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rfc:rfc4320

Network Working Group R. Sparks Request for Comments: 4320 Estacado Systems Updates: 3261 January 2006 Category: Standards Track

           Actions Addressing Identified Issues with the
     Session Initiation Protocol's (SIP) Non-INVITE Transaction

Status 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 in RFC 3261.

Table of Contents

 1. Introduction ....................................................2
 2. Improving the Situation When Responses Are Only Delayed .........2
    2.1. Action 1: Make the best use of provisional responses .......2
    2.2. Action 2: Remove the useless late-response storm ...........3
 3. Improving the Situation When an Element Is Not Going to
    Respond .........................................................4
 4. Normative Updates to RFC 3261 ...................................4
    4.1. Action 1 ...................................................4
    4.2. Action 2 ...................................................5
 5. Security Considerations .........................................5
 6. Contributors ....................................................5
 7. Normative References ............................................6

Sparks Standards Track [Page 1] RFC 4320 SIP Non-INVITE Actions January 2006

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 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 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

Sparks 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 the RFC 3261 requirement to forward such
 strays.

Sparks Standards Track [Page 3] RFC 4320 SIP Non-INVITE Actions January 2006

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 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 to RFC 3261

4.1. 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 2006

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 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 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. Contributors

 Rohan Mahy provided the Security Considerations section.

Sparks Standards Track [Page 5] RFC 4320 SIP Non-INVITE Actions January 2006

7. 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.net

Sparks Standards Track [Page 6] RFC 4320 SIP Non-INVITE Actions January 2006

Full 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.
 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.

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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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