SIPPING Working Group T. Sawada Internet Draft KDDI Corporation Expires:
JanuaryFebruary 2008 P. Kyzivat Cisco Systems, Inc. July 9,August 28, 2007 SIP (Session Initiation Protocol) Usage of Offer/Answer Model draft-ietf-sipping-sip-offeranswer-02.txtdraft-ietf-sipping-sip-offeranswer-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on November 9,28, 2007. Abstract SIP utilizes offer/answer model to establish and update multimedia sessions. The descriptions on how to use offer/answer in SIP are dispersed in the multiple RFCs. This document summarizes all the current usage of offer/answer model in SIP communication. Table of Contents 1. Summary of SIP usage of Offer/Answer Model................2Model...................2 1.1. Offer/Answer Exchange Pairs in SIP Messages...........3Messages.............3 1.2. Rejection against an Offer.........................4Offer..............................4 1.3. Session Description which is not Offer nor Answer.......5 2. Detailed Discussion on Offer/Answer Model for SIP...........5SIP............5 2.1. Offer/Answer for INVITE method with 100rel extension....5 2.1.1. INVITE Request with SDP.......................6SDP............................6 2.1.2. INVITE request without SDP.....................8SDP.........................8 2.2. Offer/Answer Exchange in Early Dialog................9Dialog...................9 2.3. Offer/Answer Exchange in Established Dialog...........9Dialog.............9 3. Exceptional Case Handling.............................10Handling...................................10 3.1. Message Crossing Case Handling.....................10Handling.........................10 3.2. Glare Case Handling..............................11Handling....................................11 4. Content of Offers and Answers..........................12Answers...............................12 4.1. General Principle for Constructing Offers and Answers..12 4.2. Choice of Media Types and Formats to Include and Exclude13 4.2.1. Sending Initial INVITE with Offer..............13Offer.................13 4.2.2. Responding with Offer when Initial INVITE has no Offer13Offer....................................................13 4.2.3. Answering Initial INVITE with Offer.............14Offer...............14 4.2.4. Answering when Initial INVITE had no Offer.......14Offer........14 4.2.5. Subsequent Offers and Answers..................14Answers.....................14 4.3. Hold and Resume of media..........................15media...............................15 5. Remaining Issues or Best Practices on Offer/Answer.........16Offer/Answer..........16 5.1. Rejecting PRACK Offer............................16Offer..................................17 5.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE Transaction........................................17Transaction.................................................17 5.3. Offer in a Reliable Response...........................19 6. Add New Offer/Answer Usage in SIP......................18SIP...........................19 6.1. Explicit Usage..................................18Usage.........................................19 6.2. Rejection of an Offer............................19Offer..................................19 6.3. Backward Compatibility...........................19Compatibility.................................20 6.4. Exceptional Case Handling.........................19Handling..............................20 7. Security Considerations..............................19Considerations.....................................20 8. References.........................................19References..................................................20 8.1. Normative References.............................19References...................................20 8.2. Informative References...........................19References.................................20 Author's Addresses.....................................20Addresses.............................................21 Full Copyright Statement................................20Statement.......................................21 Intellectual Property Statement..........................20 Acknowledgment........................................21Statement................................21 Acknowledgment.................................................22 1. Summary of SIP usage of Offer/Answer Model The offer/answer model itself is independent from the higher layer application protocols which utilize it. SIP is one of the applications using offer/answer model. RFC 3264  defines the offer/answer model, but does not specify which SIP message should convey an offer or an answer. This should be defined in the SIP core and extensions RFCs. In theory, any SIP message can include a session description in its body. But a session description in a SIP message is not necessarily an offer or an answer. Only the session description that conforms to the rules described in the standards-track RFCs can be interpreted as an offer or an answer. The rules for how to handle the offer/answer model are currently defined in several RFCs. The offer/answer model defines the update of sessions. In SIP, dialog is used to match the offer/answer exchange to the session which is to be updated with it. In other words, only the offer/answer exchange in the SIP dialog can update the session which is managed with it. 1.1. Offer/Answer Exchange Pairs in SIP Messages Currently, the rules on offer/answer model are defined in RFC 3261 , RFC 3262  and RFC 3311 . 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. Pattern 2 and pattern 4 can occur only when the INVITE request does not include an offer. 'The first reliable non-failure message' must have an offer if there is no offer in the INVITE request. This means that UA which receives the INVITE request without an offer must include an offer in the first reliable response with 100rel extension. If no reliable provisional response has been sent, the UAS must include an offer when sending 2xx response. In pattern 3, the first reliable provisional response may or may not have an answer. When a reliable provisional response contains a session description, and is the first to do so, then that session description is the answer to the offer in the INVITE request. The answer can not be updated, and a new offer can not be sent, in a subsequent reliable response for the same INVITE transaction. In pattern 5, a PRACK request can contain an offer only if the reliable response which it acknowledges contains an answer in the previous offer/answer exchange. Offer Answer RFC Ini Est Early ------------------------------------------------------------------- 1. INVITE Req. 2xx INVITE Resp. RFC 3261 O O X 2. 2xx INVITE Resp. ACK Req. RFC 3261 O O X 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 O O X 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 O O X 5. PRACK Req. 200 PRACK Resp. RFC 3262 X O O 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 X O O Table 1. Summary of SIP Usage of Offer/Answer Model In Table 1, '1xx-rel' corresponds to the reliable provisional response which contains the 100rel option defined in RFC 3262 . The 'Ini' column shows the ability to exchange the offer/answer to initiate the session. 'O' indicates that the pattern can be used in the initial offer/answer exchange, while 'X' indicates that it can not. Only the initial INVITE transaction can be used to exchange the offer/answer to establish multimedia session. 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. 1.2. Rejection against an Offer How to reject an offer when it can not be accepted is not so clear and some methods can not allow explicit rejection against an offer. Corresponding to the patterns in Table 1, how to reject an offer is shown in Table 2. When a UA receives an INVITE request with an unacceptable offer, it should respond with a 488 response, preferably with Warning header field indicating the reason of the rejectionrejection, unless another response code is more appropriate to reject it. (Pattern 1 and Pattern 3) When a UA receives an UPDATE request with an offer which it can not accept, it should respond with a 488 response preferably with Warning header field indicating the reason of the rejection, unless another response code is more appropriate to reject it. (Pattern 6) When a UA receives a PRACK request with an offer which it can not accept, it may respond with a 200 response with a syntactically correct session description followed by an UPDATE request possibly to rearrange the session parameters if both ends support UPDATE method. A UA may simply give up continuingmethod, or alternatively terminate the dialog and send an error response to the INVITE request. (Pattern 5) When a UA receives a response with an offer which it can not accept, a UA does not have a way to reject it explicitly. Therefore, a UA should respond to the offer with the correct session description and rearrange the session parameters by initiating a new offer/answer exchange, or justalternatively terminate the session. (Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA should take care not to cause a never-ending offer/answer loop. Offer Rejection ----------------------------------------------------- 1. INVITE Req. 488 INVITE Response 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer OR termination of dialog 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer OR termination of dialog 6. UPDATE Req. 488 UPDATE Response Table 2. Rejection against an Offer (*) UA should only use PRACK to send an offer when it has strong reasons to assume the receiver will accept. 1.3. Session Description which is not Offer nor Answer As previously stated, a session description in a SIP message is not necessarily an offer or an answer. For example, SIP can use a session description to describe capabilities apart from offer/answer exchange. Examples of this are 200 OK responses for OPTIONS and 488 responses for INVITE. 2. Detailed Discussion on Offer/Answer Model for SIP 2.1. Offer/Answer for INVITE method with 100rel extension The INVITE method provides the basic procedure for offer/answer exchange in SIP. Without the 100rel option, the rules are simple as described in RFC 3261 . If an INVITE request includes a session description, pattern 1 is applied and if an INVITE request does not include a session description, pattern 2 is applied. With 100rel, pattern 3 and pattern 4 are added and this makes the rules complicated. An INVITE request may cause multiple responses. Note that even if both UAs support the 100rel extension, not all the provisional responses may be sent reliably. Note also that a reliable provisional response is allowed without a session description if the UAS does not wish to send the answer yet. An unreliable provisional response may include a session description in the body if the UAS has not sent a reliable response, but its session description is neither an offer nor an answer. All the session descriptions in the unreliable responses to the INVITE request must be identical to the answer which is included in the reliable response. Session descriptions in an unreliable response that precedes a reliable response can be considered a "preview" of the session description that will be coming, and hence may be treated like an offer or an answer until the actual one arrives. NOTE: This "preview" session description rule applies to a single offer/answer exchange. In parallel offer/answer exchanges (caused by forking) a UA may obviously receive the different "preview" of answer in each dialog. UAs are expected to deal with this. 2.1.1. INVITE Request with SDP When a UAC includes an SDP body in the INVITE request as an offer, it expects the answer to be received with one of the reliable responses. Other than that, no offer/answer exchanges can occur in the INVITE 3- way handshake process. UAC UAS | F1 INVITE (SDP) | <- The offer in offer/answer model |-------------------->| | F2 1xx (SDP) | <- The SDP is not an official answer but |<--------------------| UAC acts as if it receives the answer. | | ^ | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer |<--------------------| | SDP. | F4 PRACK (no SDP) | | |-------------------->| | UAC must not send a new offer. | F5 2xx PRA (no SDP) | | |<--------------------| v | | | F6 1xx-rel (SDP) | <- The answer in offer/ answer model |<--------------------| - | F7 PRACK | | UAC can send a new offer in a PRACK |-------------------->| | request to acknowledge F6. | F8 2xx PRA | | After F7 UAC and UAS can send a new offer |<--------------------| v in an UPDATE request. | | | F9 1xx-rel | <- SDP should not be included in the |<--------------------| subsequent 1xx-rel once offer/answer | F10 PRACK | has been completed. |-------------------->| | F11 2xx PRA | |<--------------------| | | | F12 2xx INV | <- SDP should not be included in the final |<--------------------| response once offer/answer has been | F13 ACK | completed. |-------------------->| Figure 1 Example of Offer/Answer with 100rel Extension (1) For example, in Figure 1, only the SDP in F6 is the answer. The SDP in the non-reliable response (F2) is the preview of the answer and must be the same as the answer in F6, but is not officially the answer. Receiving F2, UAC should act as if it receives the answer. However, offer/answer exchange is not completed yet and UAC must not send a new offer until it receives the same SDP in the first reliable response, which is the real answer. After sending the SDP in F6, UAS must prepare to receive new offer from UAC with an UPDATE request or a PRACK request. UAS does not include an SDP in the responses F9 and F12. However, UAC should prepare to receive SDP bodies in F9 and/or F12, and just ignore them for the case that the peer does not conform to the recommended implementation. 2.1.2. INVITE request without SDP When UAC does not include an SDP body in the INVITE request, it expects the offer to be received with the first reliable response. UAC will send the answer in the request to acknowledge the response, i.e. PRACK or ACK request for the reliable response. Other than that, no offer/answer exchanges can occur in the INVITE 3-way handshake process. For example, in Figure 2, only the SDP in F3 is the offer. The SDP in the non-reliable response (F2) is the preview of the offer and must be the same as the offer in F3, but is not officially the offer. Receiving F2, UAC can act as if it receives the offer. However, the official offer is not received until it receives the first reliable response. The first reliable response (F3) must include an SDP as an offer. UAS should not include SDP in the responses F6 and F9. However, UAC should prepare to receive SDP bodies in F6 and/or F9, and just ignore them for the case that the peer does not conform to the recommended implementation. UAC UAS | F1 INVITE (no SDP) | |-------------------->| | F2 1xx (SDP) | <- SDP may be included but it is not the |<--------------------| offer. UAC may act as if it receives | | the offer. | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain an SDP |<--------------------| as the offer. | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel |-------------------->| must contain an SDP as the answer. | F5 2xx PRA (no SDP) | - |<--------------------| | | | | | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not |<--------------------| | contain an SDP. | F7 PRACK | | |-------------------->| | UAC can send a new offer in an UPDATE | F8 2xx PRA | | request after F4. |<--------------------| v | | | F9 2xx INV (no SDP) | <- The final response should not |<--------------------| contain an SDP. | F10 ACK | |-------------------->| Figure 2 Example of Offer/Answer with 100rel Extension (2) 2.2. Offer/Answer Exchange in Early Dialog When both UAs support the 100rel extension, they can update the session in the early dialog once the first offer/answer exchange has been completed. From UA sending an INVITE request: UA can send an UPDATE request with a new offer if both ends support the UPDATE method. Support for the UPDATE method must be declared in an Allow header in some prior messages in the dialog. UA can send a PRACK request with a new offer only when acknowledging the reliable provisional response with the answer to the offer in the INVITE request. Compared to using the UPDATE method, using PRACK can save messages to be exchanged between the UAs. However, as a PRACK request should not be rejected, UA is recommended to send a PRACK request only when it has strong reasons to assume the receiver will accept it. For example, the procedure used in precondition extension  is a case where a PRACK request should be used for updating the session status in the early dialog. From UA receiving an INVITE request: UA can send an UPDATE request with a new offer if both ends support UPDATE method. UAS can not send a new offer in the reliable provisional response. So the UPDATE method is the only method for UAS to update the early session. 2.3. Offer/Answer Exchange in Established Dialog The re-INVITE and UPDATE methods can be used in the established dialog to update the session. The UPDATE method is simpler and can save at least one message compared with INVITE method. But both ends must support the UPDATE method for it to be used. The INVITE method needs at least three messages to complete but no extensions are needed. Additionally, the INVITE method allows the peer to take time to decide whether it will accept a session update or not by sending provisional responses. That is, re-INVITE allows the UAS to interact with the user at the peer, while UPDATE needs to be answered automatically by the UAS. It is noted that re-INVITE should be answered immediately unless such a user interaction is needed. Otherwise, some 3pcc flows will break. 3. Exceptional Case Handling In RFC 3264 , the following restrictions are defined with regard to sending a new offer. "It MUST NOT generate a new offer if it has received an offer which it has not yet answered or rejected. It MUST NOT generate a new offer if it has generated a prior offer for which it has not yet received an answer or a rejection." Assuming that the above rules are guaranteed, there seems to be two possible 'exceptional' cases to be considered in SIP offer/answer usage, which are the 'message crossing' case and the 'glare' case. One of the reasons why the usage of a SIP method to exchange offer/answer needs to be carefully restricted in the RFCs is to make sure that UA can detect and handle appropriately the 'exceptional' cases to avoid the confusion. 3.1. Message Crossing Case Handling When message packets are crossed in the transport network, an offer may be received before the answer for the previous offer/answer exchange as described in Figure 3. In such a case, UA A must detect the session description of the offer2 is not the answer to the offer1. A B |offer1 | |----------------->| | answer1| |<------\ /-------| | \/ | | /\ offer2| |<------/ \-------| Figure 3 Message Crossing Case When offer2 is in an UPDATE request or a re-INVITE request, a session description can never be the answer. Then UA A must reject the message including offer2 with a 491 response with Retry-After header field. When offer2 is in a PRACK request, that is, when a PRACK request to acknowledge the reliable provisional response with an answer to the offer in the INVITE request contains a session description, UA A knows it is an offer. As a PRACK request should not be rejected, UA A is recommended to wait for the answer1 before sending a PRACK response with the answer to the offer2. Note that if UA A does not send a new offer until the reliable provisional response with an answer to the offer in the INVITE request is acknowledged with a PRACK request, this case never happens. Therefore, to make implementations simple, a UA acting as a UAS for an INVITE transaction is recommended not to send an UPDATE request with an offer until the reliable response with an answer to the offer in the INVITE request is acknowledged with a PRACK request. When offer2 is in a reliable provisional response or a successful final response, UA A knows it is not the answer to the offer1. For a reliable response to an initial INVITE request, this case never happens. For a reliable response to a re-INVITE request, UA A can detect the offer2 is not the answer1. In this case, UA A can not reject offer2 in a reliable response, it is recommended to wait for answer1 before sending a PRACK request with the answer to offer2. Note that this case only occurs when UA A, while waiting for an answer, sends an INVITE request without session description. 3.2. Glare Case Handling When both ends in a dialog send a new offer at nearly the same time, UA may receive a new offer before it receives the answer to the offer it sent as described in Figure 4. This case is called a 'glare' case in general. A B |offer1 offer2| |-------\ /-------| | \/ | | /\ | |<------/ \------>| Figure 4 Glare Case When offer2 is in an UPDATE request or (re-)INVITE request, it must be rejected with a 491 response. When offer2 is in a PRACK request (within the current rules, only possible if offer1 is in an UPDATE request), the PRACK may be accepted with 200 or may be rejected with a 491 response. A 491 response is valid to satisfy the offer/answer model but it may delay the completion of the reliable response transfer mechanism or, in worst case, may result in the failure to complete the SIP transaction because there is no clear retry rule when a PRACK request is rejected with a 491 response. To avoid this glare condition, UA A should not send an offer if it has already sent a reliable provisional response containing an answer to a previous offer and has not received the corresponding PRACK request. To avoid a glare condition involving an offer in a response, when UA A has sent a (re)INVITE request without session description, it should not send an offer until it has received an offer in a reliable response to the (re)INVITE, and sent an answer to that offer. 4. Content of Offers and Answers While RFCs 3264 and 3312 give some guidance, questions remain about exactly what should be included in an offer or answer. This is especially a problem when the common "hold" feature has been activated, and when there is the potential for a multimedia call. Details of behavior depend on the capabilities and state of the User Agent. The kinds of recommendations that can be made are limited by the model of device capabilities and state that is presumed to exist. This section focuses on a few key aspects of offers and answers that have been identified as troublesome, and will consider other aspects to be out of scope. This section considers: - choice of supported media types and formats to include and exclude - hold and resume of media The following are out of scope for this document: - NAT traversal and ICE - specific codecs and their parameters - the negotiation of secure media streams - grouping of media streams - preconditions 4.1. General Principle for Constructing Offers and Answers A UA should send an offer that indicates what it, and its user, are interested in using/doing at that time, without regard for what the other party in the call may have indicated previously. 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 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. 4.2. Choice of Media Types and Formats to Include and Exclude 4.2.1. Sending Initial INVITE with Offer When a UAC sends an initial INVITE with an offer, it has complete freedom to choose which media type(s) and media format(s) (payload types in the case of RTP) it should include in the offer. The media types may be all or a subset of the media the UAC is capable of supporting, with the particular subset being determined by the design and configuration  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. 4.2.2. Responding with Offer when Initial INVITE has no Offer When a UAS has received an initial INVITE without an offer, it must include an offer in the first reliable response to the INVITE. It has largely the same options as when sending an initial INVITE with an offer, but there are some differences. The choice may be governed by both static (default) selections of media types as well as dynamic selections made by a user via interaction with the device while it is alerting. NOTE: The offer may be sent in a provisional response, before the user of the device has been alerted and had an opportunity to select media options for the call. In this case the UAS cannot include any call-specific options from the user of the device. 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. 4.2.3. Answering Initial INVITE with Offer When a UAS receives an initial INVITE with an offer, what media lines the answer may contain is constrained by RFC 3264. The answer must contain the same number of m-lines as the offer, and they must contain the same media types. Each media line may be accepted, by including a non-zero port number, or rejected by including a zero port number in the answer. The media lines that are accepted should typically be those that would have been offered had the INVITE not contained an offer, but with those not offered removed. The media formats the answer may contain are constrained by RFC 3264 . 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. 4.2.4. Answering when Initial INVITE had no Offer When a UAC has sent an initial INVITE without an offer, and then receives a response with the first offer, it should answer in the same way as a UAS receiving an initial INVITE with an offer. 4.2.5. Subsequent Offers and Answers The guidelines above (sections 4.1. and 4.2.1. through 184.108.40.206.2.4. ) apply, but constraints in RFC 3264  must also be followed. The following are of particular note because they have proven troublesome: o The number of m-lines may not be reduced in a subsequent offer. Previously rejected media streams must remain, or be reused to offer the same or a different stream. o In the o-line, only the version number may change, and if it changes it must increment by one from the one previously sent as an offer or answer. If it doesn't change then the entire SDP body must be identical to what was previously sent as an offer or answer. Changing the o-line, except version number value, during the session is an error case. The behavior when receiving such a non-compliant offer/answer SDP is implementation dependent. If a UA needs to negotiate a 'new' SDP session, it should use the INVITE/Replaces method. o In the case of RTP, the mapping from a particular dynamic payload type number to a particular codec within that media stream (m- line) MUST NOT change for the duration of the session. NOTE: This may be impossible for a B2BUA to follow in some cases (e.g. 3pcc transfer) if it does not terminate media. 4.3. Hold and Resume of media RFC 3264  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  constrain what may be in an answer when the offer contains "sendonly", "recvonly", or "inactive" in an a= line. But they do not constrain what must be in a subsequent offer. The General Principle for Constructing Offers and Answers (section 4.1. ) is important here. The initiation of "hold" is a local action. It should affect the desired state of the UA. It then affects what the UA includes in offers and answers until the local state is reset. The receipt of an offer containing "a=sendonly" or "a=inactive" and the sending of a compatible answer should not change the desired state of the recipient. However, a UA that has been "placed on hold" may itself desire to initiate its own hold status, based on local input. If UA2 has previously been "placed on hold" by UA1, via receipt of "a=sendonly", then it may initiate its own hold by sending a new offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will answer with "a=inactive" because that is the only valid answer that reflects its desire not to receive media. Once in this state, to resume a two way exchange of media each side must reset its local hold status. If UA1 is first to go off hold it will then send an offer with "a=sendrecv". The UA2 will respond with its desired state of "a=sendonly" because that is a permitted response. When UA2 desires to also resume, it will send an offer with "a=sendrecv". In this case, because UA1 has the same desire it will respond "a=sendrecv". If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", and subsequently wants to initiate its own hold, 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". 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 4.1. ) 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. Remaining Issues or Best Practices on Offer/Answer This document clarifies the offer/answer usage in SIP and summarizes the correct or recommended behaviors along with the existing RFCs. To create any new normative behaviors beyond these RFCs is not the intent of this document. However, through the scrutiny of the offer/answer model in SIP, some issues are found to be unresolved within the current set of RFCs. Those remaining issues are described in this section mainly for further study. 5.1. Rejecting PRACK Offer As stated in section 1.2. and 2.2. , it is recommended not to send an offer in a PRACK request unless UAC has strong reasons to assume the receiver will accept it. Even so, there may be the cases when the UAS has to reject the offer for some reason. The current RFCs do not provide the way to reject the offer and at the same time to acknowledge the reliable response. Several candidatesideas were proposedpresented to resolve this issue, such as sending 2xx PRACK response without SDP to reject the offer.offer, or sending an SDP with decreased o-line version value. Some of the candidates may also be adapted as a way to reject an unacceptable offer in a response. Anyway, those candidates violate the current rules and lose backward compatibility to some extent.extent (e.g. section 5 of RFC 3262). It is beyond the scope of this document and remains for further study. NOTE: Deprecation of the usage of offer in PRACK may be another solution. As the precondition mechanism specification  explicitly shows a usage of sending offer in PRACK, its deprecation could cause backward compatibility issues. 5.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE Transaction When a re-INVITE transaction fails, often the dialog remains with the session bound to it. The issue here is what the session status is if offer/answer exchange has been completed (if a session description has been sent in a reliable provisional response to the re-INVITE request), or if subsequent offer/answer exchanges have taken place (using UPDATE or PRACK transactions), before the re-INVITE transaction failsis terminated with thea final failureerror response (Figure 5). One option is to take those offer/answer exchanges not committed yet and to make the session status rollback to the one before re-INVITE transaction was initiated. Another option is to take those exchanges committed and to keep the session status as it is even after re- INVITE fails. There is no clear consensus on which one is the correct behavior. There are some cases where it is useful to exchange offer(s)/answer(s) even before re-INVITE completes. The case of adding a new media (like adding video to audio only session) which requires permission from the peer through some user interaction is one example. Precondition procedures can be another case which may require several offer/answer exchanges in one re-INVITE transaction. UAC UAS | session established | |<===================>| | | | F1 re-INVITE (SDP) | |-------------------->| | F2 1xx-rel (SDP) | |<--------------------| | F3 PRACK | <- PRACK request may include new offer and |-------------------->| can complete the offer/answer with | F4 2xx PRA | the answer in 2xx PRACK response. |<--------------------| | | <- UPDATE method can update the session | | status before receiving the final | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). |<--------------------| | F6 ACK | |-------------------->| Issue: What is the correct session status | | after re-INVITE transaction. Figure 5 Commit/Rollback Issue with re-INVITE transaction To make bad things worse, if a new offer from UAC and the final response to re-INVITE are sent at nearly the same time, the UAS can not know whether this new offer was sent before or after UAC received the final failure response (Figure 6). Note that the ACK request to the failure response is sent hop-by-hop basis, therefore even after receiving the ACK request, UAS can not make sure that UPDATE request was sent after the final response had been reached to the other end. Sending a new UPDATE request from UAC to synchronize the status anytime after the re-INVITE fails may be a good option. This solution, however, requires that the UPDATE method be supported by both ends and needs care to avoid flapping when each end tries to advertise their different views of the session status. To resolveThe proper handling of this issue may beis undefined by existing standards. Resolution is beyond the scope of this documentdocument, and will require anothera new normative document. Such a document whichis the responsibility of the SIP working group, and is for further study. UAC UAS | session established | |<===================>| | | | F1 re-INVITE (SDP) | |-------------------->| | F2 1xx-rel (SDP) | |<--------------------| | F3 PRACK | |-------------------->| | F4 2xx PRA | |<--------------------| | | |UPDATE(SDP) 4xx INV | |---------\ /--------| | \/ | | /\ | |<--------/ \------->| | | Figure 6 Commit/Rollback Issue with Race Condition 5.3. Offer in a Reliable Response In RFC 3261, it is stated that when an INVITE is sent without an offer, the first reliable response MUST contain an offer. There was discussion on whether this rule can be loosened up. There is no clear explanation why this restriction is defined. However, this rule will be left as it is, unless the strong necessity to loose it up will come up in the future. 6. Add New Offer/Answer Usage in SIP It is not recommended to add new SIP methods for the offer/answer exchange beyond the ways described in this document. However, it may be requested to have new offer/answer exchange methods as SIP extensions evolve. In this clause, what should be taken into considerations is noted. 6.1. Explicit Usage New method definitions should define offer/answer usage explicitly without any ambiguity. 6.2. Rejection of an Offer New method definitions should define how to reject an offer where possible. 6.3. Backward Compatibility New methods must keep backward compatibility. 6.4. Exceptional Case Handling New methods should take care of how to handle exceptional cases, message crossing case and glare case. 7. Security Considerations There are not any security issues beyond the referenced RFCs. 8. References 8.1. Normative References  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.  Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in the Session Initiation Protocol (SIP)", RFC 3262, June 2002.  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with SDP", RFC 3264, June 2002.  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE Method", RFC 3311, September 2002.  Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002. 8.2. Informative References  Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent Profile Data Set for Media Policy", draft-ietf-sipping-media- policy-dataset-04 (work in progress), May 2007. Author's Addresses Takuya Sawada KDDI Corporation 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan Email: firstname.lastname@example.org Paul H. Kyzivat Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA 01719 USA Email: email@example.com Full Copyright Statement Copyright (C) The IETF Trust (2007). 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