13.3.1 Processing of theINVITE
The UAS core will receiveINVITErequests from the transaction layer. It first performs the request process-ing procedures of Section 8.2, which are applied for both requests inside and outside of a dialog.
Assuming these processing states are completed without generating a response, the UAS core performs the additional processing steps:
1. If the request is anINVITE that contains an Expiresheader field, the UAS core sets a timer for the number of seconds indicated in the header field value. When the timer fires, the invitation is considered to be expired. If the invitation expires before the UAS has generated a final response, a 487 (Request Terminated) responseSHOULDbe generated.
2. If the request is a mid-dialog request, the method-independent processing described in Section 12.2.2 is first applied. It might also modify the session; Section 14 provides details.
3. If the request has a tag in the To header field but the dialog identifier does not match any of the existing dialogs, the UAS may have crashed and restarted, or may have received a request for a different (possibly failed) UAS. Section 12.2.2 provides guidelines to achieve a robust behavior under such a situation.
Processing from here forward assumes that theINVITEis outside of a dialog, and is thus for the purposes of establishing a new session.
TheINVITEmay contain a session description, in which case the UAS is being presented with an offer for that session. It is possible that the user is already a participant in that session, even though theINVITEis outside of a dialog. This can happen when a user is invited to the same multicast conference by multiple other participants. If desired, the UAS MAY use identifiers within the session description to detect this duplication. For example, SDP contains a session id and version number in the origin (o) field. If the user is already a member of the session, and the session parameters contained in the session description have not changed, the UASMAYsilently accept theINVITE(that is, send a 2xx response without prompting the user).
If the INVITEdoes not contain a session description, the UAS is being asked to participate in a session, and the UAC has asked that the UAS provide the offer of the session. ItMUSTprovide the offer in its first non-failure reliable message back to the UAC. In this specification, that is a 2xx response to theINVITE.
The UAS can indicate progress, accept, redirect, or reject the invitation. In all of these cases, it formulates a response using the procedures described in Section 8.2.6.
Progress If the UAS is not able to answer the invitation immediately, it can choose to indicate some kind of progress to the UAC (for example, an indication that a phone is ringing). This is accomplished with a provisional response between 101 and 199. These provisional responses establish early dialogs and therefore follow the procedures of Section 12.1.1 in addition to those of Section 8.2.6. A UAS MAYsend as many provisional responses as it likes. Each of theseMUSTindicate the same dialog ID. However, these will not be delivered reliably.
If the UAS desires an extended period of time to answer theINVITE, it will need to ask for an “extension”
in order to prevent proxies from canceling the transaction. A proxy has the option of canceling a transaction when there is a gap of 3 minutes between responses in a transaction.Toprevent cancellation, the UASMUST
send a non-100 provisional response at every minute, to handle the possibility of lost provisional responses.
AnINVITEtransaction can go on for extended durations when the user is placed on hold, or when interworking with PSTN systems which allow communications to take place without answering the call. The latter is common in Interactive Voice Response (IVR) systems.
TheINVITEis Redirected If the UAS decides to redirect the call, a 3xx response is sent. A 300 (Multiple Choices), 301 (Moved Permanently) or 302 (Moved Temporarily) response SHOULD contain a Contact header field containing one or more URIs of new addresses to be tried. The response is passed to the INVITEserver transaction, which will deal with its retransmissions.
TheINVITEis Rejected A common scenario occurs when the callee is currently not willing or able to take additional calls at this end system. A 486 (Busy Here) SHOULDbe returned in such a scenario. If the UAS knows that no other end system will be able to accept this call, a 600 (Busy Everywhere) response
SHOULD be sent instead. However, it is unlikely that a UAS will be able to know this in general, and thus this response will not usually be used. The response is passed to theINVITEserver transaction, which will deal with its retransmissions.
A UAS rejecting an offer contained in anINVITESHOULDreturn a 488 (Not Acceptable Here) response.
Such a responseSHOULDinclude aWarningheader field value explaining why the offer was rejected.
TheINVITEis Accepted The UAS core generates a 2xx response. This response establishes a dialog, and therefore follows the procedures of Section 12.1.1 in addition to those of Section 8.2.6.
A 2xx response to an INVITE SHOULD contain the Allow header field and theSupportedheader field, and MAYcontain theAcceptheader field. Including these header fields allows the UAC to determine the features and extensions supported by the UAS for the duration of the call, without probing.
If theINVITErequest contained an offer, and the UAS had not yet sent an answer, the 2xxMUSTcontain an answer. If theINVITEdid not contain an offer, the 2xxMUSTcontain an offer if the UAS had not yet sent an offer.
Once the response has been constructed, it is passed to theINVITEserver transaction. Note, however, that theINVITEserver transaction will be destroyed as soon as it receives this final response and passes it to the transport. Therefore, it is necessary to periodically pass the response directly to the transport until the ACK arrives. The 2xx response is passed to the transport with an interval that starts at T1 seconds and doubles for each retransmission until it reaches T2 seconds (T1 and T2 are defined in Section 17). Response retransmissions cease when anACKrequest for the response is received. This is independent of whatever transport protocols are used to send the response.
Since 2xx is retransmitted end-to-end, there may be hops between UAS and UAC that are UDP.Toensure reliable delivery across these hops, the response is retransmitted periodically even if the transport at the UAS is reliable.
If the server retransmits the 2xx response for 64*T1 seconds without receiving anACK, the dialog is con-firmed, but the sessionSHOULDbe terminated. This is accomplished with aBYE, as described in Section 15.
14 Modifying an Existing Session
A successful INVITE request (see Section 13) establishes both a dialog between two user agents and a session using the offer-answer model. Section 12 explains how to modify an existing dialog using a target refresh request (for example, changing the remote target URI of the dialog). This section describes how to modify the actual session. This modification can involve changing addresses or ports, adding a media stream, deleting a media stream, and so on. This is accomplished by sending a newINVITErequest within the same dialog that established the session. AnINVITErequest sent within an existing dialog is known as a re-INVITE.
Note that a single re-INVITEcan modify the dialog and the parameters of the session at the same time.
Either the caller or callee can modify an existing session.
The behavior of a UA on detection of media failure is a matter of local policy. However, automated gen-eration of re-INVITE or BYE is NOT RECOMMENDED to avoid flooding the network with traffic when there is congestion. In any case, if these messages are sent automatically, theySHOULDbe sent after some randomized interval.
Note that the paragraph above refers to automatically generatedBYEs and re-INVITEs. If the user hangs up upon media failure, the UA would send aBYErequest as usual.
14.1 UAC Behavior
The same offer-answer model that applies to session descriptions in INVITEs (Section 13.2.1) applies to re-INVITEs. As a result, a UAC that wants to add a media stream, for example, will create a new offer that contains this media stream, and send that in anINVITErequest to its peer. It is important to note that the full description of the session, not just the change, is sent. This supports stateless session processing in various elements, and supports failover and recovery capabilities. Of course, a UACMAYsend a re-INVITEwith no session description, in which case the first reliable non-failure response to the re-INVITEwill contain the offer (in this specification, that is a 2xx response).
If the session description format has the capability for version numbers, the offerer SHOULD indicate that the version of the session description has changed.
TheTo,From,Call-ID,CSeq, andRequest-URIof a re-INVITEare set following the same rules as for regular requests within an existing dialog, described in Section 12.
A UAC MAYchoose not to add anAlert-Infoheader field or a body withContent-Disposition“alert” to re-INVITEs because UASs do not typically alert the user upon reception of a re-INVITE.
Unlike anINVITE, which can fork, a re-INVITEwill never fork, and therefore, only ever generate a single final response. The reason a re-INVITEwill never fork is that theRequest-URIidentifies the target as the UA instance it established the dialog with, rather than identifying an address-of-record for the user.
Note that a UAC MUST NOT initiate a new INVITE transaction within a dialog while another INVITE transaction is in progress in either direction.
1. If there is an ongoingINVITEclient transaction, the TUMUSTwait until the transaction reaches the completed or terminated state before initiating the newINVITE.
2. If there is an ongoingINVITEserver transaction, the TUMUSTwait until the transaction reaches the
confirmed or terminated state before initiating the newINVITE.
However, a UAMAYinitiate a regular transaction while anINVITEtransaction is in progress. A UAMAY
also initiate anINVITEtransaction while a regular transaction is in progress.
If a UA receives a non-2xx final response to a re-INVITE, the session parametersMUSTremain unchanged, as if no re-INVITEhad been issued. Note that, as stated in Section 12.2.1, if the non-2xx final response is a 481 (Call/Transaction Does Not Exist), or a 408 (Request Timeout), or no response at all is received for the re-INVITE(that is, a timeout is returned by theINVITEclient transaction), the UAC will terminate the dialog.
If a UAC receives a 491 response to a re-INVITE, itSHOULDstart a timer with a value T chosen as follows:
1. If the UAC is the owner of the Call-IDof the dialog ID (meaning it generated the value), T has a randomly chosen value between 2.1 and 4 seconds in units of 10 ms.
2. If the UAC is not the owner of theCall-IDof the dialog ID, T has a randomly chosen value of between 0 and 2 seconds in units of 10 ms.
When the timer fires, the UACSHOULDattempt the re-INVITEonce more, if it still desires for that session modification to take place. For example, if the call was already hung up with aBYE, the re-INVITEwould not take place.
The rules for transmitting a re-INVITEand for generating anACKfor a 2xx response to re-INVITEare the same as for the initialINVITE(Section 13.2.1).
14.2 UAS Behavior
Section 13.3.1 describes the procedure for distinguishing incoming re-INVITEs from incoming initial IN-VITEs and handling a re-INVITEfor an existing dialog.
A UAS that receives a second INVITEbefore it sends the final response to a first INVITEwith a lower CSeq sequence number on the same dialog MUST return a 500 (Server Internal Error) response to the secondINVITEand MUSTinclude aRetry-Afterheader field with a randomly chosen value of between 0 and 10 seconds.
A UAS that receives anINVITEon a dialog while anINVITEit had sent on that dialog is in progressMUST
return a 491 (Request Pending) response to the receivedINVITE.
If a UA receives a re-INVITEfor an existing dialog, it MUSTcheck any version identifiers in the session description or, if there are no version identifiers, the content of the session description to see if it has changed.
If the session description has changed, the UASMUSTadjust the session parameters accordingly, possibly after asking the user for confirmation.
Versioning of the session description can be used to accommodate the capabilities of new arrivals to a conference, add or delete media, or change from a unicast to a multicast conference.
If the new session description is not acceptable, the UAS can reject it by returning a 488 (Not Acceptable Here) response for the re-INVITE. This responseSHOULDinclude aWarningheader field.
If a UAS generates a 2xx response and never receives anACK, itSHOULDgenerate aBYEto terminate the dialog.
A UASMAYchoose not to generate 180 (Ringing) responses for a re-INVITEbecause UACs do not typically render this information to the user. For the same reason, UASsMAYchoose not to use anAlert-Infoheader field or a body withContent-Disposition“alert” in responses to a re-INVITE.
A UAS providing an offer in a 2xx (because the INVITEdid not contain an offer)SHOULD construct the offer as if the UAS were making a brand new call, subject to the constraints of sending an offer that updates an existing session, as described in [12] in the case of SDP. Specifically, this means that itSHOULDinclude as many media formats and media types that the UA is willing to support. 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. If, however, it is unacceptable to the UAC, the UACSHOULD generate an answer with a valid session description, and then send aBYEto terminate the session.
15 Terminating a Session
This section describes the procedures for terminating a session established by SIP. The state of the session and the state of the dialog are very closely related. When a session is initiated with anINVITE, each 1xx or 2xx response from a distinct UAS creates a dialog, and if that response completes the offer/answer exchange, it also creates a session. As a result, each session is “associated” with a single dialog - the one which resulted in its creation. If an initialINVITEgenerates a non-2xx final response, that terminates all sessions (if any) and all dialogs (if any) that were created through responses to the request. By virtue of completing the transaction, a non-2xx final response also prevents further sessions from being created as a result of the INVITE. The BYErequest is used to terminate a specific session or attempted session. In this case, the specific session is the one with the peer UA on the other side of the dialog. When aBYEis received on a dialog, any session associated with that dialogSHOULDterminate. A UAMUST NOTsend aBYEoutside of a dialog. The caller’s UAMAYsend aBYEfor either confirmed or early dialogs, and the callee’s UAMAY
send aBYEon confirmed dialogs, butMUST NOTsend aBYEon early dialogs. However, the callee’s UA
MUST NOTsend aBYEon a confirmed dialog until it has received anACKfor its 2xx response or until the server transaction times out. If no SIP extensions have defined other application layer states associated with the dialog, theBYEalso terminates the dialog.
The impact of a non-2xx final response to INVITEon dialogs and sessions makes the use of CANCEL attractive. TheCANCELattempts to force a non-2xx response to theINVITE(in particular, a 487). There-fore, if a UAC wishes to give up on its call attempt entirely, it can send aCANCEL. If theINVITEresults in 2xx final response(s) to theINVITE, this means that a UAS accepted the invitation while theCANCELwas in progress. The UACMAYcontinue with the sessions established by any 2xx responses, orMAYterminate them withBYE.
The notion of “hanging up” is not well defined within SIP. It is specific to a particular, albeit common, user interface.
Typically, when the user hangs up, it indicates a desire to terminate the attempt to establish a session, and to terminate any sessions already created. For the caller’s UA, this would imply aCANCELrequest if the initialINVITEhas not generated a final response, and aBYEto all confirmed dialogs after a final response. For the callee’s UA, it would typically imply aBYE; presumably, when the user picked up the phone, a 2xx was generated, and so hanging up would result in aBYEafter theACKis received. This does not mean a user cannot hang up before receipt of theACK, it just means that the software in his phone needs to maintain state for a short while in order to clean up
properly. If the particular UI allows for the user to reject a call before its answered, a 403 (Forbidden) is a good way to express that. As per the rules above, aBYEcan’t be sent.
15.1 Terminating a Session with aBYERequest
15.1.1 UAC Behavior
ABYErequest is constructed as would any other request within a dialog, as described in Section 12.
Once theBYEis constructed, the UAC core creates a new non-INVITEclient transaction, and passes it the BYErequest. The UAC MUSTconsider the session terminated (and therefore stop sending or listening for media) as soon as theBYErequest is passed to the client transaction. If the response for theBYEis a 481 (Call/Transaction Does Not Exist) or a 408 (Request Timeout) or no response at all is received for theBYE (that is, a timeout is returned by the client transaction), the UACMUSTconsider the session and the dialog terminated.
15.1.2 UAS Behavior
A UAS first processes theBYErequest according to the general UAS processing described in Section 8.2.
A UAS core receiving aBYErequest checks if it matches an existing dialog. If theBYEdoes not match an existing dialog, the UAS coreSHOULDgenerate a 481 (Call/Transaction Does Not Exist) response and pass that to the server transaction.
This rule means that aBYEsent without tags by a UAC will be rejected. This is a change from RFC 2543, which allowedBYEwithout tags.
A UAS core receiving aBYErequest for an existing dialog MUSTfollow the procedures of Section 12.2.2 to process the request. Once done, the UASSHOULDterminate the session (and therefore stop sending and listening for media). The only case where it can elect not to are multicast sessions, where participation is possible even if the other participant in the dialog has terminated its involvement in the session. Whether or not it ends its participation on the session, the UAS coreMUSTgenerate a 2xx response to theBYE, and
MUSTpass that to the server transaction for transmission.
The UASMUSTstill respond to any pending requests received for that dialog. It isRECOMMENDED that a 487 (Request Terminated) response be generated to those pending requests.