[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 draft-sparks-sipcore-invfix

Network Working Group                                          R. Sparks
Internet-Draft                                                   Tekelec
Updates: 3261 (if approved)                                  Jul 3, 2008
Intended status: Standards Track
Expires: January 4, 2009


  Correct transaction handling for 200 responses to Session Initiation
                        Protocol INVITE requests
                       draft-sparks-sip-invfix-02

Status of this Memo

   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 becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   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 January 4, 2009.

Abstract

   This document normatively updates RFC 3261, the Session Initiation
   Protocol (SIP), to address an error in the specified handling of
   success (200 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.





Sparks                   Expires January 4, 2009                [Page 1]

Internet-Draft                   invfix                         Jul 2008


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 Approved . . . . . . . . . . . . . . . . .  4
   6.  The Change . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   7.  Change Details . . . . . . . . . . . . . . . . . . . . . . . .  4
     7.1.  UAS Impacts  . . . . . . . . . . . . . . . . . . . . . . .  5
     7.2.  UAC Impacts  . . . . . . . . . . . . . . . . . . . . . . .  7
     7.3.  Proxy Considerations . . . . . . . . . . . . . . . . . . .  8
   8.  Exact changes to RFC3261 . . . . . . . . . . . . . . . . . . .  9
     8.1.  Page 85  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.2.  Page 107 . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.3.  Page 114 . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.4.  Pages 126 through 128  . . . . . . . . . . . . . . . . . . 10
     8.5.  Pages 134 to 135 . . . . . . . . . . . . . . . . . . . . . 13
     8.6.  Page 136 . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.7.  Page 137 . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.8.  Page 144 . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.9.  Page 146 . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.10. Page 265 . . . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     12.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18




















Sparks                   Expires January 4, 2009                [Page 2]

Internet-Draft                   invfix                         Jul 2008


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], using the process
   defined in [I-D.drage-sip-essential-correction].  The change
   addresses an error in the handling of 200 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
   200s to be processed correctly by the requester.  First, all elements
   are required to immediately destroy any INVITE client transaction
   state upon forwarding a matching 200 OK 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 core").  Second, all proxies are required to statelessly
   forward any 200 OK 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.



Sparks                   Expires January 4, 2009                [Page 3]

Internet-Draft                   invfix                         Jul 2008


   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.


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
   200 OK responses.  It adds a state to the INVITE server state machine
   to absorb retransmissions of the INVITE after a 200 OK response has
   been sent.  It also forbids forwarding stray responses to INVITE
   requests (not just 200 OK responses), which RFC3261 required.


5.  Consequences if Not Approved

   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.


6.  The Change

   An element sending or receiving a 200 OK 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 200 OK and ACK that follow.

   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.


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



Sparks                   Expires January 4, 2009                [Page 4]

Internet-Draft                   invfix                         Jul 2008


   simplify implementation.

7.1.  UAS Impacts

   To allow a UAS 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 UAS 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 UAS's TU issues a 2xx response for this transaction while the
   state machine is in the "Proceeding" state, it MUST transisition 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
   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.

   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 TU will wait to
   receive an ACK for the 2xx it is emitting before considering the
   transaction failed.

   Figure 1 graphically shows the part of the INVITE server state
   machine that has changed.  The entire new INVITE server state machine
   is shown in Figure 4.














Sparks                   Expires January 4, 2009                [Page 5]

Internet-Draft                   invfix                         Jul 2008


      BEFORE                                   AFTER

    +-----------+                         +-----------+
    |           |                         |           |
    | Proceeding|                         | Proceeding|
    |           |                         |           |
    |           |                         |           |
    |           |                         |           |
    |           |                         |           |
    +-----------+                         +-----------+
             |2xx from TU                        |2xx from TU
             |send response                      |send response
             +-------------->+                   +------------>+
                             |                                 |
                             |                                 |
                             |                                 |
                             |                                 |
                             |                   INVITE        |
                             |                   -             |
                             |                   +-----+       |
                             |                   |     |       V
                             |                   |  +------------+
                             |                   |  |            |
                             |                   +->|  Accepted  |
                             |                      |            |
                             |                      +------------+
                             |                       |   ^     |
                             |                    +--+   |     |
                             |                    |      +-----+
                             |                    |  2xx from TU
                             |                    |  send response
                             |                    |
                             |                    | Timer L fires
                             |                    | -
                             |                    |
                             |                    V
    +-----------+            |                  +------------+
    |           |            |                  |            |
    | Terminated|<-----------+                  | Terminated |
    |           |                               |            |
    +-----------+                               +------------+


     Figure 1: Changes to the INVITE server transaction state machine







Sparks                   Expires January 4, 2009                [Page 6]

Internet-Draft                   invfix                         Jul 2008


7.2.  UAC 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).

   Figure 2 graphically shows the part of the INVITE client state
   machine that has changed.  The entire new INVITE client state machine
   is shown in Figure 3.



















Sparks                   Expires January 4, 2009                [Page 7]

Internet-Draft                   invfix                         Jul 2008


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


     Figure 2: Changes to the INVITE client transaction state machine

7.3.  Proxy Considerations

   A direct consequence of the change to the UAC state machine is that a
   transaction-stateful proxy will not foward any stray INVITE
   responses.  When receiving any response SIP response, a transaction-
   stateful proxy MUST compare the transaction identifier in that
   response against its existing transaction state machines.  The proxy



Sparks                   Expires January 4, 2009                [Page 8]

Internet-Draft                   invfix                         Jul 2008


   MUST NOT forward the response if there is no 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 none is found, the element MUST NOT forward the
      response.  If a transaction is found, the response is handed to
      the client transaction.

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.





Sparks                   Expires January 4, 2009                [Page 9]

Internet-Draft                   invfix                         Jul 2008


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 in either the "Calling" or "Proceeding" states, reception of
      a 2xx response MUST cause the client transaction to transition to
      the "Accepted" state, The client MUST set Timer M to 64*T1 and 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 the 2xx



Sparks                   Expires January 4, 2009               [Page 10]

Internet-Draft                   invfix                         Jul 2008


      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





























Sparks                   Expires January 4, 2009               [Page 11]

Internet-Draft                   invfix                         Jul 2008


                                     |INVITE from TU
                   Timer A fires     |INVITE sent      Timer B fired
                   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 3: INVITE client transaction








Sparks                   Expires January 4, 2009               [Page 12]

Internet-Draft                   invfix                         Jul 2008


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





































Sparks                   Expires January 4, 2009               [Page 13]

Internet-Draft                   invfix                         Jul 2008


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



                    Figure 4: INVITE server transaction





Sparks                   Expires January 4, 2009               [Page 14]

Internet-Draft                   invfix                         Jul 2008


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 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 144

   Section 18.1.2 - Replace the second paragraph with

      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, the response MUST be silently
      discarded.

8.9.  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



Sparks                   Expires January 4, 2009               [Page 15]

Internet-Draft                   invfix                         Jul 2008


      construct a new server transaction for that request.

8.10.  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
                                                     retranmission 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.

   The changes to the state machines cause elements to hold onto each
   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 200 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.




Sparks                   Expires January 4, 2009               [Page 16]

Internet-Draft                   invfix                         Jul 2008


11.  Acknowledgments

   Pekka Pessi reported the improper handling of INVITE retransmissions.
   Bret Tate performed a careful review uncovering the need for the
   Accepted state and Timer M in the client transaction state machine.


12.  References

12.1.  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,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

12.2.  Informative References

   [I-D.drage-sip-essential-correction]
              Drage, K., "A Process for Handling Essential Corrections
              to the Session Initiation  Protocol (SIP)",
              draft-drage-sip-essential-correction-02 (work in
              progress), November 2007.


Author's Address

   Robert Sparks
   Tekelec
   17210 Campbell Road
   Suite 250
   Dallas, Texas  75254-4203
   USA

   Email: RjS@nostrum.com













Sparks                   Expires January 4, 2009               [Page 17]

Internet-Draft                   invfix                         Jul 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   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, THE IETF TRUST 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 in BCP 78 and BCP 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 at
   http://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.











Sparks                   Expires January 4, 2009               [Page 18]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/