SIPPING Working Group A. Allen, Ed. Internet-Draft Research in Motion (RIM) Expires: June 13, 2006 J. Holm Ericsson T. Hallin Motorola December 10, 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-01 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 June 13, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document describes a private Session Initiation Protocol(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 June 13, 2006 [Page 1] Internet-Draft The P-Answer-State Header December 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Background for the extension . . . . . . . . . . . . . . . . . 5 5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. The P-Answer-State header . . . . . . . . . . . . . . . . . . 7 6.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 6.2. Alternatives considered . . . . . . . . . . . . . . . . . 8 6.3. Applicability statement for the P-Answer-State header . . 9 6.4. Usage of the P-Answer-State header . . . . . . . . . . . . 10 6.4.1. Procedures at the UA (terminal) . . . . . . . . . . . 10 6.4.2. Procedures at the UA (PTT Server) . . . . . . . . . . 11 6.4.3. Procedures at the proxy server . . . . . . . . . . . . 13 7. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. P-Answer-State header syntax . . . . . . . . . . . . . . . 13 7.2. Table of the new header . . . . . . . . . . . . . . . . . 14 8. Example usage session flows . . . . . . . . . . . . . . . . . 14 8.1. Pre-arranged group call using on-demand session . . . . . 14 8.2. 1-1 Call using pre-established session . . . . . . . . . . 19 9. Security considerations . . . . . . . . . . . . . . . . . . . 25 10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 26 10.1. Registration of Header Fields . . . . . . . . . . . . . . 26 10.2. Registration of header field parameters . . . . . . . . . 26 11. Changes since previous version . . . . . . . . . . . . . . . . 27 Allen, et al. Expires June 13, 2006 [Page 2] Internet-Draft The P-Answer-State Header December 2005 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 13.1. Normative references . . . . . . . . . . . . . . . . . . . 28 13.2. Informative references . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 Intellectual Property and Copyright Statements . . . . . . . . . . 31 Allen, et al. Expires June 13, 2006 [Page 3] Internet-Draft The P-Answer-State Header December 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. Terminology The terms "PTT Server","Unconfirmed Indication", "Unconfirmed Response", "Confirmed Indication" and "Confirmed Response" are introduced in this document. A "PTT Server" as referred to here is a SIP network server that performs the network based functions for the Push to Talk service. The PTT Server may act as a SIP Proxy or back-to-back UA (B2BUA)based on the functions it needs to perform. There may be one or more PTT Servers involved in a SIP Push to Talk session. An "Unconfirmed Indication" as referred to here is an indication that Allen, et al. Expires June 13, 2006 [Page 4] Internet-Draft The P-Answer-State Header December 2005 the final target UA for the request has yet to be contacted and an intermediate SIP node is indicating that it has information that hints that the request is likely to be answered by the target UA. An "Unconfirmed Response" is a SIP response containing an "Unconfirmed Indication". A "Confirmed Indication" as referred to here is an indication that the target UA has accepted the session invitation and is ready to receive media. A "Confirmed Response" is a SIP 200 OK response containing a "Confirmed Indication" and has the usual semantics of a response containing an answer (such as an SDP answer). 4. Background for the extension 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 where she cannot use a speaker phone, etc). The OMA PoC Architecture utilizes PTT 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 PTT 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 PTT Server in order to enable buffering are defined in [6]. In addition, particularly when multiple domains are involved in the session more than one PTT server may be involved in the signaling path for the session and the PTT Server that performs the buffering may not be the PTT 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 to allow a terminal that acts as a SIP UA or a PTT 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 Allen, et al. Expires June 13, 2006 [Page 5] Internet-Draft The P-Answer-State Header December 2005 required for a PTT Server to relay the unconfirmed indication in a response back towards the originating SIP UAC. 5. Overview The purpose of this extension is to support an optimisation that makes it possible for the network to provide a faster push-to-talk experience, through an intermediate SIP agent (PTT Server) providing a 200 response before the called UA does, and a PTT Server buffering the media generated by the calling UA for replay to the called UA when it answers. However the PTT Server only can do this when there is a high probability that the called UA is in Automatic Answer mode. It is likely that PTT Servers near the called UA have up-to-date knowledge of the answering mode of the called UA, and due to the restricted bandwidth nature of the cellular network, they can pass upstream an indication of the called UA's answering mode faster than the called UA can deliver an automatically generated 200 response. Thus, when a PTT Server forwards an INVITE and knows that the called UA is likely to be in Automatic Answer mode, it also generates a 183 response containing a "P-Answer-State: Unconfirmed" header to signal to upstream PTT Servers that they may buffer the caller's media. A PTT Server that wishes to buffer the caller's media, upon seeing the provisional response containing "P-Answer-State: Unconfirmed" absorbs it and generates a 200 response for the caller's UA with an appropriate answer. When the called UA generates a 200 response, the PTT Server that generated the provisional response containing "P-Answer-State: Unconfirmed" adds to the 200 response a header "P-Answer-State: Confirmed". The 200 response is absorbed by the PTT Server that is buffering the caller's media, as it has already generated a 200 response. The buffering PTT Server then starts un-buffering the media. This document proposes a new SIP header field to support this unconfirmed indication. The new SIP header may be optionally included in a response to a SIP INVITE request or in the sipfrag of a NOTIFY sent as a result of a REFER that requests an INVITE to be sent. The header is used to provide an indication from an PTT Server acting as a SIP proxy or back-to-back UA that it has information that hints that the terminating UA will likely answer automatically. This 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. Allen, et al. Expires June 13, 2006 [Page 6] Internet-Draft The P-Answer-State Header December 2005 6. The P-Answer-State header The purpose of the P-Answer-State header field is to provide an indication from a PTT Server 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 an answer then a receiving PTT Server may send a 200 OK response containing an answe and a P-Answer-State header with the value "Unconfirmed" if the PTT Server is willing to perform media buffering. If the response containing the P-Answer-State header with the value "Unconfirmed" also contains an answer the PTT Server that inserted the header and answer 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 the sipfrag of 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 an answer the PTT Server is leaving the decision where to do buffering to other PTT Servers 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 PTT Server that acts as a B2BUA 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. 6.1. Requirements The OMA PoC service has initial setup performance requirements that can be met by a PTT Server acting as a B2BUA spooling media from the inviting PoC subscriber until one or more invited PoC subscribers have accepted the session. The specific requirements are Allen, et al. Expires June 13, 2006 [Page 7] Internet-Draft The P-Answer-State Header December 2005 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. 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. 6.2. Alternatives considered In order to meet REQ-3, a PTT 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 PTT Server 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 PTT Server to inform another PTT 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 Allen, et al. Expires June 13, 2006 [Page 8] Internet-Draft The P-Answer-State Header December 2005 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 though the UAC sees that its 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. 6.3. 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 have knowledge of the media types and codecs likely to be accepted by 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. Allen, et al. Expires June 13, 2006 [Page 9] Internet-Draft The P-Answer-State Header December 2005 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. 6.4. 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 answer in response to an 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 field in the sipfrag of 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. 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 a 200 OK response containing an answer and is included for completeness. 6.4.1. 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 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 Allen, et al. Expires June 13, 2006 [Page 10] Internet-Draft The P-Answer-State Header December 2005 request a NOTIFY request containing a P-Answer-State header field containing the parameter "Unconfirmed" 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 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. 6.4.2. Procedures at the UA (PTT Server) A UAS (PTT Server) MAY insert a P-Answer-State header field in any 1XX or 2XX response that is allowed to contain an answer in response to an 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 answer. If the response contains an answer then the sending UA MUST be ready to receive media as specified in [2]. A PTT Server 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 PTT Server UAS also includes an answer in the response along with a P-Answer-State header field containing the parameter "Unconfirmed" the PTT Server 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 a PTT Server may insert an answer in the response it generates even if the "unconfirmed response" it received did not contain an answer. A PTT Server that acts as a back-to-back UA and receives a REFER request to send an INVITE request to another UA as specified in [10] MAY insert a P-Answer-State header field containing the parameter "Unconfirmed" in the sipfrag of a 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 PTT Server that receives the REFER request receives the P-Answer-State header in a response from another node then the headers and parameters from that response are included in the sipfrag. If the PTT Server that receives the REFER request itself has the hint that the terminating UAC will automatically answer the INVITE sent as a result of the REFER request then the PTT Allen, et al. Expires June 13, 2006 [Page 11] Internet-Draft The P-Answer-State Header December 2005 Server generates a sipfrag for a 200 OK response. If the REFER was sent as part of an existing dialog established by an INVITE request and for which there has been a successful offer-answer exchange according to [2] the PTT Server 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. A PTT Server 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 PTT Server that acts as a back-to- back UA sends a NOTIFY request according to [10] then the PTT Server UAC SHOULD include the P-Answer-State header field unmodified in the sipfrag of the response included in the body of the NOTIFY request. A PTT Server UAC 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 PTT Server UAS 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. A PTT Server 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 PTT Server UAS knows that the final destination UA is ready to accept media and the PTT Server UAS did not previously send an "Unconfirmed response" the PTT Server UAS MAY insert a P-Answer-State header field containing the parameter "Confirmed" in the response. If a PTT Server 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 PTT Server did not previously include a P-Answer-State header containing the parameter "Unconfirmed" in the sipfrag of a NOTIFY request sent in response to the REFER request then the PTT Server 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. Allen, et al. Expires June 13, 2006 [Page 12] Internet-Draft The P-Answer-State Header December 2005 6.4.3. 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 answer in response to an 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 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. 7. Formal syntax The mechanisms specified in this document is described in both prose and an augmented Backus-Naur Form (BNF) defined in [4]. 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 [4] to understand this document. 7.1. 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" Allen, et al. Expires June 13, 2006 [Page 13] Internet-Draft The P-Answer-State Header December 2005 7.2. 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 [11] tables 1 and 2 in the SIP INFO method [12], tables 1 and 2 in Reliability of provisional responses in SIP [13], tables 1 and 2 in the SIP UPDATE method [14], tables 1 and 2 in the SIP extension for Instant Messaging [15], table 1 in the SIP REFER method [10], 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 - - - - - - - Figure 1 8. 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. 8.1. 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 an SDP offer 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. Allen, et al. Expires June 13, 2006 [Page 14] Internet-Draft The P-Answer-State Header December 2005 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 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 Allen, et al. Expires June 13, 2006 [Page 15] Internet-Draft The P-Answer-State Header December 2005 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 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 Allen, et al. Expires June 13, 2006 [Page 16] Internet-Draft The P-Answer-State Header December 2005 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 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 Allen, et al. Expires June 13, 2006 [Page 17] Internet-Draft The P-Answer-State Header December 2005 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. 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 Allen, et al. Expires June 13, 2006 [Page 18] Internet-Draft The P-Answer-State Header December 2005 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 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. 8.2. 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 Allen, et al. Expires June 13, 2006 [Page 19] Internet-Draft The P-Answer-State Header December 2005 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. 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: Allen, et al. Expires June 13, 2006 [Page 20] Internet-Draft The P-Answer-State Header December 2005 From: "Alice" ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (SDP not shown) 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 Allen, et al. Expires June 13, 2006 [Page 21] Internet-Draft The P-Answer-State Header December 2005 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 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) Allen, et al. Expires June 13, 2006 [Page 22] Internet-Draft The P-Answer-State Header December 2005 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 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. Allen, et al. Expires June 13, 2006 [Page 23] Internet-Draft The P-Answer-State Header December 2005 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 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 Allen, et al. Expires June 13, 2006 [Page 24] Internet-Draft The P-Answer-State Header December 2005 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 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 9. 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 Allen, et al. Expires June 13, 2006 [Page 25] Internet-Draft The P-Answer-State Header December 2005 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 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." 10. IANA considerations 10.1. 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. 10.2. 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". Allen, et al. Expires June 13, 2006 [Page 26] Internet-Draft The P-Answer-State Header December 2005 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. 11. Changes since previous version The following changes have been made since the 00 version: The term "intermediate node" has been replaced by "PTT Server". A Terminology section has been added for clarity of new terms introduced in this document. The mechanism for delivering the P-Answer_state header in a Notify when only a single PTT Server is involved has been changed to be included in a sipfrag. The term "an SDP answer" has been replaced by "an answer" to make the text compatible with other session descriptions. The term "an SDP offer" has been replaced by "an offer" to make the text compatible with other session descriptions. The text has been improved in some sections to improve the readability. Various nits and editorial corrections have been made. 12. Acknowledgements The authors would like to thank Paul Kyzivat, Dale Worley, 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. Allen, et al. Expires June 13, 2006 [Page 27] Internet-Draft The P-Answer-State Header December 2005 13. References 13.1. 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. 13.2. 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-04 (work in progress), September 2005. [7] Willis, D. and A. Allen, "Requesting Answering and Alerting Modes for the Session Initiation Protocol (SIP)", draft-willis-sip-answeralert-01 (work in progress), August 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] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003. [11] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. Allen, et al. Expires June 13, 2006 [Page 28] Internet-Draft The P-Answer-State Header December 2005 [12] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000. [13] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [14] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE Method", RFC 3311, October 2002. [15] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. [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. Rosen, "Change Process for the Session Initiation Protocol (SIP)", BCP 67, RFC 3427, December 2002. Allen, et al. Expires June 13, 2006 [Page 29] Internet-Draft The P-Answer-State Header December 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 June 13, 2006 [Page 30] Internet-Draft The P-Answer-State Header December 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 June 13, 2006 [Page 31]