SIPPING Working Group A. Allen, Ed. Internet-Draft Research in Motion (RIM) Expires: December 27, 2005 J. Holm Ericsson T. Hallin Motorola June 25, 2005 The P-Answer-State Header Extension to the Session Initiation Protocol (SIP) for the Open Mobile Alliance (OMA) Push to talk over Cellular (PoC) draft-allen-sipping-poc-p-answer-state-header-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 27, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document describes a private Session InitiationProtocol(SIP) header (P-header) used by the Open Mobile Alliance (OMA),For Push to talk over Cellular (PoC) along with its applicability, which is Allen, et al. Expires December 27, 2005 [Page 1] Internet-Draft The P-Answer-State Header June 2005 limited to the OMA PoC application. The P-Answer-State header is used for indicating the answering mode of the handset which is particular to the PoC application. Requirements Language 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 [1]. Table of Contents 1. Overall Applicability . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. The P-Answer-State header . . . . . . . . . . . . . . . . . . 5 4.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 Alternatives Considered . . . . . . . . . . . . . . . . . 7 4.3 Applicability statement for the P-Answer-State header . . 8 4.4 Usage of the P-Answer-State header . . . . . . . . . . . . 8 4.4.1 Procedures at the UA (terminal) . . . . . . . . . . . 9 4.4.2 Procedures at the UA (Intermediate Node) . . . . . . . 10 4.4.3 Procedures at the proxy server . . . . . . . . . . . . 11 5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1 P-Answer-State header syntax . . . . . . . . . . . . . . . 12 5.2 Table of the new header . . . . . . . . . . . . . . . . . 12 6. Example Usage Session Flows . . . . . . . . . . . . . . . . . 13 6.1 Pre-arranged Group Call using On-demand Session . . . . . 13 6.2 1-1 Call using Pre-established Session . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 24 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 8.1 Registration of Header Fields . . . . . . . . . . . . . . 25 8.2 Registration of Header Field Parameters . . . . . . . . . 25 9. Changes since previous version . . . . . . . . . . . . . . . . 26 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 11.1 Normative References . . . . . . . . . . . . . . . . . . . 26 11.2 Informative References . . . . . . . . . . . . . . . . . . 27 Allen, et al. Expires December 27, 2005 [Page 2] Internet-Draft The P-Answer-State Header June 2005 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28 Intellectual Property and Copyright Statements . . . . . . . . 29 Allen, et al. Expires December 27, 2005 [Page 3] Internet-Draft The P-Answer-State Header June 2005 1. Overall Applicability The SIP extension specified in this document makes certain assumptions regarding network topology, and the availability of transitive trust. These assumptions are generally NOT APPLICABLE in the Internet as a whole. The mechanism specified here was designed to satisfy the requirements specified by the Open Mobile Alliance for Push-to-talk over cellular for which either no general-purpose solution was found, where insufficient operational experience was available to understand if a general solution is needed, or where a more general solution is not yet mature. For more details about the assumptions made about this extension, consult the Applicability subsection for the extension. 2. Introduction The Open Mobile Alliance (OMA) (http://www.openmobilealliance.org) is specifying the Push-to-talk Over Cellular (PoC) service where SIP is the protocol used to establish half duplex media sessions across different participants. This document describes a private extension to address specific requirements of the PoC service and may not be applicable to the general Internet. The PoC service allows a SIP UA (PoC terminal) to establish a session to one or more SIP UAs simultaneously, usually initiated by the initiating user pushing a button. OMA has defined a collection of very stringent requirements in support of the PoC service. In order to provide the user with a satisfactory experience the initial session establishment from the time the user presses the button to the time they get an indication to speak must be minimized. 3. Overview The PoC terminal may support such hardware capabilities as a speaker phone and/or headset and software that provide the capability for the user to configure the PoC terminal to accept the session invitations immediately and play out the media as soon as it is received without requiring the intervention of the called user. This mode of operation is known as Automatic Answer mode. The user may alternatively configure the PoC terminal to first alert the user and require the user to manually accept the session invitation before media is accepted. This mode of operation is known as Manual Answer mode. The PoC terminal may support both or only one of these modes of operation. The user may change the Answer Mode (AM) configuration of the PoC terminal frequently based on their current circumstances and preference,(perhaps because the user is busy, or in a public area Allen, et al. Expires December 27, 2005 [Page 4] Internet-Draft The P-Answer-State Header June 2005 where she cannot use a speaker phone, etc). The OMA PoC Architecture utilizes SIP servers within the network that may perform such roles as a conference focus [5], a RTP translator or a network policy enforcement server. A possible optimization to minimize the delay in the providing of the caller with an indication to speak is for the SIP network server to perform buffering of media packets in order to provide an early or unconfirmed indication back to the caller and allow the caller to start speaking before the called PoC terminal has answered. An event package and mechanisms for a SIP UA to indicate its current answer mode to a SIP Server in order to enable buffering are defined in [6]. In addition, particularly when multiple domains are involved in the session more than one intermediate SIP server may be involved in the signaling path for the session and the server that performs the buffering may not be the server that has knowledge of the current answer mode of the SIP UA that is the final destination for the SIP INVITE request. A mechanism is to allow a terminal that acts as a SIP UA or a network based server that acts as a SIP UA to indicate a preference to the final destination SIP UAS to answer in a particular mode is defined in [7]. However a mechanism is required for an intermediary SIP UAS or proxy to relay the unconfirmed indication in a response back towards the originating SIP UAC. This document proposes a new SIP header field to support this unconfirmed indication. The extension may be optionally included in a response to a SIP INVITE request or in a NOTIFY sent as a result of a REFER that requests an INVITE to be sent to provide an indication from an intermediate node acting as a SIP proxy or back-to-back UA that it has information that hints that the terminating UA will likely answer automatically and therefore provides an unconfirmed indication back towards the inviting SIP UA to transmit media prior to receiving a final response from the final destination of the SIP INVITE request. The extension is described below. 4. The P-Answer-State header The purpose of the P-Answer-State header field is to provide an indication from an intermediate node acting as a SIP proxy or back- to-back UA that is has information that hints that the terminating UA identified in the Request-URI of the request will likely answer automatically and therefore provides an unconfirmed indication back towards the inviting SIP UA to transmit media prior to receiving a final response from the final destination of the SIP INVITE request. If a Provisional response contains the P-Answer-State header with the value "Unconfirmed" and does not contain SDP then a receiving intermediate node may send a 200 OK response containing SDP and a P-Answer-State header with the value "Unconfirmed" if the Allen, et al. Expires December 27, 2005 [Page 5] Internet-Draft The P-Answer-State Header June 2005 intermediate node is willing to perform media buffering. If the response containing the P-Answer-State header with the value "Unconfirmed" also contains SDP the intermediate node that inserted the header and SDP in the response is also indicating that it is willing to buffer the media until a final confirmed indication is received. The P-Answer-State header field MAY be included in a provisional or final response to a SIP INVITE request or in a NOTIFY request sent as a result of a REFER request to send an INVITE request. If the P-Answer-State header field with value "Unconfirmed" is included in a provisional response that contains SDP the intermediate node is leaving the decision where to do buffering to other nodes upstream and will forward upstream a "Confirmed indication" in a 200 OK response when the final response is received from the destination UA. The P-Answer-State header is only included in a provisional response when the node that sends the response has knowledge that there is a B2BUA node that understands this extension in the signaling path between itself and the originating UAC that will only pass the header on in either a 200 OK response or in the sipfrag of a NOTIFY request. Such a situation only occurs with specific network topologies which is another reason why use of this header is not relevant to the general internet. The originating UAC will only receive the P-Answer-state header in a 200 OK response or in the sipfrag of a NOTIFY request. 4.1 Requirements The OMA PoC service has initial setup performance requirements that can be met by an intermediate server (SIP B2BUA) spooling media from the inviting PoC subscriber until one or more invited PoC subscribers have accepted the session. The specific requirements are REQ-1: An intermediate server MAY spool media from the inviting SIP UA until one or more invited PoC SIP UAs has accepted the invitation. REQ-2: An intermediate server that is capable of spooling media MAY accept an invite request from an inviting SIP UAC even if no invited SIP UAS has accepted the invite request if it has a hint that the invited SIP UAC is likely to accept the request without requiring user intervention. Allen, et al. Expires December 27, 2005 [Page 6] Internet-Draft The P-Answer-State Header June 2005 REQ-3: An intermediate server or proxy that is incapable of spooling media or does not wish to, but has a hint that the invited SIP UAC is likely to automatically accept the session invitation MUST be able to indicate back to another intermediate server that can spool media that it has some hint that the invited UAC is likely to automatically accept the session invitation. REQ-4: An intermediate server that is willing to spool media from the inviting SIP UA until one or more invited SIP UAs have accepted the invite SHOULD indicate that it is spooling media to the inviting SIP UAC. 4.2 Alternatives Considered In order to meet REQ-3, an intermediate server needs to receive an indication back that the invited SIP UA is likely to accept the invite request without requiring user intervention. In this case, the intermediate server or proxy that has a hint that the invited SIP UAC is likely to accept the request can include an answer state indication in the 183 Session Progress or 200 OK response. A number of alternatives were considered for the intermediate server to inform the another intermediate server or the inviting SIP UAC of the invited PoC SIP UAs answer mode settings. One proposal was to create a unique reason-phrase in the 183 and 200 OK response. This was rejected because the reason phrases are normally intended for human readers and not meant to be parsed by servers for special syntactic and semantic meaning. Another proposal was to use a Reason header [8] in the 183 and 200 OK response. This was rejected because this would be inconsistent with the intended use of the reason header and its usage is not defined for these response codes and would have required creating and registering a new protocol identifier. Another proposal was to use a feature-tag in the returned Contact header as defined in [9]. This was rejected because it was not a different feature, but is an attribute of the session and can be applied to many different features. Another proposal was to use a new SDP attribute. The choice of an SDP parameter was rejected because the answer state applies to the session and not to a media stream. The P-Answer-State header was chosen to give additional information about the state of the SIP session progress and acceptance. Even Allen, et al. Expires December 27, 2005 [Page 7] Internet-Draft The P-Answer-State Header June 2005 though the UAC sees that its SDP offer has been answered and accepted, the header lets the UAC know whether invited PoC subscriber has accepted the invite or just an intermediary has done the acceptance. 4.3 Applicability statement for the P-Answer-State header The P-Answer-State header is applicable in the following circumstances: o In networks where there are UAs that engage in half-duplex communication where there is not the possibility for the invited user to verbally acknowledge the answering of the session as is normal in full duplex communication; o Where the invited UA may automatically accept the session without manual acceptance; o The network also contains intermediate network SIP servers that are trusted; o The intermediate network SIP servers have knowledge of the current answer mode setting of the terminating UAS; and, o The intermediate network SIP servers can provide buffering of the media in order to reduce the time for the inviting user to send media. o The intermediate network SIP servers assume knowledge of the network topology and the existence of similar intermediate network SIP servers in the signaling path. Such configurations are generally not applicable to the internet as a whole where such trust relationships do not exist. In addition security issues have only been considered for networks which are trusted and use hop by hop security mechanisms and security issues with usage of this mechanism in the general internet have not been evaluated. 4.4 Usage of the P-Answer-State header A UAS B2BUA or proxy MAY insert a P-Answer-State header field in any 1XX or 2XX response that is allowed to contain an SDP answer in response to an SDP offer contained in an INVITE as specified in [2]. Typically the P-Answer-State header field is inserted in either a 183 Session Progress or a 200 OK response. A UA that receives a REFER request to send an INVITE MAY also insert a P-Answer-State header Allen, et al. Expires December 27, 2005 [Page 8] Internet-Draft The P-Answer-State Header June 2005 field in a NOTIFY request it sends as a result of the implicit subscription created by the REFER request. When the P-Answer-State header field contains the parameter "Unconfirmed" the UAC or proxy is indicating that it has information that hints that the final destination UAS for the INVITE request is likely to automatically accept the session but that this is unconfirmed and it is possible that the final destination UAS will first alert the user and require manual acceptance of the session or not accept the session request. This is referred to here as an "unconfirmed response". When the P-Answer-State header field contains the parameter "Confirmed" the UAC or proxy is indicating that the destination UAS has accepted the session and is ready to receive media. The parameter value of "Confirmed" has the usual semantics of an SDP answer and is included for completeness. The usual end to end SDP answer response semantics are referred to here as a "confirmed response". 4.4.1 Procedures at the UA (terminal) A UAC (terminal) that receives a 1XX or 2XX response containing a P-Answer-State header field containing the parameter "Unconfirmed" and an SDP answer MAY send media as specified in [2], however there is no guarantee that the media will be received by the final recipient. How a UAC confirms whether the media was or was not received by the final destination when it his received a 2XX "unconfirmed response" is application specific and outside of the scope of this document. If the application is a conference then the mechanism specified in [2] could be used to determine that the invited user joined. Alternatively a BYE request could be received or the media could be placed on hold if the final destination UAS does not accept the session. A UAC (terminal) that receives a 1XX or 2XX response without a P-Answer-State Header or containing a P-Answer-State header field containing the parameter "Confirmed" SHALL treat it as a "confirmed response". A UAC (terminal) that receives in response to a REFER request a NOTIFY request containing a P-Answer-State header field containing the parameter "Unconfirmed" as either a SIP header or contained in a sipfrag in the body of the NOTIFY request received on a pre-existing dialog that was established by an INVITE request and for which there has been a successful SDP offer-answer exchange according to [2] then the UAC MAY send media, however there is no guarantee that the media will be received by the final recipient that was indicated in the Refer-To header in the original REFER request. There are no P-Answer-State procedures for a terminal acting in the UAS role. Allen, et al. Expires December 27, 2005 [Page 9] Internet-Draft The P-Answer-State Header June 2005 4.4.2 Procedures at the UA (Intermediate Node) A UAS (Intermediate Node) MAY insert a P-Answer-State header field in any 1XX or 2XX response that is allowed to contain an SDP answer in response to an SDP offer contained in an INVITE request as specified in [2]. A response containing the P-Answer-State header field containing the parameter "Unconfirmed" MAY or MAY NOT contain an SDP answer. If the response contains an SDP answer then the sending UA MUST be ready to receive media as specified in [2]. An intermediate node that acts as a back-to-back UA and returns a 1XX or 2XX response in response to an INVITE request MAY insert a P-Answer-State header field containing the parameter "Unconfirmed" in the response if it has not yet received a "confirmed response" from the final destination UA. If the intermediate node UAS also includes SDP in the response along with a P-Answer-State header field containing the parameter "Unconfirmed" the intermediate node MUST be ready to receive media as specified in [2] and MAY buffer any media it receives until it receives a "confirmed response" from the final destination UA or until the buffer is full. Such an intermediate node may insert an SDP answer in the response it generates even if the "unconfirmed response" it received did not contain an SDP answer. An intermediate node that acts as a back-to-back UA and receives a REFER request to send an INVITE request to another UA as specified in [11] MAY insert a P-Answer-State header field containing the parameter "Unconfirmed" in the initial NOTIFY request sent in response to the REFER request if it has not yet received a "confirmed response" from the final destination UA and it has information that hints that the final destination UAS for the INVITE is likely to automatically accept the session. If the REFER was sent as part of an existing dialog established by an INVITE request and for which there has been a successful SDP offer-answer exchange according to [2] the intermediate node MUST be ready to receive media as specified in [2] and MAY buffer any media it receives until it receives a "confirmed response" from the final destination UA or until its buffer is full. An intermediate node that acts as a back-to-back UA and receives a 1XX or 2XX response in response to an INVITE request containing a P-Answer-State header field in the response SHOULD include the P-Answer-State header field unmodified in the 1XX or 2XX response it sends as a result of receiving that response. If the intermediate node that acts as a back-to-back UA sends a NOTIFY request according to [11] then the intermediate node UAC SHOULD include the P-Answer- State header field unmodified in the sipfrag of the response included in the body of the NOTIFY request. Allen, et al. Expires December 27, 2005 [Page 10] Internet-Draft The P-Answer-State Header June 2005 A UAC (Intermediate Node) that receives a 1XX or 2XX response without a P-Answer-State Header or containing a P-Answer-State header field containing the parameter "Confirmed" SHALL treat it as a "confirmed response". If the UAS (Intermediate Node) knows that the final destination UA is now ready to accept media and the UAS previously sent an "Unconfirmed response" the UAS SHOULD insert a P-Answer-State header field containing the parameter "Confirmed" in the response. An intermediate node that acts as a back-to-back UA that previously sent an initial NOTIFY request containing a P-Answer-State header field containing the parameter "Unconfirmed" that subsequently receives a "confirmed response" without a P-Answer-State header field in response to the INVITE request sent as a result of the REFER request SHOULD include a P-Answer-State header containing the parameter "Confirmed" in the subsequent NOTIFY request generated as a result of the "confirmed response". If the UAS knows that the final destination UA is ready to accept media and the UAS did not previously send an "Unconfirmed response" the UAS MAY insert a P-Answer-State header field containing the parameter "Confirmed" in the response. If an intermediate node that acts as a back-to-back UA and sends an INVITE request in response to a REFER request learns by receiving a "confirmed response" that the final destination UA is ready to accept media and the back-to-back UA did not previously include a P-Answer- State header containing the parameter "Unconfirmed" in the initial NOTIFY request sent in response to the REFER request then the back- to-back UA MAY insert a P-Answer-State header field containing the parameter "Confirmed" in the response if the "confirmed response" does not contain a P-Answer-State header. 4.4.3 Procedures at the proxy server SIP proxy servers do not need to understand the semantics of the P-Answer-State header field. As part of the regular SIP rules for unknown headers, a proxy will forward unknown headers. A proxy MAY insert a P-Answer-State header field in a 1XX response that it originates compliant with [3] or add it to a 2XX response that contains an SDP answer in response to an SDP offer contained in an INVITE request as specified in [2]. A proxy that returns a 1XX response in response to an INVITE request MAY insert a P-Answer-State header field containing the parameter "Unconfirmed" in the response if it has not yet received a "confirmed response" from the final destination UA. A proxy that receives a 1XX or 2XX response without a P-Answer-State Allen, et al. Expires December 27, 2005 [Page 11] Internet-Draft The P-Answer-State Header June 2005 Header or containing a P-Answer-State header field containing the parameter "Confirmed" SHALL for the purposes of this document treat it as a "confirmed response". If the proxy knows that the final destination UA is now ready to accept media and the proxy previously sent an "Unconfirmed response" the proxy SHOULD insert a P-Answer-State header field containing the parameter "Confirmed" in the response. If the proxy knows that the final destination UA is ready to accept media and the proxy did not previously send an "Unconfirmed response" the proxy MAY insert a P-Answer-State header field containing the parameter "Confirmed" in the response. 5. Formal Syntax The mechanisms specified in this document is described in both prose and an augmented Backus-Naur Form (BNF) defined in RFC 2234 [3]. Further, several BNF definitions are inherited from SIP and are not repeated here. Implementers need to be familiar with the notation and contents of SIP [3] and RFC 2234 [3] to understand this document. 5.1 P-Answer-State header syntax The syntax of the P-Answer-State header is described as follows: P-Answer-State = "P-Answer-State" HCOLON answer-type answer-type = "Confirmed" / "Unconfirmed" 5.2 Table of the new header Table 1 extends the headers defined in this document to Table 2 in SIP [3], section 7.1 of the SIP-specific event notification [10] tables 1 and 2 in the SIP INFO method [11], tables 1 and 2 in Reliability of provisional responses in SIP [12], tables 1 and 2 in the SIP UPDATE method [13], tables 1 and 2 in the SIP extension for Instant Messaging [14], table 1 in the SIP REFER method [15], and table 2 in the SIP PUBLISH method [16]: Header field where proxy ACK BYE CAN INV OPT REG SUB _______________________________________________________________ P-Answer-State 1xx,2xx ar - - - o - - - Header field NOT PRA INF UPD MSG REF PUB _______________________________________________________________ P-Answer-State R o - - - - - - Figure 1 Allen, et al. Expires December 27, 2005 [Page 12] Internet-Draft The P-Answer-State Header June 2005 6. Example Usage Session Flows For simplicity some details such as intermediate proxies and 100 Trying responses are not shown in the following example flows. The term "policy server" is used here to mean a policy enforcement server. 6.1 Pre-arranged Group Call using On-demand Session The following flow shows Alice making a Pre-arranged Group Call using a Conference URI which has Bob on the member list. The session initiation uses the On-demand Session establishment mechanism where a SIP INVITE containing SDP is sent by Alices's terminal when Alice pushes her push to talk button. In this example Alice's Policy Server acts a Call Stateful SIP Proxy and Bob's Policy Server which is aware that the current Answer Mode setting of Bob's terminal is set to Auto Answer acts as a B2BUA. For simplicity the invitations by the Conference Focus to the other members of the group are not shown in this example. Alice's Alices's Conference Bob's Bob's Terminal Policy Server focus Policy Server Terminal | | | | | |--(1)INVITE---->| | | | | |--(2)INVITE--->| | | | | |--(3)INVITE->| | | | | |---(4)INVITE-->| | | |<--(5)183----| | | |<---(6)200-----| | | |<---(7)200------| | | | |----(8)ACK----->| | | | | |---(9)ACK----->| | | | | | | | |=======Early Media Session=====>| | | | | MEDIA | | | | BUFFERING | | | | | |<---(10)200----| | | | |---(11)ACK---->| | | |<--(12)200---| | | | |--(13)ACK--->| | | | | | | | | |========Media Session=======>| | | | | | | | | | | Figure 2 Allen, et al. Expires December 27, 2005 [Page 13] Internet-Draft The P-Answer-State Header June 2005 F1 INVITE Alice -> Alices's Policy Server INVITE sip:FriendsOfAlice@atlanta.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: "Alice's Friends" From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) F2 INVITE Alice's Policy Server -> Conference Focus INVITE sip:FriendsOfAlice@atlanta.com SIP/2.0 Via: SIP/2.0/UDP AlicesPolicyServer.atlanta.com;branch=z9hG4bK77ef4c2312983.1 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Record-Route: Max-Forwards: 69 To: "Alice's Friends" From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) The Conference Focus explodes the Conference URI and Invites Bob F3 INVITE Conference Focus -> Bob's Policy Server INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 Max-Forwards: 70 To: "Bob" From: "Alice's Friends" ;tag=2178309898 Call-ID: e60a4c784b6716 CSeq: 301166605 INVITE Contact: Content-Type: application/sdp Allen, et al. Expires December 27, 2005 [Page 14] Internet-Draft The P-Answer-State Header June 2005 Content-Length: 142 (SDP not shown) F4 INVITE Bob's Policy Server -> Bob INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/UDP BobsPolicyServer.biloxi.com;branch=z9hG4bKa27bc93 Max-Forwards: 70 To: "Bob" From: "Alice's Friends" ;tag=781299330 Call-ID: 6eb4c66a847710 CSeq: 478209 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) F5 183 Session Progress Bob's Policy Server -> Conference Focus SIP/2.0 183 Session Progress Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 To: "Bob" ;tag=a6c85cf From: "Alice's Friends" ;tag=2178309898 Call-ID: e60a4c784b6716 Contact: CSeq: 301166605 INVITE P-Answer-State: Unconfirmed Content-Length: 0 F6 200 OK Conference Focus -> Alice's Policy Server SIP/2.0 200 OK Via: SIP/2.0/UDP AlicesPolicyServer.atlanta.com;branch=z9hG4bK77ef4c2312983.1 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Record-Route: To: "Alice's Friends" ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: P-Answer-State: Unconfirmed Content-Type: application/sdp Allen, et al. Expires December 27, 2005 [Page 15] Internet-Draft The P-Answer-State Header June 2005 Content-Length: 131 (SDP not shown) F7 200 OK Alice's Policy Server -> Alice SIP/2.0 200 OK Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Record-Route: To: "Alice's Friends" ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: P-Answer-State: Unconfirmed Content-Type: application/sdp Content-Length: 131 (SDP not shown) F8 ACK Alice -> Alice's Policy Server ACK sip:AlicesConferenceFocus.atlanta.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9 Route: Max-Forwards: 70 To: "Alice's Friends" ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 ACK Content-Length: 0 F9 ACK Alice's Policy Server -> Conference Focus ACK sip:AlicesConferenceFocus.atlanta.com SIP/2.0 Via: SIP/2.0/UDP AlicesPolicyServer.atlanta.com;branch=z9hG4bK77ef4c2312983.1 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9 Max-Forwards: 69 To: "Alice's Friends" ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 ACK Content-Length: 0 The early half duplex media session between Alice and the Conference Focus is now established and the Conference Focus buffers the media it receives from Alice. Allen, et al. Expires December 27, 2005 [Page 16] Internet-Draft The P-Answer-State Header June 2005 F10 200 OK Bob -> Bob's Policy Server SIP/2.0 200 OK Via: SIP/2.0/UDP BobsPolicyServer.biloxi.com;branch=z9hG4bKa27bc93 To: "Bob" ;tag=d28119a From: "Alice's Friends" ;tag=781299330 Call-ID: 6eb4c66a847710 CSeq: 478209 INVITE Contact: Content-Type: application/sdp Content-Length: 131 (SDP not shown) F11 ACK Bob's Policy Server -> Bob ACK sip:bob@192.0.2.4 SIP/2.0 Via: SIP/2.0/UDP BobsPolicyServer.biloxi.com;branch=z9hG4bKa27bc93 Max-Forwards: 70 To: "Bob" ;tag=d28119a From: "Alice's Friends" ;tag=781299330 Call-ID: 6eb4c66a847710 CSeq: 478209 ACK Content-Length: 0 F12 200 OK Bob's Policy Server -> Conference Focus SIP/2.0 200 OK Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 To: "Bob" ;tag=a6670811 From: "Alice's Friends" ;tag=2178309898 Call-ID: e60a4c784b6716 Contact: CSeq: 301166605 INVITE P-Answer-State: Confirmed Content-Type: application/sdp Content-Length: 131 (SDP not shown) F13 ACK Conference Focus -> Bob's Policy Server ACK sip:BobsPolicyServer.biloxi.com SIP/2.0 Via: SIP/2.0/UDP Allen, et al. Expires December 27, 2005 [Page 17] Internet-Draft The P-Answer-State Header June 2005 AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 Max-Forwards: 70 To: "Bob" ;tag=a6670811 From: "Alice's Friends" ;tag=2178309898 Call-ID: e60a4c784b6716 CSeq: 301166605 ACK Content-Length: 0 The media session between Alice and Bob is now established and the Conference Focus forwards the buffered media to Bob. 6.2 1-1 Call using Pre-established Session The following flow shows Alice making a 1-1 Call to Bob using a pre- established session. A pre-established session is where a dialog is established with Alices's Policy Server using a SIP INVITE SDP offer answer exchange to pre-negotiate the codecs and other media Parameters to be used for media sessions ahead of Alice initiating a Communication. When Alice initiates a communication to Bob a SIP REFER is used to Request Alice's Policy Server to send an INVITE to Bob. In this example Bob's Terminal does not use the Pre-established Session mechanism. In this example Alice's Policy Server acts a B2BUA and also performs the Conference Focus function. Bob's Policy Server which is aware that the current Answer Mode setting of Bob's terminal is set to Auto Answer acts as a B2BUA. Allen, et al. Expires December 27, 2005 [Page 18] Internet-Draft The P-Answer-State Header June 2005 Alice's Alice's Bob's Bob's Terminal Policy Server / Policy Server Terminal Conference Focus | | | | |-------(1)INVITE-- ----->| | | |<-------(2)200-----------| | | |---------(3)ACK--------->| | | | | | | | | | | | | | | |------(4)REFER---------->| | | |<-------(5)202-----------| | | | |-----(6)INVITE---->| | | | |--(7)INVITE---->| | | | | | |<-----(8)183-------| | |<-----(9)NOTIFY----------| | | |-------(10)200---------->| | | | | | | |===Early Media Session==>| | | | MEDIA | | | BUFFERING | | | | |<---(11)200-----| | | |---(12)ACK----->| | |<-----(13)200------| | | |------(14)ACK----->| | | |============Media Session==========>| | | | | |<-----(15)NOTIFY---------| | | |-------(16)200---------->| | | | | | | Figure 3 F1 INVITE Alice -> Alices's Policy Server INVITE sip: AlicesConferenceFactoryURI.atlanta.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) Allen, et al. Expires December 27, 2005 [Page 19] Internet-Draft The P-Answer-State Header June 2005 F2 200 OK Alice's Policy Server -> Alice SIP/2.0 200 OK Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 131 (SDP not shown) F3 ACK Alice -> Alice's Policy Server ACK sip:AlicesPre-establishesSession@AlicesPolicyServer.atlanta.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9 Max-Forwards: 70 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 ACK Content-Length: 0 Alices's terminal has established a Pre-established Session with Alice's Policy Server. All the media parameters are pre-negotiated for use at communication time. Alice initiates a Communication to Bob F4 REFER Alice -> Alices's Policy Server REFER sip:AlicesPre-establishesSession@AlicesPolicyServer.atlanta.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314160 REFER Refer-To: "Bob" Contact: F5 202 ACCEPTED Alice's Policy Server -> Alice Allen, et al. Expires December 27, 2005 [Page 20] Internet-Draft The P-Answer-State Header June 2005 SIP/2.0 202 ACCEPTED Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314160 REFER Contact: F6 INVITE Conference Focus -> Bob's Policy Server INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bk4721d8 Max-Forwards: 70 To: "Bob" From: "Alice" ;tag=2178309898 Call-ID: e60a4c784b6716 CSeq: 301166605 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) F7 INVITE Bob's Policy Server -> Bob INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/UDP BobsPolicyServer.biloxi.com;branch=z9hG4bKa27bc93 Max-Forwards: 70 To: "Bob" From: "Alice" ;tag=781299330 Call-ID: 6eb4c66a847710 CSeq: 478209 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) F8 183 Session Progress Bob's Policy Server -> Conference Focus SIP/2.0 183 Session Progress Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 To: "Bob" ;tag=a6c85cf From: "Alice" ;tag=2178309898 Call-ID: e60a4c784b6716 Allen, et al. Expires December 27, 2005 [Page 21] Internet-Draft The P-Answer-State Header June 2005 Contact: CSeq: 301166605 INVITE P-Answer-State: Unconfirmed Content-Length: 0 F9 NOTIFY Alices's Policy Server -> Alice NOTIFY sip:alice@pc33.atlanta.com SIP/2.0 Via: SIP/2.0/UDP AlicesPre-establishesSession@ AlicesPolicyServer.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314161 NOTIFY Contact: Event: refer Subscription-State: Active;Expires=60 Content-Type: message/sipfrag;version=2.0 Content-Length: 99 SIP/2.0 183 Session Progress To: "Bob" ;tag=d28119a P-Answer-State: Unconfirmed F10 202 ACCEPTED Alice -> Alice's Policy Server SIP/2.0 202 ACCEPTED Via: SIP/2.0/UDP AlicesPre-establishesSession@ AlicesPolicyServer.atlanta.com;branch=z9hG4bKnashds8 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314161 NOTIFY The early half duplex media session between Alice and the Conference Focus is now established and the Conference Focus buffers the media it receives from Alice. F11 200 OK Bob -> Bob's Policy Server SIP/2.0 200 OK Via: SIP/2.0/UDP BobsPolicyServer.biloxi.com;branch=z9hG4bK927bc93 To: "Bob" ;tag=d28119a From: "Alice's Friends" ;tag=781299330 Call-ID: 6eb4c66a847710 Allen, et al. Expires December 27, 2005 [Page 22] Internet-Draft The P-Answer-State Header June 2005 CSeq: 478209 INVITE Contact: Content-Type: application/sdp Content-Length: 131 (SDP not shown) F12 ACK Bob's Policy Server -> Bob ACK sip:bob@192.0.2.4 SIP/2.0 Via: SIP/2.0/UDP BobsPolicyServer.biloxi.com;branch=z9hG4bK927bc93 Max-Forwards: 70 To: "Bob" ;tag=d28119a From: "Alice" ;tag=781299330 Call-ID: 6eb4c66a847710 CSeq: 478209 ACK Content-Length: 0 F13 200 OK Bob's Policy Server -> Conference Focus SIP/2.0 200 OK Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 To: "Bob" ;tag=a6670811 From: "Alice's Friends" ;tag=2178309898 Call-ID: e60a4c784b6716 Contact: CSeq: 301166605 INVITE P-Answer-State: Confirmed Content-Type: application/sdp Content-Length: 131 (SDP not shown) F14 ACK Conference Focus -> Bob's Policy Server ACK sip:BobsPolicyServer.biloxi.com SIP/2.0 Via: SIP/2.0/UDP AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8 Max-Forwards: 70 To: "Bob" ;tag=a6670811 From: "Alice" ;tag=1928301774 Call-ID: e60a4c784b6716 CSeq: 301166605 ACK Content-Length: 0 The media session between Alice and Bob is now established and the Allen, et al. Expires December 27, 2005 [Page 23] Internet-Draft The P-Answer-State Header June 2005 Conference Focus forwards the buffered media to Bob. F15 NOTIFY Alices's Policy Server -> Alice NOTIFY sip:alice@pc33.atlanta.com SIP/2.0 Via: SIP/2.0/UDP AlicesPre-establishesSession@ AlicesPolicyServer.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314162 NOTIFY Contact: Event: refer Subscription-State: Active;Expires=60 Content-Type: message/sipfrag;version=2.0 Content-Length: 83 SIP/2.0 200 OK To: "Bob" ;tag=d28119a P-Answer-State: Confirmed F16 202 ACCEPTED Alice -> Alice's PolicyServer SIP/2.0 202 ACCEPTED Via: SIP/2.0/UDP AlicesPre-establishesSession@ AlicesPolicyServer.atlanta.com;branch=z9hG4bKnashds8 To: ;tag=c70ef99 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314162 NOTIFY 7. Security Considerations The information returned in the P-Answer-State header is not viewed as particularly sensitive. Rather, it is informational in nature, providing an indication to the UAC that delivery of any media sent as a result of an answer in this response is not guaranteed. An eavesdropper cannot gain any useful information by obtaining the contents of this header. If end-to-end protection is not used at the SIP layer, it is possible for proxies between the UAs to remove the header or modify the contents of the header value. However end-to-end protection has not been considered as the P-Answer-State header is normally added by an intermediate node that acts either as a B2BUA or proxy. This attack either denies the caller the knowledge that the callee has yet to be Allen, et al. Expires December 27, 2005 [Page 24] Internet-Draft The P-Answer-State Header June 2005 contacted or falsely indicates that the callee has yet to be contacted when they have already answered. It is therefore RECOMMENDED that this extension is used in a secured trusted environment where transitive trust exists between the proxies and UAs. If end-to-end security mechanisms are to be used issues such as key exchange between endpoints and intermediate network nodes need to be considered." 8. IANA Considerations 8.1 Registration of Header Fields This document defines a private SIP extension header field (beginning with the prefix "P-" ) based on the registration procedures defined in RFC 3427 [17]. The following rows shall be added to the "Header Fields" section of the SIP parameter registry: +----------------+--------------+-----------+ | Header Name | Compact Form | Reference | +----------------+--------------+-----------+ | P-Answer-State | | [RFCXXXX] | +----------------+--------------+-----------+ Editor Note: [RFCXXXX] should be replaced with the designation of this document. 8.2 Registration of Header Field Parameters This document defines parameters for the header fields defined in the preceding section. The header field named "P-Answer-State" may take the values "Unconfirmed", or "Confirmed". The following rows shall be added to the "Header Field Parameters and Parameter Values" section of the SIP parameter registry: +----------------+----------------+-------------------+-----------+ | Header Field | Parameter Name | Predefined Values | Reference | +----------------+----------------+-------------------+-----------+ | P-Answer-State | Unconfirmed | Yes | [RFCXXXX] | | P-Answer-State | Confirmed | Yes | [RFCXXXX] | +----------------+----------------+-------------------+-----------+ Editor Note: [RFCXXXX] should be replaced with the designation of this document. Allen, et al. Expires December 27, 2005 [Page 25] Internet-Draft The P-Answer-State Header June 2005 9. Changes since previous version This document is based upon the definition of the P-Answer-State header in [18] with the following changes: The P-Alerting-Mode header definition has been removed as this extension will be progressed separately as a standards track RFC [7]. The text on security considerations has been improved. PUBLISH method has been added to figure 1. The procedures at the UA text has been split into UA-terminal and UA- intermediate-node for better clarity. A session flow example section has been added. Various nits and editorial corrections have been made. 10. Acknowledgements The authors would like to thank Dean Willis, Rohan Mahay, Christian Schmidt, Mike Hammer, and Miguel Garcia-Martin for their comments that contributed to the progression of this work. The authors would also like to thank the OMA POC Working Group members for their support of this document and in particular Tom Hiller for presenting the concept of the P-Answer-State header to SIPPING at IETF#62. 11. References 11.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [3] 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. [4] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. Allen, et al. Expires December 27, 2005 [Page 26] Internet-Draft The P-Answer-State Header June 2005 11.2 Informative References [5] Rosenberg, J., "A Framework for Conferencing with the Session Initiation Protocol", draft-ietf-sipping-conferencing-framework-05 (work in progress), May 2005. [6] Garcia-Martin, M., "A Session Initiation Protocol (SIP) Event Package and Data Format for various settings in support for the Push-to-talk Over Cellular (PoC) service", draft-garcia-sipping-poc-isb-am-02 (work in progress), June 2005. [7] Willis, D. and A. Allen, "Requesting Answering and Alerting Modes for the Session Initiation Protocol (SIP)", draft-willis-sip-answeralert-00 (work in progress), June 2005. [8] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason Header Field for the Session Initiation Protocol (SIP)", RFC 3326, December 2002. [9] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, August 2004. [10] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [11] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000. [12] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [13] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE Method", RFC 3311, October 2002. [14] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. [15] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003. [16] Niemi, A., "Session Initiation Protocol (SIP) Extension for Event State Publication", RFC 3903, October 2004. [17] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B. Allen, et al. Expires December 27, 2005 [Page 27] Internet-Draft The P-Answer-State Header June 2005 Rosen, "Change Process for the Session Initiation Protocol (SIP)", BCP 67, RFC 3427, December 2002. [18] Allen, A., "Private Header (P-Header) Extensions to the Session Initiation Protocol (SIP) for the Open Mobile Alliance (OMA) Push to talk over Cellular (PoC)", draft-allen-sipping-poc-p-headers-01 (work in progress), February 2005. Authors' Addresses Andrew Allen (editor) Research in Motion (RIM) 122 West John Carpenter Parkway, Suite 430 Irving, Texas 75039 USA Phone: unlisted Fax: unlisted Email: aallen@rim.com Jan Holm Ericsson Gotalandsvagen 220 Stockholm 612526 Sweden Phone: unlisted Fax: unlisted Email: Jan.Holm@ericsson.com Tom Hallin Motorola 1501 W Shure Drive Arlington Heights 60004 USA Phone: unlisted Fax: unlisted Email: thallin@motorola.com Allen, et al. Expires December 27, 2005 [Page 28] Internet-Draft The P-Answer-State Header June 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Allen, et al. Expires December 27, 2005 [Page 29]