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rfc:rfc6141

Internet Engineering Task Force (IETF) G. Camarillo, Ed. Request for Comments: 6141 C. Holmberg Updates: 3261 Ericsson Category: Standards Track Y. Gao ISSN: 2070-1721 ZTE

                                                            March 2011
           Re-INVITE and Target-Refresh Request Handling
              in the Session Initiation Protocol (SIP)

Abstract

 The procedures for handling SIP re-INVITEs are described in RFC 3261.
 Implementation and deployment experience has uncovered a number of
 issues with the original documentation, and this document provides
 additional procedures that update the original specification to
 address those issues.  In particular, this document defines in which
 situations a UAS (User Agent Server) should generate a success
 response and in which situations a UAS should generate an error
 response to a re-INVITE.  Additionally, this document defines further
 details of procedures related to target-refresh requests.

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/rfc6141.

Camarillo, et al. Standards Track [Page 1] RFC 6141 Re-INVITE Handling in SIP March 2011

Copyright Notice

 Copyright (c) 2011 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.

Camarillo, et al. Standards Track [Page 2] RFC 6141 Re-INVITE Handling in SIP March 2011

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................4
 3. Changing the Session State during a Re-INVITE ...................5
    3.1. Background on Re-INVITE Handling by UASs ...................5
    3.2. Problems with Error Responses and Already Executed Changes .9
    3.3. UAS Behavior ..............................................10
    3.4. UAC Behavior ..............................................11
    3.5. Glare Situations ..........................................11
    3.6. Example of UAS Behavior ...................................12
    3.7. Example of UAC Behavior ...................................14
    3.8. Clarifications on Canceling Re-INVITEs ....................17
 4. Refreshing a Dialog's Targets ..................................17
    4.1. Background and Terminology on a Dialog's Targets ..........17
    4.2. Background on Target-Refresh Requests .....................17
    4.3. Clarification on the Atomicity of Target-Refresh Requests .18
    4.4. UA Updating the Dialog's Local Target in a Request ........19
    4.5. UA Updating the Dialog's Local Target in a Response .......19
    4.6. A Request Updating the Dialog's Remote Target .............19
    4.7. A Response Updating the Dialog's Remote Target ............20
    4.8. Race Conditions and Target Refreshes ......................20
    4.9. Early Dialogs .............................................21
 5. A UA Losing Its Contact ........................................21
    5.1. Background on Re-INVITE Transaction Routing ...............22
    5.2. Problems with UAs Losing Their Contact ....................22
    5.3. UAS Losing Its Contact: UAC Behavior ......................22
    5.4. UAC Losing Its Contact: UAS Behavior ......................23
    5.5. UAC Losing Its Contact: UAC Behavior ......................24
 6. Security Considerations ........................................24
 7. Acknowledgements ...............................................24
 8. References .....................................................25
    8.1. Normative References ......................................25
    8.2. Informative References ....................................25

1. Introduction

 As discussed in Section 14 of RFC 3261 [RFC3261], an INVITE request
 sent within an existing dialog is known as a re-INVITE.  A re-INVITE
 is used to modify session parameters, dialog parameters, or both.
 That is, a single re-INVITE can change both the parameters of its
 associated session (e.g., changing the IP address where a media
 stream is received) and the parameters of its associated dialog
 (e.g., changing the remote target of the dialog).  A re-INVITE can
 change the remote target of a dialog because it is a target refresh
 request, as defined in Section 6 of RFC 3261 [RFC3261].

Camarillo, et al. Standards Track [Page 3] RFC 6141 Re-INVITE Handling in SIP March 2011

 A re-INVITE transaction has an offer/answer [RFC3264] exchange
 associated with it.  The UAC (User Agent Client) generating a given
 re-INVITE can act as the offerer or as the answerer.  A UAC willing
 to act as the offerer includes an offer in the re-INVITE.  The UAS
 (User Agent Server) then provides an answer in a response to the
 re-INVITE.  A UAC willing to act as answerer does not include an
 offer in the re-INVITE.  The UAS then provides an offer in a response
 to the re-INVITE becoming, thus, the offerer.
 Certain transactions within a re-INVITE (e.g., UPDATE [RFC3311]
 transactions) can also have offer/answer exchanges associated to
 them.  A UA (User Agent) can act as the offerer or the answerer in
 any of these transactions regardless of whether the UA was the
 offerer or the answerer in the umbrella re-INVITE transaction.
 There has been some confusion among implementors regarding how a UAS
 should handle re-INVITEs.  In particular, implementors requested
 clarification on which type of response a UAS should generate in
 different situations.  In this document, we clarify these issues.
 Additionally, there has also been some confusion among implementors
 regarding target refresh requests, which include but are not limited
 to re-INVITEs.  In this document, we also clarify the process by
 which remote targets are refreshed.
    Indented passages such as this one are used in this document to
    provide additional information and clarifying text.  They do not
    contain normative protocol behavior.

2. Terminology

 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].
 UA: User Agent.
 UAC: User Agent Client.
 UAS: User Agent Server.
    Note that the terms UAC and UAS are used with respect to an INVITE
    or re-INVITE transaction and do not necessarily reflect the role
    of the UA concerned with respect to any other transaction, such as
    an UPDATE transaction occurring within the INVITE transaction.

Camarillo, et al. Standards Track [Page 4] RFC 6141 Re-INVITE Handling in SIP March 2011

3. Changing the Session State during a Re-INVITE

 The following sub-sections discuss how to change the state of the
 session during a re-INVITE transaction.

3.1. Background on Re-INVITE Handling by UASs

 Eventually, a UAS receiving a re-INVITE will need to generate a
 response to it.  Some re-INVITEs can be responded to immediately
 because their handling does not require user interaction (e.g.,
 changing the IP address where a media stream is received).  The
 handling of other re-INVITEs requires user interaction (e.g., adding
 a video stream to an audio-only session).  Therefore, these
 re-INVITEs cannot be responded to immediately.
 An error response to a re-INVITE has the following semantics.  As
 specified in Section 12.2.2 of RFC 3261 [RFC3261], if a re-INVITE is
 rejected, no state changes are performed.  These state changes
 include state changes associated to the re-INVITE transaction and all
 other transactions within the re-INVITE (this section deals with
 changes to the session state; target refreshes are discussed in
 Section 4.2).  That is, the session state is the same as before the
 re-INVITE was received.  The example in Figure 1 illustrates this
 point.
               UAC                                          UAS
                |                                            |
                |-------------(1) INVITE SDP1--------------->|
                |                                            |
                |<------------(2) 200 OK SDP2----------------|
                |                                            |
                |------------------(3) ACK------------------>|
                |                                            |
                |                                            |
                |-------------(4) INVITE SDP3--------------->|
                |                                            |
                |<-----------------(5) 4xx-------------------|
                |                                            |
                |------------------(6) ACK------------------>|
                |                                            |
                  Figure 1: Rejection of a re-INVITE

Camarillo, et al. Standards Track [Page 5] RFC 6141 Re-INVITE Handling in SIP March 2011

 The UAs perform an offer/answer exchange to establish an audio-only
 session:
       SDP1:
          m=audio 30000 RTP/AVP 0
       SDP2:
          m=audio 31000 RTP/AVP 0
 At a later point, the UAC sends a re-INVITE (4) in order to add a
 video stream to the session.
       SDP3:
          m=audio 30000 RTP/AVP 0
          m=video 30002 RTP/AVP 31
 The UAS is configured to automatically reject video streams.
 Consequently, the UAS returns an error response (5).  At that point,
 the session parameters in use are still those resulting from the
 initial offer/answer exchange, which are described by SDP1 and SDP2.
 That is, the session state is the same as before the re-INVITE was
 received.
 In the previous example, the UAS rejected all the changes requested
 in the re-INVITE by returning an error response.  However, there are
 situations where a UAS wants to accept some but not all the changes
 requested in a re-INVITE.  In these cases, the UAS generates a 200
 (OK) response with a Session Description Protocol (SDP) indicating
 which changes were accepted and which were not.  The example in
 Figure 2 illustrates this point.

Camarillo, et al. Standards Track [Page 6] RFC 6141 Re-INVITE Handling in SIP March 2011

               UAC                                          UAS
                |                                            |
                |-------------(1) INVITE SDP1--------------->|
                |                                            |
                |<------------(2) 200 OK SDP2----------------|
                |                                            |
                |------------------(3) ACK------------------>|
                |                                            |
                |                                            |
                |-------------(4) INVITE SDP3--------------->|
                |                                            |
                |<------------(5) 200 OK SDP4----------------|
                |                                            |
                |------------------(6) ACK------------------>|
                |                                            |
            Figure 2: Automatic rejection of a video stream
 The UAs perform an offer/answer exchange to establish an audio-only
 session:
       SDP1:
          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.1
       SDP2:
          m=audio 31000 RTP/AVP 0
          c=IN IP4 192.0.2.5
 At a later point, the UAC moves to an access that provides a higher
 bandwidth.  Therefore, the UAC sends a re-INVITE (4) in order to
 change the IP address where it receives the audio stream to its new
 IP address and add a video stream to the session.
       SDP3:
          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.2
          m=video 30002 RTP/AVP 31
          c=IN IP4 192.0.2.2
 The UAS is automatically configured to reject video streams.
 However, the UAS needs to accept the change of the audio stream's
 remote IP address.  Consequently, the UAS returns a 200 (OK) response
 and sets the port of the video stream to zero in its SDP.

Camarillo, et al. Standards Track [Page 7] RFC 6141 Re-INVITE Handling in SIP March 2011

       SDP4:
          m=audio 31000 RTP/AVP 0
          c=IN IP4 192.0.2.5
          m=video 0 RTP/AVP 31
 In the previous example, the UAS was configured to automatically
 reject the addition of video streams.  The example in Figure 3
 assumes that the UAS requires its user's input in order to accept or
 reject the addition of a video stream and uses reliable provisional
 responses [RFC3262] (PRACK transactions are not shown for clarity).
               UAC                                          UAS
                |                                            |
                |-------------(1) INVITE SDP1--------------->|
                |                                            |
                |<------------(2) 200 OK SDP2----------------|
                |                                            |
                |------------------(3) ACK------------------>|
                |                                            |
                |                                            |
                |-------------(4) INVITE SDP3--------------->|
                |                                            |
                |<----(5) 183 Session Progress SDP4----------|
                |                                            |
                |                                            |
                |<------------(6) UPDATE SDP5----------------|
                |                                            |
                |-------------(7) 200 OK SDP6--------------->|
                |                                            |
                |<---------------(8) 200 OK------------------|
                |                                            |
                |------------------(9) ACK------------------>|
                |                                            |
       Figure 3: Manual rejection of a video stream by the user
 Everything up to (4) is identical to the previous example.  In (5),
 the UAS accepts the change of the audio stream's remote IP address
 but does not accept the video stream yet (it provides a null IP
 address instead of setting the stream to 'inactive' because inactive
 streams still need to exchange RTP Control Protocol (RTCP) traffic).
       SDP4:
          m=audio 31000 RTP/AVP 0
          c=IN IP4 192.0.2.5
          m=video 31002 RTP/AVP 31
          c=IN IP4 0.0.0.0

Camarillo, et al. Standards Track [Page 8] RFC 6141 Re-INVITE Handling in SIP March 2011

 At a later point, the UAS's user rejects the addition of the video
 stream.  Consequently, the UAS sends an UPDATE request (6) setting
 the port of the video stream to zero in its offer.
       SDP5:
          m=audio 31000 RTP/AVP 0
          c=IN IP4 192.0.2.5
          m=video 0 RTP/AVP 31
          c=IN IP4 0.0.0.0
 The UAC returns a 200 (OK) response (7) to the UPDATE with the
 following answer:
       SDP6:
          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.2
          m=video 0 RTP/AVP 31
 The UAS now returns a 200 (OK) response (8) to the re-INVITE.
 In all the previous examples, the UAC of the re-INVITE transaction
 was the offerer.  Examples with UACs acting as the answerers would be
 similar.

3.2. Problems with Error Responses and Already Executed Changes

 Section 3.1 contains examples on how a UAS rejects all the changes
 requested in a re-INVITE without executing any of them by returning
 an error response (Figure 1), and how a UAS executes some of the
 changes requested in a re-INVITE and rejects some of them by
 returning a 2xx response (Figures 2 and 3).  A UAS can accept and
 reject different sets of changes simultaneously (Figure 2) or at
 different times (Figure 3).
 The scenario that created confusion among implementors consists of a
 UAS that receives a re-INVITE, executes some of the changes requested
 in it, and then wants to reject all those already executed changes
 and revert to the pre-re-INVITE state.  Such a UAS may consider
 returning an error response to the re-INVITE (the message flow would
 be similar to the one in Figure 1), or using an UPDATE request to
 revert to the pre-re-INVITE state and then returning a 2xx response
 to the re-INVITE (the message flow would be similar to the one in
 Figure 3).  This section explains the problems associated with
 returning an error response in these circumstances.  In order to
 avoid these problems, the UAS should use the latter option (UPDATE
 request plus a 2xx response).  Sections 3.3 and 3.4 contain the
 normative statements needed to avoid these problems.

Camarillo, et al. Standards Track [Page 9] RFC 6141 Re-INVITE Handling in SIP March 2011

 The reason for not using an error response to undo already executed
 changes is that an error response to a re-INVITE for which changes
 have already been executed (e.g., as a result of UPDATE transactions
 or reliable provisional responses) is effectively requesting a change
 in the session state.  However, the UAC has no means to reject that
 change if it is unable to execute them.  That is, if the UAC is
 unable to revert to the pre-re-INVITE state, it will not be able to
 communicate this fact to the UAS.

3.3. UAS Behavior

 UASs should only return an error response to a re-INVITE if no
 changes to the session state have been executed since the re-INVITE
 was received.  Such an error response indicates that no changes have
 been executed as a result of the re-INVITE or any other transaction
 within it.
 If any of the changes requested in a re-INVITE or in any transaction
 within it have already been executed, the UAS SHOULD return a 2xx
 response.
 A change to the session state is considered to have been executed if
 an offer/answer without preconditions [RFC4032] for the stream has
 completed successfully or the UA has sent or received media using the
 new parameters.  Connection establishment messages (e.g., TCP SYN),
 connectivity checks (e.g., when using Interactive Connectivity
 Establishment (ICE) [RFC5245]), and any other messages used in the
 process of meeting the preconditions for a stream are not considered
 media.
    Normally, a UA receiving media can easily detect when the new
    parameters for the media stream are used (e.g., media is received
    on a new port).  However, in some scenarios, the UA will have to
    process incoming media packets in order to detect whether they use
    the old or new parameters.
 The successful completion of an offer/answer exchange without
 preconditions indicates that the new parameters for the media stream
 are already considered to be in use.  The successful completion of an
 offer/answer exchange with preconditions means something different.
 The fact that all mandatory preconditions for the stream are met
 indicates that the new parameters for the media stream are ready to
 be used.  However, they will not actually be used until the UAS
 decides to use them.  During a session establishment, the UAS can
 wait before using the media parameters until the callee starts being
 alerted or until the callee accepts the session.  During a session
 modification, the UAS can wait until its user accepts the changes to
 the session.  When dealing with streams where the UAS sends media

Camarillo, et al. Standards Track [Page 10] RFC 6141 Re-INVITE Handling in SIP March 2011

 more or less continuously, the UAC notices that the new parameters
 are in use because the UAC receives media that uses the new
 parameters.  However, this mechanism does not work with other types
 of streams.  Therefore, it is RECOMMENDED that when a UAS decides to
 start using the new parameters for a stream for which all mandatory
 preconditions have been met, the UAS either sends media using the new
 parameters or sends a new offer where the precondition-related
 attributes for the stream have been removed.  As indicated above, the
 successful completion of an offer/answer exchange without
 preconditions indicates that the new parameters for the media stream
 are already considered to be in use.

3.4. UAC Behavior

 A UAC that receives an error response to a re-INVITE that undoes
 already executed changes within the re-INVITE may be facing a legacy
 UAS that does not support this specification (i.e., a UAS that does
 not follow the guidelines in Section 3.3).  There are also certain
 race condition situations that get both user agents out of
 synchronization.  In order to cope with these race condition
 situations, a UAC that receives an error response to a re-INVITE for
 which changes have been already executed SHOULD generate a new
 re-INVITE or UPDATE request in order to make sure that both UAs have
 a common view of the state of the session (the UAC uses the criteria
 in Section 3.3 in order to decide whether or not changes have been
 executed for a particular stream).  The purpose of this new offer/
 answer exchange is to synchronize both UAs, not to request changes
 that the UAS may choose to reject.  Therefore, session parameters in
 the offer/answer exchange SHOULD be as close to those in the
 pre-re-INVITE state as possible.

3.5. Glare Situations

 Section 4 of RFC 3264 [RFC3264] defines glare conditions as a user
 agent receiving an offer after having sent one but before having
 received an answer to it.  That section specifies rules to avoid
 glare situations in most cases.  When, despite following those rules,
 a glare condition occurs (as a result of a race condition), it is
 handled as specified in Sections 14.1 and 14.2 of RFC 3261 [RFC3261].
 The UAS returns a 491 (Request Pending) response and the UAC retries
 the offer after a randomly selected time, which depends on which user
 agent is the owner of the Call-ID of the dialog.  The rules in RFC
 3261 [RFC3261] not only cover collisions between re-INVITEs that
 contain offers, they cover collisions between two re-INVITEs in
 general, even if they do not contain offers.  Sections 5.2 and 5.3 of
 RFC 3311 [RFC3311] extend those rules to also cover collisions
 between an UPDATE request carrying an offer and another message
 (UPDATE, PRACK, or INVITE) also carrying an offer.

Camarillo, et al. Standards Track [Page 11] RFC 6141 Re-INVITE Handling in SIP March 2011

 The rules in RFC 3261 [RFC3261] do not cover collisions between an
 UPDATE request and a non-2xx final response to a re-INVITE.  Since
 both the UPDATE request and the reliable response could be requesting
 changes to the session state, it would not be clear which changes
 would need to be executed first.  However, the procedures discussed
 in Section 3.4 already cover this type of situation.  Therefore,
 there is no need to specify further rules here.

3.6. Example of UAS Behavior

 This section contains an example of a UAS that implements this
 specification using an UPDATE request and a 2xx response to a
 re-INVITE in order to revert to the pre-re-INVITE state.  The example
 shown in Figure 4 assumes that the UAS requires its user's input in
 order to accept or reject the addition of a video stream and uses
 reliable provisional responses [RFC3262] (PRACK transactions are not
 shown for clarity).
               UAC                                          UAS
                |                                            |
                |-------------(1) INVITE SDP1--------------->|
                |                                            |
                |<------------(2) 200 OK SDP2----------------|
                |                                            |
                |------------------(3) ACK------------------>|
                |                                            |
                |                                            |
                |-------------(4) INVITE SDP3--------------->|
                |                                            |
                |<----(5) 183 Session Progress SDP4----------|
                |                                            |
                |-------------(6) UPDATE SDP5--------------->|
                |                                            |
                |<------------(7) 200 OK SDP6----------------|
                |                                            |
                |                                            |
                |<------------(8) UPDATE SDP7----------------|
                |                                            |
                |-------------(9) 200 OK SDP8--------------->|
                |                                            |
                |<--------------(10) 200 OK------------------|
                |                                            |
                |-----------------(11) ACK------------------>|
                |                                            |
           Figure 4: Rejection of a video stream by the user

Camarillo, et al. Standards Track [Page 12] RFC 6141 Re-INVITE Handling in SIP March 2011

 The UAs perform an offer/answer exchange to establish an audio-only
 session:
       SDP1:
          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.1
       SDP2:
          m=audio 31000 RTP/AVP 0
          c=IN IP4 192.0.2.5
 At a later point, the UAC sends a re-INVITE (4) in order to add a new
 codec to the audio stream and to add a video stream to the session.
       SDP3:
          m=audio 30000 RTP/AVP 0 3
          c=IN IP4 192.0.2.1
          m=video 30002 RTP/AVP 31
          c=IN IP4 192.0.2.1
 In (5), the UAS accepts the addition of the audio codec but does not
 accept the video stream yet (it provides a null IP address instead of
 setting the stream to 'inactive' because inactive streams still need
 to exchange RTCP traffic).
       SDP4:
          m=audio 31000 RTP/AVP 0 3
          c=IN IP4 192.0.2.5
          m=video 31002 RTP/AVP 31
          c=IN IP4 0.0.0.0
 At a later point, the UAC sends an UPDATE request (6) to remove the
 original audio codec from the audio stream (the UAC could have also
 used the PRACK to (5) to request this change).
       SDP5:
          m=audio 30000 RTP/AVP 3
          c=IN IP4 192.0.2.1
          m=video 30002 RTP/AVP 31
          c=IN IP4 192.0.2.1
       SDP6:
          m=audio 31000 RTP/AVP 3
          c=IN IP4 192.0.2.5
          m=video 31002 RTP/AVP 31
          c=IN IP4 0.0.0.0

Camarillo, et al. Standards Track [Page 13] RFC 6141 Re-INVITE Handling in SIP March 2011

 Yet, at a later point, the UAS's user rejects the addition of the
 video stream.  Additionally, the UAS decides to revert to the
 original audio codec.  Consequently, the UAS sends an UPDATE request
 (8) setting the port of the video stream to zero and offering the
 original audio codec in its SDP.
       SDP7:
          m=audio 31000 RTP/AVP 0
          c=IN IP4 192.0.2.5
          m=video 0 RTP/AVP 31
          c=IN IP4 0.0.0.0
 The UAC accepts the change in the audio codec in its 200 (OK)
 response (9) to the UPDATE request.
       SDP8:
          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          m=video 0 RTP/AVP 31
          c=IN IP4 192.0.2.1
 The UAS now returns a 200 (OK) response (10) to the re-INVITE.  Note
 that the media state after this 200 (OK) response is the same as the
 pre-re-INVITE media state.

3.7. Example of UAC Behavior

 Figure 5 shows an example of a race condition situation in which the
 UAs end up with different views of the state of the session.

Camarillo, et al. Standards Track [Page 14] RFC 6141 Re-INVITE Handling in SIP March 2011

a:sendrecv                                                  a:sendrecv
v:inactive                                                  v:inactive
           UA1                   Proxy                   UA2
            |                      |                      |
            |----(1) INVITE SDP1-->|                      |
            |                      |----(2) INVITE SDP1-->|
            |                      |                      |
            |                      |<----(3) 183 SDP2-----| a:sendrecv
a:sendrecv  |<----(4) 183 SDP2-----|                      | v:recvonly
v:sendonly  |                      |                      |
            |                      |<------(5) 4xx -------|
            |                      |-------(6) ACK ------>| a:sendrecv
            |           +-(7) 4xx -|                      | v:inactive
            |           |          |<---(8) UPDATE SDP3---|
            |<---(9) UPDATE SDP3---|                      |
            |           |          |                      |
a:sendonly  |---(10) 200 OK SDP4-->|                      |
v:inactive  |           |          |---(11) 200 OK SDP4-->| a:recvonly
            |<-(7) 4xx -+          |                      | v:inactive
a:sendrecv  |------(12) ACK ------>|                      |
v:inactive  |                      |                      |
                     a: status of the audio stream
                     v: status of the video stream
              Figure 5: Message flow with race condition
 The UAs in Figure 5 are involved in a session that, just before the
 message flows in the figures starts, includes a sendrecv audio stream
 and an inactive video stream.  UA1 sends a re-INVITE (1) requesting
 to make the video stream sendrecv.
       SDP1:
          m=audio 20000 RTP/AVP 0
          a=sendrecv
          m=video 20002 RTP/AVP 31
          a=sendrecv
 UA2 is configured to automatically accept incoming video streams but
 to ask for user input before generating an outgoing video stream.
 Therefore, UAS2 makes the video stream recvonly by returning a 183
 (Session Progress) response (2).

Camarillo, et al. Standards Track [Page 15] RFC 6141 Re-INVITE Handling in SIP March 2011

       SDP2:
          m=audio 30000 RTP/AVP 0
          a=sendrecv
          m=video 30002 RTP/AVP 31
          a=recvonly
 When asked for input, UA2's user chooses not to have either incoming
 or outgoing video.  In order to make the video stream inactive, UA2
 returns a 4xx error response (5) to the re-INVITE.  The ACK request
 (6) for this error response is generated by the proxy between both
 user agents.  Note that this error response undoes already executed
 changes.  So, UA2 is a legacy UA that does not support this
 specification.
 The proxy relays the 4xx response (7) towards UA1.  However, the 4xx
 response (7) takes time to arrive to UA1 (e.g., the response may have
 been sent over UDP and the first few retransmissions were lost).  In
 the meantime, UA2's user decides to put the audio stream on hold.
 UA2 sends an UPDATE request (8) making the audio stream recvonly.
 The video stream, which is inactive, is not modified and, thus,
 continues being inactive.
       SDP3:
          m=audio 30000 RTP/AVP 0
          a=recvonly
          m=video 30002 RTP/AVP 31
          a=inactive
 The proxy relays the UPDATE request (9) to UA1.  The UPDATE request
 (9) arrives at UA1 before the 4xx response (7) that had been
 previously sent.  UA1 accepts the changes in the UPDATE request and
 returns a 200 (OK) response (10) to it.
       SDP4:
          m=audio 20000 RTP/AVP 0
          a=sendonly
          m=video 30002 RTP/AVP 31
          a=inactive
 At a later point, the 4xx response (7) finally arrives at UA1.  This
 response makes the session return to its pre-re-INVITE state.
 Therefore, for UA1, the audio stream is sendrecv and the video stream
 is inactive.  However, for UA2, the audio stream is recvonly (the
 video stream is also inactive).

Camarillo, et al. Standards Track [Page 16] RFC 6141 Re-INVITE Handling in SIP March 2011

 After the message flow in Figure 5, following the recommendations in
 this section, when UA1 received an error response (7) that undid
 already executed changes, UA1 would generate an UPDATE request with
 an SDP reflecting the pre-re-INVITE state (i.e., sendrecv audio and
 inactive video).  UA2 could then return a 200 (OK) response to the
 UPDATE request making the audio stream recvonly, which is the state
 UA2's user had requested.  Such an UPDATE transaction would get the
 UAs back into synchronization.

3.8. Clarifications on Canceling Re-INVITEs

 Section 9.2 of RFC 3261 [RFC3261] specifies the behavior of a UAS
 responding to a CANCEL request.  Such a UAS responds to the INVITE
 request with a 487 (Request Terminated) at the SHOULD level.  Per the
 rules specified in Section 3.3, if the INVITE request was a re-INVITE
 and some of its requested changes had already been executed, the UAS
 would return a 2xx response instead.

4. Refreshing a Dialog's Targets

 The following sections discuss how to refresh the targets of a
 dialog.

4.1. Background and Terminology on a Dialog's Targets

 As described in Section 12 of RFC 3261 [RFC3261], a UA involved in a
 dialog keeps a record of the SIP or Session Initiation Protocol
 Secure (SIPS) URI at which it can communicate with a specific
 instance of its peer (this is called the "dialog's remote target URI"
 and is equal to the URI contained in the Contact header of requests
 and responses it receives from the peer).  This document introduces
 the complementary concept of the "dialog's local target URI", defined
 as a UA's record of the SIP or SIPS URI at which the peer can
 communicate with it (equal to the URI contained in the Contact header
 of requests and responses it sends to the peer).  These terms are
 complementary because the "dialog's remote target URI" according to
 one UA is the "dialog's local target URI" according to the other UA,
 and vice versa.

4.2. Background on Target-Refresh Requests

 A target-refresh request is defined as follows in Section 6 of RFC
 3261 [RFC3261]:
    A target-refresh request sent within a dialog is defined as a
    request that can modify the remote target of the dialog.

Camarillo, et al. Standards Track [Page 17] RFC 6141 Re-INVITE Handling in SIP March 2011

 Additionally, 2xx responses to target-refresh requests can also
 update the remote target of the dialog.  As discussed in Section 12.2
 of RFC 3261 [RFC3261], re-INVITEs are target-refresh requests.
 RFC 3261 [RFC3261] specifies the behavior of UASs receiving target-
 refresh requests and of UACs receiving a 2xx response for a target-
 refresh request.
 Section 12.2.2 of RFC 3261 [RFC3261] says:
    When a UAS receives a target refresh request, it MUST replace the
    dialog's remote target URI with the URI from the Contact header
    field in that request, if present.
 Section 12.2.1.2 of RFC 3261 [RFC3261] says:
    When a UAC receives a 2xx response to a target refresh request, it
    MUST replace the dialog's remote target URI with the URI from the
    Contact header field in that response, if present.
 The fact that re-INVITEs can be long-lived transactions and can have
 other transactions within them makes it necessary to revise these
 rules.  Section 4.3 specifies new rules for the handling of target-
 refresh requests.  Note that the new rules apply to any target-
 refresh request, not only to re-INVITEs.

4.3. Clarification on the Atomicity of Target-Refresh Requests

 The local and remote targets of a dialog are special types of state
 information because of their essential role in the exchange of SIP
 messages between UAs in a dialog.  A UA involved in a dialog receives
 the remote target of the dialog from the remote UA.  The UA uses the
 received remote target to send SIP requests to the remote UA.
 The dialog's local target is a piece of state information that is not
 meant to be negotiated.  When a UA changes its local target (i.e.,
 the UA changes its IP address), the UA simply communicates its new
 local target to the remote UA (e.g., the UA communicates its new IP
 address to the remote UA in order to remain reachable by the remote
 UA).  UAs need to follow the behavior specified in Sections 4.4, 4.5,
 4.6, and 4.7 of this specification instead of that specified in RFC
 3261 [RFC3261], which was discussed in Section 4.2.  The new behavior
 regarding target-refresh requests implies that a target-refresh
 request can, in some cases, update the remote target even if the
 request is responded to with a final error response.  This means that
 target-refresh requests are not atomic.

Camarillo, et al. Standards Track [Page 18] RFC 6141 Re-INVITE Handling in SIP March 2011

4.4. UA Updating the Dialog's Local Target in a Request

 In order to update its local target, a UA can send a target-refresh
 request.  If the UA receives an error response to the target-refresh
 request, the remote UA has not updated its remote target.
    This allows UASs to authenticate target-refresh requests (see
    Section 26.2 of RFC 3261 [RFC3261]).
 If the UA receives a reliable provisional response or a 2xx response
 to the target-refresh request, or the UA receives an in-dialog
 request on the new local target, the remote UA has updated its remote
 target.  The UA can consider the target refresh operation completed.
    Even if the target request was a re-INVITE and the final response
    to the re-INVITE was an error response, the UAS would not revert
    to the pre-re-INVITE remote target.
 A UA SHOULD NOT use the same target refresh request to refresh the
 target and to make session changes unless the session changes can be
 trivially accepted by the remote UA (e.g., an IP address change).
 Piggybacking a target refresh with more complicated session changes
 would make it unnecessarily complicated for the remote UA to accept
 the target refresh while rejecting the session changes.  Only in case
 the target refresh request is a re-INVITE and the UAS supports
 reliable provisional response or UPDATE requests, the UAC MAY
 piggyback session changes and a target refresh in the same re-INVITE.

4.5. UA Updating the Dialog's Local Target in a Response

 A UA processing an incoming target refresh request can update its
 local target by returning a reliable provisional response or a 2xx
 response to the target-refresh request.  The response needs to
 contain the updated local target URI in its Contact header field.  On
 sending the response, the UA can consider the target refresh
 operation completed.

4.6. A Request Updating the Dialog's Remote Target

 Behavior of a UA after having received a target-refresh request
 updating the remote target:
 If the UA receives a target-refresh request that has been properly
 authenticated (see Section 26.2 of RFC 3261 [RFC3261]), the UA SHOULD
 generate a reliable provisional response or a 2xx response to the
 target-refresh request.  If generating such responses is not possible
 (e.g., the UA does not support reliable provisional responses and
 needs user input before generating a final response), the UA SHOULD

Camarillo, et al. Standards Track [Page 19] RFC 6141 Re-INVITE Handling in SIP March 2011

 send an in-dialog request to the remote UA using the new remote
 target (if the UA does not need to send a request for other reasons,
 the UAS can send an UPDATE request).  On sending a reliable
 provisional response or a 2xx response to the target-refresh request,
 or a request to the new remote target, the UA MUST replace the
 dialog's remote target URI with the URI from the Contact header field
 in the target-refresh request.
    Reliable provisional responses in SIP are specified in RFC 3262
    [RFC3262].  In this document, reliable provisional responses are
    those that use the mechanism defined in RFC 3262 [RFC3262].  Other
    specifications may define ways to send provisional responses
    reliably using non-SIP mechanisms (e.g., using media-level
    messages to acknowledge the reception of the SIP response).  For
    the purposes of this document, provisional responses using those
    non-SIP mechanisms are considered unreliable responses.  Note that
    non-100 provisional responses are only applicable to INVITE
    transactions [RFC4320].
 If instead of sending a reliable provisional response or a 2xx
 response to the target-refresh request, or a request to the new
 target, the UA generates an error response to the target-refresh
 request, the UA MUST NOT update its dialog's remote target.

4.7. A Response Updating the Dialog's Remote Target

 If a UA receives a reliable provisional response or a 2xx response to
 a target-refresh request, the UA MUST replace the dialog's remote
 target URI with the URI from the Contact header field in that
 response, if present.
 If a UA receives an unreliable provisional response to a target-
 refresh request, the UA MUST NOT refresh the dialog's remote target.

4.8. Race Conditions and Target Refreshes

 SIP provides request ordering by using the Cseq header field.  That
 is, a UA that receives two requests at roughly the same time can know
 which one is newer.  However, SIP does not provide ordering between
 responses and requests.  For example, if a UA receives a 200 (OK)
 response to an UPDATE request and an UPDATE request at roughly the
 same time, the UA cannot know which one was sent last.  Since both
 messages can refresh the remote target, the UA needs to know which
 message was sent last in order to know which remote target needs to
 be used.

Camarillo, et al. Standards Track [Page 20] RFC 6141 Re-INVITE Handling in SIP March 2011

 This document specifies the following rule to avoid the situation
 just described.  If the protocol allows a UA to use a target-refresh
 request at the point in time that the UA wishes to refresh its local
 target, the UA MUST use a target-refresh request instead of a
 response to refresh its local target.  This rule implies that a UA
 only uses a response (i.e., a reliable provisional response or a 2xx
 response to a target-refresh request) to refresh its local target if
 the UA is unable to use a target-refresh request at that point in
 time (e.g., the UAS of an ongoing re-INVITE without support for
 UPDATE).

4.9. Early Dialogs

 The rules given in this section about which messages can refresh the
 target of a dialog also apply to early dialogs created by an initial
 INVITE transaction.  Additionally, as specified in Section 13.2.2.4
 of RFC 3261 [RFC3261], on receiving a 2xx response to the initial
 INVITE, the UAC recomputes the whole route set of the dialog, which
 transitions from the "early" state to the "confirmed" state.
 Section 12.1 of RFC 3261 allows unreliable provisional responses to
 create early dialogs.  However, per the rules given in this section,
 unreliable provisional responses cannot refresh the target of a
 dialog.  Therefore, the UAC of an initial INVITE transaction will not
 perform any target refresh as a result of the reception of an
 unreliable provisional response with an updated Contact value on an
 (already established) early dialog.  Note also that a given UAS can
 establish additional early dialogs, which can have different targets,
 by returning additional unreliable provisional responses with
 different To tags.

5. A UA Losing Its Contact

 The following sections discuss the case where a UA loses its
 transport address during an ongoing re-INVITE transaction.  Such a UA
 will refresh the dialog's local target so that it reflects its new
 transport address.  Note that target refreshes that do not involve
 changes in the UA's transport address are outside of the scope of
 this section.  Also, UAs losing their transport address during a
 non-re-INVITE transaction (e.g., a UA losing its transport address
 right after having sent an UPDATE request before having received a
 response to it) are out of scope as well.
 The rules given in this section are also applicable to initial INVITE
 requests that have established early dialogs.

Camarillo, et al. Standards Track [Page 21] RFC 6141 Re-INVITE Handling in SIP March 2011

5.1. Background on Re-INVITE Transaction Routing

 Re-INVITEs are routed using the dialog's route set, which contains
 all the proxy servers that need to be traversed by requests sent
 within the dialog.  Responses to the re-INVITE are routed using the
 Via entries in the re-INVITE.
 ACK requests for 2xx responses and for non-2xx final responses are
 generated in different ways.  As specified in Sections 14.1 and
 13.2.1 of RFC 3261 [RFC3261], ACK requests for 2xx responses are
 generated by the UAC core and are routed using the dialog's route
 set.  As specified in Section 17.1.1.2 of RFC 3261 [RFC3261], ACK
 requests for non-2xx final responses are generated by the INVITE
 client transaction (i.e., they are generated in a hop-by-hop fashion
 by the proxy servers in the path) and are sent to the same transport
 address as the re-INVITE.

5.2. Problems with UAs Losing Their Contact

 Refreshing the dialog's remote target during a re-INVITE transaction
 (see Section 4.3) presents some issues because of the fact that
 re-INVITE transactions can be long lived.  As described in
 Section 5.1, the way responses to the re-INVITE and ACKs for non-2xx
 final responses are routed is fixed once the re-INVITE is sent.  The
 routing of this messages does not depend on the dialog's route set
 and, thus, target refreshes within an ongoing re-INVITE do not affect
 their routing.  A UA that changes its location (i.e., performs a
 target refresh) but is still reachable at its old location will be
 able to receive those messages (which will be sent to the old
 location).  However, a UA that cannot be reachable at its old
 location any longer will not be able to receive them.
 The following sections describe the errors UAs face when they lose
 their transport address during a re-INVITE.  On detecting some of
 these errors, UAs following the rules specified in RFC 3261 [RFC3261]
 will terminate the dialog.  When the dialog is terminated, the only
 option for the UAs is to establish a new dialog.  The following
 sections change the requirements RFC 3261 [RFC3261] places on UAs
 when certain errors occur so that the UAs can recover from those
 errors.  In short, the UAs generate a new re-INVITE transaction to
 synchronize both UAs.  Note that there are existing UA
 implementations deployed that already implement this behavior.

5.3. UAS Losing Its Contact: UAC Behavior

 When a UAS that moves to a new contact and loses its old contact
 generates a non-2xx final response to the re-INVITE, it will not be
 able to receive the ACK request.  The entity receiving the response

Camarillo, et al. Standards Track [Page 22] RFC 6141 Re-INVITE Handling in SIP March 2011

 and, thus, generating the ACK request will either get a transport
 error or a timeout error, which, as described in Section 8.1.3.1 of
 RFC 3261 [RFC3261], will be treated as a 503 (Service Unavailable)
 response and as a 408 (Request Timeout) response, respectively.  If
 the sender of the ACK request is a proxy server, it will typically
 ignore this error.  If the sender of the ACK request is the UAC,
 according to Section 12.2.1.2 of RFC 3261 [RFC3261], it is supposed
 to (at the SHOULD level) terminate the dialog by sending a BYE
 request.  However, because of the special properties of ACK requests
 for non-2xx final responses, most existing UACs do not terminate the
 dialog when ACK request fails, which is fortunate.
 A UAC that accepts a target refresh within a re-INVITE MUST ignore
 transport and timeout errors when generating an ACK request for a
 non-2xx final response.  Additionally, UAC SHOULD generate a new
 re-INVITE in order to make sure that both UAs have a common view of
 the state of the session.
    It is possible that the errors ignored by the UAC were not related
    to the target refresh operation.  If that was the case, the second
    re-INVITE would fail and the UAC would terminate the dialog
    because, per the rules above, UACs only ignore errors when they
    accept a target refresh within the re-INVITE.

5.4. UAC Losing Its Contact: UAS Behavior

 When a UAC moves to a new contact and loses its old contact, it will
 not be able to receive responses to the re-INVITE.  Consequently, it
 will never generate an ACK request.
 As described in Section 16.9 of RFC 3261 [RFC3261], a proxy server
 that gets an error when forwarding a response does not take any
 measures.  Consequently, proxy servers relaying responses will
 effectively ignore the error.
 If there are no proxy servers in the dialog's route set, the UAS will
 get an error when sending a non-2xx final response.  The UAS core
 will be notified of the transaction failure, as described in Section
 17.2.1 of RFC 3261 [RFC3261].  Most existing UASs do not terminate
 the dialog on encountering this failure, which is fortunate.
 Regardless of the presence or absence of proxy servers in the
 dialog's route set, a UAS generating a 2xx response to the re-INVITE
 will never receive an ACK request for it.  According to Section 14.2
 of RFC 3261 [RFC3261], such a UAS is supposed to (at the "should"
 level) terminate the dialog by sending a BYE request.

Camarillo, et al. Standards Track [Page 23] RFC 6141 Re-INVITE Handling in SIP March 2011

 A UAS that accepts a target refresh within a re-INVITE and never
 receives an ACK request after having sent a final response to the
 re-INVITE SHOULD NOT terminate the dialog if the UA has received a
 new re-INVITE with a higher CSeq sequence number than the original
 one.

5.5. UAC Losing Its Contact: UAC Behavior

 When a UAC moves to a new contact and loses its old contact, it will
 not be able to receive responses to the re-INVITE.  Consequently, it
 will never generate an ACK request.
 Such a UAC SHOULD generate a CANCEL request to cancel the re-INVITE
 and cause the INVITE client transaction corresponding to the
 re-INVITE to enter the "Terminated" state.  The UAC SHOULD also send
 a new re-INVITE in order to make sure that both UAs have a common
 view of the state of the session.
    Per Section 14.2 of RFC 3261 [RFC3261], the UAS will accept new
    incoming re-INVITEs as soon as it has generated a final response
    to the previous INVITE request, which had a lower CSeq sequence
    number.

6. Security Considerations

 This document does not introduce any new security issue.  It just
 clarifies how certain transactions should be handled in SIP.
 Security issues related to re-INVITEs and UPDATE requests are
 discussed in RFC 3261 [RFC3261] and RFC 3311 [RFC3311].
 In particular, in order not to reduce the security level for a given
 session, re-INVITEs and UPDATE requests SHOULD be secured using a
 mechanism equivalent to or stronger than the initial INVITE request
 that created the session.  For example, if the initial INVITE request
 was end-to-end integrity protected or encrypted, subsequent
 re-INVITEs and UPDATE requests should also be so.

7. Acknowledgements

 Paul Kyzivat provided useful ideas on the topics discussed in this
 document.

Camarillo, et al. Standards Track [Page 24] RFC 6141 Re-INVITE Handling in SIP March 2011

8. References

8.1. 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.
 [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
            Provisional Responses in Session Initiation Protocol
            (SIP)", RFC 3262, June 2002.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            June 2002.
 [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
            UPDATE Method", RFC 3311, October 2002.
 [RFC4032]  Camarillo, G. and P. Kyzivat, "Update to the Session
            Initiation Protocol (SIP) Preconditions Framework",
            RFC 4032, March 2005.

8.2. Informative References

 [RFC4320]  Sparks, R., "Actions Addressing Identified Issues with the
            Session Initiation Protocol's (SIP) Non-INVITE
            Transaction", RFC 4320, January 2006.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245,
            April 2010.

Camarillo, et al. Standards Track [Page 25] RFC 6141 Re-INVITE Handling in SIP March 2011

Authors' Addresses

 Gonzalo Camarillo (editor)
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: Gonzalo.Camarillo@ericsson.com
 Christer Holmberg
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: Christer.Holmberg@ericsson.com
 Yang Gao
 ZTE
 China
 EMail: gao.yang2@zte.com.cn

Camarillo, et al. Standards Track [Page 26]

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