Correct Transaction Handling for 2xx Responses to Session Initiation Protocol (SIP) INVITE Requests
RFC 6026

Internet Engineering Task Force (IETF) R. Sparks
Request for Comments: 6026 Tekelec
Updates: 3261 T. Zourzouvillys
Category: Standards Track Skype
ISSN: 2070-1721 September 2010
 Correct Transaction Handling for 2xx Responses
 to Session Initiation Protocol (SIP) INVITE Requests
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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6026.
Sparks & Zourzouvillys Standards Track [Page 1]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
Copyright Notice
 Copyright (c) 2010 IETF Trust and the persons identified as the
 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 Simplified BSD License.
Table of Contents
 1. Introduction ....................................................3
 2. Conventions and Definitions .....................................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 RFC 3261 ......................................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 ..................................................18
 8.10. Page 146 .................................................18
 8.11. Page 265 .................................................18
 9. IANA Considerations ............................................18
 10. Security Considerations .......................................19
 11. Acknowledgments ...............................................20
 12. Normative References ..........................................20
Sparks & Zourzouvillys Standards Track [Page 2]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
1. 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.
2. 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].
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 UAs (user agents) and proxies
 (proxies forward the response upstream, the transaction layer at user
 agents forwards the response to its "UA core"). Second, all proxies
 are required to statelessly forward upstream any 2xx class responses
 that do not match an existing transaction, also called stray
 responses. 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 & Zourzouvillys Standards Track [Page 3]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 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 denial-of-service 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 retransmitted INVITE
 requests received after such an error was encountered to be processed
 as a new request instead of being absorbed as a retransmission.
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 an unrecoverable transport error when sending a
 response. It also forbids forwarding stray responses to INVITE
 requests (not just 2xx responses), which RFC 3261 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 a
 new request after an attempt to send a response results in an
 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.
Sparks & Zourzouvillys Standards Track [Page 4]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 An element encountering an unrecoverable transport 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 that 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
 being absorbed by the transaction state machine. 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 RFC 3261.
 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, the state machine MUST transition to the "Accepted" state and
 set Timer L to 64*T1, where T1 is the round-trip time estimate
 defined in Section 17.1.1.1 of [RFC3261].
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 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. RFC 3261 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 associated 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.
 Figures 1 and 2 show the parts of the INVITE server state machine
 that have changed. The entire new INVITE server state machine is
 shown in Figure 5.
Sparks & Zourzouvillys Standards Track [Page 6]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 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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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 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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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
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 that 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 shows the part of the INVITE client state machine that has
 changed. The entire new INVITE client state machine is shown in
 Figure 5.
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 +-----------+ +-----------+
 | | | |
 | 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 behavior 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 matching
 transaction state machine.
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
8. Exact Changes to RFC 3261
 This section describes exactly the same changes as above, but shows
 exactly which text in RFC 3261 is affected. This document
 intentionally does not contain a Figure 4 or Figure 6 so that the
 labels for Figures 5 and 7 are identical to the labels of the figures
 they are replacing in RFC 3261.
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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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
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 32 s 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 that
 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
Sparks & Zourzouvillys Standards Track [Page 12]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 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 the
 transaction user will wait for such messages.
 Any 2xx responses that match this client transaction and 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.
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 Replace Figure 5 with:
 |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 +------------+ |
 are labeled | | |
 with the event | Terminated |<-----------------------+
 over the action | |
 to take. +------------+
 Figure 5: INVITE client transaction
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
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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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 |INVITE
 |pass INV to TU
 INVITE V send 100 if TU won't in 200 ms
 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 7: INVITE server transaction
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
8.7. Page 137
 In 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 necessary.
 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
 In 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.
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
8.9. Page 144
 In 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, any element other than a stateless
 proxy MUST silently discard the response.
8.10. Page 146
 In 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
 IANA has updated the SIP Parameters: Method and Response Codes
 registry as follows:
 OLD:
 Methods Reference
 ------- ---------
 INVITE [RFC3261]
Sparks & Zourzouvillys Standards Track [Page 18]
RFC 6026 Correct Handling for SIP 2xx Responses September 2010
 NEW:
 Methods Reference
 ------- ---------
 INVITE [RFC3261][RFC6026]
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
 accepted INVITE transaction state 32 seconds longer than what was
 specified in RFC 3261. This will have a direct impact on the amount
 of work an attacker that is leveraging state exhaustion will have to
 exert against the system. However, this additional state is
 necessary to achieve correct operation. There is some discussion of
 avoiding state exhaustion and other denial-of-service attacks in RFC
 3261, Section 26.3.2.4.
 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 possibly 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.
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RFC 6026 Correct Handling for SIP 2xx Responses September 2010
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,
 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
Sparks & Zourzouvillys Standards Track [Page 20]