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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: August 2008                              Cisco Systems, Inc.
                                                    February 25, 2008


    SIP (Session Initiation Protocol) Usage of the Offer/Answer Model
                draft-ietf-sipping-sip-offeranswer-06.txt


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Abstract

  The Session Initiation Protocol (SIP) utilizes the offer/answer
  model to establish and update multimedia sessions using the Session
  Description Protocol (SDP). The description of the offer/answer
  model in SIP is dispersed across multiple RFCs. This document
  summarizes all the current usages of the offer/answer model in SIP
  communication.




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Table of Contents


  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 of an Offer....................................5
     2.3. Session Description which is not Offer nor Answer........6
  3. Detailed Discussion of the Offer/Answer Model for SIP.........6
     3.1. Offer/Answer for the 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 an Established Dialog..........10
  4. Exceptional Case Handling....................................11
     4.1. Message Crossing Case Handling..........................11
     4.2. Glare Case Handling.....................................14
  5. Content of Offers and Answers................................15
     5.1. General Principle for Constructing Offers and Answers...15
     5.2. Choice of Media Types and Formats to Include and Exclude16
        5.2.1. Sending an Initial INVITE with Offer...............16
        5.2.2. Responding with an Offer when the Initial INVITE has
        no Offer..................................................16
        5.2.3. Answering an Initial INVITE with Offer.............17
        5.2.4. Answering when the Initial INVITE had no Offer.....18
        5.2.5. Subsequent Offers and Answers......................18
     5.3. Hold and Resume of media................................19
     5.4. Behavior on receiving SDP with c=0.0.0.0................20
  6. Remaining Issues or Best Practices on Offer/Answer...........20
     6.1. Rejecting PRACK Offer...................................21
     6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
     Transaction..................................................22
     6.3. Offer in a Reliable Response............................23
     6.4. Requesting Hold while already on Hold...................24
  7. Add New Offer/Answer Usage in SIP............................24
     7.1. Explicit Usage..........................................24
     7.2. Rejection of an Offer...................................24
     7.3. Backward Compatibility..................................24
     7.4. Exceptional Case Handling...............................24
  8. IANA Considerations..........................................25
  9. Security Considerations......................................25
  10. References..................................................25
     10.1. Normative References...................................25
     10.2. Informative References.................................25
  Author's Addresses..............................................26
  Full Copyright Statement........................................26
  Intellectual Property Statement.................................26


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  Acknowledgment..................................................27

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 all forms 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 make normative changes. Rather, it
  makes the remaining open 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 messages 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 certain session description usages that
  conform to the rules described in 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 a mechanism for update of sessions.
  In SIP, a dialog is used to associate an offer/answer exchange with
  the session which it is to update. In other words, only the
  offer/answer exchange in the SIP dialog can update the session
  which is managed by that dialog.

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.



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  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
  exchange is carried out independently in each distinct dialog. When
  an INVITE request contains no offer, only pattern 2 or pattern 4
  apply. '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 to the
  previous offer/answer exchange.

       NOTE: It is legal to have UPDATE/2xx exchanges without
       offer/answer exchanges (pattern 6). However when re-INVITEs
       are sent for non-offer/answer purposes, an offer/answer
       exchange is required. In that case the prior SDP will
       typically be repeated.

  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.










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         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

  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 a 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 of an Offer

  It is not entirely clear how to reject an offer when it is
  unacceptable, and some methods do not allow explicit rejection of
  an offer. For each of the patterns in Table 1, Table 2 shows how to
  reject an offer.

  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


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  correct session description. This may optionally be followed by an
  UPDATE request to rearrange the session parameters if both ends
  support the UPDATE method. Alternatively the UA may terminate the
  dialog and send an error response to the INVITE request. The
  validity and consequences of a 488 response to PRACK is an open
  issue which is discussed within a sequent section. (Pattern 5)

  When a UA receives a response with an offer which it can not accept,
  the 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 an infinite offer/answer loop.

     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 of an Offer

  (*) A UA should only use PRACK to send an offer when it has strong
  reasons to expect the receiver will accept the offer.

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 of the Offer/Answer Model for SIP

3.1. Offer/Answer for the 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



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  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 complicates
  the rules. 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. A session description in an unreliable response
  that precedes a reliable response can be considered a "preview" of
  the answer that will be 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 a
       different "preview" of an answer in each dialog. UAs are
       expected to deal with this.

  Although RFC 3261 says a UA should accept media once an INVITE with
  an offer 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 neither the UAC nor the UAS
  support 100rel, the UAS should 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
    |<--------------------| v offer 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
    | F13    ACK          |    been 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, the UAC should
  act as if it receives the answer. However, offer/answer exchange is
  not completed yet and the 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, the UAS must prepare to
  receive a new offer from the UAC with an UPDATE request or a PRACK
  request.

  The UAS does not include SDP in responses F9 and F12. However, the
  UAC should prepare to receive SDP bodies in F9 and/or F12, and just



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  ignore them, to handle a peer that does not conform to the
  recommended implementation.

3.1.2. INVITE request without SDP

  When a UAC does not include an SDP body in the INVITE request, it
  expects the offer to be received with the first reliable response.
  The UAC will send the answer in the request to acknowledge the
  response, i.e. PRACK or ACK request of the reliable response. Other
  than that, no offer/answer exchanges can occur in the messages
  within the INVITE transaction.

       NOTE: The UAS should not include SDP in the responses F6 and
       F9. However, the UAC should prepare to receive SDP bodies in
       F6 and/or F9, and just ignore them to handle a peer that 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 SDP
    |<--------------------|    as the offer.
    | F4   PRACK (SDP)    | <- A PRACK request to the first 1xx-rel
    |-------------------->|    must contain SDP as the answer.
    | F5 2xx PRA (no SDP) | -
    |<--------------------| |
    |                     | |
    | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not
    |<--------------------| |  contain 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 SDP.
    | F10    ACK          |
    |-------------------->|

       Figure 2 Example of Offer/Answer with 100rel Extension (2)

  Note that in the case that the UAC needs to prompt the user to
  accept or reject the offer, the reliable provisional response with
  SDP as an offer (pattern 4) can result in the retransmission until


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  the PRACK request can be sent. The UAC should take care to avoid
  this situation when it sends the INVITE request without SDP.

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 a UA sending an INVITE request:

  A UA can send an UPDATE request with a new offer if both ends
  support the UPDATE method. Note that if the UAS needs to prompt the
  user to accept or reject the offer, the delay can result in
  retransmission of the UPDATE request.

  A UA can send a PRACK request with a new offer only when
  acknowledging the reliable provisional response carrying the answer
  to an offer in the INVITE request. Compared to using the UPDATE
  method, using PRACK can reduce the number of messages exchanged
  between the UAs. However, to avoid problems or delays caused by
  PRACK offer rejection, the UA is recommended to send a PRACK
  request only when it has strong reasons to expect 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 an early dialog. Note also that if a
  UAS needs to prompt the user to accept or reject the offer, the
  delay can result in retransmission of the PRACK request.

  From a UA receiving an INVITE request:

  A UA can send an UPDATE request with a new offer if both ends
  support the UPDATE method. A UAS can not send a new offer in the
  reliable provisional response, so the UPDATE method is the only
  method for a UAS to update an early session.

3.3. Offer/Answer Exchange in an Established Dialog

  Both the re-INVITE and UPDATE methods can be used in an established
  dialog to update the session.

  The UPDATE method is simpler and can save at least one message
  compared with the 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


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  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.

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 seem to be two
  possible 'exceptional' cases to be considered in SIP offer/answer
  usage: the 'message crossing' case, and the 'glare' case. One of
  the reasons why the usage of SIP methods to exchange offer/answer
  needs to be carefully restricted in the RFCs is to ensure that the
  UA can detect and handle appropriately the 'exceptional' cases to
  avoid incompatible behavior.

4.1. Message Crossing Case Handling

  When message packets 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
  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 cannot be the expected answer. Then UA A must



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  reject the message including offer2 with a 491 response with Retry-
  After header field.

  When offer2 is in a PRACK request(as shown in Figure 4), that is,
  when the PRACK request is acknowledging a reliable provisional
  response with an answer to an offer in an INVITE request containing
  a session description, UA A knows it is an offer. To avoid
  rejecting the PRACK or PRACK offer, UA A is recommended to wait for
  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 simplify implementations, a UA acting
  as a UAS for an INVITE transaction is recommended not to defer
  sending an UPDATE request with an offer until after 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 (as shown in Figure 5), 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 infer that offer2 is not the answer1. In
  this case, since UA A can not reject offer2 in a reliable response,
  it is recommended that it 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.


















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  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. | Delay sending response
         | 1xx-rel must have an answer, | until answer1 is received
         | not an offer.                |
  -------+------------------------------+--------------------------
  1xx-rel| Only one INVITE transaction  | Delay ACK/PRACK
  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 an offer (offer2) overtaking the previous
  answer (answer1)

   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 crossing case











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   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
                             crossing case



4.2. Glare Case Handling

  When both ends in a dialog send a new offer at nearly the same time,
  as described in Figure 6, a UA may receive a new offer before it
  receives the answer to the offer it sent. This case is usually
  called a 'glare' case.

   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


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  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

  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


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  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 an 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.

5.2.2. Responding with an Offer when the 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


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  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 an 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, excluding those not offered.

  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 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.







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5.2.4. Answering when the 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. (RFC 3264[3] section 6.)

  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. (RFC 3264[3] section 8.) 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 body 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.
     (RFC 3264[3] section 8.3.2.)

       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 supported by the UA are to be included, not just the
  ones that were negotiated by previous offer/answer exchanges. The
  same is true for 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 video, by reusing the zeroed m-line used previously.

       NOTE: The behavior above is recommended, but it is not always
       achievable - 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


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       offer any other SDP that the one currently being used, 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 reflect 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 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


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  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=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 c=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.
  c=0.0.0.0 has no special meaning for the direction attribute of the
  accepted stream in the answer.

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.


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  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 that an
  offer not be sent in a PRACK request unless UAC has strong reasons
  to assume the receiver will accept it. Even so, there may be cases
  when the UAS has to reject the offer for some reason. The current
  RFCs do not provide a way to reject the offer and at the same time
  to indicate that the PRACK adequately acknowledged the reliable
  response. It is unclear whether a non-200 response can still
  indicate an acknowledgement of 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 SDP
  with a decreased version value in the o-line. Some of the
  candidates may also be adapted as a way to reject an unacceptable
  offer in a response. Anyway, those proposals 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.

  The 488 response is another proposed solution; however the validity
  and consequences of a 488 response to PRACK is an open issue.
  Because the 488 response may be sent by a proxy, the UAC cannot
  assume the reliable transaction has been adequately acknowledged.
  If a 488 response is received, the UAC should ensure acknowledgment
  of the reliable response by sending a new PRACK with the offer
  removed or modified based upon the received 488 response. If the
  488 response is sent by UAS (open issue), it cannot assume that the
  UAC thinks that the reliable transaction has been adequately
  acknowledged even though the UAS may treat otherwise (open issue).
  If a 488 response is sent by UAS, the UAC should accommodate
  receiving the altered PRACK with higher CSeq without expecting it
  to trigger a 481 response (open issue).

       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.






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6.2. 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 is the session status
  if an 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 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




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  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.

  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.

  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


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  rule will be left as it is, unless the strong necessity to loosen
  it up is raised in the future.

6.4. Requesting Hold while already on Hold

  RFC 3264, section 8.4, contains procedures for putting a unicast
  media stream on hold. Of particular note, it states:

    "If the stream to be placed on hold was previously a recvonly
    media stream, it is placed on hold by marking it inactive."

  Section 5.3. of the current document makes a best practice
  recommentation for this case which conflicts with that, and
  explains why. Some concerns have been raised that such a
  recommendation is invalid because RFC 3264 is normative on this
  subject.

  This document takes the position that Section 8.4 of RFC 3264 is
  non-normative, and so may be overridden. It is further recommended
  that RFC 3264 be revised to avoid the confusion.

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. This
  section recommends some things that should be taken into
  considerations in that case.

7.1. Explicit Usage

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

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.


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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.

  [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-05 (work in progress), November 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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