SIPPING G. Camarillo Internet-Draft Ericsson Intended status: Informational March 3, 2009 Expires: September 4, 2009 Re-INVITE Handling in the Session Initiation Protocol (SIP) draft-camarillo-sipping-reinvite-00.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 September 4, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract In this document, we clarify the handling of re-INVITEs in SIP. We clarify in which situations a UAS (User Agent Server) should generate a success response and in which situations a UAS should generate an Camarillo Expires September 4, 2009 [Page 1] Internet-Draft Re-INVITE Handling March 2009 error response to a re-INVITE. Additionally, we clarify issues related to target refresh requests. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background on Re-INVITE Handling by UASs . . . . . . . . . . . 3 3. Clarifications on the Target Refresh Procedure . . . . . . . . 11 4. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 9. Normative References . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 Camarillo Expires September 4, 2009 [Page 2] Internet-Draft Re-INVITE Handling March 2009 1. Introduction As discussed in Section 14 of RFC 3261 [RFC3261], an INVITE request sent within an existing dialog is known as a re-INVITE. A re-INVITE is used to modify session parameters, dialog parameters, or both. That is, a single re-INVITE can change both the parameters of its associated session (e.g., changing the IP address where a media stream is received) and the parameters of its associated dialog (e.g., changing the remote target of the dialog). A re-INVITE can change the remote target of a dialog because it is a target refresh request, as defined in Section 6 of RFC 3261 [RFC3261]. There has been some confusion among implentors regarding how a UAS (User Agent Server) should handle re-INVITEs. In particular, implementors requested clarifications on which type of response a UAS should generate in different situations. In this document, we clarify these issues. OPEN ISSUE: this document is written as an informational document but could be turned into a normative document by adding a few normative statements if needed. 2. Background on Re-INVITE Handling by UASs A UAS receiving a re-INVITE will need to, eventually, generate a response to it. Some re-INVITEs can be responded to immediately because their handling does not require user interaction (e.g., changing the IP address where a media stream is received). The handling of other re-INVITEs requires user interaction (e.g., adding a video stream to an audio-only session). Therefore, these re- INVITEs cannot be responded to immediately. An error response to a re-INVITE has clear semantics. As specified in Section 12.2.2 of RFC 3261 [RFC3261], if a re-INVITE is rejected, none of the state changes requested by the re-INVITE are performed. The example in Figure 1 illustrates this point. Camarillo Expires September 4, 2009 [Page 3] Internet-Draft Re-INVITE Handling March 2009 UAC UAS | | |-------------(1) INVITE SDP1--------------->| | | |<------------(2) 200 OK SDP2----------------| | | |------------------(3) ACK------------------>| | | | | |-------------(4) INVITE SDP3--------------->| | | |<-----------------(5) 6xx-------------------| | | |------------------(6) ACK------------------>| | | Figure 1: Rejection of a re-INVITE The UAs perform an offer/answer exchange to establish an audio-only session: SDP1: m=audio 30000 RTP/AVP 0 SDP2: m=audio 31000 RTP/AVP 0 At a later point, the UAC sends a re-INVITE (4) in order to add a video stream to the session. SDP3: m=audio 30000 RTP/AVP 0 m=video 30002 RTP/AVP 31 The UAS is automatically configured to reject video streams. Consequently, the UAS returns an error response (5). At that point, the session parameters in use are still those resulting from the initial offer/answer exchange, which are described by SDP1 and SDP2. That is, the session and dialog states are the same as before the re- INVITE was received. In the previous example, the UAS wanted to reject all the changes requested in the re-INVITE. That is why generating an error response Camarillo Expires September 4, 2009 [Page 4] Internet-Draft Re-INVITE Handling March 2009 was appropriate. However, there are situations where a UAS wants to accept some but not all the changes requested in a re-INVITE. In these cases, the UAS generates a 200 (OK) response with an SDP indicating which changes were accepted and which were not. The example in Figure 2 illustrates this point. UAC UAS | | |-------------(1) INVITE SDP1--------------->| | | |<------------(2) 200 OK SDP2----------------| | | |------------------(3) ACK------------------>| | | | | |-------------(4) INVITE SDP3--------------->| | | |<------------(5) 200 OK SDP4----------------| | | |------------------(6) ACK------------------>| | | Figure 2: Automatic rejection of a video stream The UAs perform an offer/answer exchange to establish an audio only session: SDP1: m=audio 30000 RTP/AVP 0 c=IN IP4 192.0.2.1 SDP2: m=audio 31000 RTP/AVP 0 c=IN IP4 192.0.2.5 At a later point, the UAC moves to an access that provides a higher- bandwidth. Therefore, the UAC sends a re-INVITE (4) in order to change the target of the dialog to its new location, change the IP address where it receives the audio stream to its new IP address, and add a video stream to the session. Camarillo Expires September 4, 2009 [Page 5] Internet-Draft Re-INVITE Handling March 2009 SDP3: m=audio 30000 RTP/AVP 0 c=IN IP4 192.0.2.2 m=video 30002 RTP/AVP 31 c=IN IP4 192.0.2.2 The UAS is automatically configured to reject video streams. However, the UAS needs to accept the change of the remote dialog target and the change of the audio stream's remote IP address. Consequently, the UAS returns a 200 (OK) response and sets the port of the video stream to zero in its SDP. SDP4: m=audio 31000 RTP/AVP 0 c=IN IP4 192.0.2.5 m=video 0 RTP/AVP 31 c=IN IP4 192.0.2.2 In the previous example, the UAS was configured to automatically reject the addition of video streams. The example in Figure 3 assumes that the UAS requires its user's input in order to accept or reject the addition of a video stream and uses reliable provisional responses [RFC3262] (PRACK transactions are not shown for clarity). Camarillo Expires September 4, 2009 [Page 6] Internet-Draft Re-INVITE Handling March 2009 UAC UAS | | |-------------(1) INVITE SDP1--------------->| | | |<------------(2) 200 OK SDP2----------------| | | |------------------(3) ACK------------------>| | | | | |-------------(4) INVITE SDP3--------------->| | | |<----(5) 183 Session Progress SDP4----------| | | | | |<------------(6) UPDATE SDP5----------------| | | |-------------(7) 200 OK SDP6--------------->| | | |<---------------(8) 200 OK------------------| | | |------------------(9) ACK------------------>| | | Figure 3: Rejection of a video stream by the user Everything up to (4) is identical to the previous example. In (5), the UAS accepts the change of the audio stream's remote IP address but does not accept the video stream yet (it provides a null IP address instead of setting the stream to 'inactive' because inactive streams still need to exchange RTCP traffic). SDP4: m=audio 31000 RTP/AVP 0 c=IN IP4 192.0.2.5 m=video 31002 RTP/AVP 31 c=IN IP4 0.0.0.0 At a later point, the UAS's user rejects the addition of the video stream. Consequently, the UAS sends an UPDATE request setting the port of the video stream to zero in its SDP. Camarillo Expires September 4, 2009 [Page 7] Internet-Draft Re-INVITE Handling March 2009 SDP5: m=audio 31000 RTP/AVP 0 c=IN IP4 192.0.2.5 m=video 0 RTP/AVP 31 c=IN IP4 0.0.0.0 The UAS now returns a 200 (OK) response to the re-INVITE. Note that if the UAS returned an error response at this point, the media state would need to revert to the state before the re-INVITE. That is, even if the change of the IP address of the audio stream had already been accepted and executed within the re-INVITE, the audio stream would need to go back to the old IP address. Using an error response in this situation has a set of problems, which are discussed in Section 4. The example in Figure 4 also assumes that the UAS requires its user's input in order to accept or reject the addition of a video stream and uses reliable provisional responses [RFC3262] (PRACK transactions are not shown for clarity). Camarillo Expires September 4, 2009 [Page 8] Internet-Draft Re-INVITE Handling March 2009 UAC UAS | | |-------------(1) INVITE SDP1--------------->| | | |<------------(2) 200 OK SDP2----------------| | | |------------------(3) ACK------------------>| | | | | |-------------(4) INVITE SDP3--------------->| | | |<----(5) 183 Session Progress SDP4----------| | | |-------------(6) UPDATE SDP5--------------->| | | |<------------(7) 200 OK SDP6----------------| | | | | |<------------(8) UPDATE SDP7----------------| | | |-------------(9) 200 OK SDP8--------------->| | | |<--------------(10) 200 OK------------------| | | |-----------------(11) ACK------------------>| | | Figure 4: Rejection of a video stream by the user The UAs perform an offer/answer exchange to establish an audio only session: SDP1: m=audio 30000 RTP/AVP 0 c=IN IP4 192.0.2.1 SDP2: m=audio 31000 RTP/AVP 0 c=IN IP4 192.0.2.5 At a later point, the UAC sends a re-INVITE (4) in order to add a new codec to the audio stream and to add a video stream to the session. Camarillo Expires September 4, 2009 [Page 9] Internet-Draft Re-INVITE Handling March 2009 SDP3: m=audio 30000 RTP/AVP 0 3 c=IN IP4 192.0.2.1 m=video 30002 RTP/AVP 31 c=IN IP4 192.0.2.1 In (5), the UAS accepts the addition of the audio codec but does not accept the video stream yet (it provides a null IP address instead of setting the stream to 'inactive' because inactive streams still need to exchange RTCP traffic). SDP4: m=audio 31000 RTP/AVP 0 3 c=IN IP4 192.0.2.5 m=video 31002 RTP/AVP 31 c=IN IP4 0.0.0.0 At a later point, the UAC sends an UPDATE request (6) to remove the original audio codec from the audio stream (the UAC could have also used the PRACK to (5) to request this change). SDP5: m=audio 30000 RTP/AVP 3 c=IN IP4 192.0.2.1 m=video 30002 RTP/AVP 31 c=IN IP4 192.0.2.1 SDP6: m=audio 31000 RTP/AVP 3 c=IN IP4 192.0.2.5 m=video 31002 RTP/AVP 31 c=IN IP4 0.0.0.0 Yet at a later point, the UAS's user rejects the addition of the video stream. Additionally, the UAS decides to revert to the original audio codec. Consequently, the UAS sends an UPDATE request (8) setting the port of the video stream to zero and offering the original audio codec in its SDP. SDP7: m=audio 31000 RTP/AVP 0 c=IN IP4 192.0.2.5 m=video 0 RTP/AVP 31 c=IN IP4 0.0.0.0 Camarillo Expires September 4, 2009 [Page 10] Internet-Draft Re-INVITE Handling March 2009 The UAC accepts the change in the audio codec in its 200 (OK) response (9) to the UPDATE request. SDP5: m=audio 30000 RTP/AVP 0 c=IN IP4 192.0.2.1 m=video 0 RTP/AVP 31 c=IN IP4 192.0.2.1 The UAS now returns a 200 (OK) response (10) to the re-INVITE. Note that the media state after this 200 (OK) response is the same as the pre-re-INVITE media state. Section 4 discusses why the UAS uses an UPDATE request (8) and a 200 (OK) response (10) to the re-INVITE instead of an error response to the re-INVITE in order to go back to the pre-re-INVITE media state. 3. Clarifications on the Target Refresh Procedure Both re-INVITEs and UPDATE [RFC3311] requests are target refresh requests for INVITE-initiated dialogs. There is some confusion regarding how target refresh requests are handled within a re-INVITE. Section 5.1 of [RFC3311] specifies how to handle UPDATE requests within re-INVITEs but does not describe all possible situations in detail. In this section, we clarify those situations that are more likely to be unclear from the specification. When a UAC sends a re-INVITE refreshing the target of the dialog, the UAS needs to start using the new target immediately. For example, if the UAS needs to send an UPDATE request within the re-INVITE, the UAS will use the new target. When the UAS sends a request to the new target, it is implicitly accepting the refresh of the target. Therefore, as discussed in Section 4, the UAS will respond to the re- INVITE with a 200 (OK) response, even if it needs to reject media changes in the response's SDP. A UAC can send a re-INVITE refreshing the target of the dialog and later send an UPDATE request within the re-INVITE refreshing the target again. In this case, a 200 (OK) response to the re-INVITE would accept the latest target refresh within the re-INVITE. That is, the target refresh performed by the UPDATE request. 4. UAS Behavior UASs should only return an error response to a re-INVITE if no changes to the session or to the dialog state have been performed Camarillo Expires September 4, 2009 [Page 11] Internet-Draft Re-INVITE Handling March 2009 since the re-INVITE was received. Such an error response indicates that no changes have been made as a result of the re-INVITE or any other transaction within it. If changes requested by a re-INVITE or any transaction within it have already been performed, the UAS should always return a 200 (OK) response. Even if the UAS would like to revert to the pre-INVITE state, it would still return a 200 (OK) to the INVITE request. In order to undo changes that were already executed, the UAS uses a new offer/answer exchange or a target refresh request. The reason for not using an error response to undo already executed changes is that an error response to a re-INVITE for which changes have already been executed is effectively requesting a change in the session or the dialog state. However, the UAC has no means to reject those changes if it is unable to execute them. That is, if the UAC is unable to revert to the pre-re-INVITE state, it will not be able to communicate this fact to the UAS. On the other hand, if the UAS uses an offer or a target refresh request to request changes in the session, the UAC will able to reject the changes if it is unable to execute them. Using an error response to undo already executed changes presents an additional problem. SIP includes rules to avoid offer/answer collisions in race conditions. Even when both UAs generate an offer at the same time, there are rules to determine which one should be processed first. However, there are no rules to avoid a collision between an offer in an UPDATE request and an error response for a re- INVITE. Since both the UPDATE request and the error response would request changes, it would not be clear which changes would need to be executed first. This is yet another reason why UASs should not use error responses to undo already executed changes. A change to the session state is considered to have been executed when the new media parameters are being used. Therefore, a change to a stream subject to preconditions [RFC4032] is considered to have been executed when the new media parameters start being used, not when the preconditions for the stream are met. See [draft-camarillo-sipping-precons-00.txt] for a more-detailed discussion on this issue. 5. UAC Behavior The behavior of a UAC communicating with a well-behaved UAS (a UAS that follows the guidelines in Section 4 is straight forward. However, a UAC may face a legacy UAS that uses an error response to undo already executed changes within a re-INVITE. In order to cope with this type of UAS, a UAC that receives an error Camarillo Expires September 4, 2009 [Page 12] Internet-Draft Re-INVITE Handling March 2009 response to a re-INVITE but cannot revert to the media state before the re-INVITE for some reason should perform a new offer/answer exchange in order to set the parameters of the session and the dialog so that the session can continue. This new offer/answer exchange should contain the minimum set of changes needed to continue the session in order to minimize the chances of the UAS rejecting it as well. If the error response reverted an already executed target refresh, the UAC should use a new target refresh request in order to set the target to an appropriate value. 6. Security Considerations This document does not introduce any new security issue. Security issues related to re-INVITEs and UPDATE requests are discussed in RFC 3261 [RFC3261] and RFC 3311 [RFC3311]. 7. IANA Considerations There are no IANA actions associated with this document. 8. Acknowledgements Paul Kyzivat, Christer Holmberg, and Yang Gao provided useful ideas on the topics discussed in this document. 9. Normative References [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE Method", RFC 3311, October 2002. [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session Initiation Protocol (SIP) Preconditions Framework", RFC 4032, March 2005. Camarillo Expires September 4, 2009 [Page 13] Internet-Draft Re-INVITE Handling March 2009 Author's Address Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: Gonzalo.Camarillo@ericsson.com Camarillo Expires September 4, 2009 [Page 14]