Network Working Group R. Sparks Internet-Draft Xten Expires: July 2, 2005 Jan 2005 Actions addressing identified issues with the Session Initiation Protocol's non-INVITE Transaction draft-sparks-sip-nit-actions-03 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 July 2, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This draft describes modifications to the Session Initiation Protocol (SIP) to address problems that have been identified with the SIP non-INVITE transaction. These modifications reduce the probability of messages losing the race condition inherent in the non-INVITE transaction and reduce useless network traffic. They also improve the robustness of SIP networks when elements stop responding. These changes update behavior defined in RFCs 3261. Sparks Expires July 2, 2005 [Page 1] Internet-Draft SIP non-INVITE Actions Jan 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Improving the situation when responses are only delayed . . . 3 2.1 Action 1: Make the best use of provisional responses . . . 3 2.2 Action 2: Remove the useless late-response storm . . . . . 4 3. Improving the situation when an element is not going to respond . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Normative Updates to RFC 3261 . . . . . . . . . . . . . . . . 5 4.1 Action 1 . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2 Action 2 . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . 8 Sparks Expires July 2, 2005 [Page 2] Internet-Draft SIP non-INVITE Actions Jan 2005 1. Introduction There are a number of unpleasant edge conditions created by the SIP non-INVITE transaction (NIT) model's fixed duration. The negative aspects of some of these are exacerbated by the effect provisional responses have on the non-INVITE transaction state machines. These problems are documented in [3]. In summary: A non-INVITE transaction must complete immediately or risk losing a race Losing the race will cause the requester to stop sending traffic to the responder (the responder will be temporarily blacklisted) Provisional responses can delay recovery from lost final responses The 408 response is useless for the non-INVITE transaction As non-INVITE transactions through N proxies time-out, there can be an O(N^2) storm of the useless 408 responses This draft specifies updates to RFC 3261 [1] to improve the behavior of SIP elements when these edge conditions arise. 2. Improving the situation when responses are only delayed There are two goals to achieve when we constrain the problem to those cases where all elements are ultimately responsive and networks ultimately deliver messages: o Reduce the probability of losing the race, preferably to the point that it is negligible o Reduce or eliminate useless messaging 2.1 Action 1: Make the best use of provisional responses o Disallow non-100 provisionals to non-INVITE requests o Disallow 100 Trying to non-INVITE requests before Timer E reaches T2 (for UDP hops) o Allow 100 Trying after Timer E reaches T2 (for UDP hops) o Allow 100 Trying for hops over reliable transports Since non-INVITE transactions must complete rapidly ([3]), any Sparks Expires July 2, 2005 [Page 3] Internet-Draft SIP non-INVITE Actions Jan 2005 information beyond "I'm here" (which can be provided by a 100 Trying) can be just as usefully delayed to the final response. Sending non-100 provisionals wastes bandwidth. As shown in [3], sending any provisional response inside a NIT before Timer E reaches T2 damages recovery from failure of an unreliable transport. Without a provisional, a late final response is the same as no response at all and will likely result in blacklisting the late responding element ([3]). If an element is delaying its final response at all, sending a 100 Trying after Timer E reaches T2 prevents this blacklisting without damaging recovery from unreliable transport failure. Blacklisting on a late response occurs even over reliable transports. Thus, if an element processing a request received over a reliable transport is delaying its final response at all, sending a 100 Trying well in advance of the timeout will prevent blacklisting. Sending a 100 Trying immediately will not harm the transaction as it would over UDP, but a policy of always sending such a message results in unneccessary traffic. A policy of sending a 100 Trying after the period of time in which Timer E reaches T2 had this been a UDP hop is one reasonable compromise. 2.2 Action 2: Remove the useless late-response storm o Disallow 408 to non-INVITE requests o Absorb stray non-INVITE responses at proxies A 408 to non-INVITE will always arrive too late to be useful ([3]), The client already has full knowledge of the timeout. The only information this message would convey is whether or not the server believed the transaction timed out. However, with the current design of the NIT, a client can't do anything with this knowledge. Thus the 408 simply wasting network resources and contributes to the response bombardment illustrated in [3]. Late non-INVITE responses by definition arrive after the client transaction's Timer F has fired and the client transaction has entered the Terminated state. Thus, these responses cannot be distinguished from strays. Changing the protocol behavior to prohibit forwarding non-INVITE stray responses stops the late response storm. It also improves the proxy's defenses against malicious users counting on the RFC 3261 requirement to forward such strays. Sparks Expires July 2, 2005 [Page 4] Internet-Draft SIP non-INVITE Actions Jan 2005 3. Improving the situation when an element is not going to respond When we expand the scope of the problem to also deal with element or network failure, we have more goals to achieve: o Identifying when an element is non-responsive o Minimizing or eliminating falsely identifying responsive elements as non-responsive o Avoiding non-responsive elements with future requests Action 1 helps with the first two goals, dramatically improving an element's ability to distinguish between failure and delayed response from the next downstream element. Some response, either provisional or final, will almost certainly be received before the transaction times out. So, an element can more safely assume that no response at all indicates the peer is not available and follow the existing requirements in [1] and [2] for that case. Achieving the third goal requires more agressive changes to the protocol. As noted in [3], future non-invite transactions are likely to fail again unless the implementation takes steps beyond what is defined in [1] and [2] to remember non-responsive destinations between transactions. Standardizing these extra steps is left to future work. 4. Normative Updates to RFC 3261 4.1 Action 1 A SIP element MUST NOT send any provisional response with a Status-Code other than 100 to a non-INVITE request. A SIP element MUST NOT respond to a non-INVITE request with a Status-Code of 100 over any unreliable transport, such as UDP, before the amount of time it takes a client transaction's Timer E to be reset to T2. A SIP element MAY respond to a non-INVITE request with a Status-Code of 100 over a reliable transport at any time. Without regard to transport, a SIP element MUST respond to a non-INVITE request with a Status-Code of 100 if it has not otherwise responed after the amount of time it takes a client transaction's Timer E to be reset to T2. Sparks Expires July 2, 2005 [Page 5] Internet-Draft SIP non-INVITE Actions Jan 2005 4.2 Action 2 A transaction-stateful SIP element MUST NOT send a response with Status-Code of 408 to a non-INVITE request. As a consequence, an element that can not respond before the transaction expires will not send a final response at all. A transaction-stateful SIP proxy MUST NOT send any response to a non-INVITE request unless it has a matching server transaction that is not in the Terminated state. As a consequence, this proxy will not forward any "late" non-INVITE response. 5. Security Considerations This document makes a number of small changes to the core SIP specification [1] to improve the robustness of SIP non-INVITE transactions. Many of these actions also prevent flooding and denial-of-service attacks. One change prohibits proxies and User Agents from sending 408 responses to non-INVITE transactions. Without this change, proxies automatically generate a storm of useless responses as described in [3]. An attacker could capitalize on this by enticing User Agents to send non-INVITE requests to a black hole (through social engineering or DNS poisoning) or by selectively dropping responses. Another change prohibits proxies from forwarding late responses. Without this change, an attacker could easily forge messages which appear to be late responses. All proxies compliant with RFC 3261 are required to forward these responses, wasting bandwidth and CPU and potentially overwhelming target User Agents (especially those with low speed connections). The remainder of these changes do not affect the security of the SIP protocol. 6. IANA Considerations This document requires no action by IANA. 7. Contributors Rohan Mahy provided the Security Considerations section. 8 References [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Sparks Expires July 2, 2005 [Page 6] Internet-Draft SIP non-INVITE Actions Jan 2005 Session Initiation Protocol", RFC 3261, June 2002. [2] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol (SIP): Locating SIP Servers", RFC 3263, June 2002. [3] Sparks, R., "Problems identified associated with the Session Initiation Protocol's non-INVITE Transaction", draft-sparks-sip-nit-problems (work in progress), February 2004. Author's Address Robert J. Sparks Xten 5100 Tennyson Parkway Suite 1000 Plano, TX 75024 EMail: RjS@xten.com Sparks Expires July 2, 2005 [Page 7] Internet-Draft SIP non-INVITE Actions Jan 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. Sparks Expires July 2, 2005 [Page 8]