BLISS M. Procter
Internet-Draft VoIP.co.uk
Intended status: Informational April 22, 2009
Expires: October 24, 2009
Implementing Call Park and Retrieve using the Session Initiation
Protocol (SIP)
draft-procter-bliss-call-park-extension-04
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Abstract
Call Park and Call Retrieve are useful telephony services that are
familiar to many users. Existing implementations using the Session
Initiation Protocol (SIP) show that a variety of approaches can be
taken, with varying degrees of interoperability. This draft
discusses a number of feature variations, and how they may be
implemented using existing techniques. An additional URI parameter
is also described, which enables further common use-cases to be
implemented.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Parking a call . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Parking a call without an orbit . . . . . . . . . . . . . 6
2.2. Parking a call with an orbit specified by the UA . . . . . 6
2.3. Parking a call with an orbit specified by the Park
Server . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. A failed attempt to park a call . . . . . . . . . . . . . 10
3. Retrieving a Parked Call . . . . . . . . . . . . . . . . . . . 10
4. User Agent Considerations . . . . . . . . . . . . . . . . . . 13
5. Park Server Considerations . . . . . . . . . . . . . . . . . . 14
6. Other Implementations and Interoperability . . . . . . . . . . 14
6.1. Parking by blind transfer . . . . . . . . . . . . . . . . 15
6.2. Parking by blind transfer with central allocation . . . . 15
6.3. Parking with orbit parameters . . . . . . . . . . . . . . 15
6.4. Parking on the User Agent . . . . . . . . . . . . . . . . 16
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Overview
Call Park is a feature that enables UAs to make a call inactive but
not terminated, in such a way as to allow the call to be resumed by
the UA that parked the call, or by a different UA.
This feature is typically used when User A wishes to transfer a call
in progress to User B, but doesn't necessarily know how to reach User
B's UA directly. In this situation, User A parks the call, and then
tells User B where the call is parked. User B may then retrieve the
call using a convenient UA.
Other uses include allowing multiple calls to be parked at the same
'location', and forming a queue. In this way, a simple 'ACD'
(Automatic Call Distribution) system can be implemented that permits
calls to be initially sorted and placed in one of a number of queues,
ready to be handled when an appropriate agent becomes available (and
retrieves the next call from the queue).
In all cases, the parked call is subsequently identifiable by a short
(typically 3 or 4 digit) label known as an 'orbit'. This orbit is
often allocated by the user parking the call, but some environments
favour allocation of the orbit by a Park Server. Both approaches are
described in this document.
Multiple Park Servers can be beneficial in some enviroments for a
variety of reasons including load-sharing and administrative
policies. This document shows how support for multiple servers can
easily be achieved whilst still permitting a single 'well-known' Park
Server URI to be advertised for configuration.
2. Parking a call
A basic call flow for Call Park is given in [RFC5359] (section 2.15),
and this forms the basis of the feature. The flow shows Alice and
Bob in a call, when Bob decides to park the call by sending a REFER
to the Park Server.
It is worth noting that whilst the flow is conceptually similar to an
Unattended Transfer [RFC5359] (section 2.4), the REFER is sent to
different endpoints in the two cases. For Unattended Transfer, the
Transferor sends the REFER to the Transferee, instructing him to call
the Transfer Target. For Call Park, the Transferor (Bob) sends the
REFER to the Transfer Target (Park Server), instructing it to call
the Transferee (Alice).
By following the Call Park model, we ensure that Bob has visibility
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over the success or failure of the park attempt. We also ensure that
Bob does not rely on Alice to correctly pass the orbit parameter back
from the Park Server for the centrally-allocated orbit number
situation. Finally, because Bob sends the REFER to the Park Server,
we give the Park Server the opportunity to challenge Bob and ensure
that appropriate authorisation exists for the feature.
Alice Bob Park Server Carol
| | | |
| INVITE F1 | | |
|------------->| | |
|180 Ringing F2| | |
|<-------------| | |
| 200 OK F3 | | |
|<-------------| | |
| ACK F4 | | |
|------------->| | |
| RTP Media | | |
|<============>| | |
| Bob Parks Call | |
| | REFER Refer-To: A F5 |
| |------------->| |
| | 202 F6 | |
| |<-------------| |
| | NOTIFY F7 | |
| |<-------------| |
| | 200 F8 | |
| |------------->| |
| INVITE F9 Replaces: B | |
|<----------------------------| |
| 200 OK F10 | |
|---------------------------->| |
| ACK F11 | |
|<----------------------------| |
|(Music-on-Hold or other RTP?)| |
|<===========================>| |
| BYE F12 | | |
|------------->| NOTIFY F14 | |
| 200 OK F13 |<-------------| |
|<-------------| 200 OK F15 | |
| |------------->| |
The basic call flow described above uses the SIP dialog ID between
the parked endpoint and the Park Server itself as the unique parked
call identifier. Using the dialog ID has a number of advantages
since it is unique and allocated by both the parked user and the Park
Server. However, it is also long, which can lead to problems when
trying to identify parked calls by verbal or human-written
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mechanisms.
Traditional PBX users have become accustomed to calls being parked
against a short number (typically 3 or 4 digits), and then using this
identifier to communicate to the retrieving party which call to
retrieve. This information may be passed verbally, or by means of
small paper notes. Whilst collisions may occur, they are generally
avoided satisfactorily by administrative policies.
This draft attempts to reconcile these two models by allowing a short
label to be attached to a parked call (the 'orbit'). The retrieving
party can then use the same label to locate the relevant dialog ID in
order to retrieve the parked call. Note that the orbit may be
allocated by the User Agent parking the call or centrally by the Park
Server.
2.1. Parking a call without an orbit
Certain environments do not require an 'orbit' to be used, either
because calls are parked in a single queue, or the dialog identifiers
are readily passed between concerned UAs. In this scenario, the flow
described in [RFC5359] (section 2.15) is followed without deviation.
2.2. Parking a call with an orbit specified by the UA
The message flow of parking a call in this scenario is identical to
that illustrated in [RFC5359] (section 2.15). The difference that
this document introduces is in the REFER message to the Park Server.
In this scenario, it is assumed that Bob has entered a parking orbit
in some manner appropriate to his UA. Once this is done, the REFER
is sent to the URI <sips:park@server.example.com;orbit=1234> instead
of simply directing the request to the URI
<sips:park@server.example.com>. The addition of the orbit parameter
to the URI effectively labels the parked call with a short memorable
code entered by the user.
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F5 REFER Bob -> Park Server
REFER sips:park@server.example.com;orbit=1234 SIP/2.0
Via: SIP/2.0/TLS client.biloxi.example.com:5061
;branch=z9hG4bKnashds9
Max-Forwards: 70
From: Bob <sips:bob@biloxi.example.com>;tag=02134
To: Park Server <sips:park@server.example.com;orbit=1234>
Call-ID: 4802029847@biloxi.example.com
CSeq: 1 REFER
<allOneLine>
Refer-To: <sips:alice@client.atlanta.example.com?Replaces=
12345601%40atlanta.example.com%3Bfrom-tag%3D314159
%3Bto-tag%3D1234567>
</allOneLine>
Referred-By: <sips:bob@biloxi.example.com>
Contact: <sips:bob@client.biloxi.example.com>
Content-Length: 0
2.3. Parking a call with an orbit specified by the Park Server
Sometimes an orbit number assignment policy needs to be implemented.
This may be to ensure that all orbit numbers are a particular length,
or have a form that means that they can be dialled directly (given
suitable extensions to an Application Server). It may also be
implemented to eliminate the problem of trying to park more than one
call on the same orbit.
To enforce a policy, we ensure that the orbit number is not allocated
by the UA (entered by the user, or by configuration etc.) but is
instead allocated by the Park Server, and relayed to the UA. The
approach taken here is analogous to the Conference Factory approach
described in [RFC4579]. Bob sends a REFER to the preconfigured Park
Server URI, but without any 'orbit' parameter added. The Park Server
then responds by redirecting Bob to the correct orbit by using a '302
Moved Temporarily' response. The orbit can then be found by
inspecting this new target.
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Alice Bob Park Server Carol
| | | |
| Active Call | | |
|<============>| | |
| Bob Parks Call | |
| | REFER Refer-To: A F5 |
| |------------->| |
| |302 Orbit allocated F6 |
| |<-------------| |
| | REFER Refer-To: A F7 |
| |------------->| |
| |202 Accepted F8 |
| |<-------------| |
| | NOTIFY | |
| |<-------------| |
| | 200 OK | |
| |------------->| |
F5 REFER Bob -> Park Server
REFER sips:park@server.example.com SIP/2.0
Via: SIP/2.0/TLS client.biloxi.example.com:5061
;branch=z9hG4bKnashdsB
Max-Forwards: 70
From: Bob <sips:bob@biloxi.example.com>;tag=22134
To: Park Server <sips:park-server@example.com>
Call-ID: 4802029847@biloxi.example.com
CSeq: 1 REFER
<allOneLine>
Refer-To: <sips:alice@client.atlanta.example.com?Replaces=
12345601%40atlanta.example.com%3Bfrom-tag%3D314159
%3Bto-tag%3D1234567>
</allOneLine>
Referred-By: <sips:bob@biloxi.example.com>
Contact: <sips:bob@client.biloxi.example.com>
Content-Length: 0
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F6 302 Orbit Allocated Park Server -> Bob
SIP/2.0 202 Orbit Allocated
Via: SIP/2.0/TLS client.biloxi.example.com:5061
;branch=z9hG4bKnashdsB
;received=192.0.2.105
From: Bob <sips:bob@biloxi.example.com>;tag=22134
To: Park Server <sips:park-server@example.com>;tag=56324
Call-ID: 4802029848@biloxi.example.com
CSeq: 1 REFER
Contact: <sips:park@server.example.com;orbit=1234>
Content-Length: 0
This is also the means by which multiple Park Servers can be
deployed. A REFER to <sips:park@server.example.com> might result in
a 302 response, nominating
<sips:park@server-1.example.com;orbit=1234> as the desired target.
Different network architectures may result in different behaviours as
seen by Bob. In particular, whether Bob sees the 302 response will
depend on whether or not an intermediate proxy recurses on it.
Therefore, Bob's UA must be prepared to extract the orbit parameter
from either the 302 response (if one is seen) or the Contact header
of the 2xx response to his REFER.
Since this technique may also be used to resolve the problem of
parking multiple calls on the same orbit, Bob's UA must be prepared
to extract the orbit even if it provided one in the initial request.
If the orbit differs to the one requested, the extracted orbit should
be rendered to Bob in an appropriate manner.
F8 202 Accepted Park Server -> Bob
SIP/2.0 202 Accepted
Via: SIP/2.0/TLS client.biloxi.example.com:5061
;branch=z9hG4bKnashds9
;received=192.0.2.105
From: Bob <sips:bob@biloxi.example.com>;tag=02134
To: Park Server <sips:park@server.example.com>;tag=56323
Call-ID: 4802029847@biloxi.example.com
Contact: <sips:park@server.example.com;orbit=1234>
CSeq: 1 REFER
Content-Length: 0
This variation is only possible on Park Servers capable of generating
Contact URIs of the correct form, i.e. with an 'orbit' URI parameter,
in either a 302 response, or in a 2xx response to the REFER. Park
Servers unable to generate URIs of this form are therefore confined
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to environments that don't require centrally allocated parking
orbits.
2.4. A failed attempt to park a call
A Park Server may choose to reject a park attempt for many reasons,
including prohibiting multiple calls being parked against the same
orbit, or prohibiting certain users from parking calls on certain
orbits. Whatever the reason, the response sent to Bob will enable
Bob to take appropriate action. The following example shows the Park
Server rejecting a call due to the orbit already being in use.
Alice Bob Park Server Carol
| | | |
| INVITE F1 | | |
|------------->| | |
|180 Ringing F2| | |
|<-------------| | |
| 200 OK F3 | | |
|<-------------| | |
| ACK F4 | | |
|------------->| | |
| RTP Media | | |
|<============>| | |
| Bob Parks Call | |
| | REFER Refer-To: A F5 |
| |------------->| |
| |486 Busy Here | |
| |<-------------| |
When Bob's parking attempt is rejected, Bob may choose to attempt to
park the call again, but using a different orbit number. The ability
for Bob to recover from failed parking attempts such as this without
dropping the call to Alice is an important consequence of Bob sending
the REFER to the Park Server, rather than sending the REFER to Alice
so that she can park herself.
3. Retrieving a Parked Call
In order to retrieve a parked call, Carol needs to obtain the dialog
identifiers for the dialog between Alice and wherever Alice is
parked.
The dialog identifiers can be obtained by issuing a SUBSCRIBE for the
dialog event package [RFC4235]. The resulting NOTIFY will contain
details of all pertinent calls, including the dialog identifiers.
Carol may (if presented with multiple dialogs) choose which call to
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retrieve. Many implementations choose the first dialog listed,
although some use the <duration> element to identify which call has
been parked for the longest time. Obtaining the dialog information
in this way follows the flow described in [RFC5359] (section 2.15).
By subscribing to the dialog event package [RFC4235] at the same URI
used for parking the call, i.e.
<sips:park-server@example.com;orbit=1234>, all the information that
is required for the call to be retrieved by C is delivered in the
corresponding NOTIFY.
Similarly, if the call was parked in an environment that does not
require 'orbit' parameters, subscribing to the URI used for parking
the call, i.e. <sips:park-server@example.com>, will still result in
the necessary information being provided for the call to be
retrieved.
Alice Bob Park Server Carol
| | | |
| | | SUBSCRIBE F1 |
| | |<-------------|
| | | 200 OK F2 |
| | |------------->|
| | | NOTIFY F3 |
| | |------------->|
| | | 200 OK F4 |
| | |<-------------|
| | | |
| | | |
| INVITE Replaces: Park Server F5 |
|<-------------------------------------------|
| | | 200 F6 |
|------------------------------------------->|
| | | ACK F7 |
|<-------------------------------------------|
| RTP Media |
|<==========================================>|
| BYE F8 | |
|---------------------------->| |
| 200 OK F9 | |
|<----------------------------| |
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F1 SUBSCRIBE Carol -> Park Server
SUBSCRIBE sips:park@server.example.com;orbit=1234 SIP/2.0
Via: SIP/2.0/TLS chicago.example.com:5061;branch=z9hG4bK92bz
Max-Forwards: 70
From: Carol <sips:carol@chicago.example.com>;tag=8672349
To: <sips:park@server.example.com;orbit=1234>
Call-ID: xt4653gs2ham@chicago.example.com
CSeq: 1 SUBSCRIBE
Contact: <sips:carol@client.chicago.example.com>
Event: dialog
Subscription-State: active;expires=0
Accept: application/dialog-info+xml
Content-Length: 0
F2 200 OK Park Server -> Carol
SIP/2.0 200 OK
Via: SIP/2.0/TLS chicago.example.com:5061;branch=z9hG4bK92bz
;received=192.0.2.114
Max-Forwards: 70
From: Carol <sips:carol@chicago.example.com>;tag=8672349
To: <sips:park@server.example.com;orbit=1234>;tag=1234567
Call-ID: xt4653gs2ham@chicago.example.com
CSeq: 1 SUBSCRIBE
Content-Length: 0
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F3 NOTIFY Park Server -> Carol
NOTIFY sips:carol@client.chicago.example.com SIP/2.0
Via: SIP/2.0/TLS chicago.example.com:5061;branch=z9hG4bK93ca
Max-Forwards: 70
To: Carol <sips:carol@chicago.example.com>;tag=8672349
From: <sips:park@server.example.com;orbit=1234>;tag=1234567
Call-ID: xt4653gs2ham@chicago.example.com
CSeq: 2 NOTIFY
Contact: <sips:park@server.example.com;orbit=1234>
Event: dialog
Subscription-State: terminated
Content-Type: application/dialog-info+xml
Content-Length: ...
<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0" state="full"
entity="sips:park@server.example.com;orbit=1234">
<dialog id="94992014524" call-id="12345600@atlanta.example.com"
local-tag="3145678" remote-tag="1234567" direction="recipient"
remote-uri="alice@atlanta.example.com"
remote-target="alice@client.atlanta.example.com">
<state>confirmed</state>
</dialog>
</dialog-info>
F4 200 OK Carol -> Park Server
SIP/2.0 200 OK
Via: SIP/2.0/TLS chicago.example.com:5061;branch=z9hG4bK93ca
To: Carol <sips:carol@chicago.example.com>;tag=8672349
From: <sips:park@server.example.com;orbit=1234>;tag=1234567
Call-ID: xt4653gs2ham@chicago.example.com
CSeq: 2 NOTIFY
Contact: <sips:carol@client.chicago.example.com>
Content-Length: 0
The remainder of the frames are the same as the corresponding frames
from [RFC5359], since the required dialog ID has been obtained
through the SUBSCRIBE / NOTIFY cycle from the Park Server.
4. User Agent Considerations
For Bob and Carol to be able to park and retrieve calls using a Park
Server, both need to be configured with the URI of the Park Server.
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In addition, Bob and Carol should be configured to understand whether
or not an orbit will be required for park and retrieve. Finally, Bob
also needs to be configured to determine whether Bob should provide
the orbit or whether the orbit will be allocated by the Park Server.
Any orbit received from the Park Server, either in the Contact URI of
a 302 or 2xx response to REFER, should be rendered to the user in an
appropriate manner, even if an orbit was provided in the initial
REFER. *** FIXME UNLESS it is the same as requested? This is to
allow Park Servers to implement various policy decisions and allocate
orbits as required. Failure to do this may lead to a call being
parked on a different orbit to the expected one, and hence being
effectively lost.
If the UA provided an orbit in the REFER request, and no orbit is
received from the Park Server, then the UA may assume that the call
has been parked against the requested orbit correctly.
5. Park Server Considerations
It is expected that Park Servers will not necessarily support all the
feature variations described in this document, at least not
simultaneously. Therefore Park Servers should offer the set that is
most appropriate for the target environment. For example, some Park
Servers may offer centrally allocated orbits, some may not, and some
may be configurable. Other policy-related decisions include how to
handle more than one call being parked on a particular orbit. ***
FIXME more!
6. Other Implementations and Interoperability
Several vendors already implement Call Park/Retrieve in different
ways. Many of these approaches use non-standard/proprietary
extensions to achieve some of the goals that this document addresses.
Interoperability is rarely a driving concern with proprietary
extensions, since a non-interoperable implementation is simply
regarded as not fully implemented.
This section describes some of the approaches that have been
implemented. It is not intended to be an exhaustive review of all
actual or possible implementations, nor is it intended to give
sufficient detail for new implementations. Instead, it aims to give
an overview sufficient to let the reader compare the different
tradeoffs that have been seen in the field.
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6.1. Parking by blind transfer
Some implementations support call park by performing a Blind Transfer
to a URI of the form 'sip:xxxx@example.com', where 'xxxx' represents
the park number, or orbit. To retrieve a parked call, users
generally dial something of the form 'sip:*4xxxx@example.com'. The
park number and the retrieval code both exist in the local dialling
domain, and tend to be easy for UAs to dial.
This approach is generally supported by most UAs, since performing a
blind transfer is a commonly implemented feature. Dialling the
retrieval code is also commonly supported as it resembles a 'star
code', or 'Vertical Service Code'.
This approach doesn't permit park numbers to be centrally allocated,
as the user is required to select one when parking. The park numbers
must also be chosen so as to not clash with local extension numbers.
6.2. Parking by blind transfer with central allocation
At least one implementation supports parking a call by performing a
Blind Transfer to a preconfigured URI. The allocated parking orbit
is returned to the UA in the 202 response to the REFER, in a custom
header. The UA performing the park must understand this header, and
render the contents to the user. To retrieve the call, the park
number is simply dialled as though it were a local extension.
This approach requires explicit support in the UA used to park calls,
as the park number needs to be extracted from the response to the
REFER, and displayed to the user. Retrieving a call can be performed
by most UAs.
6.3. Parking with orbit parameters
Some implementations choose an approach that is similar to the one
described in this document. A Blind Transfer to a URI of the form
'sip:callpark@example.com;orbit=7001' is performed. Rather than
retrieving the call as described above, a call is made to the
corresponding URI 'sip:pickup@example.com;orbit=7001'.
This approach requires support in the UA used to park calls, since
the target of the Blind Transfer requires a uri parameter to be
added. Support is also required in the UA used to retrieve calls,
since again the URI in question requires a parameter.
No central allocation of park numbers is supported in this approach.
However, the parked numbers may overlap with the local extension plan
if desired, since calls are not directly placed to the park numbers.
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6.4. Parking on the User Agent
Some implementations require the User Agents to perform all park-
related functionality. To do this, each UA that is expected to park
calls is configured with a range of park numbers. Should a UA wish
to park a call, it keeps the call on hold locally, and registers
itself against one of its park numbers. Any other UA can then
retrieve the call by dialling the park number. The call arrives at
the parking UA, which can connect it to the held call using 3rd party
call control.
This approach requires explicit support in the UA used to park calls.
Retrieving a call can be performed by most UAs.
Whilst attractive from the perspective of not needing any centralised
server support, this approach suffers from the requirement to
preconfigure all the UAs with park numbers, in case they need to park
a call. Failure to provision enough numbers will prevent calls from
being parked on a particular phone. Conversely, provisioning too
many numbers will rapidly deplete the local extension numbering
space.
7. Acknowledgements
The following individuals were part of the Call Park Design Team, and
have helped to shape this document:
Francois Audet, Jason Fischl, Derek Macdonald, Shida Schubert, Sanjay
Sinha, Dale Worley and Theo Zourzouvillys.
8. Security Considerations
None.
9. IANA Considerations
Open issue: According to [RFC3969], defining a URI parameter can only
be done in a standards-track RFC. That doesn't sound like the sort
of thing this document will do, nor the sort of thing BLISS will do
either. However, [RFC4240] ('netann') defines values that are
included in the current registry, and it is most definately
'Informational'.
This specification adds a new value to the IANA "SIP/SIPS URI
Parameters" registry as defined in [RFC3969].
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Parameter Name Predefined Values Reference
____________________________________________
orbit No RFCxxxx
The ABNF [RFC5234] grammar for this parameter is shown below.
orbit-param = "orbit" EQUAL orbit-value
orbit-value = 1*DIGIT
10. References
10.1. Normative References
[RFC5359] Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Service
Examples", BCP 144, RFC 5359, October 2008.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC3969] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Uniform Resource Identifier (URI) Parameter
Registry for the Session Initiation Protocol (SIP)",
BCP 99, RFC 3969, December 2004.
10.2. Informative References
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, August 2006.
[RFC4240] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
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Author's Address
Michael Procter
VoIP.co.uk
Commerce House
Telford Road
Bicester, Oxfordshire OX26 4LD
UK
Email: michael@voip.co.uk
URI: http://voip.co.uk
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