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Network Working Group                                          R. SparksRequest for Comments: 4320                              Estacado SystemsUpdates:3261                                               January 2006Category: Standards TrackActions Addressing Identified Issues with theSession Initiation Protocol's (SIP) Non-INVITE TransactionStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   This document 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 inRFC 3261.Table of Contents1. Introduction ....................................................22. Improving the Situation When Responses Are Only Delayed .........22.1. Action 1: Make the best use of provisional responses .......22.2. Action 2: Remove the useless late-response storm ...........3   3. Improving the Situation When an Element Is Not Going to      Respond .........................................................44. Normative Updates toRFC 3261 ...................................44.1. Action 1 ...................................................44.2. Action 2 ...................................................55. Security Considerations .........................................56. Contributors ....................................................57. Normative References ............................................6Sparks                      Standards Track                     [Page 1]

RFC 4320                 SIP Non-INVITE Actions             January 20061.  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 that   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 document specifies updates toRFC 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 messaging2.1.  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 transportsSparks                      Standards Track                     [Page 2]

RFC 4320                 SIP Non-INVITE Actions             January 2006   Since non-INVITE transactions must complete rapidly ([3]), any   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   unnecessary 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 cannot do anything with this knowledge.  Thus,   the 408 is 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 theRFC 3261 requirement to forward such   strays.Sparks                      Standards Track                     [Page 3]

RFC 4320                 SIP Non-INVITE Actions             January 20063.  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 that the peer is not available and follow the existing   requirements in [1] and [2] for that case.   Achieving the third goal requires more aggressive 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 toRFC 32614.1.  Action 1   An SIP element MUST NOT send any provisional response with a Status-   Code other than 100 to a non-INVITE request.   An 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.   An 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, an SIP element MUST respond to a non-   INVITE request with a Status-Code of 100 if it has not otherwise   responded after the amount of time it takes a client transaction's   Timer E to be reset to T2.Sparks                      Standards Track                     [Page 4]

RFC 4320                 SIP Non-INVITE Actions             January 20064.2.  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 cannot 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 responses.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 that   appear to be late responses.  All proxies compliant withRFC 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.  Contributors   Rohan Mahy provided the Security Considerations section.Sparks                      Standards Track                     [Page 5]

RFC 4320                 SIP Non-INVITE Actions             January 20067.  Normative References   [1]  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.   [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 (SIP) Non-INVITE Transaction",RFC 4321,        January 2006.Author's Address   Robert J. Sparks   Estacado Systems   17210 Campbell Road   Suite 250   Dallas, TX 75252-4203   EMail: rjsparks@estacado.netSparks                      Standards Track                     [Page 6]

RFC 4320                 SIP Non-INVITE Actions             January 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Sparks                      Standards Track                     [Page 7]

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