Network Working Group R. Sparks Internet-Draft dynamicsoft Expires: August 6, 2004 February 6, 2004 Actions addressing identified issues with the Session Initiation Protocol's non-INVITE Transaction draft-sparks-sip-nit-actions-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 6, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 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 and 3263. Sparks Expires August 6, 2004 [Page 1] Internet-Draft SIP non-INVITE Actions February 2004 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 3.1 Action 3: Strengthen specification of caching success and failures in RFC 3263 . . . . . . . . . . . . . . . . . . . . . 5 4. Normative Updates to RFC 3261 . . . . . . . . . . . . . . . . 5 4.1 Action 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2 Action 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Normative Updates to RFC 3263 . . . . . . . . . . . . . . . . 6 5.1 Action 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . 8 Sparks Expires August 6, 2004 [Page 2] Internet-Draft SIP non-INVITE Actions February 2004 1. Introduction There are a number of unpleasant edge conditions created by the SIP non-INVITE transaction 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] and RFC 3263 [2] 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 Sparks Expires August 6, 2004 [Page 3] Internet-Draft SIP non-INVITE Actions February 2004 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 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 August 6, 2004 [Page 4] Internet-Draft SIP non-INVITE Actions February 2004 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 dramatically improves an elements ability to distinguish between failure and delayed response from the next downstream element. Ssome 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] (as amended by this memo) for that case. As [3] discusses, behavior once an element is identified as non-responsive is currently underspecified. [2] speaks only non-normatively about caching the addresses of servers that have successfully been communicated with for an unspecified period of time. 3.1 Action 3: Strengthen specification of caching success and failures in RFC 3263 o Make the caching recommendation normative for servers successfully reached o Add failures due to non-responsiveness to that cache This cache will also be used to remember servers that have issued a 503 with or without a Retry-After. 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 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. Sparks Expires August 6, 2004 [Page 5] Internet-Draft SIP non-INVITE Actions February 2004 A SIP element MAY respond to a request with a Status-Code of 100 over an unreliable transport after the amount of time it takes a client transaction's Timer E to be reset to T2. A SIP element MAY respond to a request with a Status-Code of 100 over a reliable transport at any time. 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. Normative Updates to RFC 3263 5.1 Action 3 (Note that RFC 3263 uses "client" for "any SIP element wishing to send a request".) Once a client identifies an available server for a domain name using the algorithms defined in RFC 3263, it SHOULD cache the identity of that server in an available-cache. This identity MUST be periodically removed from the cache, and its time-to-live in that cache SHOULD be short. If the server with that identity becomes unavailable, the identity MUST be immediately removed from the cache and SHOULD be placed in an unavailable-cache. The next attempt to reach that domain name MUST invoke the algorithms in RFC 3263. If any attempt to contact a server based on the output of the algorithms of RFC 3263 yeilds that the server is unavailable (the request times out or the server returns a 503 Status-Code), the identity of that server SHOULD be placed in an unavailable-cache. This identity MUST be periodically removed from that cache, and its time-to-live in that cache SHOULD be short. If information about the period of unavailability is present (such as in a Retry-After header field in a 503 response), the time-to-live in this cache SHOULD reflect that information. If the algorithms of RFC 3263 yeild a server identity that is in an unavailable-cache, that identity MUST be discarded and the algorithm MUST be continued to search for another candidate. Sparks Expires August 6, 2004 [Page 6] Internet-Draft SIP non-INVITE Actions February 2004 OPEN ISSUE: Can we strengthen placing identities in an unavailable-cache to MUST? RFC 3263 failover for non-INVITE will not work without it. OPEN ISSUE: Is it possible to recommend a time more specific than "short" in these requirements? 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 non-INVITE Transaction", draft-sparks-sip-nit-problems (work in progress), February 2004. Author's Address Robert J. Sparks dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 EMail: rsparks@dynamicsoft.com Sparks Expires August 6, 2004 [Page 7] Internet-Draft SIP non-INVITE Actions February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication 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 implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION Sparks Expires August 6, 2004 [Page 8] Internet-Draft SIP non-INVITE Actions February 2004 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Sparks Expires August 6, 2004 [Page 9]