REFER will fail with a "420 Bad
   Extension" response.  The REFER-Issuer will be confused as no "Refer-
   Sub" was in the request, and the resulting request will fail.  Should
   the REFER-Recipient support the extension, the 2xx response will
   contain the "Refer-Sub" header field set to "false".  In any case,
   the REFER-Recipient will not establish a new dialog and send NOTIFYs.
   As a result the REFER-Recipient will not learn the outcome of the
   operation on the Refer-To URI.

   Should an intermediary maliciously remove a "Refer-Sub" header field
   set to "false", the REFER-Recipient will try to sent notifications
   over the "explicitly established" dialog.  It may confuse the REFER-
   Issuer, unless the Man in the Middle (MitM) has the motivation and
   the ability to intercept the notifications.

   To protect against these kinds of MitM attacks, integrity protection
   should be used.  For example, the REFER-Issuer could use S/MIME as
   discussed in RFC 3261 [2] to protect against these kinds of attacks.


9.  Acknowledgements

   The SIP community would like to thank Sriram Parameswar for his ideas
   being originally presented in draft-parameswar-sipping-norefersub-00
   and served as the basis for this specification.


10.  References




Levin                     Expires July 17, 2006                 [Page 7]

Internet-Draft       SIP REFER without Subscription         January 2006


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-06 (work in progress), October 2005.

   [6]  Lonnfors, M. and K. Kiss, "User Agent Capability Extension to
        Presence Information Data Format (PIDF)",
        draft-ietf-simple-prescaps-ext-05 (work in progress),
        October 2005.

   [7]  Sparks, R., "Session Initiation Protocol Torture Test Messages",
        draft-ietf-sipping-torture-tests-09 (work in progress),
        November 2005.






















Levin                     Expires July 17, 2006                 [Page 8]

Internet-Draft       SIP REFER without Subscription         January 2006


Author's Address

   Orit Levin
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: 425-722-2225
   Email: oritl@microsoft.com









































Levin                     Expires July 17, 2006                 [Page 9]

Internet-Draft       SIP REFER without Subscription         January 2006


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




Levin                     Expires July 17, 2006                [Page 10]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/