SIP (Session Initiation Protocol) Usage of the Offer/Answer Model

SIP utilizes the offer/answer model to establish and update sessions. The rules to govern the offer/answer behaviors in SIP are described in the several RFCs. (, , , and .) The primary purpose of this document is to describe all forms of SIP usage of the offer/answer model in one document to help the readers to fully understand it. Also, this document tries to incorporate the results of the discussions on the controversial issues to avoid repeating the same discussions later. This document is not intended to make normative changes. Rather, it makes the remaining open issues clear and leaves them for further study.

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 RFC 2119 . This document only uses these key words when referencing normative statements in existing RFCs.

The offer/answer model itself is independent from the higher layer application protocols which utilize it. SIP is one of the applications using the offer/answer model. defines the offer/answer model, but does not specify which SIP messages should convey an offer or an answer. This should be defined in the SIP core and extensions RFCs. In theory, any SIP message can include a session description in its body. But a session description in a SIP message is not necessarily an offer or an answer. Only certain session description usages that conform to the rules described in standards-track RFCs can be interpreted as an offer or an answer. The rules for how to handle the offer/answer model are currently defined in several RFCs. The offer/answer model defines a mechanism for update of sessions. In SIP, a dialog is used to associate an offer/answer exchange with the session which it is to update. In other words, only the offer/answer exchange in the SIP dialog can update the session which is managed by that dialog.

Currently, the rules on the offer/answer model are defined in , , , and . 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. 'The first reliable non-failure message' must have an offer if there is no offer in the INVITE request. This means that UA which receives the INVITE request without an offer must include an offer in the first reliable response with 100rel extension. If no reliable provisional response has been sent, the UAS must include an offer when sending 2xx response. In pattern 3, the first reliable provisional response may or may not have an answer. When a reliable provisional response contains a session description, and is the first to do so, then that session description is the answer to the offer in the INVITE request. The answer can not be updated, and a new offer can not be sent in a subsequent reliable response for the same INVITE transaction. In pattern 5, a PRACK request can contain an offer only if the reliable response which it acknowledges contains an answer to the previous offer/answer exchange. NOTE: It is legal to have UPDATE/2xx exchanges without offer/answer exchanges (pattern 6). However when re-INVITEs are sent for non-offer/answer purposes, an offer/answer exchange is required. In that case the prior SDP will typically be repeated. There may be ONLY ONE offer/answer negotiation in progress for a single dialog at any point in time. 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" . That is not addressed by this draft.

Offer Answer RFC Ini Est Early ------------------------------------------------------------------- 1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N 2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N 5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y In Table 1, '1xx-rel' corresponds to the reliable provisional response which contains the 100rel option defined in . The 'Ini' column shows the ability to exchange the offer/answer to initiate the session. 'Y' indicates that the pattern can be used in the initial offer/answer exchange, while 'N' indicates that it can not. Only the initial INVITE transaction can be used to exchange the offer/answer to establish a multimedia session. The 'Est' column shows the ability to update the established session. The 'Early' column indicates which patterns may be used to modify the established session in an early dialog. There are two ways to exchange a subsequent offer/answer in an early dialog.

It is not entirely clear how to reject an offer when it is unacceptable, and some methods do not allow explicit rejection of an offer. For each of the patterns in Table 1, Table 2 shows how to reject an offer. When a UA receives an INVITE request with an unacceptable offer, it should respond with a 488 response, preferably with Warning header field indicating the reason of the rejection, unless another response code is more appropriate to reject it. (Pattern 1 and Pattern 3) When a UA receives an UPDATE request with an offer which it can not accept, it should respond with a 488 response preferably with Warning header field indicating the reason of the rejection, unless another response code is more appropriate to reject it. (Pattern 6) When a UA receives a PRACK request with an offer which it can not accept, it may respond with a 200 response with a syntactically correct session description. This may optionally be followed by an UPDATE request to rearrange the session parameters if both ends support the UPDATE method. Alternatively the UA may terminate the dialog and send an error response to the INVITE request. The validity and consequences of a 488 response to PRACK is an open issue which is discussed in . (Pattern 5) When a UA receives a response with an offer which it can not accept, the UA does not have a way to reject it explicitly. Therefore, a UA should respond to the offer with the correct session description and rearrange the session parameters by initiating a new offer/answer exchange, or alternatively terminate the session. (Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA should take care not to cause an infinite offer/answer loop.

Offer Rejection ----------------------------------------------------- 1. INVITE Req. 488 INVITE Response 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer OR termination of dialog 3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer OR termination of dialog 6. UPDATE Req. 488 UPDATE Response (*) A UA should only use PRACK to send an offer when it has strong reasons to expect the receiver will accept the offer.

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.

The INVITE method provides the basic procedure for offer/answer exchange in SIP. Without the 100rel option, the rules are simple as described in . 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 complicates the rules. An INVITE request may cause multiple responses. Note that even if both UAs support the 100rel extension, not all the provisional responses may be sent reliably. Note also that a reliable provisional response is allowed without a session description if the UAS does not wish to send the answer yet. An unreliable provisional response may include a session description in the body if the UAS has not sent a reliable response, but its session description is neither an offer nor an answer. All the session descriptions in the unreliable responses to the INVITE request must be identical to the answer which is included in the reliable response. A session description in an unreliable response that precedes a reliable response can be considered a "preview" of the answer that will be coming, and hence may be treated like an answer until the actual one arrives. NOTE: This "preview" session description rule applies to a single offer/answer exchange. In parallel offer/answer exchanges (caused by forking) a UA may obviously receive a different "preview" of an answer in each dialog. UAs are expected to deal with this. Although RFC 3261 says a UA should accept media once an INVITE with an offer has been sent, in many cases, an answer (or, at least a preview of it) is required in order for media to be accepted. Two examples of why this might be required are: 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. 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.

When a UAC includes an SDP body in the INVITE request as an offer, it expects the answer to be received with one of the reliable responses. Other than that, no offer/answer exchanges can occur in the messages within the INVITE transaction.

| | F2 1xx (SDP) | <- The offer/answer exchange is not |<--------------------| closed yet, but UAC acts as if it | | ^ receives the answer. | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer |<--------------------| | SDP. | F4 PRACK (no SDP) | | |-------------------->| | UAC must not send a new offer. | F5 2xx PRA (no SDP) | | |<--------------------| v | | | F6 1xx-rel (SDP) | <- The answer in the offer/ answer model |<--------------------| - | F7 PRACK | | UAC can send a new offer in a PRACK |-------------------->| | request to acknowledge F6. | F8 2xx PRA | | After F7 UAC and UAS can send a new |<--------------------| v offer in an UPDATE request. | | | F9 1xx-rel | <- SDP should not be included in the |<--------------------| subsequent 1xx-rel once offer/answer | F10 PRACK | has been completed. |-------------------->| | F11 2xx PRA | |<--------------------| | | | F12 2xx INV | <- SDP should not be included in the |<--------------------| final response once offer/answer has | F13 ACK | been completed. |-------------------->| ]]> For example, in Figure 1, only the SDP in F6 is the answer. The SDP in the non-reliable response (F2) is the preview of the answer and must be the same as the answer in F6. Receiving F2, the UAC should act as if it receives the answer. However, offer/answer exchange is not completed yet and the UAC must not send a new offer until it receives the same SDP in the first reliable response, which is the real answer. After sending the SDP in F6, the UAS must prepare to receive a new offer from the UAC with an UPDATE request or a PRACK request. The UAS does not include SDP in responses F9 and F12. However, the UAC should prepare to receive SDP bodies in F9 and/or F12, and just ignore them, to handle a peer that does not conform to the recommended implementation.

When a UAC does not include an SDP body in the INVITE request, it expects the offer to be received with the first reliable response. The UAC will send the answer in the request to acknowledge the response, i.e. PRACK or ACK request of the reliable response. Other than that, no offer/answer exchanges can occur in the messages within the INVITE transaction. NOTE: The UAS should not include SDP in the responses F6 and F9. However, the UAC should prepare to receive SDP bodies in F6 and/or F9, and just ignore them to handle a peer that does not conform to the recommended implementation.

| | F2 1xx | |<--------------------| | | | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP |<--------------------| as the offer. | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel |-------------------->| must contain SDP as the answer. | F5 2xx PRA (no SDP) | - |<--------------------| | | | | | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not |<--------------------| | contain SDP. | F7 PRACK | | |-------------------->| | UAC can send a new offer in an UPDATE | F8 2xx PRA | | request after F4. |<--------------------| v | | | F9 2xx INV (no SDP) | <- The final response should not |<--------------------| contain SDP. | F10 ACK | |-------------------->| ]]> 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.

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 is a case where a PRACK request should be used for updating the session status in an early dialog. Note also that if a UAS needs to prompt the user to accept or reject the offer, the delay can result in retransmission of the PRACK request. From a UA receiving an INVITE request: A UA can send an UPDATE request with a new offer if both ends support the UPDATE method. A UAS can not send a new offer in the reliable provisional response, so the UPDATE method is the only method for a UAS to update an early session.

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 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.

In , 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. 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.

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.

| |SDP-2 (answer1)| M2 |<------\ /-------| | \/ | |SDP-3 /\(offer2)| M3 |<------/ \-------| ]]> 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.) There is a variant, shown in Figure 4, which is dependent on an INVITE (Mx) that contains no offer. This case should be extremely rare - it is easily avoided by delaying Mx until answer1 is received. It adds another possibility to Table 3.

| |re-INV (no offer) | Mx |------------------------------>| --+ |SDP-2 answer1 (2xx-UPD)| | M2 |<===========\ /===============| | first reliable | \/ offer2| | response |SDP-3 /\ (1xx-rel/2xx)| | M3 |<===========/ \===============| <-+ |SDP-4 answer2 (PRACK/ACK)| My |------------------------------>| Wait until answer1 | | ]]>

Table 3 shows that there are only two ambiguous cases when an answer is expected and an arriving message M2 containing SDP could be either the expected answer or an offer. These are a reliable 1xx response to an INVITE, or an UPDATE. When message M2 is an UPDATE request or a (re)INVITE request, then message M1 must also have been an UPDATE or INVITE. There may have been message crossing, or not. If not then it is a glare case. Either way, the remedy is for UA A to reject message M2 with a 491 response with Retry-After header field. When M2 is a reliable provisional response or a successful final response, and M1 was an UPDATE, then SDP-2 cannot be the expected answer1. In this case, since UA A can not reject offer2 in reliable response M2, it is recommended that it wait for answer1 before sending a PRACK request with the answer to offer2. Note that this case only occurs when UA A, while waiting for an answer, sends an INVITE request without session description. When M2 is a PRACK request Table 3 shows that it cannot be an offer out of order, so UA A may infer SDP-2 is an answer. Table 4 summarizes the discussions above.

When both ends in a dialog send a new offer at nearly the same time, as described in Figure 5, 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.

| ]]> 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.

While and 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

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 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.

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 ) 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.

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.

When a UAS receives an initial INVITE with an offer, what media lines the answer may contain is constrained by . The answer must contain the same number of m-lines as the offer, and they must contain the same media types. Each media line may be accepted, by including a non-zero port number, or rejected by including a zero port number in the answer. The media lines that are accepted should typically be those that would have been offered had the INVITE not contained an offer, excluding those not offered. The media formats the answer may contain are constrained by . 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.

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.

The guidelines above ( and through ) apply, but constraints in must also be followed. The following are of particular note because they have proven troublesome: 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 .) 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 .) 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. 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 .) NOTE: This may be impossible for a B2BUA to follow in some cases (e.g. 3pcc transfer) if it does not terminate media. When the new offer is sent in response to an offerless (re)INVITE, all codecs supported by the UA are to be included, not just the ones that were negotiated by previous offer/answer exchanges. The same is true for media types - so if UA A initially offered audio and video to UA B, and they end up with only audio, and UA B sends an offerless (re)INVITE to UA A, A's resulting offer should re- attempt video, by reusing the zeroed m-line used previously. NOTE: The behavior above is recommended, but it is not always achievable - for example in some interworking scenarios. Or, the offerer may simply not have enough resources to offer "everything" at that point. Even if the UAS is not able to 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.

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 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 () is important here. The initiation of "hold" is a local action. It should reflect the desired state of the UA. It then affects what the UA includes in offers and answers until the local state is reset. The receipt of an offer containing "a=sendonly" or "a=inactive" and the sending of a compatible answer should not change the desired state of the recipient. However, a UA that has been "placed on hold" may itself desire to initiate its own hold status, based on local input. If UA2 has previously been "placed on hold" by UA1, via receipt of "a=sendonly", then it may initiate its own hold by sending a new offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will answer with "a=inactive" because that is the only valid answer that reflects its desire not to receive media. Once in this state, to resume a two way exchange of media each side must reset its local hold status. If UA1 is first to go off hold it will then send an offer with "a=sendrecv". The UA2 will respond with its desired state of "a=sendonly" because that is a permitted response. When UA2 desires to also resume, it will send an offer with "a=sendrecv". In this case, because UA1 has the same desire it will respond with "a=sendrecv". In the same case, when UA2 receives the offer with "a=sendrecv", if it has decided it wants to reset its local hold but has not yet signaled the intent, it may send "a=sendrecv" in the answer. If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", and subsequently wants to initiate its own hold, also using "a=inactive", it need not send a new offer, since the only valid response is "a=inactive" and that is already in effect. However, its local desired state will now be either "inactive" or "a=sendonly". This affects what it will send in future offers and answers. If a UA has occasion to send another offer in the session, without any desire to change the hold status (e.g. in response to a re- INVITE without an offer, or when sending a re-INVITE to refresh the session timer) it should follow the General Principle for Constructing Offers and Answers (). 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.

specifies that an agent MUST be capable of receiving SDP with a connection address of 0.0.0.0, in which case it means that neither RTP nor RTCP should be sent to the peer. If a UA generates an answer to the offer received with c=0.0.0.0, the direction attribute of the accepted media stream in the answer must be based on direction attribute of the offered stream and rules specified in RFC 3264 to form the a-line in the answer. c=0.0.0.0 has no special meaning for the direction attribute of the accepted stream in the answer.

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.

As stated in and , it is recommended that an offer not be sent in a PRACK request unless UAC has strong reasons to assume the receiver will accept it. Even so, there may be cases when the UAS has to reject the offer for some reason. The current RFCs do not provide a way to reject the offer and at the same time to indicate that the PRACK adequately acknowledged the reliable response. It is unclear whether a non-200 response can still indicate an acknowledgement of the reliable response. Several ideas were presented to resolve this issue, such as sending 2xx PRACK response without SDP to reject the offer, or sending SDP with a decreased version value in the o-line. Some of the candidates may also be adapted as a way to reject an unacceptable offer in a response. Anyway, those proposals violate the current rules and lose backward compatibility to some extent (e.g. section 5 of ). It is beyond the scope of this document and remains for further study. The 488 response is another proposed solution; however the validity and consequences of a 488 response to PRACK is an open issue. Because the 488 response may be sent by a proxy, the UAC cannot assume the reliable transaction has been adequately acknowledged. If a 488 response is received, the UAC should ensure acknowledgment of the reliable response by sending a new PRACK with the offer removed or modified based upon the received 488 response. If the 488 response is sent by UAS (open issue), it cannot assume that the UAC thinks that the reliable transaction has been adequately acknowledged even though the UAS may treat otherwise (open issue). If a 488 response is sent by UAS, the UAC should accommodate receiving the altered PRACK with higher CSeq without expecting it to trigger a 481 response (open issue). NOTE: Deprecation of the usage of offer in PRACK may be another solution. As the precondition mechanism specification explicitly shows a usage of sending offer in PRACK, its deprecation could cause backward compatibility issues.

When a re-INVITE transaction fails, the dialog remains with the session bound to it. The issue here is: what is the session status if an offer/answer exchange has been completed (if a session description has been sent in a reliable provisional response to the re-INVITE request), or if subsequent offer/answer exchanges have taken place (using UPDATE or PRACK transactions), before the re- INVITE transaction is terminated with a final error response (Figure 6). 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.

| | | | 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. ]]> 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 7). Note that the ACK request to the failure response is sent hop-by-hop basis, therefore even after receiving the ACK request, UAS can not make sure that UPDATE request was sent after the final response had been reached to the other end. Sending a new UPDATE request from UAC to synchronize the status anytime after the re-INVITE fails may be a good option. This solution, however, requires that the UPDATE method be supported by both ends and needs care to avoid flapping when each end tries to advertise their different views of the session status. The proper handling of this issue is undefined by existing standards. Resolution is beyond the scope of this document, and will require a new normative document.

| | | | F1 re-INVITE (SDP) | |-------------------->| | F2 1xx-rel (SDP) | |<--------------------| | F3 PRACK | |-------------------->| | F4 2xx PRA | |<--------------------| | | |UPDATE(SDP) 4xx INV | |---------\ /--------| | \/ | | /\ | |<--------/ \------->| | | ]]>

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 loosen it up is raised in the future.

RFC 3264, section 8.4, contains procedures for putting a unicast media stream on hold. Of particular note, it states: "If the stream to be placed on hold was previously a recvonly media stream, it is placed on hold by marking it inactive." of the current document makes a recommendation for this case which conflicts with that, and explains why. Some concerns have been raised that such a recommendation is invalid because RFC 3264 is normative on this subject. This document takes the position that Section 8.4 of RFC 3264 is non-normative, and so may be overridden. It is further recommended that RFC 3264 be revised to avoid the confusion.

This document recommends against the addition of new offer/answer methods using SIP. However, it may be necessary to define new offer/answer exchange methods as SIP extensions evolve. This section recommends some things that should be taken into considerations in that case.

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

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

New methods must keep backward compatibility.

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

This document has no actions for IANA.

There are not any security issues beyond the referenced RFCs.

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