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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
Intended status: Informational                               P. Kyzivat
Expires: April 2008                                 Cisco Systems, Inc.
                                                       October 24, 2007


     SIP (Session Initiation Protocol) Usage of the Offer/Answer Model
                 draft-ietf-sipping-sip-offeranswer-04.txt


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Abstract

   SIP utilizes the 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 the offer/answer model in SIP communication.

Table of Contents






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   1. Introduction...................................................3
   2. Summary of SIP usage of the Offer/Answer Model.................3
      2.1. Offer/Answer Exchange Pairs in SIP Messages...............3
      2.2. Rejection against an Offer................................5
      2.3. Session Description which is not Offer nor Answer.........6
   3. Detailed Discussion on the Offer/Answer Model for SIP..........6
      3.1. Offer/Answer for INVITE method with 100rel extension......6
         3.1.1. INVITE Request with SDP..............................7
         3.1.2. INVITE request without SDP...........................9
      3.2. Offer/Answer Exchange in Early Dialog....................10
      3.3. Offer/Answer Exchange in Established Dialog..............10
   4. Exceptional Case Handling.....................................11
      4.1. Message Crossing Case Handling...........................11
      4.2. Glare Case Handling......................................13
   5. Content of Offers and Answers.................................14
      5.1. General Principle for Constructing Offers and Answers....15
      5.2. Choice of Media Types and Formats to Include and Exclude.15
         5.2.1. Sending Initial INVITE with Offer...................15
         5.2.2. Responding with Offer when Initial INVITE has no Offer16
         5.2.3. Answering Initial INVITE with Offer.................16
         5.2.4. Answering when Initial INVITE had no Offer..........17
         5.2.5. Subsequent Offers and Answers.......................17
      5.3. Hold and Resume of media.................................18
      5.4. Behavior on receiving SDP with c=IN IP4 0.0.0.0..........19
   6. Remaining Issues or Best Practices on Offer/Answer............20
      6.1. Rejecting PRACK Offer....................................20
      6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
      Transaction...................................................20
      6.3. Offer in a Reliable Response.............................22
   7. Add New Offer/Answer Usage in SIP.............................22
      7.1. Explicit Usage...........................................22
      7.2. Rejection of an Offer....................................23
      7.3. Backward Compatibility...................................23
      7.4. Exceptional Case Handling................................23
   8. IANA Considerations...........................................23
   9. Security Considerations.......................................23
   10. Acknowledgement..............................................23
   11. References...................................................23
      11.1. Normative References....................................23
      11.2. Informative References..................................24
   Author's Addresses...............................................24
   Full Copyright Statement.........................................24
   Intellectual Property Statement..................................25
   Acknowledgment...................................................25





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1. Introduction

   SIP utilizes the offer/answer model to establish and update the
   session. The rules to govern the offer/answer behaviors in SIP are
   described in the several RFCs.

   The primary purpose of this document is to describe the whole set of
   SIP usage of the offer/answer model in one document to help the
   readers to fully understand it. Also, this document tries to
   incorporate the results of the discussions on the controversial
   issues to avoid repeating the same discussions later.

   This document is not intended to create any new normative methods.
   Rather, it makes the remaining issues clear and leaves them for
   further study.

2. Summary of SIP usage of the 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 the offer/answer model. RFC 3264 [3] defines the
   offer/answer model, but does not specify which SIP message should
   convey an offer or an answer. This should be defined in the SIP core
   and extensions RFCs.

   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 standards-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.

2.1. Offer/Answer Exchange Pairs in SIP Messages

   Currently, the rules on the 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. When an initial
   INVITE causes multiple dialogs due to forking, an offer/answer


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   exchange is carried out independently in each distinct dialog.
   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. The
   answer can not be updated, and a new offer can not be sent, in a
   subsequent reliable response for the same INVITE transaction.

   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.

        NOTE: It is legal to have UPDATE/2xx exchanges without
        offer/answer exchanges (pattern 6). As opposed to that, when re-
        INVITEs are sent without a desire for an offer/answer exchange,
        an offer/answer exchange cannot be avoided. In that case it is
        often possible to use the current SDP. (See section 5. for
        details.)

   There may be ONLY ONE offer/answer negotiation in progress for a
   single dialog at any point in time. Section 4 explains how to ensure
   this. When an INVITE results in multiple dialogs each has a separate
   offer/answer negotiation.

        NOTE: This is when using a Content-Disposition of "session".
        There may be a second offer/answer negotiation in progress using
        a Content-Disposition of "early-session" [6]. That is not
        addressed by this draft.

          Offer                Answer             RFC    Ini Est Early
   -------------------------------------------------------------------
   1. INVITE Req.          2xx INVITE Resp.     RFC 3261  Y   Y    N
   2. 2xx INVITE Resp.     ACK Req.             RFC 3261  Y   Y    N
   3. INVITE Req.          1xx-rel INVITE Resp. RFC 3262  Y   Y    N
   4. 1xx-rel INVITE Resp. PRACK Req.           RFC 3262  Y   Y    N
   5. PRACK Req.           200 PRACK Resp.      RFC 3262  N   Y    Y
   6. UPDATE Req.          2xx UPDATE Resp.     RFC 3311  N   Y    Y

   Table 1. Summary of SIP Usage of the Offer/Answer Model


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   In Table 1, '1xx-rel' corresponds to the reliable provisional
   response which contains the 100rel option defined in RFC 3262 [2].

   The 'Ini' column shows the ability to exchange the offer/answer to
   initiate the session. 'Y' indicates that the pattern can be used in
   the initial offer/answer exchange, while 'N' indicates that it can
   not. Only the initial INVITE transaction can be used to exchange the
   offer/answer to establish multimedia session.

   The 'Est' column shows the ability to update the established session.

   The 'Early' column indicates which patterns may 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.

2.2. 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 unacceptable offer, 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,
   or alternatively terminate 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 alternatively 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.



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      Offer                Rejection
   -----------------------------------------------------
   1. INVITE Req.          488 INVITE Response
   2. 2xx INVITE Resp.     Answer in ACK Req. followed by new offer
                           OR termination of dialog
   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
                           OR termination of dialog
   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.

2.3. 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.

3. Detailed Discussion on the Offer/Answer Model for SIP

3.1. 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
   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 answer that will be


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   coming, and hence may be treated like an answer until the actual one
   arrives.

        NOTE: This "preview" session description rule applies to a
        single offer/answer exchange. In parallel offer/answer exchanges
        (caused by forking) a UA may obviously receive the different
        "preview" of answer in each dialog. UAs are expected to deal
        with this.

   Although RFC 3261 says a UA should accept media once the INVITE has
   been sent, in many cases, an answer (or, at least a preview of it) is
   required in order for media to be accepted. Therefore, a UAS should
   send an SDP answer reliably (if possible) before it starts sending
   media. And, if either the UAC or the UAS does not support 100rel, it
   should still send a preview of the answer before it starts sending
   media.

3.1.1. 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 messages
   within the INVITE transaction.


























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    UAC                   UAS
     | F1  INVITE (SDP)    | <- The offer in the offer/answer model
     |-------------------->|
     | F2     1xx (SDP)    | <- The offer/answer exchange is not closed
     |<--------------------|    yet, 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 the 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. 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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3.1.2. 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 messages within the INVITE
   transaction.

        NOTE: 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          |
     |<--------------------|
     |                     |
     | 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)

   Note that in the case that UAC needs to prompt the user to accept or
   reject the offer, the reliable provisional response with an SDP as an
   offer (pattern 4) can result in the retransmission until the PRACK
   request can be sent. UAC should take care to avoid this situation
   when it sends the INVITE request without an SDP.



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3.2. 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. Note that in the case that UAS needs to prompt the
   user to accept or reject the offer, an UPDATE request may need to be
   retransmitted.

   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. Note also that in the case that
   UAS needs to prompt the user to accept or reject the offer, a PRACK
   request may need to be retransmitted.

   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.

3.3. 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


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   should be answered immediately unless such a user interaction is
   needed. Otherwise, some 3pcc flows will break.

4. 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.

4.1. 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


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   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
   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 this case only occurs when UA A, while waiting for an
   answer, sends an INVITE request without session description.

   Table 3 summarizes the discussions above.

   offer2 | How to know it's not answer1 | Actions to take
   -------+------------------------------+--------------------------
   INVITE | Never be an answer           | 491 response
   UPDATE | Glare case for UA A          | with Retry-After
   -------+------------------------------+--------------------------
   PRACK  | Not a pattern 4. in Table 1. | Wait sending response
          | 1xx-rel must have an answer, | until answer1 is received
          | not an offer.                |
   -------+------------------------------+--------------------------
   1xx-rel| Only one INVITE transaction  | Wait sending acknowledge
   2xx    | at a time. Then UA can know  | until answer1 is received
   -------+------------------------------+--------------------------

   NOTE: PRACK and 1xx-rel/2xx case is extremely rare case and easily
   avoidable. See Figure 4 and Figure 5.

   Table 3. UA's action to the offer (offer2) overtaken the previous
   answer (answer1)








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    A                               B
    |                               |
    |                   INV (offer0)|
    |<------------------------------|
    | 1xx-rel (answer0)             |
    |------------------------------>| --+
    |offer1(e.g. UPD)               |   |
    |==============================>|   |
    |              answer1 (2xx-UPD)|   | Acknowledge
    |<===========\  /===============|   |
    |             \/                |   |
    |             /\     offer2(PRA)|   |
    |<===========/  \===============| <-+
    | answer2 (2xx-PRA)             |
    |------------------------------>| Wait until answer1
    |                               |

       Figure 4 PRACK as a message with offer2 in message cross case


    A                               B
    |                               |
    |offer1(e.g. UPD)               |
    |==============================>|
    |re-INV (no offer)              |
    |------------------------------>| --+
    |              answer1 (2xx-UPD)|   |
    |<===========\  /===============|   | The first reliable response
    |             \/          offer2|   |
    |             /\   (1xx-rel/2xx)|   |
    |<===========/  \===============| <-+
    | answer2 (PRACK/ACK)           |
    |------------------------------>| Wait until answer1
    |                               |

   Figure 5 Reliable response as a message with offer2 in message cross
                                   case



4.2. 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 6. This case is called a 'glare' case
   in general.


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    A                  B
    |offer1      offer2|
    |-------\  /-------|
    |        \/        |
    |        /\        |
    |<------/  \------>|

                            Figure 6 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 (within the current rules, only
   possible if offer1 is in an UPDATE request), the PRACK may be
   accepted with 200 or may be rejected with a 491 response. A 491
   response is valid to satisfy the 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 A should not
   send an offer if it has already sent a reliable provisional response
   containing an answer to a previous offer and has not received the
   corresponding PRACK request.

   To avoid a glare condition involving an offer in a response, when UA
   A has sent a (re)INVITE request without session description, it
   should not send an offer until it has received an offer in a reliable
   response to the (re)INVITE, and sent an answer to that offer.

5. 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


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   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

5.1. 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. This is the
   case even when the offer is sent in response to an INVITE or re-
   INVITE that contains no offer. (However in the case of re-INVITE the
   constraints of RFCs 3261 and 3264 must be observed.)

   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.

        NOTE: "at that time" is important. The device may permit the
        user to configure which supported media are to be used by
        default.

   In some cases a UA may not have direct knowledge of what it is
   interested in doing at a particular time. If it is an intermediary it
   may be able to delegate the decision. In the worst case it may apply
   a default, such as assuming it wants to use all of its capabilities.

5.2. Choice of Media Types and Formats to Include and Exclude

5.2.1. 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.


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   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.

5.2.2. 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. If there is a possibility that the user of the device
        will wish to change what is offered before answering the call,
        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.

5.2.3. 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 are constrained by RFC 3264
   [3]. For each accepted m-line in the answer, there must be at least

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   one media format in common with the corresponding m-line of the
   offer. 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.

5.2.4. 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.

5.2.5. Subsequent Offers and Answers

   The guidelines above (sections 5.1. and 5.2.1. through 5.2.4.) 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.

   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. Changing the o-line, except version number value, during
      the session is an error case. The behavior when receiving such a
      non-compliant offer/answer SDP is implementation dependent. If a
      UA needs to negotiate a 'new' SDP session, it should use the
      INVITE/Replaces method.

   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.

   When the new offer is sent in response to an offerless (re)INVITE,
   all codecs the UA wishes to use at this time are to be included, not
   just the ones that were negotiated by previous offer/answer
   exchanges. Same with media types - so if UA A initially offered audio
   and video to UA B, and they end up with only audio, and UA B sends an
   offerless (re)INVITE to UA A, A's resulting offer should re-attempt

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   video, by reusing the zeroed m-line used previously if it still
   wishes to use video.

        NOTE: The behavior above is recommended, but it is not always to
        achieve - for example in some interworking scenarios. Or, the
        offerer may simply not have enough resources to offer
        "everything" at that point. Even if the UAS is not able to offer
        any other SDP that the one currently using, it should not reject
        the re-INVITE. Instead, it should generate an offer with the
        currently used SDP with o- line unchanged.

5.3. 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 5.1. )
   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
   offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will


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   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 with "a=sendrecv".In the same case, when UA2 receives the
   offer with "a=sendrecv", if it has decided it wants to reset its
   local hold but has not yet signaled the intent, it may send
   "a=sendrecv" in the answer.

   If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive",
   and subsequently wants to initiate its own hold, also using
   "a=inactive", it need not send a new offer, since the only valid
   response is "a=inactive" and that is already in effect. However, its
   local desired state will now be either "inactive" or "a=sendonly".
   This affects what it will send in future offers and answers.

   If a UA has occasion to send another offer in the session, without
   any desire to change the hold status (e.g. in response to a re-INVITE
   without an offer, or when sending a re-INVITE to refresh the session
   timer) it should follow the General Principle for Constructing Offers
   and Answers (section 5.1. ). If it previously initiated a "hold" by
   sending "a=sendonly" or "a=inactive" then it should offer that again.
   If it had not previously initiated "hold" then it should offer
   "a=sendrecv", even if it had previously been forced to answer
   something else. Without this behavior it is possible to get "stuck on
   hold" in some cases, especially when a third-party call controller is
   involved.

5.4. Behavior on receiving SDP with c=IN IP4 0.0.0.0

   RFC 3264[3] specifies that An agent MUST be capable of receiving SDP
   with a connection address of 0.0.0.0, in which case it means that
   neither RTP nor RTCP should be sent to the peer.

   If a UA generates an answer to the offer received with a connection
   address of 0.0.0.0, the direction attribute of the accepted media
   stream in the answer must be based on direction attribute of the
   offered stream and rules specified in RFC 3264 to form the a-line in
   the answer. A connection address of 0.0.0.0 has no special meaning
   for the direction attribute of the accepted stream in the answer.




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6. 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.

6.1. Rejecting PRACK Offer

   As stated in section 2.2. and 3.2. , 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 ideas were presented to resolve this issue, such as sending
   2xx PRACK response without SDP to reject the offer, or sending an SDP
   with decreased o-line version value. 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 (e.g. section 5 of RFC 3262). It is
   beyond the scope of this document and remains for further study.

        NOTE: Deprecation of the usage of offer in PRACK may be another
        solution. As the precondition mechanism specification [2]
        explicitly shows a usage of sending offer in PRACK, its
        deprecation could cause backward compatibility issues.

6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
   Transaction

   When a re-INVITE transaction fails, often 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 (if a session description
   has been sent in a reliable provisional response to the re-INVITE
   request), or if subsequent offer/answer exchanges have taken place
   (using UPDATE or PRACK transactions), before the re-INVITE
   transaction is terminated with a final error response (Figure 7). 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


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   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 7 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 8). Note that the ACK request to
   the failure response is sent hop-by-hop basis, therefore even after
   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.



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   The proper handling of this issue is undefined by existing standards.
   Resolution is beyond the scope of this document, and will require a
   new normative document. Such a document is the responsibility of the
   SIP working group, and 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 8 Commit/Rollback Issue with Race Condition

6.3. Offer in a Reliable Response

   In RFC 3261, it is stated that when an INVITE is sent without an
   offer, the first reliable response MUST contain an offer. There was
   discussion on whether this rule can be loosened up. There is no clear
   explanation why this restriction is defined. However, this rule will
   be left as it is, unless the strong necessity to loose it up will
   come up in the future.

7. Add New Offer/Answer Usage in SIP

   This document recommends against the addition of new offer/answer
   methods using SIP. However, it may be necessary to define new
   offer/answer exchange methods as SIP extensions evolve. In this
   section, what should be taken into considerations is noted.

7.1. Explicit Usage

   New method definitions should define offer/answer usage explicitly
   without any ambiguity.


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7.2. Rejection of an Offer

   New method definitions should define how to reject an offer where
   possible.

7.3. Backward Compatibility

   New methods must keep backward compatibility.

7.4. Exceptional Case Handling

   New methods should take care of how to handle exceptional cases,
   message crossing case and glare case.

8. IANA Considerations

   This document has no actions for IANA.

9. Security Considerations

   There are not any security issues beyond the referenced RFCs.

10. Acknowledgement

   The authors would like to thank Christer Holmberg, Rajeev Seth,
   Nataraju A B, Byron Campen and Jonathan Rosenberg for their thorough
   reviews and comments. Many of their suggestions and ideas are
   incorporated to complete this document.

11. References

11.1. 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.



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   [5]   Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
         Resource Management and Session Initiation Protocol (SIP)", RFC
         3312, October 2002.

11.2. Informative References

   [6]   G. Camarillo, "The Early Session Disposition Type for the
         Session Initiation Protocol (SIP)", RFC 3959, December 2004.

   [7]   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.

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

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



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