SIPPING Working Group A. Allen, Ed.
Internet-Draft Research in Motion (RIM)
Expires: December 27, 2005 J. Holm
Ericsson
T. Hallin
Motorola
June 25, 2005
The P-Answer-State Header Extension to the Session Initiation Protocol
(SIP) for the Open Mobile Alliance (OMA) Push to talk over Cellular
(PoC)
draft-allen-sipping-poc-p-answer-state-header-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a private Session InitiationProtocol(SIP)
header (P-header) used by the Open Mobile Alliance (OMA),For Push to
talk over Cellular (PoC) along with its applicability, which is
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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. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. The P-Answer-State header . . . . . . . . . . . . . . . . . . 5
4.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Alternatives Considered . . . . . . . . . . . . . . . . . 7
4.3 Applicability statement for the P-Answer-State header . . 8
4.4 Usage of the P-Answer-State header . . . . . . . . . . . . 8
4.4.1 Procedures at the UA (terminal) . . . . . . . . . . . 9
4.4.2 Procedures at the UA (Intermediate Node) . . . . . . . 10
4.4.3 Procedures at the proxy server . . . . . . . . . . . . 11
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1 P-Answer-State header syntax . . . . . . . . . . . . . . . 12
5.2 Table of the new header . . . . . . . . . . . . . . . . . 12
6. Example Usage Session Flows . . . . . . . . . . . . . . . . . 13
6.1 Pre-arranged Group Call using On-demand Session . . . . . 13
6.2 1-1 Call using Pre-established Session . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
8.1 Registration of Header Fields . . . . . . . . . . . . . . 25
8.2 Registration of Header Field Parameters . . . . . . . . . 25
9. Changes since previous version . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1 Normative References . . . . . . . . . . . . . . . . . . . 26
11.2 Informative References . . . . . . . . . . . . . . . . . . 27
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . 29
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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. Overview
The PoC terminal may support such hardware capabilities as a speaker
phone and/or headset and software that provide the capability for the
user to configure the PoC terminal to accept the session invitations
immediately and play out the media as soon as it is received without
requiring the intervention of the called user. This mode of
operation is known as Automatic Answer mode. The user may
alternatively configure the PoC terminal to first alert the user and
require the user to manually accept the session invitation before
media is accepted. This mode of operation is known as Manual Answer
mode. The PoC terminal may support both or only one of these modes
of operation. The user may change the Answer Mode (AM) configuration
of the PoC terminal frequently based on their current circumstances
and preference,(perhaps because the user is busy, or in a public area
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where she cannot use a speaker phone, etc).
The OMA PoC Architecture utilizes SIP servers within the network that
may perform such roles as a conference focus [5], a RTP translator or
a network policy enforcement server. A possible optimization to
minimize the delay in the providing of the caller with an indication
to speak is for the SIP network server to perform buffering of media
packets in order to provide an early or unconfirmed indication back
to the caller and allow the caller to start speaking before the
called PoC terminal has answered. An event package and mechanisms
for a SIP UA to indicate its current answer mode to a SIP Server in
order to enable buffering are defined in [6]. In addition,
particularly when multiple domains are involved in the session more
than one intermediate SIP server may be involved in the signaling
path for the session and the server that performs the buffering may
not be the server that has knowledge of the current answer mode of
the SIP UA that is the final destination for the SIP INVITE request.
A mechanism is to allow a terminal that acts as a SIP UA or a network
based server that acts as a SIP UA to indicate a preference to the
final destination SIP UAS to answer in a particular mode is defined
in [7]. However a mechanism is required for an intermediary SIP UAS
or proxy to relay the unconfirmed indication in a response back
towards the originating SIP UAC.
This document proposes a new SIP header field to support this
unconfirmed indication. The extension may be optionally included in
a response to a SIP INVITE request or in a NOTIFY sent as a result of
a REFER that requests an INVITE to be sent to provide an indication
from an intermediate node acting as a SIP proxy or back-to-back UA
that it has information that hints that the terminating UA will
likely answer automatically and therefore provides an unconfirmed
indication back towards the inviting SIP UA to transmit media prior
to receiving a final response from the final destination of the SIP
INVITE request. The extension is described below.
4. The P-Answer-State header
The purpose of the P-Answer-State header field is to provide an
indication from an intermediate node acting as a SIP proxy or back-
to-back UA that is has information that hints that the terminating UA
identified in the Request-URI of the request will likely answer
automatically and therefore provides an unconfirmed indication back
towards the inviting SIP UA to transmit media prior to receiving a
final response from the final destination of the SIP INVITE request.
If a Provisional response contains the P-Answer-State header with the
value "Unconfirmed" and does not contain SDP then a receiving
intermediate node may send a 200 OK response containing SDP and a
P-Answer-State header with the value "Unconfirmed" if the
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intermediate node is willing to perform media buffering. If the
response containing the P-Answer-State header with the value
"Unconfirmed" also contains SDP the intermediate node that inserted
the header and SDP in the response is also indicating that it is
willing to buffer the media until a final confirmed indication is
received.
The P-Answer-State header field MAY be included in a provisional or
final response to a SIP INVITE request or in a NOTIFY request sent as
a result of a REFER request to send an INVITE request. If the
P-Answer-State header field with value "Unconfirmed" is included in a
provisional response that contains SDP the intermediate node is
leaving the decision where to do buffering to other nodes upstream
and will forward upstream a "Confirmed indication" in a 200 OK
response when the final response is received from the destination UA.
The P-Answer-State header is only included in a provisional response
when the node that sends the response has knowledge that there is a
B2BUA node that understands this extension in the signaling path
between itself and the originating UAC that will only pass the header
on in either a 200 OK response or in the sipfrag of a NOTIFY request.
Such a situation only occurs with specific network topologies which
is another reason why use of this header is not relevant to the
general internet. The originating UAC will only receive the
P-Answer-state header in a 200 OK response or in the sipfrag of a
NOTIFY request.
4.1 Requirements
The OMA PoC service has initial setup performance requirements that
can be met by an intermediate server (SIP B2BUA) spooling media from
the inviting PoC subscriber until one or more invited PoC subscribers
have accepted the session. The specific requirements are
REQ-1: An intermediate server MAY spool media from the inviting SIP
UA until one or more invited PoC SIP UAs has accepted the
invitation.
REQ-2: An intermediate server that is capable of spooling media MAY
accept an invite request from an inviting SIP UAC even if no
invited SIP UAS has accepted the invite request if it has a hint
that the invited SIP UAC is likely to accept the request without
requiring user intervention.
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REQ-3: An intermediate server or proxy that is incapable of spooling
media or does not wish to, but has a hint that the invited SIP UAC
is likely to automatically accept the session invitation MUST be
able to indicate back to another intermediate server that can
spool media that it has some hint that the invited UAC is likely
to automatically accept the session invitation.
REQ-4: An intermediate server that is willing to spool media from the
inviting SIP UA until one or more invited SIP UAs have accepted
the invite SHOULD indicate that it is spooling media to the
inviting SIP UAC.
4.2 Alternatives Considered
In order to meet REQ-3, an intermediate server needs to receive an
indication back that the invited SIP UA is likely to accept the
invite request without requiring user intervention. In this case,
the intermediate server or proxy that has a hint that the invited SIP
UAC is likely to accept the request can include an answer state
indication in the 183 Session Progress or 200 OK response.
A number of alternatives were considered for the intermediate server
to inform the another intermediate server or the inviting SIP UAC of
the invited PoC SIP UAs answer mode settings.
One proposal was to create a unique reason-phrase in the 183 and 200
OK response. This was rejected because the reason phrases are
normally intended for human readers and not meant to be parsed by
servers for special syntactic and semantic meaning.
Another proposal was to use a Reason header [8] in the 183 and 200 OK
response. This was rejected because this would be inconsistent with
the intended use of the reason header and its usage is not defined
for these response codes and would have required creating and
registering a new protocol identifier.
Another proposal was to use a feature-tag in the returned Contact
header as defined in [9]. This was rejected because it was not a
different feature, but is an attribute of the session and can be
applied to many different features.
Another proposal was to use a new SDP attribute. The choice of an
SDP parameter was rejected because the answer state applies to the
session and not to a media stream.
The P-Answer-State header was chosen to give additional information
about the state of the SIP session progress and acceptance. Even
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though the UAC sees that its SDP offer has been answered and
accepted, the header lets the UAC know whether invited PoC subscriber
has accepted the invite or just an intermediary has done the
acceptance.
4.3 Applicability statement for the P-Answer-State header
The P-Answer-State header is applicable in the following
circumstances:
o In networks where there are UAs that engage in half-duplex
communication where there is not the possibility for the invited
user to verbally acknowledge the answering of the session as is
normal in full duplex communication;
o Where the invited UA may automatically accept the session
without manual acceptance;
o The network also contains intermediate network SIP servers that
are trusted;
o The intermediate network SIP servers have knowledge of the
current answer mode setting of the terminating UAS; and,
o The intermediate network SIP servers can provide buffering of
the media in order to reduce the time for the inviting user to
send media.
o The intermediate network SIP servers assume knowledge of the
network topology and the existence of similar intermediate network
SIP servers in the signaling path.
Such configurations are generally not applicable to the internet as a
whole where such trust relationships do not exist.
In addition security issues have only been considered for networks
which are trusted and use hop by hop security mechanisms and security
issues with usage of this mechanism in the general internet have not
been evaluated.
4.4 Usage of the P-Answer-State header
A UAS B2BUA or proxy MAY insert a P-Answer-State header field in any
1XX or 2XX response that is allowed to contain an SDP answer in
response to an SDP offer contained in an INVITE as specified in [2].
Typically the P-Answer-State header field is inserted in either a 183
Session Progress or a 200 OK response. A UA that receives a REFER
request to send an INVITE MAY also insert a P-Answer-State header
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field in a NOTIFY request it sends as a result of the implicit
subscription created by the REFER request.
When the P-Answer-State header field contains the parameter
"Unconfirmed" the UAC or proxy is indicating that it has information
that hints that the final destination UAS for the INVITE request is
likely to automatically accept the session but that this is
unconfirmed and it is possible that the final destination UAS will
first alert the user and require manual acceptance of the session or
not accept the session request. This is referred to here as an
"unconfirmed response". When the P-Answer-State header field
contains the parameter "Confirmed" the UAC or proxy is indicating
that the destination UAS has accepted the session and is ready to
receive media. The parameter value of "Confirmed" has the usual
semantics of an SDP answer and is included for completeness. The
usual end to end SDP answer response semantics are referred to here
as a "confirmed response".
4.4.1 Procedures at the UA (terminal)
A UAC (terminal) that receives a 1XX or 2XX response containing a
P-Answer-State header field containing the parameter "Unconfirmed"
and an SDP answer MAY send media as specified in [2], however there
is no guarantee that the media will be received by the final
recipient. How a UAC confirms whether the media was or was not
received by the final destination when it his received a 2XX
"unconfirmed response" is application specific and outside of the
scope of this document. If the application is a conference then the
mechanism specified in [2] could be used to determine that the
invited user joined. Alternatively a BYE request could be received
or the media could be placed on hold if the final destination UAS
does not accept the session.
A UAC (terminal) that receives a 1XX or 2XX response without a
P-Answer-State Header or containing a P-Answer-State header field
containing the parameter "Confirmed" SHALL treat it as a "confirmed
response". A UAC (terminal) that receives in response to a REFER
request a NOTIFY request containing a P-Answer-State header field
containing the parameter "Unconfirmed" as either a SIP header or
contained in a sipfrag in the body of the NOTIFY request received on
a pre-existing dialog that was established by an INVITE request and
for which there has been a successful SDP offer-answer exchange
according to [2] then the UAC MAY send media, however there is no
guarantee that the media will be received by the final recipient that
was indicated in the Refer-To header in the original REFER request.
There are no P-Answer-State procedures for a terminal acting in the
UAS role.
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4.4.2 Procedures at the UA (Intermediate Node)
A UAS (Intermediate Node) MAY insert a P-Answer-State header field in
any 1XX or 2XX response that is allowed to contain an SDP answer in
response to an SDP offer contained in an INVITE request as specified
in [2]. A response containing the P-Answer-State header field
containing the parameter "Unconfirmed" MAY or MAY NOT contain an SDP
answer. If the response contains an SDP answer then the sending UA
MUST be ready to receive media as specified in [2].
An intermediate node that acts as a back-to-back UA and returns a 1XX
or 2XX response in response to an INVITE request MAY insert a
P-Answer-State header field containing the parameter "Unconfirmed" in
the response if it has not yet received a "confirmed response" from
the final destination UA. If the intermediate node UAS also includes
SDP in the response along with a P-Answer-State header field
containing the parameter "Unconfirmed" the intermediate node MUST be
ready to receive media as specified in [2] and MAY buffer any media
it receives until it receives a "confirmed response" from the final
destination UA or until the buffer is full. Such an intermediate
node may insert an SDP answer in the response it generates even if
the "unconfirmed response" it received did not contain an SDP answer.
An intermediate node that acts as a back-to-back UA and receives a
REFER request to send an INVITE request to another UA as specified in
[11] MAY insert a P-Answer-State header field containing the
parameter "Unconfirmed" in the initial NOTIFY request sent in
response to the REFER request if it has not yet received a "confirmed
response" from the final destination UA and it has information that
hints that the final destination UAS for the INVITE is likely to
automatically accept the session. If the REFER was sent as part of
an existing dialog established by an INVITE request and for which
there has been a successful SDP offer-answer exchange according to
[2] the intermediate node MUST be ready to receive media as specified
in [2] and MAY buffer any media it receives until it receives a
"confirmed response" from the final destination UA or until its
buffer is full.
An intermediate node that acts as a back-to-back UA and receives a
1XX or 2XX response in response to an INVITE request containing a
P-Answer-State header field in the response SHOULD include the
P-Answer-State header field unmodified in the 1XX or 2XX response it
sends as a result of receiving that response. If the intermediate
node that acts as a back-to-back UA sends a NOTIFY request according
to [11] then the intermediate node UAC SHOULD include the P-Answer-
State header field unmodified in the sipfrag of the response included
in the body of the NOTIFY request.
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A UAC (Intermediate Node) that receives a 1XX or 2XX response without
a P-Answer-State Header or containing a P-Answer-State header field
containing the parameter "Confirmed" SHALL treat it as a "confirmed
response". If the UAS (Intermediate Node) knows that the final
destination UA is now ready to accept media and the UAS previously
sent an "Unconfirmed response" the UAS SHOULD insert a P-Answer-State
header field containing the parameter "Confirmed" in the response.
An intermediate node that acts as a back-to-back UA that previously
sent an initial NOTIFY request containing a P-Answer-State header
field containing the parameter "Unconfirmed" that subsequently
receives a "confirmed response" without a P-Answer-State header field
in response to the INVITE request sent as a result of the REFER
request SHOULD include a P-Answer-State header containing the
parameter "Confirmed" in the subsequent NOTIFY request generated as a
result of the "confirmed response".
If the UAS knows that the final destination UA is ready to accept
media and the UAS did not previously send an "Unconfirmed response"
the UAS MAY insert a P-Answer-State header field containing the
parameter "Confirmed" in the response.
If an intermediate node that acts as a back-to-back UA and sends an
INVITE request in response to a REFER request learns by receiving a
"confirmed response" that the final destination UA is ready to accept
media and the back-to-back UA did not previously include a P-Answer-
State header containing the parameter "Unconfirmed" in the initial
NOTIFY request sent in response to the REFER request then the back-
to-back UA MAY insert a P-Answer-State header field containing the
parameter "Confirmed" in the response if the "confirmed response"
does not contain a P-Answer-State header.
4.4.3 Procedures at the proxy server
SIP proxy servers do not need to understand the semantics of the
P-Answer-State header field. As part of the regular SIP rules for
unknown headers, a proxy will forward unknown headers. A proxy MAY
insert a P-Answer-State header field in a 1XX response that it
originates compliant with [3] or add it to a 2XX response that
contains an SDP answer in response to an SDP offer contained in an
INVITE request as specified in [2].
A proxy that returns a 1XX response in response to an INVITE request
MAY insert a P-Answer-State header field containing the parameter
"Unconfirmed" in the response if it has not yet received a
"confirmed response" from the final destination UA.
A proxy that receives a 1XX or 2XX response without a P-Answer-State
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Header or containing a P-Answer-State header field containing the
parameter "Confirmed" SHALL for the purposes of this document treat
it as a "confirmed response".
If the proxy knows that the final destination UA is now ready to
accept media and the proxy previously sent an "Unconfirmed response"
the proxy SHOULD insert a P-Answer-State header field containing the
parameter "Confirmed" in the response.
If the proxy knows that the final destination UA is ready to accept
media and the proxy did not previously send an "Unconfirmed response"
the proxy MAY insert a P-Answer-State header field containing the
parameter "Confirmed" in the response.
5. Formal Syntax
The mechanisms specified in this document is described in both prose
and an augmented Backus-Naur Form (BNF) defined in RFC 2234 [3].
Further, several BNF definitions are inherited from SIP and are not
repeated here. Implementers need to be familiar with the notation
and contents of SIP [3] and RFC 2234 [3] to understand this document.
5.1 P-Answer-State header syntax
The syntax of the P-Answer-State header is described as follows:
P-Answer-State = "P-Answer-State" HCOLON answer-type
answer-type = "Confirmed" / "Unconfirmed"
5.2 Table of the new header
Table 1 extends the headers defined in this document to Table 2 in
SIP [3], section 7.1 of the SIP-specific event notification [10]
tables 1 and 2 in the SIP INFO method [11], tables 1 and 2 in
Reliability of provisional responses in SIP [12], tables 1 and 2 in
the SIP UPDATE method [13], tables 1 and 2 in the SIP extension for
Instant Messaging [14], table 1 in the SIP REFER method [15], and
table 2 in the SIP PUBLISH method [16]:
Header field where proxy ACK BYE CAN INV OPT REG SUB
_______________________________________________________________
P-Answer-State 1xx,2xx ar - - - o - - -
Header field NOT PRA INF UPD MSG REF PUB
_______________________________________________________________
P-Answer-State R o - - - - - -
Figure 1
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6. Example Usage Session Flows
For simplicity some details such as intermediate proxies and 100
Trying responses are not shown in the following example flows. The
term "policy server" is used here to mean a policy enforcement
server.
6.1 Pre-arranged Group Call using On-demand Session
The following flow shows Alice making a Pre-arranged Group Call using
a Conference URI which has Bob on the member list. The session
initiation uses the On-demand Session establishment mechanism where a
SIP INVITE containing SDP is sent by Alices's terminal when Alice
pushes her push to talk button.
In this example Alice's Policy Server acts a Call Stateful SIP Proxy
and Bob's Policy Server which is aware that the current Answer Mode
setting of Bob's terminal is set to Auto Answer acts as a B2BUA.
For simplicity the invitations by the Conference Focus to the other
members of the group are not shown in this example.
Alice's Alices's Conference Bob's Bob's
Terminal Policy Server focus Policy Server Terminal
| | | | |
|--(1)INVITE---->| | | |
| |--(2)INVITE--->| | |
| | |--(3)INVITE->| |
| | | |---(4)INVITE-->|
| | |<--(5)183----| |
| |<---(6)200-----| | |
|<---(7)200------| | | |
|----(8)ACK----->| | | |
| |---(9)ACK----->| | |
| | | | |
|=======Early Media Session=====>| | |
| | MEDIA | |
| | BUFFERING | |
| | | |<---(10)200----|
| | | |---(11)ACK---->|
| | |<--(12)200---| |
| | |--(13)ACK--->| |
| | | | |
| | |========Media Session=======>|
| | | | |
| | | | |
Figure 2
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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
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
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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
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
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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
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.
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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
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
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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.
6.2 1-1 Call using Pre-established Session
The following flow shows Alice making a 1-1 Call to Bob using a pre-
established session. A pre-established session is where a dialog is
established with Alices's Policy Server using a SIP INVITE SDP offer
answer exchange to pre-negotiate the codecs and other media
Parameters to be used for media sessions ahead of Alice initiating a
Communication. When Alice initiates a communication to Bob a SIP
REFER is used to Request Alice's Policy Server to send an INVITE to
Bob. In this example Bob's Terminal does not use the Pre-established
Session mechanism.
In this example Alice's Policy Server acts a B2BUA and also performs
the Conference Focus function. Bob's Policy Server which is aware
that the current Answer Mode setting of Bob's terminal is set to Auto
Answer acts as a B2BUA.
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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>
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)
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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
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
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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)
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
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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.
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
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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
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
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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
7. Security Considerations
The information returned in the P-Answer-State header is not viewed
as particularly sensitive. Rather, it is informational in nature,
providing an indication to the UAC that delivery of any media sent as
a result of an answer in this response is not guaranteed. An
eavesdropper cannot gain any useful information by obtaining the
contents of this header.
If end-to-end protection is not used at the SIP layer, it is possible
for proxies between the UAs to remove the header or modify the
contents of the header value. However end-to-end protection has not
been considered as the P-Answer-State header is normally added by an
intermediate node that acts either as a B2BUA or proxy. This attack
either denies the caller the knowledge that the callee has yet to be
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contacted or falsely indicates that the callee has yet to be
contacted when they have already answered. It is therefore
RECOMMENDED that this extension is used in a secured trusted
environment where transitive trust exists between the proxies and
UAs.
If end-to-end security mechanisms are to be used issues such as key
exchange between endpoints and intermediate network nodes need to be
considered."
8. IANA Considerations
8.1 Registration of Header Fields
This document defines a private SIP extension header field (beginning
with the prefix "P-" ) based on the registration procedures defined
in RFC 3427 [17].
The following rows shall be added to the "Header Fields" section of
the SIP parameter registry:
+----------------+--------------+-----------+
| Header Name | Compact Form | Reference |
+----------------+--------------+-----------+
| P-Answer-State | | [RFCXXXX] |
+----------------+--------------+-----------+
Editor Note: [RFCXXXX] should be replaced with the designation of
this document.
8.2 Registration of Header Field Parameters
This document defines parameters for the header fields defined in the
preceding section. The header field named "P-Answer-State" may take
the values "Unconfirmed", or "Confirmed".
The following rows shall be added to the "Header Field Parameters and
Parameter Values" section of the SIP parameter registry:
+----------------+----------------+-------------------+-----------+
| Header Field | Parameter Name | Predefined Values | Reference |
+----------------+----------------+-------------------+-----------+
| P-Answer-State | Unconfirmed | Yes | [RFCXXXX] |
| P-Answer-State | Confirmed | Yes | [RFCXXXX] |
+----------------+----------------+-------------------+-----------+
Editor Note: [RFCXXXX] should be replaced with the designation of
this document.
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9. Changes since previous version
This document is based upon the definition of the P-Answer-State
header in [18] with the following changes:
The P-Alerting-Mode header definition has been removed as this
extension will be progressed separately as a standards track RFC
[7].
The text on security considerations has been improved.
PUBLISH method has been added to figure 1.
The procedures at the UA text has been split into UA-terminal and
UA- intermediate-node for better clarity.
A session flow example section has been added.
Various nits and editorial corrections have been made.
10. Acknowledgements
The authors would like to thank Dean Willis, Rohan Mahay, Christian
Schmidt, Mike Hammer, and Miguel Garcia-Martin for their comments
that contributed to the progression of this work. The authors would
also like to thank the OMA POC Working Group members for their
support of this document and in particular Tom Hiller for presenting
the concept of the P-Answer-State header to SIPPING at IETF#62.
11. References
11.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[4] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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11.2 Informative References
[5] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol",
draft-ietf-sipping-conferencing-framework-05 (work in
progress), May 2005.
[6] Garcia-Martin, M., "A Session Initiation Protocol (SIP) Event
Package and Data Format for various settings in support for
the Push-to-talk Over Cellular (PoC) service",
draft-garcia-sipping-poc-isb-am-02 (work in progress),
June 2005.
[7] Willis, D. and A. Allen, "Requesting Answering and Alerting
Modes for the Session Initiation Protocol (SIP)",
draft-willis-sip-answeralert-00 (work in progress), June 2005.
[8] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason Header
Field for the Session Initiation Protocol (SIP)", RFC 3326,
December 2002.
[9] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[10] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[11] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[12] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262,
June 2002.
[13] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[14] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[15] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[16] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.
[17] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B.
Allen, et al. Expires December 27, 2005 [Page 27]
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Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002.
[18] Allen, A., "Private Header (P-Header) Extensions to the Session
Initiation Protocol (SIP) for the Open Mobile Alliance (OMA)
Push to talk over Cellular (PoC)",
draft-allen-sipping-poc-p-headers-01 (work in progress),
February 2005.
Authors' Addresses
Andrew Allen (editor)
Research in Motion (RIM)
122 West John Carpenter Parkway, Suite 430
Irving, Texas 75039
USA
Phone: unlisted
Fax: unlisted
Email: aallen@rim.com
Jan Holm
Ericsson
Gotalandsvagen 220
Stockholm 612526
Sweden
Phone: unlisted
Fax: unlisted
Email: Jan.Holm@ericsson.com
Tom Hallin
Motorola
1501 W Shure Drive
Arlington Heights 60004
USA
Phone: unlisted
Fax: unlisted
Email: thallin@motorola.com
Allen, et al. Expires December 27, 2005 [Page 28]
Internet-Draft The P-Answer-State Header June 2005
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Allen, et al. Expires December 27, 2005 [Page 29]
