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rfc:rfc6337

Internet Engineering Task Force (IETF) S. Okumura Request for Comments: 6337 Softfront Category: Informational T. Sawada ISSN: 2070-1721 KDDI Corporation

                                                            P. Kyzivat
                                                           August 2011
 Session Initiation Protocol (SIP) Usage of the Offer/Answer Model

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.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6337.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must

Okumura, et al. Informational [Page 1] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
 2. Summary of SIP Usage of the Offer/Answer Model ..................3
    2.1. Terminology ................................................3
    2.2. Offer/Answer Exchange Pairs in SIP Messages ................4
    2.3. Rejection of an Offer ......................................5
    2.4. Session Description That Is Not an Offer or an Answer ......7
 3. Detailed Discussion of the Offer/Answer Model for SIP ...........8
    3.1. Offer/Answer for the INVITE method with 100rel Extension ...8
         3.1.1. INVITE Request with SDP .............................8
         3.1.2. INVITE Request without SDP .........................11
    3.2. Offer/Answer Exchange in Early Dialog .....................12
    3.3. Offer/Answer Exchange in an Established Dialog ............12
    3.4. Recovering from a Failed Re-INVITE ........................13
 4. Exceptional Case Handling ......................................13
    4.1. Message Crossing Case Handling ............................13
    4.2. Glare Case Handling .......................................18
    4.3. Interworking of UPDATE and Re-INVITE ......................21
 5. Content of Offers and Answers ..................................25
    5.1. General Principle for Constructing Offers and Answers .....26
    5.2. Choice of Media Types and Formats to Include and Exclude ..26
         5.2.1. Sending an Initial INVITE with Offer ...............26
         5.2.2. Responding with an Offer When the Initial
                INVITE Has No Offer ................................27
         5.2.3. Answering an Initial INVITE with Offer .............27
         5.2.4. Answering When the Initial INVITE Had No Offer .....28
         5.2.5. Subsequent Offers and Answers ......................28
    5.3. Hold and Resume of Media ..................................29
    5.4. Behavior on Receiving SDP with c=0.0.0.0 ..................31
 6. Security Considerations ........................................31
 7. Acknowledgements ...............................................31
 8. References .....................................................32
    8.1. Normative References ......................................32
    8.2. Informative References ....................................33

Okumura, et al. Informational [Page 2] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

1. Introduction

 SIP utilizes the offer/answer model to establish and update sessions.
 The rules that govern the offer/answer behaviors in SIP are described
 in several RFCs: [RFC3261], [RFC3262], [RFC3264], [RFC3311], and
 [RFC6141].
 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 describes ambiguities in the current specifications and
 the authors' understanding of the correct interpretation of these
 specifications.  This document is not intended to make any changes to
 those specifications, but rather is intended to provide a reference
 for future standards development work on the SIP offer/answer model
 and to developers looking for advice on how to implement in
 compliance with the standards.

2. Summary of SIP Usage of the Offer/Answer Model

 The offer/answer model itself is independent from the higher layer
 application protocols that utilize it.  SIP is one of the
 applications using the offer/answer model.  [RFC3264] 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 extension 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 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 that it is to update.  In other words, only the offer/
 answer exchange in the SIP dialog can update the session that is
 managed by that dialog.

2.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

Okumura, et al. Informational [Page 3] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 The following abbreviations are used in this document.
 UA:  User Agent.
 UAC: User Agent Client.
 UAS: User Agent Server.
 SDP: Session Description Protocol [RFC4566].

2.2. Offer/Answer Exchange Pairs in SIP Messages

 Currently, the rules on the offer/answer model are defined in
 [RFC3261], [RFC3262], [RFC3264], [RFC3311], and [RFC6141].  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
 exchange is carried out independently in each distinct dialog.  When
 an INVITE request contains no offer, only Pattern 2 or Pattern 4
 apply.  According to Section 13.2.1 of [RFC3261], 'The first reliable
 non-failure message' must have an offer if there is no offer in the
 INVITE request.  This means that the User Agent (UA) that 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 User Agent Server (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 cannot be updated, and a new offer cannot be sent in a
 subsequent reliable response for the same INVITE transaction.
 In Pattern 5, a Provisional Response ACKnowledgement (PRACK) request
 can contain an offer only if the reliable response that 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.

Okumura, et al. Informational [Page 4] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 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" [RFC3959].  That is not
    addressed by this document.
          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 that contains the 100rel option defined in [RFC3262].
 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
 cannot.  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.

2.3. Rejection of an Offer

 It is not always 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).

Okumura, et al. Informational [Page 5] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 If this is a re-INVITE, extra care must be taken, as detailed in
 [RFC6141].  Specifically, if the offer contains any changes or
 additions to media stream properties, and those have already been
 used to transmit/receive media before the final response is sent,
 then a 2xx response should be sent, with a syntactically 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 send an error response to
 the (re-)INVITE request to terminate the dialog or to roll back the
 offer/answer status before sending re-INVITE request.  In this case,
 the UAS should not continue to retransmit the unacknowledged reliable
 provisional response; the User Agent Client (UAC) should not continue
 to retransmit a PRACK request.
 When a UA receives an UPDATE request with an offer that it cannot
 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 that it cannot
 accept, it may respond with a 200 response with a syntactically
 correct session description.  Optionally, this may 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 (Pattern 5).
 In addition, there is a possibility for UAC to receive a 488 response
 for an PRACK request.  In that case, UAC may send again a PRACK
 request without an offer or send a CANCEL request to terminate the
 INVITE transaction.
    NOTE: In [RFC3262], the following restriction is defined with
    regard to responding to a PRACK request.
    "If the PRACK does match an unacknowledged reliable provisional
    response, it MUST be responded to with a 2xx response."
    This restriction is not clear.  There are cases where it is
    unacceptable to send a 2xx response.  For example, the UAS may
    need to send an authentication challenge in a 401 response.  This
    is an open issue and out of scope for this document.
 When a UA receives a response with an offer that it cannot 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

Okumura, et al. Informational [Page 6] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 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.
 Section 14.2 of [RFC3261], "UAS Behavior", states:
    The UAS MUST ensure that the session description overlaps with its
    previous session description in media formats, transports, or
    other parameters that require support from the peer.  This is to
    avoid the need for the peer to reject the session description.
 This is a rule for an offer within 2xx response to a re-INVITE.  This
 rule should be applied to an offer within a reliable provisional
 response and a PRACK request.
      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
 (*) If this was a re-INVITE, a failure response should not be sent if
 media has already been exchanged using the new offer.
 (**) A UA should only use PRACK to send an offer when it has strong
 reasons to expect the receiver will accept the offer.
                    Table 2: Rejection of an Offer

2.4. Session Description That Is Not an Offer or an 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 a 200 OK response for OPTIONS and a
 488 response for INVITE.

Okumura, et al. Informational [Page 7] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

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 [RFC3261].  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, Patterns 3, 4, and 5 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.

3.1.1. INVITE Request with SDP

 When a UAC includes an SDP body in the INVITE request as an offer,
 only the first SDP in a reliable non-failure response to the INVITE
 request is the real answer.  No other offer/answer exchanges can
 occur within the messages (other responses and ACK) of the INVITE
 transaction.
 In [RFC3261] there are some descriptions about an offer/answer
 exchange, but those cause a little confusion.  We interpret those
 descriptions as follows,
 UAC behavior:
    1.  If the first SDP that the UAC received is included in an
        unreliable provisional response to the INVITE request,
        [RFC3261] (Section 13.2.1, second bullet) requires that this
        be treated as an answer.  However, because that same section
        states that the answer has to be in a reliable non-failure
        message, this SDP is not the true answer and therefore the
        offer/answer exchange is not yet completed.
    2.  After the UAC has received the answer in a reliable
        provisional response to the INVITE, [RFC3261] requires that
        any SDP in subsequent responses be ignored.
    3.  If the second and subsequent SDP (including a real answer) is
        different from the first SDP, the UAC should consider that the
        SDP is equal to the first SDP.  Therefore, the UAC should not
        switch to the new SDP.

Okumura, et al. Informational [Page 8] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 UAS behavior:
    1.  [RFC3261] requires all SDP in the responses to the INVITE
        request to be identical.
    2.  After the UAS has sent the answer in a reliable provisional
        response to the INVITE, the UAS should not include any SDPs in
        subsequent responses to the INVITE.
    3.  [RFC3261] permits the UAS to send any provisional response
        without SDP regardless of the transmission of the answer.
 A session description in an unreliable response that precedes a
 reliable response can be considered a "preview" of the answer that
 will be coming.
    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 [RFC3261] 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.  Two
 examples of why this might be required are as follows:
 o  To avoid receiving media from undesired sources, some User Agents
    assume symmetric RTP will be used, ignore all incoming media
    packets until an address/port has been received from the other
    end, and then use that address/port to filter incoming media
    packets.
 o  In some networks, an intermediate node must authorize a media
    stream before it can flow and requires a confirming answer to the
    offer before doing so.
 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.

Okumura, et al. Informational [Page 9] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

   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) | |
    |-------------------->| | The 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          | | The UAC can send a new offer in a PRACK
    |-------------------->| | request to acknowledge F6.
    | F8 2xx PRA          | | After F7, the 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 a reliable non-failure response, which is
 the real answer.  After sending the SDP in F6, the UAS must prepare
 to receive a new offer from the UAC in a PRACK request or in an
 UPDATE request if the UAS supports UPDATE.
 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
 ignore them, to handle a peer that does not conform to the
 recommended implementation.

Okumura, et al. Informational [Page 10] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

3.1.2. INVITE Request without SDP

 When a UAC does not include an SDP body in the INVITE request,
 [RFC3261] (Section 13.2.1, first bullet) requires that the UAS
 include an offer in the first reliable non-failure response.
 However, a UAC might not expect an SDP in the other responses to the
 INVITE request because RFC 3261 simply does not anticipate the
 possibility.  Therefore, the UAS ought not include any SDP in the
 other responses to the INVITE request.
    NOTE: In Figure 2, 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          | |
   |-------------------->| | The 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 the PRACK
 request can be sent.  The UAC should take care to avoid this
 situation when it sends the INVITE request without 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 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
 [RFC3312] 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 cannot 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
 peer to take time to decide whether or not it will accept a session
 update 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

Okumura, et al. Informational [Page 12] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 should be answered immediately unless such a user interaction is
 needed.  Otherwise, some Third Party Call Control (3PCC) [RFC3725]
 flows will break.

3.4. Recovering from a Failed Re-INVITE

 Section 14.1 of [RFC3261] requires that the session parameters in
 effect prior to a re-INVITE remain unchanged if the re-INVITE fails,
 as if no re-INVITE had been issued.  This remains the case even if
 multiple offer/answer exchanges have occurred between the sending of
 the re-INVITE and its failure, and even if media has been exchanged
 using the proposed changes in the session.  Because this can be
 difficult to achieve in practice, a newer specification [RFC6141]
 recommends the UAS to send a 2xx response to a re-INVITE in cases
 where rolling back changes would be problematic.
 Nevertheless, a UAC may receive a failure response to a re-INVITE
 after proposed changes that must be rolled back have already been
 used.  In such a case, the UAC should send an UPDATE offering the SDP
 that has been reinstated.  (See [RFC6141] for details.)

4. Exceptional Case Handling

 In [RFC3264], the following restrictions are defined with regard to
 sending a new offer.
    At any time, either agent MAY generate a new offer that updates
    the session.  However, it MUST NOT generate a new offer if it has
    received an offer which it has not yet answered or rejected.
    Furthermore, 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 cross in the transport network, an offer may be
 received before the answer for the previous offer/answer exchange, as
 shown in Figure 3.  In such a case, UA A must detect that the session
 description SDP-2 is not the answer to offer1.

Okumura, et al. Informational [Page 13] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

                          A                  B
                          |SDP-1     (offer1)|
                       M1 |----------------->|
                          |SDP-2    (answer1)|
                       M2 |<------\  /-------|
                          |        \/        |
                          |SDP-3   /\(offer2)|
                       M3 |<------/  \-------|
                    Figure 3: Message Crossing Case
 Because of the restrictions on placement of offers and answers
 (summarized in Table 1), there are a limited number of valid
 exchanges of messages that may lead to this message crossing case.
 These are enumerated in Table 3.  (This table only shows messages
 containing offers or answers.  There could be other messages, without
 session descriptions, which are not shown.)
 When a response to an UPDATE request crosses a reliable response to
 an INVITE request, there are variants shown in Figures 4 and 5, which
 are dependent on an INVITE (Mx) that contains no offer.  These are
 also included in Table 3.
                 A                               B
                 |                               |
                 |UPDATE(offer1)                 |
              M1 |==============================>|
                 |re-INVITE(no offer)            |
              Mx |------------------------------>| --+
                 |               2xx-UPD(answer1)|   |
              M2 |<===========\  /===============|   | first reliable
                 |             \/ 1xx-rel/2xx-INV|   | response
                 |             /\        (offer2)|   |
              M3 |<===========/  \===============| <-+
                 |PRACK/ACK(answer2)             |
              My |------------------------------>|
                 |                               |
              Figure 4: Avoidable Message Crossing Cases
 To avoid the message crossing condition shown in Figure 4, UA A
 should not send this re-INVITE request until an UPDATE transaction
 has been completed.  If UA B encounters this message crossing
 condition, it should reject this re-INVITE request with a 500
 response.

Okumura, et al. Informational [Page 14] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

                 A                               B
                 |                               |
                 |re-INVITE(no offer)            |
              Mx |------------------------------>| --+
                 |UPDATE(offer1)                 |   |
              M1 |==============================>|   |
                 |               2xx-UPD(answer1)|   |
              M2 |<===========\  /===============|   | first reliable
                 |             \/ 1xx-rel/2xx-INV|   | response
                 |             /\        (offer2)|   |
              M3 |<===========/  \===============| <-+
                 |PRACK/ACK(answer2)             |
              My |------------------------------>|
                 |                               |
              Figure 5: Avoidable Message Crossing Cases
 To avoid the message crossing condition shown in Figure 5, UA A
 should not send this UPDATE request until an ACK or a PRACK
 transaction associated with an offer/answer has been completed.  If
 UA B encounters this message crossing condition, it should reject
 this UPDATE request with a 500 response.
 The situation when a PRACK request crosses UPDATE request is shown in
 Figure 6.
                 A                               B
                 |                               |
                 |           re-INVITE (no offer)|
 1st reliable+-- |<------------------------------|
 response    | M1|1xx-rel(offer1)                |
             +-> |==============================>| --+
                 |                 PRACK(answer1)| M3| Acknowledge
                 |<===========\  /===============| <-+
                 |             \/                |
                 |             /\  UPDATE(offer2)|
                 |<===========/  \===============| M2
                 |500-UPD                        |
                 |------------------------------>|
                 |2xx-PRA                        |
                 |------------------------------>|
                 |                               |
              Figure 6: Avoidable Message Crossing Cases

Okumura, et al. Informational [Page 15] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 To avoid the message crossing condition shown in Figure 6, UA B
 should not send this UPDATE request until a PRACK transaction
 associated with an offer/answer has been completed.  If UA A
 encounters this message crossing condition, it should reject this
 UPDATE request with a 500 response.
 The situation when a reliable provisional response to an INVITE
 request crosses UPDATE request is shown in Figure 7.
                 A                               B
                 |                               |
                 |re-INVITE(offer1)              |
              M1 |==============================>|
                 |               1xx-rel(answer1)|
                 |<===========\  /===============| M3
                 |             \/                |
                 |             /\  UPDATE(offer2)|
             +-- |<===========/  \===============| M2
             |   |491-UPD                        |
 Acknowledge |   |------------------------------>|
             |   |PRACK                          |
             +-> |------------------------------>|
                 |                               |
              Figure 7: Avoidable Message Crossing Cases
 To avoid the message crossing condition shown in Figure 7, UA B
 should not send this UPDATE request until a PRACK transaction
 associated with an offer/answer has been completed.  If UA A
 encounters this message crossing condition, it should reject this
 UPDATE request with a 491 response.
 The situation when a 2xx response to an INVITE request crosses UPDATE
 request is shown in Figure 8.

Okumura, et al. Informational [Page 16] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

                 A                               B
                 |                               |
                 |re-INVITE(offer1)              |
                 |==============================>|
                 |               2xx-INV(answer1)|
                 |<===========\  /===============|
                 |             \/                |
                 |             /\  UPDATE(offer2)|
             +-- |<===========/  \===============|
             |   |491-UPD                        |
 Acknowledge |   |------------------------------>|
             |   |ACK                            |
             +-> |------------------------------>|
                 |                               |
              Figure 8: Avoidable Message Crossing Cases
 This is a true glare.  To avoid the message crossing condition shown
 in Figure 8, UA B should not send the UPDATE request until it has
 received an ACK request.  But there is no problem even if UA B sends
 it.  If UA A encounters this message crossing condition, it should
 reject this UPDATE request with a 491 response.
 The situation when a response to an UPDATE request crosses a PRACK
 request is shown in Figure 9.
                 A                               B
                 |                               |
                 |              re-INVITE(offer0)|
                 |<------------------------------|
                 |1xx-rel(answer0)               |
                 |------------------------------>| --+
                 |UPDATE(offer1)                 |   |
              M1 |==============================>|   |
                 |               2xx-UPD(answer1)|   | Acknowledge
                 |<===========\  /===============| M3|
                 |             \/                |   |
                 |             /\   PRACK(offer2)| M2|
                 |<===========/  \===============| <-+
                 |                               |
               Figure 9: Avoidable Message Crossing Case
 To avoid the message crossing condition shown in Figure 9, UA A
 should not send this UPDATE request until a PRACK transaction
 associated with an offer/answer has been completed.  If UA B
 encounters this message crossing condition, it should reject this
 UPDATE request with a 491 response.

Okumura, et al. Informational [Page 17] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 Table 3 summarizes this section.  Each action is described in
 Section 4.3.
      | M1     | M3       | M2        |Action |Action |Figure|
      |(offer1)|(answer1) |(offer2)   | of A  | of B  |      |
      +--------+----------+-----------+-------+-------+------+
      | UPDATE | 2xx-UPD  | UPDATE    |UAS-UcU|       |      |
      |        |          +-----------+-------+ -     |      |
      |        |          | INVITE    |UAS-UcI|       |      |
      |        |          +-----------+-------+-------+------+
      |        |          | 1xx-INV   |       |       |      |
      |        |          +-----------+UAC-UI,|UAS-UsI| 4,5  |
      |        |          | 2xx-INV   |UAC-IU |UAS-IsU|      |
      |        |          +-----------+-------+-------+------+
      |        |          | PRACK  (*)|UAC-IU |UAS-IcU|  9   |
      +--------+----------+-----------+-------+-------+------+
      | PRACK  | 2xx-PRA  | UPDATE    |UAS-IcU|       |      |
      +--------+----------+-----------+-------+       |      |
      | 2xx-INV| ACK      | UPDATE    |UAS-IsU| -     |      |
      |        |          +-----------+-------+       |      |
      |        |          | INVITE    |UAS-IsI|       |      |
      +--------+----------+-----------+-------+-------+------+
      | 1xx-rel| PRACK    | UPDATE    |UAS-IsU|       |  6   |
      +--------+----------+-----------+-------+UAC-IU +------+
      | INVITE | 1xx-rel  | UPDATE (*)|       |       |  7   |
      |        +----------+-----------+UAS-IcU+-------+------+
      |        | 2xx-INV  | UPDATE (*)|       | -     |  8   |
      +--------+----------+-----------+-------+-------+------+
      (*) invalid sequences if INVITE request is an initial one
           Table 3: Offer/Answer Crossing Message Sequences

4.2. Glare Case Handling

 When both ends in a dialog send a new offer at nearly the same time,
 as described in Figure 10, 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|
                       M1 |-------\  /-------| M2
                          |        \/        |
                          |        /\        |
                          |<------/  \------>|
                         Figure 10: Glare Case

Okumura, et al. Informational [Page 18] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 When offer2 is in an UPDATE request or (re-)INVITE request, it must
 be rejected with a 491 or 500 response.
 There is a variant of Figure 7.  When offer2 is in a PRACK request
 (within the current rules, only possible if offer1 is in an UPDATE
 request), as shown in Figure 11, UA A has a dilemma.
                 A                               B
                 |                               |
                 |              re-INVITE(offer0)|
                 |<------------------------------|
                 |1xx-rel(answer0)               |
                 |------------------------------>| --+
                 |UPDATE(offer1)    PRACK(offer2)| M2| Acknowledge
              M1 |============\  /===============| <-+
                 |             \/                |
                 |             /\                |
                 |<===========/  \==============>|
                 |                        491-UPD|
                 |<------------------------------|
                 |                               |
                    Figure 11: Avoidable Glare Case
 All PRACKs are supposed to be accepted with a 200 response, yet there
 is no way to indicate the problem with a 200 response.  At best, it
 could proceed on the assumption that the UPDATE will be rejected with
 a 491.  To avoid the glare condition shown in Figure 11, UA A should
 not send this UPDATE request until a PRACK transaction associated
 with an offer/answer has been completed.  If UA B encounters this
 glare condition, it should reject this UPDATE request with a 491
 response.
 Glare can also occur when offer2 is in a 1xx or 2xx response.  This
 is a variant of Figure 5, as shown in Figure 12.

Okumura, et al. Informational [Page 19] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

                 A                               B
                 |                               |
                 |re-INVITE(no offer)            |
                 |------------------------------>| --+
                 |                1xx-rel/2xx-INV|   | 1st reliable
                 |UPDATE(offer1)         (offer2)| M2| response
              M1 |============\  /===============| <-+
                 |             \/                |
                 |             /\                |
                 |<===========/  \==============>|
                 |                        500-UPD|
                 |<------------------------------|
                 |                               |
                    Figure 12: Avoidable Glare Case
 To avoid the glare condition shown in Figure 12, UA A should not send
 this UPDATE request until an ACK or a PRACK transaction associated
 with an offer/answer has been completed.  If UA B encounters this
 glare condition, it should reject this UPDATE request with a 500
 response.
 There is a variant of Figure 4, as shown in Figure 13.
                 A                               B
                 |                               |
                 |UPDATE(offer1)                 |
                 |==========\                    |
                 |re-INVITE  \  (no offer)       |
                 |------------\----------------->| --+
                 |             \  1xx-rel/2xx-INV|   | 1st reliable
                 |              \        (offer2)|   | response
                 |<==============\===============| <-+
                 |                \              |
                 |                 \============>|
                 |                        500-UPD|
                 |<------------------------------|
                 |                               |
                    Figure 13: Avoidable Glare Case
 To avoid the glare condition shown in Figure 13, UA A should not send
 this re-INVITE request until an UPDATE transaction has been
 completed.  If UA B encounters this glare condition, it should reject
 this UPDATE request with a 500 response.

Okumura, et al. Informational [Page 20] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 Table 4 summarizes this section.  Each action is described in
 Section 4.3.
     | offer1    | offer2    |Action |Action |Figure|
     |  M1       |  M2       | of A  | of B  |      |
     +-----------+-----------+-------+-------+------+
     |           | re-INVITE |UAS-IcI|UAS-IcI|      |
     | re-INVITE +-----------+-------+-------+      |
     |           | UPDATE    |UAS-IcU|UAS-UcI|      |
     +-----------+-----------+-------+-------+      |
     |           | UPDATE    |UAS-UcU|UAS-UcU|      |
     |           +-----------+-------+-------+------+
     |           | 1xx-rel   |       |       |      |
     | UPDATE    +-----------+UAC-IU,|UAS-IsU|12,13 |
     |           | 2xx-INV   |UAC-UI |       |      |
     |           +-----------+-------+-------+------+
     |           | PRACK (*) |UAC-IU |UAS-IcU|  11  |
     +-----------+-----------+-------+-------+------+
      (*) invalid sequences if INVITE request is an initial one
             Table 4: Offer/Answer Glare Message Sequences

4.3. Interworking of UPDATE and Re-INVITE

 Almost all exceptional cases are caused by an interworking of UPDATE
 and re-INVITE.  The interworking is described in Section 5 of
 [RFC3311].  And UAC behavior sending an UPDATE is described in
 Section 5.1 of [RFC3311].  There are two concerns in this section:
 1.  It seems to describe different rules for each of initial INVITE
     and re-INVITE.  But there is no particular reason why the rules
     are separated.  The lack of restrictions for sending a re-INVITE
     request cause a lot of problems shown in Section 4.1.
 2.  It seems to describe that a UA may send an UPDATE request after
     sending or receiving a PRACK request.  But it should be "after
     PRACK transaction is completed by 2xx response", because it
     causes the message-crossing case shown in Figure 6.
 Since it is assumed that the language in this section itself is non-
 normative and is justified as a corollary of [RFC3261], we interpret
 it as follows:
 UAC-II:   While an INVITE transaction is incomplete or ACK
           transaction associated with an offer/answer is incomplete,
           a UA must not send another INVITE request.

Okumura, et al. Informational [Page 21] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 UAC-UU:   While an UPDATE transaction is incomplete, a UA must not
           send another UPDATE request.
 UAC-UI:   While an UPDATE transaction is incomplete, a UA should not
           send a re-INVITE request.
 UAC-IU:   While an INVITE transaction is incomplete, and an ACK or a
           PRACK transaction associated with an offer/answer is
           incomplete, a UA should not send an UPDATE request.
 When a 2xx response to an INVITE includes an offer, the ACK
 transaction is considered to be associated with an offer/answer.
 When a reliable provisional response to an INVITE includes an offer
 or an answer, the PRACK transaction is considered to be associated
 with an offer/answer.
 UAS behavior receiving an UPDATE is described in Section 5.2 of
 [RFC3311].  There are two concerns in this section:
 1.  There is no description about the interworking of an UPDATE
     request and an INVITE request without an offer.
 2.  There is no description about the interworking of an UPDATE
     request and reliable response to an INVITE with an offer.
 We interpret this section as follows:
 UAS-IcI:  While an INVITE client transaction is incomplete or ACK
           transaction associated with an offer/answer is incomplete,
           a UA must reject another INVITE request with a 491
           response.
 UAS-IsI:  While an INVITE server transaction is incomplete or ACK
           transaction associated with an offer/answer is incomplete,
           a UA must reject another INVITE request with a 500
           response.
 UAS-UcU:  While an UPDATE client transaction is incomplete, a UA must
           reject another UPDATE request with a 491 response.
 UAS-UsU:  While an UPDATE server transaction is incomplete, a UA must
           reject another UPDATE request with a 500 response.
 UAS-UcI:  While an UPDATE client transaction is incomplete, a UA
           should reject a re-INVITE request with a 491 response.

Okumura, et al. Informational [Page 22] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 UAS-UsI:  While an UPDATE server transaction is incomplete, a UA
           should reject a re-INVITE request with a 500 response.
 UAS-IcU:  While an INVITE client transaction is incomplete, and an
           ACK or a PRACK transaction associated with an offer/answer
           is incomplete, a UA should reject an UPDATE request with a
           491 response.
 UAS-IsU:  While an INVITE server transaction is incomplete, and an
           ACK or a PRACK transaction associated with an offer/answer
           is incomplete, a UA should reject an UPDATE request with a
           500 response.
 These rules are shown in following figures.
             A                               B
             |                               |
             |                         UPDATE|
             |<------------------------------|
             |UPDATE                         |
             |==============================>|
             |                            491|
             |<==============================|
             |                               |
          Figure 14: Example of UAC-UU and UAS-UcU
             A                               B
             |                               |
             |UPDATE CSeq:m                  |
             |------------------------------>|
             |UPDATE CSeq:n(>m)              |
             |==============================>|
             |            500 (UPDATE CSeq:n)|
             |<==============================|
             |                               |
          Figure 15: Example of UAC-UU and UAS-UsU

Okumura, et al. Informational [Page 23] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

             A                               B
             |                               |
             |                 UPDATE(offer1)|
             |<------------------------------|
             |reINVITE(no offer)             |
             |==============================>|
             |                   491 (INVITE)|
             |<==============================|
             |                               |
          Figure 16: Example of UAC-UI and UAS-UcI
             A                               B
             |                               |
             |UPDATE(offer1)                 |
             |------------------------------>|
             |reINVITE(no offer)             |
             |==============================>|
             |                   500 (INVITE)|
             |<==============================|
             |                               |
          Figure 17: Example of UAC-UU and UAS-UsI
             A                               B
             |                               |
             |             reINVITE(no offer)|
             |<------------------------------|
             |1xx-rel(offer0)                |
             |------------------------------>|
             |UPDATE(offer1)                 |
             |==============================>|
             |                   491 (UPDATE)|
             |<==============================|
             |                               |
          Figure 18: Example of UAC-IU and UAS-IcU

Okumura, et al. Informational [Page 24] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

             A                               B
             |                               |
             |reINVITE(no offer)             |
             |------------------------------>|
             |                1xx-rel(offer0)|
             |<------------------------------|
             |UPDATE(offer1)                 |
             |==============================>|
             |                   500 (UPDATE)|
             |<==============================|
             |                               |
          Figure 19: Example of UAC-IU and UAS-IsU
 In addition, it is assumed that the UPDATE request in this section
 includes an offer.  The interworking of a re-INVITE and an UPDATE
 without an offer is out of scope for this document.

5. Content of Offers and Answers

 While [RFC3264] and [RFC3312] 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:
 o  choice of supported media types and formats to include and exclude
 o  hold and resume of media
 The following are out of scope for this document:
 o  NAT traversal and Interactive Connectivity Establishment (ICE)
 o  specific codecs and their parameters
 o  the negotiation of secure media streams
 o  grouping of media streams
 o  preconditions

Okumura, et al. Informational [Page 25] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

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 [RFC3261] and [RFC3264] 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 [RFC3264] 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 (e.g., via [RFC6080]) 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 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.

Okumura, et al. Informational [Page 26] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

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
 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 reliable 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, the offer in a 200 response for the initial INVITE should
    include only the media types and formats believed to be acceptable
    to the user.

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 [RFC3264].  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 with types and formats the UAS would have included
 if it were the offerer.
 The media formats the answer may contain are constrained by
 [RFC3264].  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.  Doing so establishes an asymmetric
 media format situation, where these "other" media formats may only be
 sent from the offerer to the answerer.  This asymmetric media
 situation is also limited because it cannot be sustained if there is
 a subsequent offer/answer exchange in the opposite direction.  Also,
 there is limited value in including these other media formats because
 there is no assurance that the offerer will be able to use them.

Okumura, et al. Informational [Page 27] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 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.
 When the UAS wishes to reject all of the media lines in the offer, it
 may send a 488 failure response.  Alternatively, it may send a
 reliable non-failure response including all media lines with port
 numbers set to zero.

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.
 Because the offer arrives in a response to the INVITE, the UAC cannot
 reject the message containing the offer.  If the UAC wishes to reject
 the entire offer, it must send a PRACK or ACK request including all
 the media lines with ports set to zero.  Then, if it does not wish to
 continue the session, it may send a CANCEL or BYE request to
 terminate the dialog.

5.2.5. Subsequent Offers and Answers

 The guidelines above (Sections 5.1 and 5.2.1 through Section 5.2.4)
 apply, but constraints in [RFC3264] 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.  (Section 6 of [RFC3264].)
 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.  (Section 8 of [RFC3264].)  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.  (Section
    8.3.2 of [RFC3264].)

Okumura, et al. Informational [Page 28] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

       NOTE: This may be impossible for a back-to-back user agent
       (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,
 it should be constructed according to the General Principle for
 Constructing Offers and Answers (Section 5.1 ): all codecs the UA is
 currently willing and able to use should 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 most likely
 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 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

 [RFC3264] specifies (using non-normative language) that "hold" should
 be indicated in an established session by sending a new offer
 containing "a=sendonly" attribute for each media stream to be held.
 An answerer is then to respond with "a=recvonly" attribute 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"
 attribute 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" attribute if
 it does not intend to transmit any media while in hold status.
 The rules of [RFC3264] 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.

Okumura, et al. Informational [Page 29] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 The receipt of an offer containing "a=sendonly" attribute or
 "a=inactive" attribute 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" attribute, then it may initiate its own hold by sending
 a new offer containing "a=sendonly" attribute to UA1.  Upon receipt
 of that, UA1 will answer with "a=inactive" attribute because that is
 the only valid answer that reflects its desire not to receive media.
    NOTE: Section 8.4 of [RFC3264] contains a conflicting
    recommendation that the offer contain "a=inactive" attribute in
    this case.  We interpret that recommendation to be non-normative.
    The use of "a=sendonly" attribute in this case will never produce
    a worse outcome, and can produce a better outcome in useful cases.
 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" attribute.  The UA2 will
 respond with its desired state of "a=sendonly" attribute because that
 is a permitted response.  When UA2 desires to also resume, it will
 send an offer with "a=sendrecv" attribute.  In this case, because UA1
 has the same desire it will respond with "a=sendrecv" attribute.  In
 the same case, when UA2 receives the offer with "a=sendrecv"
 attribute, if it has decided it wants to reset its local hold but has
 not yet signaled the intent, it may send "a=sendrecv" attribute in
 the answer.
 If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive"
 attribute, and subsequently wants to initiate its own hold, also
 using "a=inactive" attribute, it need not send a new offer, since the
 only valid response is "a=inactive" attribute and that is already in
 effect.  However, its local desired state will now be either
 "inactive" or "a=sendonly" attribute.  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" attribute or "a=inactive"
 attribute, then it should offer that again.  If it had not previously
 initiated "hold", then it should offer "a=sendrecv" attribute, even

Okumura, et al. Informational [Page 30] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

 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 3pcc is involved.

5.4. Behavior on Receiving SDP with c=0.0.0.0

 [RFC3264] requires that an agent 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=IN IP4
 0.0.0.0", the direction attribute of the accepted media stream in the
 answer must still be based on direction attribute of the offered
 stream and rules specified in [RFC3264] to form the direction "a="
 line in the answer.  There is no clear rule about the use of "c=IN
 IP4 0.0.0.0" in the answer; it may be used or "c=" line with a valid
 IP address may be used.  RTP/RTCP will not be sent toward an address
 of 0.0.0.0 because it is an invalid address.

6. Security Considerations

 This document clarifies ambiguities in the intended behavior of the
 two SIP User Agents engaged in a dialog.  The primary specification
 of offer/answer behavior that is being clarified resides in [RFC3261]
 and [RFC3264], with extensions in [RFC3311], [RFC3312], and
 [RFC6141].  The focus of this document is on cases where ambiguities
 can result failed or degraded calls when there is no attacker.  The
 clarifications exclude call flows that lead to difficulties, without
 legitimizing any formerly invalid call flows.  Thus, the security
 considerations of the above mentioned documents continue to apply and
 need not be extended to handle any additional cases.
 The offer/answer process can be disrupted in numerous ways by an
 attacker.  SIP provides mechanisms to protect the offer/answer
 exchange from tampering by third parties.  Of note is "Enhancements
 for Authenticated  Identity Management in the Session Initiation
 Protocol (SIP)" [RFC4474], as well as Section 26.3.2, "Security
 Solutions", of [RFC3261].

7. Acknowledgements

 The authors would like to thank Christer Holmberg, Rajeev Seth,
 Nataraju A B, Byron Campen, Jonathan Rosenberg, Gonzalo Camarillo,
 and Gao Yang for their thorough reviews and comments.  Many of their
 suggestions and ideas have been incorporated in this document.

Okumura, et al. Informational [Page 31] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            June 2002.
 [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
            Provisional Responses in Session Initiation Protocol
            (SIP)", RFC 3262, June 2002.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            June 2002.
 [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
            UPDATE Method", RFC 3311, October 2002.
 [RFC3312]  Camarillo, G., Marshall, W., and J. Rosenberg,
            "Integration of Resource Management and Session Initiation
            Protocol (SIP)", RFC 3312, October 2002.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.
 [RFC6141]  Camarillo, G., Holmberg, C., and Y. Gao, "Re-INVITE and
            Target-Refresh Request Handling in the Session Initiation
            Protocol (SIP)", RFC 6141, March 2011.

Okumura, et al. Informational [Page 32] RFC 6337 SIP Usage of the Offer/Answer Model August 2011

8.2. Informative References

 [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
            Camarillo, "Best Current Practices for Third Party Call
            Control (3pcc) in the Session Initiation Protocol (SIP)",
            BCP 85, RFC 3725, April 2004.
 [RFC3959]  Camarillo, G., "The Early Session Disposition Type for the
            Session Initiation Protocol (SIP)", RFC 3959,
            December 2004.
 [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
            Authenticated Identity Management in the Session
            Initiation Protocol (SIP)", RFC 4474, August 2006.
 [RFC6080]  Petrie, D. and S. Channabasappa, "A Framework for Session
            Initiation Protocol User Agent Profile Delivery",
            RFC 6080, March 2011.

Authors' Addresses

 OKUMURA Shinji
 Softfront
 28-196, Noth9, West15, Chuo-ku
 Sapporo, Hokkaido  060-0009
 Japan
 EMail: shinji.okumura@softfront.jp
 Takuya Sawada
 KDDI Corporation
 3-10-10, Iidabashi, Chiyoda-ku
 Tokyo
 Japan
 EMail: tu-sawada@kddi.com
 Paul H. Kyzivat
 Hudson, MA  01749
 USA
 EMail: pkyzivat@alum.mit.edu

Okumura, et al. Informational [Page 33]

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