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Versions: (draft-sparks-sipcore-invfix) 00 01 RFC 6026

Network Working Group                                          R. Sparks
Internet-Draft                                                   Tekelec
Updates: 3261 (if approved)                             T. Zourzouvillys
Intended status: Standards Track                                   Skype
Expires: September 9, 2010                                   Mar 8, 2010


  Correct transaction handling for 2xx responses to Session Initiation
                     Protocol (SIP) INVITE requests
                      draft-ietf-sipcore-invfix-01

Abstract

   This document normatively updates RFC 3261, the Session Initiation
   Protocol (SIP), to address an error in the specified handling of
   success (2xx class) responses to INVITE requests.  Elements following
   RFC 3261 exactly will misidentify retransmissions of the request as a
   new, unassociated, request.  The correction involves modifying the
   INVITE transaction state machines.  The correction also changes the
   way responses that cannot be matched to an existing transaction are
   handled to address a security risk.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 9, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Conventions and Definitions  . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Reason for Change  . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Summary of Change  . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Consequences if Not Implemented  . . . . . . . . . . . . . . .  4
   6.  The Change . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   7.  Change Details . . . . . . . . . . . . . . . . . . . . . . . .  5
     7.1.  Server Transaction Impacts . . . . . . . . . . . . . . . .  5
     7.2.  Client Transaction Impacts . . . . . . . . . . . . . . . .  9
     7.3.  Proxy Considerations . . . . . . . . . . . . . . . . . . . 10
   8.  Exact changes to RFC3261 . . . . . . . . . . . . . . . . . . . 11
     8.1.  Page 85  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.2.  Page 107 . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.3.  Page 114 . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.4.  Pages 126 through 128  . . . . . . . . . . . . . . . . . . 12
     8.5.  Pages 134 to 135 . . . . . . . . . . . . . . . . . . . . . 15
     8.6.  Page 136 . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.7.  Page 137 . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.8.  Page 141 . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.9.  Page 144 . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.10. Page 146 . . . . . . . . . . . . . . . . . . . . . . . . . 18
     8.11. Page 265 . . . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20









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1.  Conventions and Definitions

   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 [RFC2119].


2.  Introduction

   This document describes an essential correction to the Session
   Initiation Protocol (SIP), defined in [RFC3261].  The change
   addresses an error in the handling of 2xx class responses to INVITE
   requests that leads to retransmissions of the INVITE being treated as
   new requests and forbids forwarding stray INVITE responses.


3.  Reason for Change

   One use of the INVITE method in SIP is to establish new sessions.
   These "initial" INVITEs may fork at intermediaries, and more than one
   receiving endpoint may choose to accept the request.  SIP is designed
   such that the requester receives all of these success responses.

   Two sets of requirements in [RFC3261] work together to allow multiple
   2xx responses to be processed correctly by the requester.  First, all
   elements are required to immediately destroy any INVITE client
   transaction state upon forwarding a matching 2xx class response.
   This requirement applies to both proxies and user agents (proxies
   forward the response upstream, the transaction layer at user agents
   forward the response to its "UA (User-Agent) core").  Second, all
   proxies are required to statelessly forward any 2xx class responses
   that do not match an existing transaction, also called stray
   responses, upstream.  The transaction layer at user agents is
   required to forward these responses to its UA core.  Logic in the UA
   core deals with acknowledging each of these responses.

   This technique for specifying the behavior was chosen over adjusting
   INVITE client transaction state machines as a simpler way to specify
   the correct behavior.

   Over time, implementation experience demonstrated the existing text
   is in error.  Once any element with a server transaction (say, a
   proxy in the path of the INVITE) deletes that transaction state, any
   retransmission of the INVITE will be treated as a new request,
   potentially forwarded to different locations than the original.  Many
   implementations in the field have made proprietary adjustments to
   their transaction logic to avoid this error.




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   The requirement to statelessly forward stray responses has also been
   identified as a security risk.  Through it, elements compliant to
   [RFC3261] are compelled to do work (forward packets) that is not
   protected by the admission policies applied to requests.  This can be
   leveraged to, for instance, use a SIP proxy as an anonymizing
   forwarder of packets in a distributed DOS attack.  General internet
   endpoints can also collude to tunnel non-SIP content through such
   proxies by wrapping them in an SIP response envelope.

   Additionally, [RFC3261] requires that if an unrecoverable transport
   error is encountered while sending a response in a client
   transaction, that the transaction moves immediately into the
   Terminated state.  This will result in any re-transmitted INVITE
   requests received after such an error was encountered be processed as
   a new request instead of being absorbed as a re-transmission.


4.  Summary of Change

   This correction document updates [RFC3261], adding a state and
   changing the transitions in the INVITE client state machine such that
   the INVITE client transaction remains in place to receive multiple
   2xx responses.  It adds a state to the INVITE server state machine to
   absorb retransmissions of the INVITE after a 2xx response has been
   sent.  It modifies state transitions in the INVITE server state
   machine to absorb retransmissions of the INVITE request after
   encountering a unrecoverable transport error when sending a response.
   It also forbids forwarding stray responses to INVITE requests (not
   just 2xx responses), which RFC3261 requires.


5.  Consequences if Not Implemented

   Implementations strictly conformant to [RFC3261] will process
   retransmitted initial INVITE requests as new requests.  Proxies may
   forward them to different locations than the original.  Proxies may
   also be used as anonymizing forwarders of bulk traffic.
   Implementations will process any retransmitted INVITE request as new
   request after an attempt to send a response resulted in a
   unrecoverable error.


6.  The Change

   An element sending or receiving a 2xx to an INVITE transaction MUST
   NOT destroy any matching INVITE transaction state.  This state is
   necessary to ensure correct processing of retransmissions of the
   request and the retransmission of the 2xx and ACK that follow.



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   An element encountering an unrecoverable tranport error when trying
   to send a response to an INVITE request MUST NOT immediately destroy
   the associated INVITE server transaction state.  This state is
   necessary to ensure correct processing of retransmissions of the
   request.

   When receiving any SIP response, a transaction-stateful proxy MUST
   compare the transaction identifier in that response against its
   existing transaction state machines.  The proxy MUST NOT forward the
   response if there is no matching transaction state machine.

   When receiving an ACK that matches an existing INVITE server
   transaction, and the ACK does not contain a branch parameter
   containing the magic cookie defined in RFC 3261, the matching
   transaction MUST be checked to see if it is in the "Accepted" state.
   If it is, then the ACK must be passed directly to the transaction
   user instead of absorbing it in the transaction.  This is necessary
   as requests from RFC 2543 clients will not include a unique branch
   parameter, and the mechanisms for calculating the transaction id from
   such a request will be the same for both INVITE and ACKs.


7.  Change Details

   These changes impact requirements in several sections of RFC3261.
   The exact effect on that text is detailed in Section 8.  This section
   describes the details of the change, particularly the impact on the
   INVITE state machines, more succinctly to facilitate review and
   simplify implementation.

7.1.  Server Transaction Impacts

   To allow a SIP element to recognize retransmissions of an INVITE as
   retransmissions instead of new requests, a new state, "Accepted", is
   added to the INVITE server transaction state machine.  A new timer,
   Timer L, is also added to ultimately allow the state machine to
   terminate.  A server transaction in the "Proceeding" state will
   transition to the "Accepted" state when it issues a 2xx response, and
   will remain in that state just long enough to absorb any
   retransmissions of the INVITE.

   If the SIP element's TU (Transaction User) issues a 2xx response for
   this transaction while the state machine is in the "Proceeding"
   state, it MUST transition to the "Accepted" state and set Timer L to
   64*T1.

   While in the "Accepted" state, any retransmissions of the INVITE
   received will match this transaction state machine and will be



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   absorbed by the machine without changing its state.  These
   retransmissions are not passed onto the TU.  RFC3261 requires the TU
   to periodically retransmit the 2xx response until it receives an ACK.
   The server transaction MUST NOT generate 2xx retransmissions on its
   own.  Any retransmission of the 2xx response passed from the TU to
   the transaction while in the "Accepted" state MUST be passed to the
   transport layer for transmission.  Any ACKs received from the network
   while in the "Accepted" state MUST be passed directly to the TU and
   not absorbed.

   When Timer L fires and the state machine is in the "Accepted" state,
   the machine MUST transition to the "Terminated" state.  Once the
   transaction is in the "Terminated" state, it MUST be destroyed
   immediately.  Timer L reflects the amount of time the server
   transaction could receive 2xx responses for retransmission from the
   TU while it is waiting to receive an ACK.

   A server transaction MUST NOT discard transaction state based only on
   encountering a non-recoverable transport error when sending a
   response.  Instead the assocated INVITE server transaction state
   machine MUST remain in its current state.  (Timers will eventually
   cause it to transition to the Terminated state).  This allows
   retransmissions of the INVITE to be absorbed instead of being
   processed as a new request.

   Figure 1 and Figure 2 graphically show the parts of the INVITE server
   state machine that has changed.  The entire new INVITE server state
   machine is shown in Figure 5.























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    BEFORE                                 AFTER

  +-----------+                       +-----------+
  |           |                       |           |
  | Proceeding|                       | Proceeding|
  |           |                       |           |
  |           |                       |           |
  |           |                       |           |
  |           |                       |           |
  +-----------+                       +-----------+
           |2xx from TU                      |2xx from TU
           |send response                    |send response
           +-------------->+                 +------->+
                           |                          |
                           |                          |
                           |                          |
                           |                          |  Transport
                           |                 INVITE   |  Error
                           |                 -        |  Inform TU
                           |                 +-----+  |  +--+
                           |                 |     |  V  |  v
                           |                 |  +------------+
                           |                 |  |            |<--+
                           |                 +->|  Accepted  |   | ACK
                           |                    |            |---+ to TU
                           |                    +------------+
                           |                     |   ^     |
                           |                  +--+   |     |
                           |                  |      +-----+
                           |                  |  2xx from TU
                           |                  |  send response
                           |                  |
                           |                  | Timer L fires
                           |                  | -
                           |                  |
                           |                  V
  +-----------+            |                +------------+
  |           |            |                |            |
  | Terminated|<-----------+                | Terminated |
  |           |                             |            |
  +-----------+                             +------------+


   Figure 1: Changes to the INVITE server transaction state machine when
                                sending 2xx






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       BEFORE                                  AFTER

    +-----------+                          +------------+
    |           |                          |            |
    | Proceeding|                          | Proceeding | Transport Err.
    |           |                          |            | Inform TU
    |           |   Transport Err.         |            |----------+
    |           |   Inform TU              |            |          |
    |           |--------------->+         |            |<---------+
    +-----------+                |         +------------+
                                 |
                                 |
                                 |
                                 |
                                 |                       Transport Err.
    +-----------+                |         +-----------+ Inform TU
    |           |                |         |           |---------+
    | Completed |                |         | Completed |         |
    |           |                |         |           |<--------+
    +-----------+                |         +-----------+
             |                   |
             |                   |
             +------------------>+
                   Transport Err.|
                   Inform TU     |
                                 |
                                 |
                                 |
                                 |
                                 |
                                 |
                                 |
                                 |
                                 |
    +-----------+                |
    |           |                |
    | Terminated|<---------------+
    |           |
    +-----------+



    Figure 2: Changes to the INVITE server transaction state machine on
                       encountering transport error







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7.2.  Client Transaction Impacts

   In order to correctly distinguish retransmissions of 2xx responses
   from stray 2xx responses, the INVITE client state machine is modified
   to not transition immediately to "Terminated" on receipt of a 2xx
   response.  Instead, the machine will transition to a new "Accepted"
   state, and remain there just long enough, determined by a new timer
   M, to receive and pass to the TU any retransmissions of the 2xx
   response or any additional 2xx responses from other branches of a
   downstream fork of the matching request.  If a 2xx response is
   received while the client INVITE state machine is in the "Calling" or
   "Proceeding" states, it MUST transition to the "Accepted" state, pass
   the 2xx response to the TU, and set Timer M to 64*T1.  A 2xx response
   received while in the "Accepted" state MUST be passed to the TU and
   the machine remains in the "Accepted" state.  The client transaction
   MUST NOT generate an ACK to any 2xx response on its own.  The TU
   responsible for the transaction will generate the ACK.

   When Timer M fires and the state machine is in the "Accepted" state,
   the machine MUST transition to the "Terminated" state.  Once the
   transaction is in the "Terminated" state, it MUST be destroyed
   immediately.

   Any response received which does not match an existing client
   transaction state machine is simply dropped.  (Implementations are,
   of course, free to log or do other implementation specific things
   with such responses, but the implementer should be sure to consider
   the impact of large numbers of malicious stray responses).

   Note that it is not necessary to preserve client transaction state
   upon the detection of unrecoverable transport errors.  Existing
   requirements ensure the TU has been notified, and the new
   requirements in this document ensure that any received retransmitted
   response will be dropped since there will no longer be any matching
   transaction state.

   Figure 3 graphically shows the part of the INVITE client state
   machine that has changed.  The entire new INVITE client state machine
   is shown in Figure 4.












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    +-----------+                        +-----------+
    |           |                        |           |
    |  Calling  |                        |  Calling  |
    |           |----------->+           |           |-----------+
    +-----------+ 2xx        |           +-----------+ 2xx       |
                  2xx to TU  |                         2xx to TU |
                             |                                   |
                             |                                   |
                             |                                   |
                             |                                   |
    +-----------+            |           +-----------+           |
    |           |            |           |           |           |
    |Proceeding |----------->|           |Proceeding |---------->|
    |           | 2xx        |           |           | 2xx       |
    +-----------+ 2xx to TU  |           +-----------+ 2xx to TU |
                             |                                   |
                             |                                   |
                             |                                   |
                             |                                   V
                             |                            +-----------+
                             |                            |           |
                             |                            | Accepted  |
                             |                        +---|           |
                             |              2xx       |   +-----------+
                             |              2xx to TU |     ^    |
                             |                        |     |    |
                             |                        +-----+    |
                             |                                   |
                             |                 +-----------------+
                             |                 | Timer M fires
                             |                 | -
                             |                 V
    +-----------+            |           +-----------+
    |           |            |           |           |
    | Terminated|<-----------+           | Terminated|
    |           |                        |           |
    +-----------+                        +-----------+


     Figure 3: Changes to the INVITE client transaction state machine

7.3.  Proxy Considerations

   This document changes the behaviour of transaction-stateful proxies
   to not forward stray INVITE responses.  When receiving any SIP
   response, a transaction-stateful proxy MUST compare the transaction
   identifier in that response against its existing transaction state
   machines.  The proxy MUST NOT forward the response if there is no



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   matching transaction state machine.


8.  Exact changes to RFC3261

   This section describes exactly the same changes as above, but shows
   exactly which text in RFC3261 is affected.

8.1.  Page 85

   Section 13.3.1.4 paragraph 4 is replaced entirely by

      Once the response has been constructed, it is passed to the INVITE
      server transaction.  In order to ensure reliable end-to-end
      transport of the response, it is necessary to periodically pass
      the response directly to the transport until the ACK arrives.  The
      2xx response is passed to the transport with an interval that
      starts at T1 seconds and doubles for each retransmission until it
      reaches T2 seconds (T1 and T2 are defined in Section 17).
      Response retransmissions cease when an ACK request for the
      response is received.  This is independent of whatever transport
      protocols are used to send the response.

8.2.  Page 107

   Section 16.7 paragraphs 1 and 2 are replaced entirely by

      When a response is received by an element, it first tries to
      locate a client transaction (Section 17.1.3) matching the
      response.  If a transaction is found, the response is handed to
      the client transaction.  If none is found, the element MUST NOT
      forward the response.

8.3.  Page 114

   Section 16.7, part 9, first paragraph.  Replace this sentence

      If the server transaction is no longer available to handle the
      transmission, the element MUST forward the response statelessly by
      sending it to the server transport.

   with

      If the server transaction is no longer available to handle the
      transmission, the response is simply discarded.






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8.4.  Pages 126 through 128

   Section 17.1.1.2.  Replace paragraph 7 (starting "When in either")
   through the end of the section with

      When in either the "Calling" or "Proceeding" states, reception of
      a response with status code from 300-699 MUST cause the client
      transaction to transition to "Completed".  The client transaction
      MUST pass the received response up to the TU, and the client
      transaction MUST generate an ACK request, even if the transport is
      reliable (guidelines for constructing the ACK from the response
      are given in Section 17.1.1.3) and then pass the ACK to the
      transport layer for transmission.  The ACK MUST be sent to the
      same address, port, and transport to which the original request
      was sent.

      The client transaction MUST start timer D when it enters the
      "Completed" state for any reason, with a value of at least 32
      seconds for unreliable transports, and a value of zero seconds for
      reliable transports.  Timer D reflects the amount of time that the
      server transaction can remain in the "Completed" state when
      unreliable transports are used.  This is equal to Timer H in the
      INVITE server transaction, whose default is 64*T1, and is also
      equal to the time a UAS core will wait for an ACK once it sends a
      2xx response.  However, the client transaction does not know the
      value of T1 in use by the server transaction or any downstream UAS
      cores, so an absolute minimum of 32s is used instead of basing
      Timer D on T1.

      Any retransmissions of a response with status code 300-699 that
      are received while in the "Completed" state MUST cause the ACK to
      be re-passed to the transport layer for retransmission, but the
      newly received response MUST NOT be passed up to the TU.

      A retransmission of the response is defined as any response which
      would match the same client transaction based on the rules of
      Section 17.1.3.

      If timer D fires while the client transaction is in the
      "Completed" state, the client transaction MUST move to the
      "Terminated" state.

      When a 2xx response is received while in either the "Calling" or
      "Proceeding" states, the client transaction MUST transition to the
      "Accepted" state, and Timer M MUST be started with a value of
      64*T1.  The 2xx response MUST be passed up to the TU.  The client
      transaction MUST NOT generate an ACK to the 2xx response - its
      handling is delegated to the TU.  A UAC core will send an ACK to



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      the 2xx response using a new transaction.  A proxy core will
      always forward the 2xx response upstream.

      The purpose of the "Accepted" state is to allow the client
      transaction to continue to exist to receive, and pass to the TU,
      any retransmissions of the 2xx response and any additional 2xx
      responses from other branches of the INVITE if it forked
      downstream.  Timer M reflects the amount of time that transaction
      user will wait for such messages.

      Any 2xx responses matching this client transaction that are
      received while in the "Accepted" state MUST be passed up to the
      TU.  The client transaction MUST NOT generate an ACK to the 2xx
      response.  The client transaction takes no further action.

      If timer M fires while the client transaction is in the "Accepted"
      state, the client transaction MUST move to the "Terminated" state.

      The client transaction MUST be destroyed the instant it enters the
      "Terminated" state.

   Replace Figure 5 with





























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                                     |INVITE from TU
                   Timer A fires     |INVITE sent      Timer B fires
                   Reset A,          V                 or Transport Err.
                   INVITE sent +-----------+           inform TU
                     +---------|           |--------------------------+
                     |         |  Calling  |                          |
                     +-------->|           |-----------+              |
    300-699                    +-----------+ 2xx       |              |
    ACK sent                      |  |       2xx to TU |              |
    resp. to TU                   |  |1xx              |              |
    +-----------------------------+  |1xx to TU        |              |
    |                                |                 |              |
    |                1xx             V                 |              |
    |                1xx to TU +-----------+           |              |
    |                +---------|           |           |              |
    |                |         |Proceeding |           |              |
    |                +-------->|           |           |              |
    |                          +-----------+ 2xx       |              |
    |         300-699             |    |     2xx to TU |              |
    |         ACK sent,  +--------+    +---------------+              |
    |         resp. to TU|                             |              |
    |                    |                             |              |
    |                    V                             V              |
    |              +-----------+                   +----------+       |
    +------------->|           |Transport Err.     |          |       |
                   | Completed |Inform TU          | Accepted |       |
                +--|           |-------+           |          |-+     |
        300-699 |  +-----------+       |           +----------+ |     |
        ACK sent|    ^  |              |               |  ^     |     |
                |    |  |              |               |  |     |     |
                +----+  |              |               |  +-----+     |
                        |Timer D fires |  Timer M fires|    2xx       |
                        |-             |             - |    2xx to TU |
                        +--------+     |   +-----------+              |
       NOTE:                     V     V   V                          |
    transitions                 +------------+                        |
    labeled with                |            |                        |
    the event                   | Terminated |<-----------------------+
    over the action             |            |
    to take                     +------------+


                    Figure 4: INVITE client transaction








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8.5.  Pages 134 to 135

   Section 17.2.1 paragraph 4 is replaced with

      If, while in the "Proceeding" state, the TU passes a 2xx response
      to the server transaction, the server transaction MUST pass this
      response to the transport layer for transmission.  It is not
      retransmitted by the server transaction; retransmissions of 2xx
      responses are handled by the TU.  The server transaction MUST then
      transition to the "Accepted" state.

8.6.  Page 136

   Replace Figure 7 with





































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                                      |INVITE
                                      |pass INV to TU
                   INVITE             V send 100 if TU won't in 200ms
                   send response+------------+
                       +--------|            |--------+ 101-199 from TU
                       |        |            |        | send response
                       +------->|            |<-------+
                                | Proceeding |
                                |            |--------+ Transport Err.
                                |            |        | Inform TU
                                |            |<-------+
                                +------------+
                   300-699 from TU |    |2xx from TU
                   send response   |    |send response
                    +--------------+    +------------+
                    |                                |
   INVITE           V          Timer G fires         |
   send response +-----------+ send response         |
        +--------|           |--------+              |
        |        |           |        |              |
        +------->| Completed |<-------+      INVITE  |  Transport Err.
                 |           |               -       |  Inform TU
        +--------|           |----+          +-----+ |  +---+
        |        +-----------+    | ACK      |     | v  |   v
        |          ^   |          | -        |  +------------+
        |          |   |          |          |  |            |---+ ACK
        +----------+   |          |          +->|  Accepted  |   | to TU
        Transport Err. |          |             |            |<--+
        Inform TU      |          V             +------------+
                       |      +-----------+        |  ^     |
                       |      |           |        |  |     |
                       |      | Confirmed |        |  +-----+
                       |      |           |        |  2xx from TU
         Timer H fires |      +-----------+        |  send response
         -             |          |                |
                       |          | Timer I fires  |
                       |          | -              | Timer L fires
                       |          V                | -
                       |        +------------+     |
                       |        |            |<----+
                       +------->| Terminated |
                                |            |
                                +------------+



                    Figure 5: INVITE server transaction




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8.7.  Page 137

   Section 17.2.1 - Replace the last paragraph (starting "Once the
   transaction") with

      The purpose of the "Accepted" state is to absorb retransmissions
      of an accepted INVITE request.  Any such retransmissions are
      absorbed entirely within the server transaction.  They are not
      passed up to the TU since any downstream UAS cores that accepted
      the request have taken responsibility for reliability and will
      already retransmit their 2xx responses if neccessary.

      While in the "Accepted" state, if the TU passes a 2xx response,
      the server transaction MUST pass the response to the transport
      layer for transmission.

      When the INVITE server transaction enters the "Accepted" state,
      Timer L MUST be set to fire in 64*T1 for all transports.  This
      value matches both Timer B in the next upstream client state
      machine (the amount of time the previous hop will wait for a
      response when no provisionals have been sent) and the amount of
      time this (or any downstream) UAS core might be retransmitting the
      2xx while waiting for an ACK.  If an ACK is received while the
      INVITE server transaction is in the "Accepted" state, then the ACK
      must be passed up to the TU.  If Timer L fires while the INVITE
      server transaction is in the "Accepted" state, the transaction
      MUST transition to the "Terminated" state.

      Once the transaction is in the "Terminated" state, it MUST be
      destroyed immediately.

8.8.  Page 141

   Section 17.2.4 - Replace the second paragraph with

      First, the procedures in [4] are followed, which attempt to
      deliver the response to a backup.  If those should all fail, based
      on the definition of failure in [4], the server transaction SHOULD
      inform the TU that a failure has occurred, and MUST remain in the
      current state.

8.9.  Page 144

   Section 18.1.2 - Replace the second paragraph with







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      The client transport uses the matching procedures of Section
      17.1.3 to attempt to match the response to an existing
      transaction.  If there is a match, the response MUST be passed to
      that transaction.  Otherwise, any element other than a stateless
      proxy MUST silently discard the response.

8.10.  Page 146

   Section 18.2.1 - Replace the last paragraph with

      Next, the server transport attempts to match the request to a
      server transaction.  It does so using the matching rules described
      in Section 17.2.3.  If a matching server transaction is found, the
      request is passed to that transaction for processing.  If no match
      is found, the request is passed to the core, which may decide to
      construct a new server transaction for that request.

8.11.  Page 265

   Add to Table 4:

   Timer L  64*T1             Section 17.2.1         Wait time for
                                                     accepted INVITE
                                                     request retransmits

   Timer M  64*T1             Section 17.1.1         Wait time for
                                                     retransmission of
                                                     2xx to INVITE or
                                                     additional 2xx from
                                                     other branches of
                                                     a forked INVITE



9.  IANA Considerations

   None.


10.  Security Considerations

   This document makes two changes to the Session Initiation Protocol to
   address the error discussed in Section 3.  It changes the behavior of
   both the client and server INVITE transaction state machines, and it
   changes the way "stray" responses (those that don't match any
   existing transaction) are handled at transaction stateful elements.

   The changes to the state machines cause elements to hold onto each



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   accepted INVITE transaction state longer (32 seconds) than what was
   specified in RFC 3261.  This will have a direct impact on the amount
   of work an attacker leveraging state exhaustion will have to exert
   against the system.  However, this additional state is necessary to
   achieve correct operation.

   RFC 3261 required SIP proxies to forward any stray 2xx class
   responses to an INVITE request upstream statelessly.  As a result,
   conformant proxies can be forced to forward packets (that look
   sufficiently like SIP responses) to destinations of the sender's
   choosing.  Section 3 discusses some of the malicious behavior this
   enables.  This document reverses the stateless forwarding
   requirement, making it a violation of the specification to forward
   stray responses.

   RFC 3261 defines a "stateless proxy" which forwards requests and
   responses without creating or maintaining any transaction state.  The
   requirements introduced in this document do not change the behavior
   of these elements in any way.  Stateless proxies are inherently
   vulnerable to the abuses discussed in Section 3.  One way operators
   might mitigate this vulnerability is to carefully control which peer
   elements can present traffic to a given stateless proxy.

   The changes introduced by this document are backward-compatible.
   Transaction behavior will be no less correct, and possible more
   correct, when only one peer in a transaction implements these
   changes.  Except for the considerations mentioned earlier in this
   section, introducing elements implementing these changes into
   deployments with RFC 3261 implementations adds no additional security
   concerns.


11.  Acknowledgments

   Pekka Pessi reported the improper handling of INVITE retransmissions.
   Brett Tate performed a careful review uncovering the need for the
   Accepted state and Timer M in the client transaction state machine.
   Jan Kolomaznik noticed that a server transaction should let a TU know
   about transport errors when it attempts to send a 2xx-class response.
   Michael Procter corrected several nits.


12.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,



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              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.


Authors' Addresses

   Robert Sparks
   Tekelec
   17210 Campbell Road
   Suite 250
   Dallas, Texas  75252
   USA

   Email: RjS@nostrum.com


   Theo Zourzouvillys
   Skype
   3rd Floor
   8000 Marina Blvd
   Brisbane, California  84005
   US

   Email: theo@crazygreek.co.uk


























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