BLISS                                                         M. Procter
Intended status: Informational                             June 28,                          October 14, 2009
Expires: December 30, 2009 April 17, 2010

    Implementing Call Park and Retrieve using the Session Initiation
                             Protocol (SIP)

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   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

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 . . . . . . . . . . 15
     6.1.  Parking by blind transfer  . . . . . . . . . . . . . . . . 15
     6.2.  Parking by blind transfer with central allocation  . . . . 15
     6.3.  Parking with orbit parameters  . . . . . . . . . . . . . . 16
     6.4.  Parking on the User Agent  . . . . . . . . . . . . . . . . 16
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 18

1.  Overview

   Call Park is a feature that enables User Agents (UA) 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
   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 service.

              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           |              |
                |<----------------------------|              |
                |(optional Music-on-Hold 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

   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

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 <;orbit=1234> instead
   of simply directing the request to the URI
   <>.  The addition of the orbit parameter
   to the URI effectively labels the parked call with a short memorable
   code entered by the user.

      F5 REFER Bob -> Park Server

      REFER;orbit=1234 SIP/2.0
      Via: SIP/2.0/TLS
      Max-Forwards: 70
      From: Bob <>;tag=02134
      To: Park Server <;orbit=1234>
      CSeq: 1 REFER
      Refer-To: <
      Referred-By: <>
      Contact: <>
      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.

              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 SIP/2.0
      Via: SIP/2.0/TLS
      Max-Forwards: 70
      From: Bob <>;tag=22134
      To: Park Server <>
      CSeq: 1 REFER
      Refer-To: <
      Referred-By: <>
      Contact: <>
      Content-Length: 0
      F6 302 Orbit Allocated Park Server -> Bob

      SIP/2.0 202 Orbit Allocated
      Via: SIP/2.0/TLS
      From: Bob <>;tag=22134
      To: Park Server <>;tag=56324
      CSeq: 1 REFER
      Contact: <;orbit=1234>
      Content-Length: 0

   This is also the means by which multiple Park Servers can be
   deployed.  A REFER to <> might result in
   a 302 response, nominating
   <;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
      From: Bob <>;tag=02134
      To: Park Server <>;tag=56323
      Contact: <;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
   to environments that don't require centrally allocated parking

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

   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
   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.
   <;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. <>, will still result in
   the necessary information being provided for the call to be

              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          |              |
                |<----------------------------|              |

   F1 SUBSCRIBE  Carol -> Park Server

   SUBSCRIBE;orbit=1234 SIP/2.0
   Via: SIP/2.0/TLS;branch=z9hG4bK92bz
   Max-Forwards: 70
   From: Carol <>;tag=8672349
   To: <;orbit=1234>
   Contact: <>
   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;branch=z9hG4bK92bz
   Max-Forwards: 70
   From: Carol <>;tag=8672349
   To: <;orbit=1234>;tag=1234567
   Content-Length: 0
   F3 NOTIFY  Park Server -> Carol

   Via: SIP/2.0/TLS;branch=z9hG4bK93ca
   Max-Forwards: 70
   To: Carol <>;tag=8672349
   From: <;orbit=1234>;tag=1234567
   CSeq: 2 NOTIFY
   Contact: <;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"
      <dialog id="94992014524" call-id=""
         local-tag="3145678" remote-tag="1234567" direction="recipient"

   F4 200 OK  Carol -> Park Server

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS;branch=z9hG4bK93ca
   To: Carol <>;tag=8672349
   From: <;orbit=1234>;tag=1234567
   CSeq: 2 NOTIFY
   Contact: <>
   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.

   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, especially if a different orbit was provided in
   the initial REFER.  This is to allow Park Servers to implement
   various policy decisions and allocate orbits as required.  Failure to
   render the final orbit 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.

   Alice's UA needs to support certain key pieces of protocol in order
   to allow itself to be parked and retrieved.  In addition to
   [RFC3261], support for 'Replaces' is also required [RFC3891].  As
   required by [RFC3261], the Contact URI should be globally routable,
   so that an initial INVITE with a Replaces header may be received and
   processed correctly.  To help achieve this, [I-D.ietf-sip-gruu] may
   be useful.  Should the UA wish to mask the Contact URI for privacy
   reasons, following the advice in [I-D.ietf-sip-ua-privacy] might
   prove beneficial, although other solutions also exist.

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 their target environment.  For example, some
   Park Servers may offer centrally allocated orbits, some may not, and
   some may be configurable.

   Park Servers will typically implement additional functionality at a
   policy level.  Examples of policy-related decisions include what
   media to provide to parked calls, how to handle more than one call
   being parked on a particular orbit, and how to handle a call that has
   been parked for an excessive length of time.

   If the Park Server chooses to allocate a parking orbit for a call,
   consideration should be given to the format used in the target
   environment.  In particular, a short digit string should be used when
   it is likely that the parked call will be retrieved by a UA with a
   more limited user interface.  This restriction may be relaxed for
   more advanced parking applications.

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.

6.1.  Parking by blind transfer

   Some implementations support call park by performing a Blind Transfer
   to a URI of the form '', where 'xxxx' represents
   the park number, or orbit.  To retrieve a parked call, users
   generally dial something of the form 'sip:*'.  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
   ';orbit=7001' is performed.  Rather than
   retrieving the call as described above, a call is made to the
   corresponding URI ';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.

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

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.

   Additional feedback was received from Scott Orton, Andy Hutton and
   John Elwell.

8.  Security Considerations

   The underlying call flows in this document are drawn from [RFC5359],
   so the security considerations in that document apply.  This document
   introduces the 'orbit' parameter, which is used during communications
   between the UA parking the call and the Park Server.  Later, the UA
   retrieving the parked call also uses the orbit when communicating
   with the Park Server.

   The 'orbit' may be thought of as a short token identifying a
   particular parked dialog.  Due to its length, and also to common
   administrative policies for orbit allocation, it is likely to be
   easily guessable.  Because of this, knowledge of the 'orbit' should
   not be considered to be an indication of authorisation to manipulate
   a parked call.  The Park Server should ensure that any UA attempting
   to park a call or retrieve a parked call is suitably authorised,
   regardless of whether or not a valid orbit is present in the SIP

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

   This specification adds a new value to the IANA "SIP/SIPS URI
   Parameters" registry as defined in [RFC3969].

     Parameter Name  Predefined Values  Reference
     orbit           No                 RFCxxxx

   The ABNF [RFC5234] grammar for this parameter is shown below.  The
   definition of 'pvalue' is given in [RFC3261].

     orbit-param     = "orbit" EQUAL orbit-value
     orbit-value     = pvalue

10.  References
10.1.  Normative References

   [RFC3261]  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.

   [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.

   [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
              Protocol (SIP) "Replaces" Header", RFC 3891,
              September 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.

              Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
              (SIP)", draft-ietf-sip-gruu-15 (work in progress),
              October 2007.

              Munakata, M., Schubert, S., and T. Ohba, "UA-Driven
              Privacy Mechanism for SIP", draft-ietf-sip-ua-privacy-08
              (work in progress), May 2009.

Author's Address

   Michael Procter
   Commerce House
   Telford Road
   Bicester, Oxfordshire  OX26 4LD