SIPCORE Working Group C. Holmberg Internet-Draft Ericsson Expires: June 12, 2010 December 9, 2009 Response Code for Indication of Terminated Dialog draft-ietf-sipcore-199-02.txt Abstract This specification defines a new SIP response code, 199 Early Dialog Terminated, which a SIP forking proxy and a UAS can use to indicate upstream towards the UAC that an early dialog has been terminated, before a final response is sent towards the UAC. 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 June 12, 2010. 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Holmberg Expires June 12, 2010 [Page 1] Internet-Draft 199 December 2009 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. User Agent Client behavior . . . . . . . . . . . . . . . . . . 4 4.1. Examples of resource types . . . . . . . . . . . . . . . . 5 4.2. Examples of policy procedures . . . . . . . . . . . . . . 6 5. User Agent Server behavior . . . . . . . . . . . . . . . . . . 7 6. Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Backward compatibility . . . . . . . . . . . . . . . . . . . . 9 8. Usage with SDP offer/answer . . . . . . . . . . . . . . . . . 9 9. Usage with 100rel . . . . . . . . . . . . . . . . . . . . . . 10 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10.1. Example with a forking proxy which generates 199 . . . . . 10 10.2. Example with a forking proxy which receives 200 OK . . . . 11 10.3. Example with two forking proxies, of which one generates 199 . . . . . . . . . . . . . . . . . . . . . . 12 11. Security Considerations . . . . . . . . . . . . . . . . . . . 13 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 12.1. IANA Registration of the 199 response code . . . . . . . . 14 12.2. IANA Registration of the 199 Option Tag . . . . . . . . . 14 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 14.1. Normative References . . . . . . . . . . . . . . . . . . . 14 14.2. Informational References . . . . . . . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 Holmberg Expires June 12, 2010 [Page 2] Internet-Draft 199 December 2009 1. Introduction As defined in SIP (Session Initiation Protocol) specification [RFC3261], an early SIP dialog is created when a non-100 provisional response is sent to the dialog initiation request (e.g. INVITE). The dialog is considered to be in early state until a final response is sent. When a proxy receives an initial request (outside an existing dialog, and without a pre-defined route set), it can forward it towards multiple remote destinations. When the proxy does that, it performs forking. When a forking proxy receives non-100 provisional responses, it forwards the responses upstream towards the sender of the associated request. When a forking proxy receives a 2xx final response, it forwards the response upstream towards the sender of the associated request. At that point the proxy normally sends a CANCEL request downstream towards all remote destinations where it previously sent the request associated with the 2xx final response, and from which it has yet not received a final response, in order to terminate associated outstanding early dialogs. It is possible to receive multiple 2xx final responses. When SIP entities upstream receive the first 2xx final response, and they do not to intend to accept subsequent 2xx final responses, they will automatically terminate other associated outstanding early dialogs. If additional 2xx final responses are received, those SIP entities will normally send a BYE request using the dialog identifier retrieved from the subsequent 2xx final response. NOTE: A UAC can use the Request-Disposition header [RFC3841] to request that proxies do not send CANCEL requests downstream once they have received the first final 2xx response. When a forking proxy receives a non-2xx final response, it does not always immediately forward the response upstream towards the sender of the associated request. Instead, the forking proxy "stores" it and waits for further final responses from remote destinations where the forked request was forwarded. At some point the proxy uses a specified mechanism to determine the "best" final response code, and forwards that final response upstream towards the sender of the associated request. When SIP entities upstream receive the non-2xx final response they will release resources associated with the session, and the UAC will terminate, or retry, the session setup. Since the forking proxy does not always immediately forward non-2xx final responses, SIP entities upstream (including the UAC that initiated the request) do not know that a specific early dialog has Holmberg Expires June 12, 2010 [Page 3] Internet-Draft 199 December 2009 been terminated, and the SIP entities keep possible resources associated with the early dialog until they receive a final response from the forking proxy. This specification defines a new SIP response code, 199 Early Dialog Terminated, which a forking proxy and a UAS can use to indicate upstream that an early dialog has been terminated. The 199 response can also be sent by a UAS, prior to sending a non-2xx final response. SIP entities that receive the 199 provisional response can release resources associated with the specific early dialog. The SIP entities can also use the 199 provisional response to make policy related decisions related to early dialogs. The 199 response code is an optimization, which allows the UAC to be informed about terminated early dialogs. However, since the support of the 199 response is optional, a UAC cannot assume that it will always receive a 199 provisional response for all terminated early dialogs. 2. Conventions 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 BCP 14, RFC 2119 [RFC2119]. 3. Requirements REQ 1: It must be possible to indicate to the UAC that an early dialog has been terminated before a final response is sent. 4. User Agent Client behavior When a UAC sends an initial request, and if it wants to receive 199 responses, it MUST insert the 199 option-tag in the Supported header, which indicates that the client supports the 199 Early Dialog Terminated response code. The UAC SHOULD NOT insert the 199 option- tag in the Require header, unless the particular session usage requires the UAS to support the response code. Also, the UAC SHOULD NOT insert the 199 option-tag in the Proxy-Require header, unless the particular session usage requires every proxy on the path to support the response code. Using Require or Proxy-Require with the 199 option-tag will in many cases result in unnecessary session establishment failures. Holmberg Expires June 12, 2010 [Page 4] Internet-Draft 199 December 2009 When a UAC receives a 199 response it MAY release resources and procedures associated with the early dialog on which the 199 response is received. Examples of resources and procedures are e.g. procedures for the establishment of media plane resources (bandwidth, radio, codecs etc), media security procedures or procedures related to NAT traversal. In addition, the UAC may use the 199 response for policy decisions related to early dialogs, e.g. when choosing to process media associated with a particular early dialog. If multiple usages [RFC5057] are used within an early dialog, and it is not clear which dialog usage the 199 response terminates, SIP entities that keep dialog state SHALL NOT release resources associated with the early dialog when they receive the 199 response. If a client receives an unreliable 199 response on a dialog which has not previously been created (this can happen if a 199 response reaches the client before a 18x response) the client SHALL discard the 199 responses. If a client receives a reliable 199 response on a dialog which has not previously been created the UAC SHOULD acknowledge the 199 response, as described in [RFC3262]. 4.1. Examples of resource types Examples which benefit from resource-release are: 1. Codec release - when resources for a specific codec has been reserved only for the stream that is terminated. In that case the resources associated with that codec can be released. 2. Pre-conditions - when the dialog is terminated, procedures and resources associated to the pre-conditions for that dialog can be released. 3. In-band security negotiation - when the dialog is terminated, procedures and resources associated with the in-band security negotiation for that dialog can be released. 4. ICE [I-D.ietf-mmusic-ice] mechanism - when the dialog is terminated, procedures and resources associated with the ICE related in-band procedures for that dialog can be released. 5. Limited access resources - in case of forking and multiple stream it may not be possible to allow early media on all dialogs, so media sessions associated with some dialogs may e.g. be set to "inactive". When a dialog is terminated, media sessions associated with other dialogs can be allowed. 6. Secure media selection - when SRTP is used to encrypt the media. Holmberg Expires June 12, 2010 [Page 5] Internet-Draft 199 December 2009 In some cases SIP entities are only able to render media associated with a single early dialog. If a 199 response is received on a dialog, and media associated with that media has been rendered, the SIP entity can start rendering media associated with another early dialog. If the client is able to associate the 199 response with a specific media stream, it MAY choose to discard media on that specific media stream, it MAY release all resources associated with that media stream and it MAY start to process media streams received on other early dialogs. When the P-Early-Media header [RFC5009] is used, a UA MAY trigger different actions depending on whether the header has been used for the terminated dialog. How the association between the dialog and the associated media stream is done is outside the scope of this document. NOTE: When using SRTP [RFC3711], the secure media stream is bound to the crypto context setup for the dialog, and can be identified using the MKI (if used) of SRTP. If the client only has a single early dialog (other early dialogs MAY not have been established, or they MAY have been established and later terminated) and a 199 response is received for that early dialog, the client terminates the early dialog. Afterwards, the client SHOULD act as before the first early dialog was established. 4.2. Examples of policy procedures 1. UAC early media selection - when media associated with multiple early dialogs is received, SIP endpoint normally chooses to process media associated with a single early dialog (e.g. the recently established early dialog). If a 199 response is received on such early dialog, the SIP endpoint can start processing media associated with another early dialog. For example, an early dialog may be used for an announcement message, and when the message is finished a 199 response will be sent on that dialog, in order for the SIP endpoint to stop processing media associated with that early dialog. This kind of policy is normal especially in PSTN gateways, where the calling user cannot control which media is processed. 2. SBC early media selection - when an SBC is used to control which media is processed and forwarded. In many cases, the SBC only processes media associated with a single early dialog. Typical for NAT traversal, the SBC often "latches" onto a media stream. If a 199 response is received, the SBC can choose to start processing media associated with another dialog. If the SBC performs latching, it can trigger a "re-latch" onto a new media stream when the 199 response is received. Holmberg Expires June 12, 2010 [Page 6] Internet-Draft 199 December 2009 3. UAC ringing tone generation - when a UAC receives a 180 response, it may choose to generate a local ringing tone. If early media is received, the UAC may stop the local ringing tone generation and play the incoming early media packets. If a 199 response is received on the early dialog associated with the early media, and the UAC has previously received a 180 response for another early dialog, it can start to generate local ringing tone again. Having knowledge that the early dialog associated with early media has been terminated, the UAC can also start generating local ringing tone if a 180 is received on another early dialog after the early dialog has been terminated. 5. User Agent Server behavior If the received initial request contains an 199 option tag, the UAS SHOULD NOT send a 199 response on a dialog on which it intends to send a final response, unless it e.g. has been configured to do so due to lack of 199 support by forking proxies or other intermediate SIP entities. NOTE: If the UAS has created multiple early dialogs (the UAS is acting similar to a forking proxy), it does not always intend to send a final response for all of those dialogs. When a UAS generates a 199 response, the response MUST contain a To header tag parameter, which identifies the early dialog that has been terminated. The UAS MUST also insert a Reason header [RFC3326] which contains a response code which describes the reason why the dialog was terminated. If the UAS intends to send 199 responses, and if it supports the procedures defined in [RFC3840], it MAY during the registration procedure use the sip.extensions feature tag [RFC3840] to indicate support of the 199 response code. A 199 response SHOULD NOT contain an SDP offer/answer message body, unless required by the rules in [RFC3264]. If the INVITE request did not contain an SDP offer, and the 199 response is the first reliably sent response, the 199 response is required to contain an SDP offer. In this case the UAS SHOULD send the 199 response unreliable, or include an SDP offer with no m- lines in the reliable 199 response. When a 199 response is sent by a UAS, since the provisional response is only used for information purpose, the UAS SHOULD send it unreliably even if the 100rel option tag [RFC3262] is present in the Require header of the associated request. Holmberg Expires June 12, 2010 [Page 7] Internet-Draft 199 December 2009 6. Proxy behavior When a proxy receives a 199 response, it MUST process the response as any other non-100 provisional responses. The proxy will forward the response upstream towards the sender of the associated request. The proxy MAY release resources it has reserved associated with the dialog on which the response is received. If a proxy receives a 199 response out of dialog, it processes it as other non-100 provisional responses received out of dialog. When a forking proxy receives a non-2xx final response that it recognizes as terminating one or more early dialogs, it SHOULD generate and send a 199 response upstream for each of the terminated early dialogs that satisfy each of the following conditions: - the forking proxy does not intend to forward the final response immediately (in accordance with rules for a forking proxy) - the UAC has indicated support (using the 199 option tag) for the 199 response code - the forking proxy has not already received and forwarded a 199 response for that early dialog - the forking proxy has not already sent a final response for any of the early dialogs As a consequence, once a final response to the INVITE has been issued by the proxy, no further 199 responses associated with the INVITE request will be generated or forwarded by the proxy. When the forking proxy forks the initial request, it generates a unique Via header branch parameter value for each forked leg. The proxy can determine whether additional forking has occurred downstream of the proxy by storing the top Via branch value from each response which creates an early dialog. If the same top Via branch value is received for multiple early dialogs, the proxy knows that additional forking has occured downstream of the proxy. A non-2xx final response received for a specific early dialog also terminates all other early dialog for which the same top Via branch value was received in the responses which created those early dialogs. Based on implementation policy, the forking proxy MAY wait before sending the 199 response, e.g. if it expects to receive a 2xx final response on another dialog shortly after it received the non-2xx final response which triggered the 199 response. When a forking proxy generates a 199 response, the response MUST Holmberg Expires June 12, 2010 [Page 8] Internet-Draft 199 December 2009 contain a To header tag parameter, which identifies the early dialog that has been terminated. The proxy MUST also insert a Reason header [RFC3326] which contains the response code of the response that triggered the 199 response. A forking proxy which supports generating of 199 responses MUST keep track of early dialogs, in order to determine whether to generate a 199 response when the proxy receives a non-2xx final response. In addition, the proxy MUST keep track on which early dialogs it has received and forwarded 199 responses, in order to not generate additional 199 responses for those early dialogs. If a forking proxy receives a reliably sent 199 response for a dialog, for which it has previously generated and sent a 199 response, it MUST forward the 199 response. In case of a unreliably sent 199 response, the proxy MAY forward the 199 response, or it MAY discard it. When a forking proxy generates a 199 response, the response MUST NOT be sent reliably. 7. Backward compatibility Since all SIP entities involved in a session setup do not necessarily support the specific meaning of the 199 Early Dialog Terminated provisional response, the sender of the response MUST be prepared to receive SIP requests and responses associated with the dialog for which the 199 response was sent (a proxy can receive SIP messages from either direction). If such request is received by a UA, it MUST act in the same way as if it had received the request after sending the final non-2xx response to the INVITE, as specified in [RFC3261]. A UAC that receives a 199 response for an early dialog MUST NOT send any further requests on that dialog, except for requests which acknowledge reliable responses. A proxy MUST forward requests according to [RFC3261], even if the proxy has knowledge that the early dialog has been terminated. The 199 Early Dialog Terminated response code does not "replace" a final response. RFC 3261 [RFC3261] specifies when a final response is sent. 8. Usage with SDP offer/answer A 199 Early Dialog Terminated provisional response SHOULD NOT contain an SDP offer/answer message body, unless required by the rules in [RFC3264]. Holmberg Expires June 12, 2010 [Page 9] Internet-Draft 199 December 2009 If the INVITE request did not contain an SDP offer, and the 199 response is the first reliably sent response, the 199 response is required to contain an SDP offer. In this case the UAS SHOULD send the 199 response unreliable, or include an SDP offer with no m- lines in the reliable 199 response. 9. Usage with 100rel When a 199 Early Dialog Terminated provisional response is sent by a UAS, since the provisional response is only used for information purpose, the UAS SHOULD send it unreliably even if the 100rel option tag [RFC3262] is present in the Require header of the associated request. When a forking proxy generates a 199 response, the response MUST NOT be sent reliably. 10. Examples 10.1. Example with a forking proxy which generates 199 The figure shows an example, where a proxy (P1) forks an INVITE received from UAC. The forked INVITE reaches UAS_2, UAS_3 and UAS_4, which send 18x provisional responses in order to create early dialogs between themselves and the UAC. UAS_2 and UAS_3 reject the INVITE by sending a 4xx error response each. When P1 receives the 4xx responses it immediately sends 199 responses, associated with the dialogs where the 4xx responses were received, towards the UAC. The early dialog leg is shown in parenthesis. Holmberg Expires June 12, 2010 [Page 10] Internet-Draft 199 December 2009 UAC P1 UAS_2 UAS_3 UAS_4 | | | | | |-- INVITE -->| | | | | |--- INVITE (2) ->| | | | |--- INVITE (3) --------->| | | |--- INVITE (4) ----------------->| | |<-- 18x (2) -----| | | |<- 18x (2) --| | | | | |<-- 18x (3) -------------| | |<- 18x (3) --| | | | | |<-- 18x (4) ---------------------| |<- 18x (4) --| | | | | |<-- 4xx (2) -----| | | | |--- ACK (2) ---->| | | |<- 199 (2) --| | | | | |<-- 4xx (3) -------------| | | |--- ACK (3) ------------>| | |<- 199 (3) --| | | | | |<-- 200 (4) ---------------------| |<- 200 (4) --| | | | |-- ACK (4) ->| | | | | |--- ACK (4) -------------------->| | | | | | Figure 1: Example call flow 10.2. Example with a forking proxy which receives 200 OK The figure shows an example, where a proxy (P1) forks an INVITE received from UAC. The forked INVITE reaches UAS_2, UAS_3 and UAS_4, which send 18x provisional responses in order to create early dialogs between themselves and the UAC. UAS_4 accepts the session by sending a 200 OK final response. When P1 receives the 200 OK responses it immediately forwards it towards the UAC. P1 does not send 199 responses for the early dialogs from UAS_2 and UAS_3, since P1 has yet not received any final responses on those early dialogs (even if P1 sends CANCEL request to UAS_2 and UAS_3 P1 may still receive 200 OK final response from UAS_2 or UAS_3, which P1 would have to forward towards the UAC. The early dialog leg is shown in parenthesis. Holmberg Expires June 12, 2010 [Page 11] Internet-Draft 199 December 2009 UAC P1 UAS_2 UAS_3 UAS_4 | | | | | |-- INVITE -->| | | | | |--- INVITE (2) ->| | | | |--- INVITE (3) --------->| | | |--- INVITE (4) ----------------->| | |<-- 18x (2) -----| | | |<- 18x (2) --| | | | | |<-- 18x (3) -------------| | |<- 18x (3) --| | | | | |<-- 18x (4) ---------------------| |<- 18x (4) --| | | | | |<-- 200 (4) ---------------------| |<- 200 (4) --| | | | |-- ACK (4) ->| | | | | |--- ACK (4) -------------------->| | | | | | Figure 2: Example call flow 10.3. Example with two forking proxies, of which one generates 199 The figure shows an example, where a proxy (P1) forks an INVITE received from UAC. One of the forked INVITEs reaches UAS_2. The other forked INVITE reaches another proxy (P2), which forks the INVITE to UAS_3 and UAS_4, which send 18x provisional responses in order to create early dialogs between themselves and the UAC. UAS_3 and UAS_4 reject the INVITE by sending a 4xx error response each. P2 does not support the 199 response code, and forwards a single 4xx response. When P1 receives the 4xx responses from P2, it manages to associate the response with the early dialogs from both UAS_3 and UAS_4, so it generates and sends two 199 response to indicate that the early dialogs from UAS_3 and UAS_4 have been terminated. The early dialog leg is shown in parenthesis. Holmberg Expires June 12, 2010 [Page 12] Internet-Draft 199 December 2009 UAC P1 P2 UAS_2 UAS_3 UAS_4 | | | | | | |-- INVITE -->| | | | | | |-- INVITE (2) ------------------>| | | | |-- INVITE ---->| | | | | | |--- INVITE (3) --------->| | | | |--- INVITE (4) ----------------->| | | |<-- 18x (3) -------------| | | |<- 18x (3) ----| | | | |<- 18x (3) --| | | | | | | |<-- 18x (4) ---------------------| | |<- 18x (4) ----| | | | |<- 18x (4) --| | | | | | | |<-- 4xx (3) -------------| | | | |--- ACK (3) ------------>| | | | |<-- 4xx (4) ---------------------| | | |--- ACK (4) -------------------->| | |<- 4xx (3) ----| | | | | |-- ACK (3) --->| | | | |<- 199 (3) --| | | | | |<- 199 (4) --| | | | | | |<- 200 (2) ----------------------| | | |<- 200 (2) --| | | | | |-- ACK (2) ->| | | | | | |-- ACK (2) --------------------->| | | | | | | | | Figure 3: Example call flow 11. Security Considerations General security issues related to SIP responses are described in [RFC3261]. Due to the nature of the 199 response, it may be attractive to use it for launching attacks in order to terminate specific early dialogs (other early dialogs will not be affected). In addition, if a man-in-the-middle sends a 199 response to the UAC, which terminates a specific dialog, it can take a while until the UAS finds out that the UAC, and possbile stateful intermediates, have terminated the dialog. SIP security mechanisms (e.g. hop-to-hop TLS) can be used to minimize, or eliminate, the risk for such attacks. 12. IANA Considerations This section registers a new SIP response code and a new option tag, according to the procedures of RFC 3261. Holmberg Expires June 12, 2010 [Page 13] Internet-Draft 199 December 2009 12.1. IANA Registration of the 199 response code This section registers a new SIP response code, 199. The required information for this registration, as specified in RFC 3261, is: RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the RFC number of this specification]] Response Code Number: 199 Default Reason Phrase: Early Dialog Terminated 12.2. IANA Registration of the 199 Option Tag This section registers a new SIP option tag, 199. The required information for this registration, as specified in RFC 3261, is: Name: 199 Description: This option tag is for indicating support of the 199 Early Dialog Terminated provisional response code. When present in a Supported header, it indicates that the UA supports the response code. When present in a Require header in a request, it indicates that the UAS MUST support the sending of the response code. 13. Acknowledgements Thanks to Paul Kyzivat, Dale Worley, Gilad Shaham, Francois Audet, Attila Sipos, Robert Sparks, Brett Tate, Ian Elz, Hadriel Kaplan, Timothy Dwight, Dean Willis, Serhad Doken, John Elwell, Gonzalo Camarillo, Adam Roach, Bob Penfield,Tom Taylor, Ya Ching Tan, Keith Drage and Hans Erik van Elburg for their feedback and suggestions. 14. References 14.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of Holmberg Expires June 12, 2010 [Page 14] Internet-Draft 199 December 2009 Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason Header Field for the Session Initiation Protocol (SIP)", RFC 3326, December 2002. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, August 2004. [RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller Preferences for the Session Initiation Protocol (SIP)", RFC 3841, August 2004. [I-D.ietf-mmusic-ice] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", draft-ietf-mmusic-ice-19 (work in progress), October 2007. 14.2. Informational References [RFC5057] Sparks, R., "Multiple Dialog Usages in the Session Initiation Protocol", RFC 5057, November 2007. [RFC5009] Ejza, R., "Private Header (P-Header) Extension to the Session Initiation Protocol (SIP) for Authorization of Early Media", RFC 5009, September 2007. Holmberg Expires June 12, 2010 [Page 15] Internet-Draft 199 December 2009 Author's Address Christer Holmberg Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: christer.holmberg@ericsson.com Holmberg Expires June 12, 2010 [Page 16]