SIP O. Levin Internet-Draft Microsoft Corporation Expires: August 24, 2005 February 20, 2005 Suppression of Session Initiation Protocol REFER Method Implicit Subscription draft-ietf-sip-refer-with-norefersub-01.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with RFC 3668. 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 August 24, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This specification defines a way to suppress an implicit subscription with the Session Initiation Protocol (SIP) REFER method. A new SIP extension tag 'norefersub' is defined to indicate support for this extension. Levin Expires August 24, 2005 [Page 1] Internet-Draft SIP REFER with norefersub February 2005 Table of Contents 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Preventing Forking of REFER Requests . . . . . . . . . . . . . 4 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 10.1 Normative References . . . . . . . . . . . . . . . . . . . 6 10.2 Informational References . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . 8 Levin Expires August 24, 2005 [Page 2] Internet-Draft SIP REFER with norefersub February 2005 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 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. 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 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. Levin Expires August 24, 2005 [Page 3] Internet-Draft SIP REFER with norefersub February 2005 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. Note that when using this extension, the REFER remains a target refresh request (as in the default case - when the option tag "norefersub" is not used). 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 (i.e. does not insert the "norefersub" option tag neither in the Supported nor in the Require header), 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 by sending "420 Bad Extension" response back to the REFER-Issuer. If the REFER-Recipient is willing to grant the "norefersub" behavior for the issued REFER request, it MUST insert a Require: norefersub header 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. 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]. 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 "norefsub" 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 "norefersub" to ensure that no forking will result. Levin Expires August 24, 2005 [Page 4] Internet-Draft SIP REFER with norefersub February 2005 The best way that the REFER-Issuer can ensure that REFER doesn't get forked is by only sending a REFER with "norefersub" with a Request-URI which has GRUU properties according to definitions of [5]. If this is not known, the only other way to ensure that forking will not occur is to ensure that there are no proxies between the REFER-Issuer and the REFER-Recipient. This could be done by sending the REFER with a Max-Forwards: 0 header field. Any proxy receiving this request will return a "483 Too Many Hops" response, indicating that it is not safe to use the "norefersub" extension. 6. Example An example of REFER which suppresses the implicit subscription is shown below: REFER sip:pc-b@example.com SIP/2.0 Via: SIP/2.0/TCP issuer.example.com;branch=z9hG4bK-a-1 From: ;tag=1a To: Call-ID: 1@issuer.example.com CSeq: 234234 REFER Max-Forwards: 70 Refer-To: Require: norefersub Contact: sip:a@issuer.example.com Content-Length: 0 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 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 Require:norefersub header field 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 Require:norefsub header Levin Expires August 24, 2005 [Page 5] Internet-Draft SIP REFER with norefersub February 2005 field, 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 Require was in the request, and the resulting request will fail. Should the REFER-Recipient support the extension, the 2xx response will contain the Supported: norefsub header field. 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 Require:norefsub header field, 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 incorporated in this document. 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)", Internet-Draft draft-ietf-sip-gruu-02, July 2004. Levin Expires August 24, 2005 [Page 6] Internet-Draft SIP REFER with norefersub February 2005 Author's Address Orit Levin Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: 425-722-2225 Email: oritl@microsoft.com Levin Expires August 24, 2005 [Page 7] Internet-Draft SIP REFER with norefersub February 2005 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. 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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 (2005). 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 August 24, 2005 [Page 8]