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Versions: (draft-sawada-sipping-sip-offeranswer) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 6337

    SIPPING Working Group                                       T. Sawada
    Internet Draft                                        KDDI Corporation
    Expires: November 2007                                     P. Kyzivat
                                                       Cisco Systems, Inc.
                                                              May 28, 2007
    
    
           SIP (Session Initiation Protocol) Usage of Offer/Answer Model
                     draft-ietf-sipping-sip-offeranswer-01.txt
    
    
    Status of this Memo
    
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       This Internet-Draft will expire on November 28, 2007.
    
    
    Abstract
    
       SIP utilizes offer/answer model to establish and update multimedia
       sessions. The descriptions on how to use offer/answer in SIP are
       dispersed in the multiple RFCs. This document summarizes all the
       current usage of offer/answer model in SIP communication.
    
    Table of Contents
    
    
       1. Summary of SIP usage of Offer/Answer Model...................2
    
    

    
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          1.1. Offer/Answer Exchange Pairs in SIP Messages.............3
          1.2. Rejection against an Offer..............................4
          1.3. Session Description which is not Offer nor Answer........5
       2. Detailed Discussion on Offer/Answer Model for SIP............5
          2.1. Offer/Answer for INVITE method with 100rel extension.....5
             2.1.1. INVITE Request with SDP............................6
             2.1.2. INVITE request without SDP.........................8
          2.2. Offer/Answer Exchange in Early Dialog...................9
          2.3. Offer/Answer Exchange in Established Dialog.............9
       3. Exceptional Case Handling...................................10
          3.1. Message Crossing Case Handling.........................10
          3.2. Glare Case Handling....................................11
       4. Content of Offers and Answers...............................12
          4.1. General Principle for Constructing Offers and Answers...12
          4.2. Choice of Media Types and Formats to Include and Exclude13
             4.2.1. Sending Initial INVITE with Offer.................13
             4.2.2. Responding with Offer when Initial INVITE has no Offer13
             4.2.3. Answering Initial INVITE with Offer...............14
             4.2.4. Answering when Initial INVITE had no Offer.........14
             4.2.5. Subsequent Offers and Answers.....................14
          4.3. Hold and Resume of media...............................15
       5. Remaining Issues or Best Practices on Offer/Answer..........16
          5.1. Rejecting PRACK Offer..................................16
          5.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
          Transaction................................................17
       6. Add New Offer/Answer Usage in SIP...........................18
          6.1. Explicit Usage........................................18
          6.2. Rejection of an Offer..................................19
          6.3. Backward Compatibility.................................19
          6.4. Exceptional Case Handling..............................19
       7. Security Considerations.....................................19
       8. References.................................................19
          8.1. Normative References...................................19
          8.2. Informative References.................................19
       Author's Addresses............................................20
       Full Copyright Statement.......................................20
       Intellectual Property Statement................................20
       Acknowledgment................................................21
    
    1. Summary of SIP usage of Offer/Answer Model
    
       The offer/answer model itself is independent from the higher layer
       application protocols which utilize it. SIP is one of the
       applications using offer/answer model. In RFC 3264 [3], which defines
       the offer/answer model, which SIP message should convey an offer or
       an answer is not defined. This should be defined in the SIP core and
       extensions RFCs.
    
    
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       In theory, any SIP message can include a session description in its
       body. But a session description in a SIP message is not necessarily
       an offer or an answer. Only the session description that conforms to
       the rules described in the standard track RFCs can be interpreted as
       an offer or an answer. The rules for how to handle the offer/answer
       model are currently defined in several RFCs.
    
       The offer/answer model defines the update of sessions. In SIP, dialog
       is used to match the offer/answer exchange to the session which is to
       be updated with it. In other words, only the offer/answer exchange in
       the SIP dialog can update the session which is managed with it.
    
    1.1. Offer/Answer Exchange Pairs in SIP Messages
    
       Currently, the rules on offer/answer model are defined in RFC 3261
       [1], RFC 3262 [2] and RFC 3311 [4]. In these RFCs, only the six
       patterns shown in Table 1 are defined for exchanging an offer and an
       answer with SIP messages.
    
       Note that an offer/answer exchange initiated by an INVITE request
       must follow exactly one of the patterns 1, 2, 3, 4. Only one of them,
       one for each dialog if multiple dialogs are created, must occur in an
       INVITE 3-way handshake process. Pattern 2 and pattern 4 can occur
       only when the INVITE request does not include an offer. 'The first
       reliable non-failure message' must have an offer if there is no offer
       in the INVITE request. This means that UA which receives the INVITE
       request without an offer must include an offer in the first reliable
       response with 100rel extension. If no reliable provisional response
       has been sent, the UAS must include an offer when sending 2xx
       response.
    
       In pattern 3, the first reliable provisional response may or may not
       have an answer. When a reliable provisional response contains a
       session description, and is the first to do so, then that session
       description is the answer to the offer in the INVITE request.
    
       In pattern 5, a PRACK request can contain an offer only if the
       reliable response which it acknowledges contains an answer in the
       previous offer/answer exchange.
    
    
    
    
    
    
    
    
    
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              Offer                Answer             RFC    Ini Est Early
       -------------------------------------------------------------------
       1. INVITE Req.          2xx INVITE Resp.     RFC 3261  O   O    X
       2. 2xx INVITE Resp.     ACK Req.             RFC 3261  O   O    X
       3. INVITE Req.          1xx-rel INVITE Resp. RFC 3262  O   O    X
       4. 1xx-rel INVITE Resp. PRACK Req.           RFC 3262  O   O    X
       5. PRACK Req.           200 PRACK Resp.      RFC 3262  X   O    O
       6. UPDATE Req.          2xx UPDATE Resp.     RFC 3311  X   O    O
    
       Table 1. Summary of SIP Usage of Offer/Answer Model
    
       In Table 1, '1xx-rel' corresponds to the reliable provisional
       response which contains the 100rel option defined in RFC 3262 [2].
    
       'Ini' column shows the ability to exchange the offer/answer to
       initiate the session. 'O' indicates that the pattern can be used in
       the initial offer/answer exchange, while 'X' indicates that it can
       not. Only the initial INVITE transaction can be used to exchange the
       offer/answer to establish multimedia session.
    
       'Est' column shows the ability to update the established session.
    
       'Early' column shows the ability to be used to modify the established
       session in an early dialog. There are two ways to exchange a
       subsequent offer/answer in an early dialog.
    
    1.2. Rejection against an Offer
    
       How to reject an offer when it can not be accepted is not so clear
       and some methods can not allow explicit rejection against an offer.
       Corresponding to the patterns in Table 1, how to reject an offer is
       shown in Table 2.
    
       When a UA receives an INVITE request with an offer which it can not
       accept, it should respond with a 488 response, preferably with
       Warning header field indicating the reason of the rejection unless
       another response code is more appropriate to reject it. (Pattern 1
       and Pattern 3)
    
       When a UA receives an UPDATE request with an offer which it can not
       accept, it should respond with a 488 response preferably with Warning
       header field indicating the reason of the rejection, unless another
       response code is more appropriate to reject it. (Pattern 6)
    
       When a UA receives a PRACK request with an offer which it can not
       accept, it may respond with a 200 response with a syntactically
       correct session description followed by an UPDATE request possibly to
       rearrange the session parameters if both ends support UPDATE method.
    
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       A UA may simply give up continuing the dialog and send an error
       response to the INVITE request. (Pattern 5)
    
       When a UA receives a response with an offer which it can not accept,
       a UA does not have a way to reject it explicitly. Therefore, a UA
       should respond to the offer with the correct session description and
       rearrange the session parameters by initiating a new offer/answer
       exchange, or just terminate the session. (Pattern 2 and Pattern 4)
       When initiating a new offer/answer, a UA should take care not to
       cause a never-ending offer/answer loop.
    
          Offer                Rejection
       -----------------------------------------------------
       1. INVITE Req.          488 INVITE Response
       2. 2xx INVITE Resp.     Answer in ACK Req. followed by new offer
       3. INVITE Req.          488 INVITE Response (same as Pattern 1.)
       4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer
       5. PRACK Req. (*)       200 PRACK Resp. followed by new offer
       6. UPDATE Req.          488 UPDATE Response
    
       Table 2. Rejection against an Offer
    
       (*) UA should only use PRACK to send an offer when it has strong
       reasons to assume the receiver will accept.
    
    1.3. Session Description which is not Offer nor Answer
    
       As previously stated, a session description in a SIP message is not
       necessarily an offer or an answer. For example, SIP can use a session
       description to describe capabilities apart from offer/answer exchange.
       Examples of this are 200 OK responses for OPTIONS and 488 responses
       for INVITE.
    
    2. Detailed Discussion on Offer/Answer Model for SIP
    
    2.1. Offer/Answer for INVITE method with 100rel extension
    
       The INVITE method provides the basic procedure for offer/answer
       exchange in SIP. Without the 100rel option, the rules are simple as
       described in RFC 3261 [1]. If an INVITE request includes a session
       description, pattern 1 is applied and if an INVITE request does not
       include a session description, pattern 2 is applied.
    
       With 100rel, pattern 3 and pattern 4 are added and this makes the
       rules complicated. An INVITE request may cause multiple responses.
       Note that even if both UAs support the 100rel extension, not all the
       provisional responses may be sent reliably. Note also that a reliable
       provisional response is allowed without a session description if the
    
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       UAS does not wish to send the answer yet. An unreliable provisional
       response may include a session description in the body if the UAS has
       not sent a reliable response, but its session description is neither
       an offer nor an answer. All the session descriptions in the
       unreliable responses to the INVITE request must be identical to the
       answer which is included in the reliable response. Session
       descriptions in an unreliable response that precedes a reliable
       response can be considered a "preview" of the session description
       that will be coming, and hence may be treated like an offer or an
       answer until the actual one arrives.
    
    2.1.1. INVITE Request with SDP
    
       When a UAC includes an SDP body in the INVITE request as an offer, it
       expects the answer to be received with one of the reliable responses.
       Other than that, no offer/answer exchanges can occur in the INVITE 3-
       way handshake process.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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        UAC                   UAS
         | F1  INVITE (SDP)    | <- The offer in offer/answer model
         |-------------------->|
         | F2     1xx (SDP)    | <- The SDP is not an official answer but
         |<--------------------|    UAC acts as if it receives the answer.
         |                     | ^
         | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer
         |<--------------------| |   SDP.
         | F4   PRACK (no SDP) | |
         |-------------------->| | UAC must not send a new offer.
         | F5 2xx PRA (no SDP) | |
         |<--------------------| v
         |                     |
         | F6 1xx-rel (SDP)    | <- The answer in offer/ answer model
         |<--------------------| -
         | F7   PRACK          | | UAC can send a new offer in a PRACK
         |-------------------->| | request to acknowledge F6.
         | F8 2xx PRA          | | After F7 UAC and UAS can send a new offer
         |<--------------------| v in an UPDATE request.
         |                     |
         | F9 1xx-rel          | <- SDP should not be included in the
         |<--------------------|    subsequent 1xx-rel once offer/answer
         | F10  PRACK          |    has been completed.
         |-------------------->|
         | F11 2xx PRA         |
         |<--------------------|
         |                     |
         | F12 2xx INV         | <- SDP should not be included in the final
         |<--------------------|    response once offer/answer has been
         | F13    ACK          |    completed.
         |-------------------->|
    
             Figure 1 Example of Offer/Answer with 100rel Extension (1)
    
       For example, in Figure 1, only the SDP in F6 is the answer. The SDP
       in the non-reliable response (F2) is the preview of the answer and
       must be the same as the answer in F6, but is not officially the
       answer. Receiving F2, UAC should act as if it receives the answer.
       However, offer/answer exchange is not completed yet and UAC must not
       send a new offer until it receives the same SDP in the first reliable
       response, which is the real answer. After sending the SDP in F6, UAS
       must prepare to receive new offer from UAC with an UPDATE request or
       a PRACK request.
    
       UAS does not include an SDP in the responses F9 and F12. However, UAC
       should prepare to receive SDP bodies in F9 and/or F12, and just
       ignore them for the case that the peer does not conform to the
       recommended implementation.
    
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    2.1.2. INVITE request without SDP
    
       When UAC does not include an SDP body in the INVITE request, it
       expects the offer to be received with the first reliable response.
       UAC will send the answer in the request to acknowledge the response,
       i.e. PRACK or ACK request for the reliable response. Other than that,
       no offer/answer exchanges can occur in the INVITE 3-way handshake
       process.
    
       For example, in Figure 2, only the SDP in F3 is the offer. The SDP in
       the non-reliable response (F2) is the preview of the offer and must
       be the same as the offer in F3, but is not officially the offer.
       Receiving F2, UAC can act as if it receives the offer. However, the
       official offer is not received until it receives the first reliable
       response. The first reliable response (F3) must include an SDP as an
       offer.
    
       UAS should not include SDP in the responses F6 and F9. However, UAC
       should prepare to receive SDP bodies in F6 and/or F9, and just ignore
       them for the case that the peer does not conform to the recommended
       implementation.
    
        UAC                   UAS
         | F1  INVITE (no SDP) |
         |-------------------->|
         | F2     1xx (SDP)    | <- SDP may be included but it is not the
         |<--------------------|    offer. UAC may act as if it receives
         |                     |    the offer.
         | F3 1xx-rel (SDP)    | <- The first 1xx-rel must contain an SDP
         |<--------------------|    as the offer.
         | F4   PRACK (SDP)    | <- A PRACK request to the first 1xx-rel
         |-------------------->|    must contain an SDP as the answer.
         | F5 2xx PRA (no SDP) | -
         |<--------------------| |
         |                     | |
         | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not
         |<--------------------| |  contain an SDP.
         | F7   PRACK          | |
         |-------------------->| | UAC can send a new offer in an UPDATE
         | F8 2xx PRA          | | request after F4.
         |<--------------------| v
         |                     |
         | F9 2xx INV (no SDP) | <- The final response should not
         |<--------------------|    contain an SDP.
         | F10    ACK          |
         |-------------------->|
    
             Figure 2 Example of Offer/Answer with 100rel Extension (2)
    
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    2.2. Offer/Answer Exchange in Early Dialog
    
       When both UAs support the 100rel extension, they can update the
       session in the early dialog once the first offer/answer exchange has
       been completed.
    
       From UA sending an INVITE request:
    
       UA can send an UPDATE request with a new offer if both ends support
       the UPDATE method. Support for the UPDATE method must be declared in
       an Allow header in some prior messages in the dialog.
    
       UA can send a PRACK request with a new offer only when acknowledging
       the reliable provisional response with the answer to the offer in the
       INVITE request. Compared to using the UPDATE method, using PRACK can
       save messages to be exchanged between the UAs. However, as a PRACK
       request should not be rejected, UA is recommended to send a PRACK
       request only when it has strong reasons to assume the receiver will
       accept it. For example, the procedure used in precondition extension
       [5] is a case where a PRACK request should be used for updating the
       session status in the early dialog.
    
       From UA receiving an INVITE request:
    
       UA can send an UPDATE request with a new offer if both ends support
       UPDATE method. UAS can not send a new offer in the reliable
       provisional response. So the UPDATE method is the only method for UAS
       to update the early session.
    
    2.3. Offer/Answer Exchange in Established Dialog
    
       The re-INVITE and UPDATE methods can be used in the established
       dialog to update the session.
    
       The UPDATE method is simpler and can save at least one message
       compared with INVITE method. But both ends must support the UPDATE
       method for it to be used.
    
       The INVITE method needs at least three messages to complete but no
       extensions are needed. Additionally, the INVITE method allows the
       peer to take time to decide whether it will accept a session update
       or not by sending provisional responses. That is, re-INVITE allows
       the UAS to interact with the user at the peer, while UPDATE needs to
       be answered automatically by the UAS. It is noted that re-INVITE
       should be answered immediately unless such a user interaction is
       needed. Otherwise, some 3pcc flows will break.
    
    
    
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    3. Exceptional Case Handling
    
       In RFC 3264 [3], the following restrictions are defined with regard
       to sending a new offer.
    
          "It MUST NOT generate a new offer if it has received an offer
          which it has not yet answered or rejected. It MUST NOT generate a
          new offer if it has generated a prior offer for which it has not
          yet received an answer or a rejection."
    
       Assuming that the above rules are guaranteed, there seems to be two
       possible 'exceptional' cases to be considered in SIP offer/answer
       usage, which are the 'message crossing' case and the 'glare' case.
       One of the reasons why the usage of a SIP method to exchange
       offer/answer needs to be carefully restricted in the RFCs is to make
       sure that UA can detect and handle appropriately the 'exceptional'
       cases to avoid the confusion.
    
    3.1. Message Crossing Case Handling
    
       When message packets are crossed in the transport network, an offer
       may be received before the answer for the previous offer/answer
       exchange as described in Figure 3. In such a case, UA A must detect
       the session description of the offer2 is not the answer to the offer1.
    
        A                  B
        |offer1            |
        |----------------->|
        |           answer1|
        |<------\  /-------|
        |        \/        |
        |        /\  offer2|
        |<------/  \-------|
    
                          Figure 3 Message Crossing Case
    
       When offer2 is in an UPDATE request or a re-INVITE request, a session
       description can never be the answer. Then UA A must reject the
       message including offer2 with a 491 response with Retry-After header
       field.
    
       When offer2 is in a PRACK request, that is, when a PRACK request to
       acknowledge the reliable provisional response with an answer to the
       offer in the INVITE request contains a session description, UA A
       knows it is an offer. As a PRACK request should not be rejected, UA A
       is recommended to wait for the answer1 before sending a PRACK
       response with the answer to the offer2. Note that if UA A does not
       send a new offer until the reliable provisional response with an
    
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       answer to the offer in the INVITE request is acknowledged with a
       PRACK request, this case never happens. Therefore, to make
       implementations simple, a UA acting as a UAS for an INVITE
       transaction is recommended not to send an UPDATE request with an
       offer until the reliable response with an answer to the offer in the
       INVITE request is acknowledged with a PRACK request.
    
       When offer2 is in a reliable provisional response or a successful
       final response, UA A knows it is not the answer to the offer1. For a
       reliable response to an initial INVITE request, this case never
       happens. For a reliable response to a re-INVITE request, UA A can
       detect the offer2 is not the answer1. In this case, UA A can not
       reject offer2 in a reliable response, it is recommended to wait for
       answer1 before sending a PRACK request with the answer to offer2.
       Note that if UA A does not send an INVITE request without session
       description if it has sent the offer which has not yet received the
       answer to it, this case never happens.
    
    3.2. Glare Case Handling
    
       When both ends in a dialog send a new offer at nearly the same time,
       UA may receive a new offer before it receives the answer to the offer
       it sent as described in Figure 4. This case is called a 'glare' case
       in general.
    
        A                  B
        |offer1      offer2|
        |-------\  /-------|
        |        \/        |
        |        /\        |
        |<------/  \------>|
    
                                Figure 4 Glare Case
    
       When offer2 is in an UPDATE request or (re-)INVITE request, it must
       be rejected with a 491 response.
    
       When offer2 is in a PRACK request, it may be accepted with 200 or may
       be rejected with a 491 response. A 491 response may be adequate for
       offer/answer model but it may delay the completion of the reliable
       response transfer mechanism or, in worst case, may result in the
       failure to complete the SIP transaction because there is no clear
       retry rule when a PRACK request is rejected with a 491 response. To
       avoid this glare condition, UA is recommended not to send an offer,
       which currently must be in an UPDATE request, if it has generated the
       reliable provisional response with the answer to the offer in the
       INVITE request which is not acknowledged with a PRACK request.
    
    
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       To avoid glare condition for offer2 in the response, UA A is
       recommended not to send a new offer if it has sent a (re)INVITE
       request without session description and has not received the reliable
       response which includes the offer.
    
    4. Content of Offers and Answers
    
       While RFCs 3264[3] and 3312[5] give some guidance, questions remain
       about exactly what should be included in an offer or answer. This is
       especially a problem when the common "hold" feature has been
       activated, and when there is the potential for a multimedia call.
    
       Details of behavior depend on the capabilities and state of the User
       Agent. The kinds of recommendations that can be made are limited by
       the model of device capabilities and state that is presumed to exist.
    
       This section focuses on a few key aspects of offers and answers that
       have been identified as troublesome, and will consider other aspects
       to be out of scope. This section considers:
    
       - choice of supported media types and formats to include and exclude
    
       - hold and resume of media
    
       The following are out of scope for this document:
    
       - NAT traversal and ICE
    
       - specific codecs and their parameters
    
       - the negotiation of secure media streams
    
       - grouping of media streams
    
       - preconditions
    
    4.1. General Principle for Constructing Offers and Answers
    
       A UA should send an offer that indicates what it, and its user, are
       interested in using/doing at that time, without regard for what the
       other party in the call may have indicated previously.
    
       A UA should send an answer that includes as close an approximation to
       what the UA and its user are interested in doing at that time, while
       remaining consistent with the offer/answer rules of RFC 3264[3] and
       other RFCs.
    
    
    
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            NOTE: "at that time" is important. The device may permit the
            user to configure which supported media are to be used by
            default.
    
       Some UAs may not have an understanding of what it is interested in
       doing at a particular time. (E.g. a gateway to a different protocol.)
       In this case the UA could delegate the decision to the other protocol,
       if the situation can be represented. Or it can make some assumptions.
       This may result in a limitation in what works through the gateway.
    
    4.2. Choice of Media Types and Formats to Include and Exclude
    
    4.2.1. Sending Initial INVITE with Offer
    
       When a UAC sends an initial INVITE with an offer, it has complete
       freedom to choose which media type(s) and media format(s) (payload
       types in the case of RTP) it should include in the offer.
    
       The media types may be all or a subset of the media the UAC is
       capable of supporting, with the particular subset being determined by
       the design and configuration [6] of the UAC combined with input from
       the user interface of the UAC.
    
       The media formats may be all or a subset of the media formats the UAC
       is capable of supporting for the corresponding media type, with the
       particular subset being determined by the design and configuration
       [6] of the UAC combined with input from the user interface of the UAC.
    
       Including all supported media formats will maximize the possibility
       that the other party will have a supported format in common. But
       including many can result in an unacceptably large SDP body.
    
    4.2.2. Responding with Offer when Initial INVITE has no Offer
    
       When a UAS has received an initial INVITE without an offer, it must
       include an offer in the first reliable response to the INVITE. It has
       largely the same options as when sending an initial INVITE with an
       offer, but there are some differences. The choice may be governed by
       both static (default) selections of media types as well as dynamic
       selections made by a user via interaction with the device while it is
       alerting.
    
            NOTE: The offer may be sent in a provisional response, before
            the user of the device has been alerted and had an opportunity
            to select media options for the call. In this case the UAS
            cannot include any call-specific options from the user of the
            device. It there is a possibility that the user of the device
            may wish to change what is offered before answering the call,
    
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            then special care should be taken. If PRACK and UPDATE are
            supported by caller and callee then an initial offer can be sent
            reliably, and changed with an UPDATE if the user desires a
            change. If PRACK and UPDATE are not supported then the initial
            offer cannot be changed until the call is fully established. In
            that case either the offer should be delayed until the 200 is
            sent, or else the offer should include the minimum set of media
            the user is able to select.
    
    4.2.3. Answering Initial INVITE with Offer
    
       When a UAS receives an initial INVITE with an offer, what media lines
       the answer may contain is constrained by RFC 3264.[3] The answer must
       contain the same number of m-lines as the offer, and they must
       contain the same media types. Each media line may be accepted, by
       including a non-zero port number, or rejected by including a zero
       port number in the answer. The media lines that are accepted should
       typically be those that would have been offered had the INVITE not
       contained an offer, but with those not offered removed.
    
       The media formats the answer may contain is constrained by RFC 3264
       [3]. For each accepted m-line in the answer, there must be at least
       one media format in common with those in the request. The UAS may
       also include other media formats it is able to support at this time.
       However there is little benefit to including added types.
    
       If the UAS does not wish to indicate support for any of the media
       types in a particular media line of the offer it must reject the
       corresponding media line, by setting the port number to zero.
    
    4.2.4. Answering when Initial INVITE had no Offer
    
       When a UAC has sent an initial INVITE without an offer, and then
       receives a response with the first offer, it should answer in the
       same way as a UAS receiving an initial INVITE with an offer.
    
    4.2.5. Subsequent Offers and Answers
    
       The guidelines above (sections 4.1. and 4.2.1. through 4.2.5.) apply,
       but constraints in RFC 3264 [3] must also be followed. The following
       are of particular note because they have proven troublesome:
    
       o The number of m-lines may not be reduced in a subsequent offer.
          Previously rejected media streams must remain, or be reused to
          offer the same or a different stream.
    
    
    
    
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       o In the o-line, only the version number may change, and if it
          changes it must increment by one from the one previously sent as
          an offer or answer. If it doesn't change then the entire SDP body
          must be identical to what was previously sent as an offer or
          answer.
    
       o In the case of RTP, the mapping from a particular dynamic payload
          type number to a particular codec within that media stream (m-
          line) MUST NOT change for the duration of the session.
    
            NOTE: This may be impossible for a B2BUA to follow in some cases
            (e.g. 3pcc transfer) if it does not terminate media.
    
    4.3. Hold and Resume of media
    
       RFC 3264 [3] specifies (non-normatively) that "hold" should be
       indicated in an established session by sending a new offer containing
       "a=sendonly" for each media stream to be held. An answerer is then to
       respond with "a=recvonly" to acknowledge that the hold request has
       been understood.
    
       Note that the use of sendonly/recvonly is not limited to hold. These
       may be used for other reasons, such as devices that are only capable
       of sending or receiving. So receiving an offer with "a=sendonly" must
       not be treated as a certain indication that the offerer has placed
       the media stream on hold.
    
       This model is based on an assumption that the UA initiating the hold
       will want to play Music on Hold, which is not always the case. A UA
       may, if desired, initiate hold by offering "a=inactive" if it does
       not intend to transmit any media while in hold status.
    
       The rules of RFC 3264 [3] constrain what may be in an answer when the
       offer contains "sendonly", "recvonly", or "inactive" in an a= line.
       But they do not constrain what must be in a subsequent offer. The
       General Principle for Constructing Offers and Answers (section 4.1.)
       is important here. The initiation of "hold" is a local action. It
       should affect the desired state of the UA. It then affects what the
       UA includes in offers and answers until the local state is reset.
    
       The receipt of an offer containing "a=sendonly" or "a=inactive" and
       the sending of a compatible answer should not change the desired
       state of the recipient. However, a UA that has been "placed on hold"
       may itself desire to initiate its own hold status, based on local
       input.
    
       If UA2 has previously been "placed on hold" by UA1, via receipt of
       "a=sendonly", then it may initiate its own hold by sending a new
    
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       offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will
       answer with "a=inactive" because that is the only valid answer that
       reflects its desire not to receive media.
    
       Once in this state, to resume a two way exchange of media each side
       must reset its local hold status. If UA1 is first to go off hold it
       will then send an offer with "a=sendrecv". The UA2 will respond with
       its desired state of "a=sendonly" because that is a permitted
       response. When UA2 desires to also resume, it will send an offer with
       "a=sendrecv". In this case, because UA1 has the same desire it will
       respond "a=sendrecv".
    
       If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive",
       and subsequently wants to initiate its own hold, it need not send a
       new offer, since the only offer it could make would be "a=inactive"
       and that is already in effect in both directions. However, its local
       desired state will now be either "sendonly" or "inactive" according
       to how it desires to send Music on Hold. This affects what it will
       send in future offers and answers.
    
    5. Remaining Issues or Best Practices on Offer/Answer
    
       This document clarifies the offer/answer usage in SIP and summarizes
       the correct or recommended behaviors along with the existing RFCs. To
       create any new normative behaviors beyond these RFCs is not the
       intent of this document.
    
       However, through the scrutiny of the offer/answer model in SIP, some
       issues are found to be unresolved within the current set of RFCs.
       Those remaining issues are described in this section mainly for
       further study.
    
    5.1. Rejecting PRACK Offer
    
       As stated in section 1.2. and 2.2., it is recommended not to send an
       offer in a PRACK request unless UAC has strong reasons to assume the
       receiver will accept it. Even so, there may be the cases when the UAS
       has to reject the offer for some reason. The current RFCs do not
       provide the way to reject the offer and at the same time to
       acknowledge the reliable response.
    
       Several candidates were proposed to resolve this issue, such as
       sending 2xx PRACK response without SDP to reject the offer. Some of
       the candidates may also be adapted as a way to reject an unacceptable
       offer in a response. Anyway, those candidates violate the current
       rules and lose backward compatibility to some extent. It is beyond
       the scope of this document and remains for further study.
    
    
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    5.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
       Transaction
    
       When a re-INVITE transaction fails, the dialog remains with the
       session bound to it. The issue here is what the session status is if
       offer/answer exchange has been completed before the re-INVITE
       transaction fails with the final failure response (Figure 5). One
       option is to take those offer/answer exchanges not committed yet and
       to make the session status rollback to the one before re-INVITE
       transaction was initiated. Another option is to take those exchanges
       committed and to keep the session status as it is even after re-
       INVITE fails. There is no clear consensus on which one is the correct
       behavior.
    
       There are some cases where it is useful to exchange
       offer(s)/answer(s) even before re-INVITE completes. The case of
       adding a new media (like adding video to audio only session) which
       requires permission from the peer through some user interaction is
       one example. Precondition procedures can be another case which may
       require several offer/answer exchanges in one re-INVITE transaction.
    
        UAC                   UAS
         | session established |
         |<===================>|
         |                     |
         | F1  re-INVITE (SDP) |
         |-------------------->|
         | F2 1xx-rel (SDP)    |
         |<--------------------|
         | F3   PRACK          | <- PRACK request may include new offer and
         |-------------------->|    can complete the offer/answer with
         | F4 2xx PRA          |    the answer in 2xx PRACK response.
         |<--------------------|
         |                     | <- UPDATE method can update the session
         |                     |    status before receiving the final
         | F5 4xx/5xx/6xx INV  |    response to re-INVITE request (F1).
         |<--------------------|
         | F6     ACK          |
         |-------------------->|  Issue: What is the correct session status
         |                     |         after re-INVITE transaction.
    
             Figure 5 Commit/Rollback Issue with re-INVITE transaction
    
       To make bad things worse, if a new offer from UAC and the final
       response to re-INVITE are sent at nearly the same time, the UAS can
       not know whether this new offer was sent before or after UAC received
       the final failure response (Figure 6). Note that the ACK request to
       the failure response is sent hop-by-hop basis, therefore even after
    
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       receiving the ACK request, UAS can not make sure that UPDATE request
       was sent after the final response had been reached to the other end.
    
       Sending a new UPDATE request from UAC to synchronize the status
       anytime after the re-INVITE fails may be a good option. This solution,
       however, requires that the UPDATE method be supported by both ends
       and needs care to avoid flapping when each end tries to advertise
       their different views of the session status.
    
       To resolve this issue may be beyond the scope of this document and
       require another normative document which is for further study.
    
       UAC                   UAS
         | session established |
         |<===================>|
         |                     |
         | F1  re-INVITE (SDP) |
         |-------------------->|
         | F2 1xx-rel (SDP)    |
         |<--------------------|
         | F3   PRACK          |
         |-------------------->|
         | F4 2xx PRA          |
         |<--------------------|
         |                     |
         |UPDATE(SDP)  4xx INV |
         |---------\  /--------|
         |          \/         |
         |          /\         |
         |<--------/  \------->|
         |                     |
    
                 Figure 6 Commit/Rollback Issue with Race Condition
    
    
    
    6. Add New Offer/Answer Usage in SIP
    
       It is not recommended to add new SIP methods for the offer/answer
       exchange beyond the ways described in this document. However, it may
       be requested to have new offer/answer exchange methods as SIP
       extensions evolve. In this clause, what should be taken into
       considerations is noted.
    
    6.1. Explicit Usage
    
       New method definitions should define offer/answer usage explicitly
       without any ambiguity.
    
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    6.2. Rejection of an Offer
    
       New method definitions should define how to reject an offer where
       possible.
    
    6.3. Backward Compatibility
    
       New methods must keep backward compatibility.
    
    6.4. Exceptional Case Handling
    
       New methods should take care of how to handle exceptional cases,
       message crossing case and glare case.
    
    7. Security Considerations
    
       There are not any security issues beyond the referenced RFCs.
    
    8. References
    
    8.1. Normative References
    
       [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
             Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
             Session Initiation Protocol", RFC 3261, June 2002.
    
       [2]  Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
             Responses in the Session Initiation Protocol (SIP)", RFC 3262,
             June 2002.
    
       [3]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
             SDP", RFC 3264, June 2002.
    
       [4]  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
             Method", RFC 3311, September 2002.
    
       [5]  Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
             Resource Management and Session Initiation Protocol (SIP)", RFC
             3312, October 2002.
    
    8.2. Informative References
    
       [6]  Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent
             Profile Data Set for Media Policy", draft-ietf-sipping-media-
             policy-dataset-04 (work in progress), May 2007.
    
    
    
    
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    Author's Addresses
    
       Takuya Sawada
       KDDI Corporation
       3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan
    
       Email: tu-sawada@kddi.com
    
    
       Paul H. Kyzivat
       Cisco Systems, Inc.
       1414 Massachusetts Avenue
       Boxborough, MA  01719
       USA
    
       Email: pkyzivat@cisco.com
    
    
    Full Copyright Statement
    
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