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Versions: (draft-allen-sipping-poc-p-headers) 00 01 02 03 04 05 RFC 4964

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
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   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



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   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




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   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











































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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



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   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



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   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.



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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





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   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



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   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.




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   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



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   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



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   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.



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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"






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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.









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      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" <sip:FriendsOfAlice@atlanta.com>
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:alice@pc33.atlanta.com>
   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



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   Record-Route: <sip:AlicesPolicyServer.atlanta.com>
   Max-Forwards: 69
   To: "Alice's Friends" <sip:FriendsOfAlice@atlanta.com>
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:alice@pc33.atlanta.com>
   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" <sip:bob@biloxi.com>
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=2178309898
   Call-ID: e60a4c784b6716
   CSeq: 301166605 INVITE
   Contact: <sip:AlicesConferenceFocus.atlanta.com>
   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" <sip:bob@biloxi.com>
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=781299330
   Call-ID: 6eb4c66a847710
   CSeq: 478209 INVITE
   Contact: <sip:BobsPolicyServer.biloxi.com>
   Content-Type: application/sdp
   Content-Length: 142

   (SDP not shown)

   F5 183 Session Progress Bob's Policy Server -> Conference Focus



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   SIP/2.0 183 Session Progress
   Via: SIP/2.0/UDP
   AlicesConferenceFocus.atlanta.com;branch=z9hG4bK4721d8
   To: "Bob" <sip:bob@biloxi.com>;tag=a6c85cf
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=2178309898
   Call-ID: e60a4c784b6716
   Contact: <sip:BobsPolicyServer.biloxi.com>
   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: <sip:AlicesPolicyServer.atlanta.com>
   To: "Alice's Friends" <sip:FriendsOfAlice@atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:AlicesConferenceFocus.atlanta.com>
   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: <sip:AlicesPolicyServer.atlanta.com>
   To: "Alice's Friends" <sip:FriendsOfAlice@atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:AlicesConferenceFocus.atlanta.com>
   P-Answer-State: Unconfirmed
   Content-Type: application/sdp
   Content-Length: 131

   (SDP not shown)

   F8 ACK Alice -> Alice's Policy Server




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   ACK sip:AlicesConferenceFocus.atlanta.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9
   Route: <sip:AlicesPolicyServer.atlanta.com>
   Max-Forwards: 70
   To: "Alice's Friends" <sip:FriendsOfAlice@atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;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" <sip:FriendsOfAlice@atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;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" <sip:bob@biloxi.com>;tag=d28119a
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=781299330
   Call-ID: 6eb4c66a847710
   CSeq: 478209 INVITE
   Contact: <sip:bob@192.0.2.4>
   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" <sip:bob@biloxi.com>;tag=d28119a



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   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;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" <sip:bob@biloxi.com>;tag=a6670811
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=2178309898 Call-ID:
   e60a4c784b6716
   Contact: <sip:BobsPolicyServer.biloxi.com>
   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" <sip:bob@biloxi.com>;tag=a6670811
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;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



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   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: <sip:AlicesConferenceFactoryURI.atlanta.com>



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   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:alice@pc33.atlanta.com>
   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: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:AlicesPre-establishesSession@
   AlicesPolicyServer.atlanta.com>
   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: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;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



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   To: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 REFER
   Refer-To: "Bob" <sip:bob@biloxi.com>
   Contact: <sip:alice@pc33.atlanta.com>

   F5 202 ACCEPTED Alice's Policy Server -> Alice

   SIP/2.0 202 ACCEPTED
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
   To: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 REFER
   Contact: <sip:AlicesPre-establishesSession@
   AlicesPolicyServer.atlanta.com>

   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" <sip:bob@biloxi.com>
   From: "Alice" <sip:Alice@atlanta.com>;tag=2178309898
   Call-ID: e60a4c784b6716
   CSeq: 301166605 INVITE
   Contact: <sip:AlicesConferenceFocus.atlanta.com>
   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" <sip:bob@biloxi.com>
   From: "Alice" <sip:Alice@atlanta.com>;tag=781299330
   Call-ID: 6eb4c66a847710
   CSeq: 478209 INVITE
   Contact: <sip:BobsPolicyServer.biloxi.com>
   Content-Type: application/sdp
   Content-Length: 142

   (SDP not shown)



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   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" <sip:bob@biloxi.com>;tag=a6c85cf
   From: "Alice" <sip:Alice@atlanta.com>;tag=2178309898
   Call-ID: e60a4c784b6716
   Contact: <sip:BobsPolicyServer.biloxi.com>
   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: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314161 NOTIFY
   Contact: <sip:AlicesPre-establishesSession@
   AlicesPolicyServer.atlanta.com>
   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" <sip:bob@biloxi.com>;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: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;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.




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   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" <sip:bob@biloxi.com>;tag=d28119a
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=781299330
   Call-ID: 6eb4c66a847710
   CSeq: 478209 INVITE
   Contact: <sip:bob@192.0.2.4>
   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" <sip:bob@biloxi.com>;tag=d28119a
   From: "Alice" <sip:Alice@atlanta.com>;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" <sip:bob@biloxi.com>;tag=a6670811
   From: "Alice's Friends"
   <sip:FriendsOfAlice@atlanta.com>;tag=2178309898
   Call-ID: e60a4c784b6716
   Contact: <sip:BobsPolicyServer.biloxi.com>
   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



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   Max-Forwards: 70
   To: "Bob" <sip:bob@biloxi.com>;tag=a6670811
   From: "Alice" <sip:Alice@atlanta.com>;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: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314162 NOTIFY
   Contact: <sip:AlicesPre-establishesSession@
   AlicesPolicyServer.atlanta.com>
   Event: refer
   Subscription-State: Active;Expires=60
   Content-Type: message/sipfrag;version=2.0
   Content-Length: 83

   SIP/2.0 200 OK
   To: "Bob" <sip:bob@biloxi.com>;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: <sip:AlicesConferenceFactoryURI.atlanta.com>;tag=c70ef99
   From: "Alice" <sip:alice@atlanta.com>;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



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   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".






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   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.



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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.



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   [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.
































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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


















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