    Network Working Group                                        B. Campbell
    Request for Comments: 3087                                     R. Sparks
    Category: Informational                                      dynamicsoft
                                                                  April 2001

                Control of Service Context using SIP Request-URI

    Status of this Memo

       This memo provides information for the Internet community.  It does
       not specify an Internet standard of any kind.  Distribution of this
       memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

    Abstract

       This memo provides information for the Internet community.  It
       describes a useful way to conceptualize the use of the standard SIP
       (Session Initiation Protocol) Request-URI (Uniform Resource
       Identifier) that the authors and many members of the SIP community
       think is suitable as a convention.  It does not define any new
       protocol with respect to RFC 2543.

       In a conventional telephony environment, extended service
       applications often use call state information, such as calling party,
       called party, reason for forward, etc, to infer application context.
       In a SIP/2.0 call, much of this information may be either non-
       existent or unreliable.  This document proposes a mechanism to
       communicate context information to an application.  Under this
       proposal, a client or proxy can communicate context through the use
       of a distinctive Request-URI.  This document continues with examples
       of how this mechanism could be used in a voice mail application.

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    RFC 3087                  SIP Service Control                 April 2001

    Table of Contents

       1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
       2.      Example Application  . . . . . . . . . . . . . . . . . . .  3
       2.1     Using URIs to Control Voice Mail Service Behavior  . . . .  3
       3.      Voice Mail Scenario Descriptions . . . . . . . . . . . . .  5
       3.1     Deposits . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1   Direct Request to Deposit to a particular mailbox  . . . .  5
       3.1.1.1 SIP source . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.2 Arbitrary PSTN source  . . . . . . . . . . . . . . . . . .  5
       3.1.1.3 Recognized PSTN source . . . . . . . . . . . . . . . . . .  6
       3.1.2   Direct Request to Deposit, mailbox to be determined  . . .  6
       3.1.2.1 SIP source . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2.2 PSTN source  . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2.3 Indirect Request to Deposit, due to find-me proxy decision  6
       3.2     Retrievals . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.1   Request to Retrieve from a particular mailbox  . . . . . .  7
       3.2.1.1 Trusted SIP source . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.2 Authenticated SIP source . . . . . . . . . . . . . . . . .  7
       3.2.1.3 Unauthenticated SIP source . . . . . . . . . . . . . . . .  7
       3.2.1.4 PSTN source  . . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2   Request to Retrieve, mailbox to be determined  . . . . . .  7
       3.2.2.1 SIP source . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2.2 Arbitrary PSTN source  . . . . . . . . . . . . . . . . . .  8
       3.2.2.3 Recognized PSTN source . . . . . . . . . . . . . . . . . .  8
       4.      Voice Mail Call Flow Examples  . . . . . . . . . . . . . .  8
       4.1     Generic Scenario . . . . . . . . . . . . . . . . . . . . .  8
       4.1.1   Direct call to the voice mail system . . . . . . . . . . .  8
       4.2     Message Deposit Scenarios  . . . . . . . . . . . . . . . . 13
       4.2.1   Call to known subscriber forwarded on no answer  . . . . . 13
       4.2.2   Call to known subscriber forwarded on busy . . . . . . . . 19
       4.2.3   Direct call to a subscriber's mailbox  . . . . . . . . . . 24
       4.3     Message Retrieval Scenarios  . . . . . . . . . . . . . . . 29
       4.3.1   Call to retrieve messages believed to be from a known
               subscriber . . . . . . . . . . . . . . . . . . . . . . . . 29
       4.3.2   Call to retrieve messages from an authenticated subscriber 33
       5.      Security Considerations  . . . . . . . . . . . . . . . . . 38
       6.      Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 38
               References . . . . . . . . . . . . . . . . . . . . . . . . 38
               Authors' Addresses . . . . . . . . . . . . . . . . . . . . 38
               Full Copyright Statement . . . . . . . . . . . . . . . . . 39

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    RFC 3087                  SIP Service Control                 April 2001

    1. Introduction

       A communication service should make use of the information it has at
       hand when being accessed.  For example, in most current voice mail
       implementations, a subscriber retrieving messages from his own desk
       does not have to reenter his voice mailbox number - the service
       assumes that the store being accessed is the one associated with the
       endpoint being used to access the service.  Some services allow the
       user to validate this assumption using IVR techniques before
       prompting for a PIN.

       This concept of context-awareness can be captured in a voice mail
       service implementing SIP as defined in RFC 2543[1], without
       modification, through the standard use of that protocol's Request-
       URI.  Furthermore, the concept is applicable to any SIP-based service
       where initial application state should be determined from context.

       This concept is a usage convention of standard SIP as defined in RFC
       2543[1] and does not modify or extend that protocol in any way.

    2. Example Application

       In this document, we use the example of voice mail to illustrate the
       technique.  One motivation for applying this technique to this
       problem is allowing a proxy or location server to control the initial
       state of a voice service.  For example, a voice client might register
       a contact list ending with the URL that would accept voice messages
       for the client.

    2.1 Using URIs to Control Voice Mail Service Behavior

       Many conventional voice mail systems use call state information, such
       as the calling party, called party, reason for forward, etc, to
       decide the initial application state.  For example, it might play one
       outgoing message if the call reached voice mail because the called
       party did not answer and another if the line was busy.  It decides
       whom the message is for based on the called party information.  If
       the call originated from a subscriber's phone number, it might
       authenticate the caller and then go directly to the message retrieval
       and account maintenance menu.

       When a new subscriber is added to a system, a set of identities could
       be generated, each given a unique sip URI.  The following tables show
       some of the identities that might be generated (it is not
       exhaustive).  The example schemes show that the URIs could, but don't
       necessarily have to, have mnemonic value.

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       In practical applications, it is important that an application does
       not apply semantic rules to the various URIs.  Instead, it should
       allow any arbitrary string to be provisioned, and map the string to
       the desired behavior.  The owner of the system may choose to
       provision mnemonic strings, but the application should not require
       it.  In any large installation, the system owner is likely to have
       pre-existing rules for mnemonic URIs, and any attempt by an
       application to define its own rules may create a conflict.  For our
       example, this means a voice mail system should allow an arbitrary mix
       of URLs from these schemes, or any other scheme that renders valid
       SIP URIs to be provisioned, rather than enforce one particular
       scheme.

          URI Identity       Example Scheme 1
                                  Example Scheme 2
                                       Example Scheme 3

          Deposit with       sip:sub-rjs-deposit@vm.wcom.com
          standard greeting       sip:677283@vm.wcom.com
                                       sip:rjs@vm.wcom.com;mode=deposit

          Deposit with on    sip:sub-rjs-deposit-busy.vm.wcom.com
          phone greeting          sip:677372@vm.wcom.com
                                       sip:rjs@vm.wcom.com;mode=3991243

          Deposit with       sip:sub-rjs-deposit-sg@vm.wcom.com
          special greeting        sip:677384@vm.wcom.com
                                       sip:rjs@vm.wcom.com;mode=sg

          Retrieve - SIP     sip:sub-rjs-retrieve@vm.wcom.com
          authentication          sip:677405@vm.wcom.com
                                       sip:rjs@vm.wcom.com;mode=retrieve

          Retrieve - prompt  sip:sub-rjs-retrieve-inpin.vm.wcom.com
          for PIN in-band         sip:677415@vm.wcom.com
                                       sip:rjs@vm.wcom.com;mode=inpin

       When a service is first set up, identities such as the following
       could be created.

          URI Identity       Example Scheme 1
                                  Example Scheme 2
                                       Example Scheme 3

          Deposit -          sip:deposit@vm.wcom.com
          identify target         sip:670001@vm.wcom.com
          mailbox by To:               sip:deposit@vm.wcom.com

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          Retrieve -         sip:retrieve@vm.wcom.com
          identify target         sip:670002@vm.wcom.com
          mailbox by SIP               sip:retrieve@vm.wcom.com
          authentication

          Deposit - prompt   sip:deposit-in@vm.wcom.com
          for target              sip:670003@vm.wcom.com
          mailbox in-band              sip:deposit@vm.wcom.com;mode=inband

          Retrieve - prompt  sip:retrieve-in@vm.wcom.com
          for target              sip:670004@vm.wcom.com
          mailbox and PIN              sip:retrieve@vm.wcom.com;mode=inband
          in-band

       In addition to providing this set of URIs to the subscriber (to use
       as he sees fit), an integrated service provider could add these to
       the set of contacts in a find-me proxy.  The proxy could then route
       calls to the appropriate URI based on the origin of the request, the
       subscriber's preferences and current state.

    3. Voice Mail Scenario Descriptions

       In each of these scenarios, the PSTN gateway is configured to
       communicate only with a particular proxy-registrar.

    3.1 Deposits

    3.1.1 Direct Request to Deposit to a particular mailbox

    3.1.1.1 SIP source

       A SIP client that knew the URI for a particular deposit mailbox
       (sip:sub-rjs-deposit@vm.wcom.com) could place a direct invitation to
       the voicemail service, or through a protecting proxy.  The proxy
       could restrict access to deposit identities with special greetings by
       authenticating the requester.

    3.1.1.2 Arbitrary PSTN source

       The gateway's proxy would map a call from an unrecognized PSTN number
       to a number associated with a subscriber's mailbox into an invite to
       the deposit with standard greeting URI (sip:sub-rjs-
       deposit@vm.wcom.com).

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    3.1.1.3 Recognized PSTN source

       The gateway's proxy would map a call from a recognized (exact or
       pattern match) PSTN number to a number associated with a subscriber's
       mailbox into an invite to the appropriate special greeting URI
       (sip:sub-rjs-deposit-sg@vm.wcom.com).  The gateway's ability to
       identify the calling party (using calling party number) is trusted,
       so no further authentication of the requester is performed.

    3.1.2 Direct Request to Deposit, mailbox to be determined

    3.1.2.1 SIP source

       A voice mail service or its protecting proxy could expose a generic
       deposit URL for use when a caller wished to go directly to voice
       mail.  The service would likely play an IVR dialog to determine what
       message store to deposit a message into.

       An application designer may be tempted to attempt to match the To:
       and From: headers on a call to infer information.  However, this
       approach could cause complications when multiple proxy forwards occur
       in a call.  For example, A calls B, who has all calls forwarded to C.
       C forwards the call to her voice mail service.  If the voice mail
       service matches the To: header to determine the message store, it
       will get the information for B instead of C.  But there is no reason
       to assume that C's voice mail service has any knowledge of B.

    3.1.2.2 PSTN source

       The gateway's proxy would map a call from an unrecognized PSTN number
       to the top level voice mail service access number to an invite to the
       Deposit - prompt for target mailbox in-band URI (sip:deposit-
       in@vm.wcom.com for example).  Getting the call to the target mailbox
       would proceed as in the SIP source case.

    3.1.2.3 Indirect Request to Deposit, due to find-me proxy decision

       A find-me proxy could map an invitation to a subscriber
       (sip:rjs@wcom.com) to the appropriate voice mail service URI
       depending on the subscriber's current state.  The normal deposit URI
       could be chosen if the subscriber's contact list has been otherwise
       exhausted with no answer.  The busy-announcement URI would be chosen
       when a busy everywhere response is received from one of the contacts.
       A DND announcement URI could be selected if the subscriber had
       activated DND. Calls to sip:receptionist@wcom.com could be configured
       to roll to sip:deposit@wcom.com

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    3.2 Retrievals

    3.2.1 Request to Retrieve from a particular mailbox

    3.2.1.1 Trusted SIP source

       A request to retrieve the contents of a particular mailbox (sip:sub-
       rjs-retrieve@vm.wcom.com) coming from a trusted source could be
       honored without further authentication checks.  A trusted source is
       one with which the voice mail service has secure communications, and
       to which it is willing to delegate authentication.  This could be the
       service's protecting proxy for example.

    3.2.1.2 Authenticated SIP source

       A service, or its protecting proxy, could choose to honor a retrieve
       request for a particular mailbox (sip:sub-rjs-retrieve@vm.wcom.com)
       based on SIP authentication.  If SIP level authentication failed, the
       service or proxy could be configured to send the call to the in-band
       pin prompting URI (sip:sub-rjs-retrieve-inpin@vm.wcom.com).

    3.2.1.3 Unauthenticated SIP source

       A service, or its protecting proxy, receiving a retrieve request for
       a particular mailbox (sip:sub-rjs-retrieve@vm.wcom.com) with no other
       method of authenticating the requestor could send the request to the
       in-band pin prompting URI (sip:sub-rjs-retrieve-inpin@vm.wcom.com).

    3.2.1.4 PSTN source

       This scenario assumes that the service provider's network has been
       configured such that a PSTN number could be dialed explicitly for
       retrieving messages from a particular mailbox.  Such services
       currently exist, but are not common.  In such a network, the
       gateway's proxy would map the call to the in-band pin prompting URI
       (sip:sub-rjs-retrieve-inpin@vm.wcom.com).

    3.2.2 Request to Retrieve, mailbox to be determined

    3.2.2.1 SIP source

       As in the Request to Deposit scenario, when a service receives a
       request for the top level retrieve URI it would most likely need to
       use in-band IVR techniques to determine the target mailbox and
       authenticate the caller.

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    3.2.2.2 Arbitrary PSTN source

       This scenario assumes there is a single PSTN number that subscribers
       dial to access the voice mail service to retrieve messages.  This is
       the most common access method provided by current voice mail
       services.

       The gateway's proxy would map a call to the top level PSTN number to
       the top level retrieve in-band prompting URI (sip:retrieve-
       in@vm.wcom.com).  Once the system identifies the target mailbox, the
       call would be transferred to the appropriate in-band pin prompting
       URI (sip:sub-rjs-retrieve-inpin@vm.wcom.com).

    3.2.2.3 Recognized PSTN source

       This scenario also assumes there is a single PSTN number that
       subscribers dial to access the voice mail service to retrieve
       messages.

       The gateway's proxy would recognize the calling party number as a
       subscriber, and map the call to the subscriber's in-band prompting
       URI (sip:sub-rjs-retrieve-inpin@vm.wcom.com)

    4. Voice Mail Call Flow Examples

       The following section describes some example call flows for a
       hypothetical voice mail service, with the host name of vm.wcom.com.
       All the call flows assume that a proxy protects the voice mail
       service and that a trust relationship exists between the voice mail
       service and the proxy.

    4.1 Generic Scenario

    4.1.1 Direct call to the voice mail system

       User A calls the voice mail system directly.  The voice mail system
       invokes the top-level menu, which might prompt the caller for an
       extension or the first few letters of a name.

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    RFC 3087                  SIP Service Control                 April 2001

                User A              Proxy               VM Service
                  |                   |                       |
                  |  INVITE F1        |                       |
                  |------------------>|                       |
                  |                   |  INVITE F2            |
                  | (100 Trying) F3   |---------------------->|
                  |<------------------|                       |
                  |                   |  180 Ringing F4       |
                  |  180 Ringing F5   |<----------------------|
                  |<------------------|                       |
                  |                   |  200 OK F6            |
                  |  200 OK F6        |<----------------------|
                  |<------------------|                       |
                  |                   |                       |
                  |  ACK F8           |                       |
                  |------------------>|  ACK F9               |
                  |                   |---------------------->|
                  |                   |                       |
                  |      RTP Established- Play top level menu |
                  |<-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m->|
                  |                   |                       |
                  |  BYE F10          |                       |
                  |------------------>|  BYE F11              |
                  |                   |---------------------->|
                  |                   |                       |
                  |                   |  200 OK F12           |
                  |                   |<----------------------|
                  |  200 OK F13       |                       |
                  |<------------------|                       |
                  |                   |                       |

         Flow Id                            Comments

        INVITE F1     INVITE sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Proxy-Authorization:Digest username="UserA",
                      realm="MCI WorldCom SIP",
                      nonce="ea9c8e88df84f1cc4e341ae6cbe5a359", opaque="",
                      uri="sip:VoiceMail@wcom.com", response=<appropriately
                      calculated hash goes here>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

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                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

                      /*Client for A prepares to receive data on port 49170
                      from the network. */

        INVITE F2     INVITE sip:top@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        (100 Trying   SIP/2.0 100 Trying
        F3            Via: SIP/2.0/UDP here.com:5060
        Proxy->A)     From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        180 Ringing   SIP/2.0 180 Ringing
        F4            Via: SIP/2.0/UDP wcom.com:5060; branch=1
        VM->Proxy     Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

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        180 Ringing   SIP/2.0 180 Ringing
        F5            Via: SIP/2.0/UDP here.com:5060
        Proxy->A      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        200 OK F6     SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: VoiceMailSystem <sip:top@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F7     SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact VoiceMailSystem <sip:top@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

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    RFC 3087                  SIP Service Control                 April 2001

        ACK F8        ACK sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:top@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        ACK F9        ACK sip:top@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>; tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

                      /* RTP streams are established between A and VM.  VM
                      system starts IVR dialog for top level menu */

                      /* User A Hangs Up with VM system.  Alternatively, the
                      VM system could initiate the BYE*/

        BYE F10       BYE sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip: top@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        BYE F11       BYE sip: top@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F12    SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com

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    RFC 3087                  SIP Service Control                 April 2001

                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F13    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq:  2 BYE
                      Content-Length: 0

    4.2 Message Deposit Scenarios

    4.2.1 Call to known subscriber forwarded on no answer

       User A attempts to call UserB, who does not answer.  The call is
       forwarded to UserB's mailbox, and the voice mail system plays UserB's
       outgoing message for a ring-no-answer.  The flow assumes that the URL
       of "sip:UserB-dep-fna@vm.wcom.com maps" to the desired behavior for
       depositing a message on a forward-no-answer.

    Campbell & Sparks            Informational                     [Page 13]

    RFC 3087                  SIP Service Control                 April 2001

          User A            Proxy              User B         VM System
            |                 |                  |                |
            |  INVITE F1      |                  |                |
            |---------------->|  INVITE F2       |                |
            |                 |----------------->|                |
            | (100 Trying) F3 |                  |                |
            |<----------------| 180 Ringing F4   |                |
            |                 |<-----------------|                |
            |  180 Ringing F5 |                  |                |
            |<----------------| (Request Timeout)|                |
            |                 |                  |                |
            |                 | Cancel F6        |                |
            |                 |----------------->|                |
            |                 |                  |                |
            |                 | 200 OK F7        |                |
            |                 |<-----------------|                |
            |                 |                  |                |
            |                 |         INVITE F8                 |
            |                 |---------------------------------->|
            |                 |                  |                |
            |                 |         200 OK F9|                |
            |  200 OK F10     |<----------------------------------|
            |<----------------|                  |                |
            |                 |                  |                |
            |  ACK F11        |                  |                |
            |---------------->|         ACK F12  |                |
            |                 |---------------------------------->|
            |                 |                  |                |
            |    RTP Established Both Ways-Deposit Msg for B      |
            |<-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m->|
            |                 |                  |                |
            |  BYE F13        |                  |                |
            |---------------->|         BYE F14  |                |
            |                 |---------------------------------->|
            |                 |                  |                |
            |                 |         OK F15   |                |
            |  OK F16         |<----------------------------------|
            |<----------------|                  |                |
            |                 |                  |                |

         Flow Id                            Comments

        INVITE F1     INVITE sip:UserB@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com

    Campbell & Sparks            Informational                     [Page 14]

    RFC 3087                  SIP Service Control                 April 2001

                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Proxy-Authorization:Digest username="UserA",
                      realm="MCI WorldCom SIP",
                      nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
                      uri="sip:UserB@wcom.com", response=<appropriately
                      calculated hash goes here>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

                      /*Client for A prepares to receive data on port 49170
                      from the network. */

        INVITE F2     INVITE sip:UserB1@somewhere.wcom.com SIP/2.0
        Proxy->B1     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        (100 Trying   SIP/2.0 100 Trying
        F3            Via: SIP/2.0/UDP here.com:5060
        Proxy->A)     From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

    Campbell & Sparks            Informational                     [Page 15]

    RFC 3087                  SIP Service Control                 April 2001

        180 Ringing   SIP/2.0 180 Ringing
        F4            Via: SIP/2.0/UDP wcom.com:5060; branch=1
        B1->Proxy     Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        180 Ringing   SIP/2.0 180 Ringing
        F5            Via: SIP/2.0/UDP here.com:5060
        Proxy->A      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

                      /* B1 rings for 9 seconds, this duration is a
                      configurable parameter in the Proxy Server.  Proxy
                      sends Cancel and proceeds down the list of routes,
                      eventually hitting the voice mail URI for forward no
                      answer */

        CANCEL F6     CANCEL sip:UserB1@wcom.com SIP/2.0
        Proxy->B1     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 CANCEL
                      Content-Length: 0

        200 OK F7     SIP/2.0 200 OK
        B1->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 CANCEL
                      Content-Length: 0

        INVITE F8     INVITE sip:UserB-dep-fna@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060;branch=2
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE

    Campbell & Sparks            Informational                     [Page 16]

    RFC 3087                  SIP Service Control                 April 2001

                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F9     SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=2
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheLittleGuyVoiceMail <sip:UserB-dep-
                      fna@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F10    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheLittleGuyVoiceMail <sip:UserB-dep-
                      fna@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com

    Campbell & Sparks            Informational                     [Page 17]

    RFC 3087                  SIP Service Control                 April 2001

                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        ACK F11       ACK sip:UserB@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip: UserB-dep-fna@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        ACK F12       ACK sip:UserB-dep-fna@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

                      /* RTP streams are established between A and B2.  VM
                      system starts IVR dialog for message-deposit on no-
                      answer for UserB */

                      /* User A Hangs Up with VM system.  Alternatively, the
                      VM system could initiate the BYE*/

        BYE F13       BYE sip:UserB@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip: UserB-dep-fna@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        BYE F14       BYE sip: UserB-dep-fna@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

    Campbell & Sparks            Informational                     [Page 18]

    RFC 3087                  SIP Service Control                 April 2001

        200 OK F15    SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F16    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq:  2 BYE
                      Content-Length: 0

    4.2.2 Call to known subscriber forwarded on busy

       User A attempts to call UserB, who is busy.  The call is forwarded to
       UserB's mailbox, and the voice mail system plays UserB's outgoing
       message for a busy.  This flow assumes that "sip:UserB-dep-
       fb@vm.wcom.com" maps to UserB's mailbox and the behavior of "deposit
       message on busy."

    Campbell & Sparks            Informational                     [Page 19]

    RFC 3087                  SIP Service Control                 April 2001

          User A            Proxy              User B         VM System
            |                 |                  |                |
            |  INVITE F1      |                  |                |
            |---------------->|  INVITE F2       |                |
            |                 |----------------->|                |
            | (100 Trying) F3 |                  |                |
            |<----------------| 486 Busy Here F4 |                |
            |                 |<-----------------|                |
            |                 |                  |                |
            |                 |  ACK F5          |                |
            |                 |----------------->|                |
            |                 |                  |                |
            |                 |         INVITE F6                 |
            |                 |---------------------------------->|
            |                 |                  |                |
            |                 |         200 OK F7|                |
            |  200 OK F8      |<----------------------------------|
            |<----------------|                  |                |
            |                 |                  |                |
            |  ACK F9         |                  |                |
            |---------------->|         ACK F10  |                |
            |                 |---------------------------------->|
            |                 |                  |                |
            |    RTP Established Both Ways-Deposit Msg for B      |
            |<-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m->|
            |                 |                  |                |
            |  BYE F11        |                  |                |
            |---------------->|         BYE F12  |                |
            |                 |---------------------------------->|
            |                 |                  |                |
            |                 |         OK F13   |                |
            |  OK F14         |<----------------------------------|
            |<----------------|                  |                |
            |                 |                  |                |

         Flow Id                            Comments

        INVITE F1     INVITE sip:UserB@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Proxy-Authorization:Digest username="UserA",
                      realm="MCI WorldCom SIP",
                      nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
                      uri="sip:UserB@wcom.com", response=<appropriately

    Campbell & Sparks            Informational                     [Page 20]

    RFC 3087                  SIP Service Control                 April 2001

                      calculated hash goes here>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

                      /*Client for A prepares to receive data on port 49170
                      from the network. */

        INVITE F2     INVITE sip:UserB1@somewhere.wcom.com SIP/2.0
        Proxy->B1     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        (100 Trying   SIP/2.0 100 Trying
        F3            Via: SIP/2.0/UDP here.com:5060
        Proxy->A)     From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        486 Busy      SIP/2.0 486 Busy Here
        Here F4       Via: SIP/2.0/UDP wcom.com:5060;branch=1
        B1->Proxy     Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456

    Campbell & Sparks            Informational                     [Page 21]

    RFC 3087                  SIP Service Control                 April 2001

                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        ACK F5        ACK sip: UserB1@wcom.com SIP/2.0
        Proxy->B      Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=123456
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        INVITE F6     INVITE sip:UserB-dep-fb@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060;branch=2
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F7     SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=2
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheLittleGuyVoiceMail <sip:UserB-dep-
                      fb@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP

    Campbell & Sparks            Informational                     [Page 22]

    RFC 3087                  SIP Service Control                 April 2001

                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F8     SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact TheLittleGuyVoiceMail <sip:UserB-dep-
                      fb@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        ACK F9        ACK sip:UserB@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:UserB-dep-fb@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        ACK F10       ACK sip:UserB-dep-fb@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

                      /* RTP streams are established between A and B2.  VM
                      system starts IVR dialog for message-deposit on busy
                      for UserB */

    Campbell & Sparks            Informational                     [Page 23]

    RFC 3087                  SIP Service Control                 April 2001

                      /* User A Hangs Up with VM system.  Alternatively, the
                      VM system could initiate the BYE*/

        BYE F11       BYE sip:UserB@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route: <sip:UserB-dep-fnb@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        BYE F12       BYE sip: UserB-dep-fnb@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F13    SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F14    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuy <sip:UserB@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq:  2 BYE
                      Content-Length: 0

    4.2.3 Direct call to a subscriber's mailbox

       User A calls UserB's mailbox directly.  This flow assumes that
       "sip:UserB-dep@vm.wcom.com" maps to UserB's mailbox and the behavior
       of "generic message deposit"

    Campbell & Sparks            Informational                     [Page 24]

    RFC 3087                  SIP Service Control                 April 2001

                User A              Proxy                VM Service
                  |                   |                       |
                  |  INVITE F1        |                       |
                  |------------------>|                       |
                  |                   |  INVITE F2            |
                  | (100 Trying) F3   |---------------------->|
                  |<------------------|                       |
                  |                   |  200 OK F4            |
                  |  200 OK F5        |<----------------------|
                  |<------------------|                       |
                  |                   |                       |
                  |  ACK F6           |                       |
                  |------------------>|  ACK F7               |
                  |                   |---------------------->|
                  |                   |                       |
                  |      RTP Both Ways - Deposit Msg for B    |
                  |<-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m->|
                  |                   |                       |
                  |  BYE F8           |                       |
                  |------------------>|  BYE F9               |
                  |                   |---------------------->|
                  |                   |                       |
                  |                   |  200 OK F10           |
                  |                   |<----------------------|
                  |  200 OK F11       |                       |
                  |<------------------|                       |
                  |                   |                       |

         Flow Id                            Comments

        INVITE F1     INVITE sip:UserB-VM@vm.wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-VM@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Proxy-Authorization:Digest username="UserA",
                      realm="MCI WorldCom SIP",
                      nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
                      uri="sip:UserB-VM@wcom.com", response=<appropriately
                      calculated hash goes here>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP

    Campbell & Sparks            Informational                     [Page 25]

    RFC 3087                  SIP Service Control                 April 2001

                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

                      /*Client for A prepares to receive data on port 49170
                      from the network. */

        INVITE F2     INVITE sip:UserB-dep@vm.wcom.com SIP/2.0
        Proxy->B1     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB-VM@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-VM@vm.wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        (100 Trying   SIP/2.0 100 Trying
        F3            Via: SIP/2.0/UDP here.com:5060
        Proxy->A)     From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-VM@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        200 OK F4     SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB-VM@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheLittleGuyVoiceMail <sip:UserB-
                      dep@vm.wcom.com>
                      Content-Type: application/sdp

    Campbell & Sparks            Informational                     [Page 26]

    RFC 3087                  SIP Service Control                 April 2001

                      Content-Length: <appropriate value>
                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F5     SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:UserB-VM@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact TheLittleGuyVoiceMail <sip:UserB-
                      dep@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        ACK F6        ACK sip:UserB-VM@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:UserB-dep@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        ACK F7        ACK sip:UserB-dep@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK

    Campbell & Sparks            Informational                     [Page 27]

    RFC 3087                  SIP Service Control                 April 2001

                      Content-Length: 0
                      /* RTP streams are established between A and VM.  VM
                      system starts IVR dialog for generic message-deposit
                      for UserB */

                      /* User A Hangs Up with VM system.  Alternatively, the
                      VM system could initiate the BYE*/

        BYE F8        BYE sip:UserB-VM@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:UserB-dep@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        BYE F9        BYE sip: UserB-dep@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F10    SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F11    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: TheLittleGuyVoiceMail <sip:UserB-
                      VM@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq:  2 BYE
                      Content-Length: 0

    Campbell & Sparks            Informational                     [Page 28]

    RFC 3087                  SIP Service Control                 April 2001

    4.3 Message Retrieval Scenarios

    4.3.1 Call to retrieve messages believed to be from a known subscriber

       Some user uses a SIP client on UserA's desk to call the voice mail
       system to retrieve messages.  The SIP client has authenticated itself
       to the proxy using credentials assigned to the device.  The proxy can
       make a weak assumption that the caller is the device owner.  The URI
       of "sip:UserA-retrieve@vm.wcom.com" maps to UserA's mailbox and the
       behavior of "retrieve messages after prompting for and verifying
       PIN." The VM System trusts the proxy, and will not accept calls from
       an untrusted source.  The proxy will not allow direct calls to
       UserA-retrieve@vm.wcom.com.  The proxy will forward calls placed to
       VoiceMail@wcom.com to UserA-retrieve@vm.wcom.com only for calls
       placed from a client device assigned to UserA.

                User A              Proxy                VM Service
                  |                   |                       |
                  |  INVITE F1        |                       |
                  |------------------>|                       |
                  |                   |  INVITE F2            |
                  | (100 Trying) F3   |---------------------->|
                  |<------------------|                       |
                  |                   |  200 OK F4            |
                  |  200 OK F5        |<----------------------|
                  |<------------------|                       |
                  |                   |                       |
                  |  ACK F6           |                       |
                  |------------------>|  ACK F7               |
                  |                   |---------------------->|
                  |                   |                       |
                  |      RTP Both Ways - VM prompts for PIN
                  |<-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m->|
                  |                   |                       |
                  |  BYE F8           |                       |
                  |------------------>|  BYE F9               |
                  |                   |---------------------->|
                  |                   |                       |
                  |                   |  200 OK F10           |
                  |                   |<----------------------|
                  |  200 OK F11       |                       |
                  |<------------------|                       |
                  |                   |                       |

    Campbell & Sparks            Informational                     [Page 29]

    RFC 3087                  SIP Service Control                 April 2001

         Flow Id                            Comments

        INVITE F1     INVITE sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Proxy-Authorization:Digest username="UserAPhone",
                      realm="MCI WorldCom SIP",
                      nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
                      uri="sip:VoiceMail@wcom.com", response=<appropriately
                      calculated hash goes here>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

                      /*Client for A prepares to receive data on port 49170
                      from the network. */

        INVITE F2     INVITE sip:UserA-retrieve@vm.wcom.com SIP/2.0
        Proxy->B1     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

    Campbell & Sparks            Informational                     [Page 30]

    RFC 3087                  SIP Service Control                 April 2001

        (100 Trying   SIP/2.0 100 Trying
        F3            Via: SIP/2.0/UDP here.com:5060
        Proxy->A)     From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        200 OK F4     SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: VoiceMailSystem <sip:UserA-
                      retrieve@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F5     SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact VoiceMailSystem <sip: UserA-
                      retrieve@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

    Campbell & Sparks            Informational                     [Page 31]

    RFC 3087                  SIP Service Control                 April 2001

        ACK F6        ACK sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:UserA-retrieve@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        ACK F7        ACK sip:UserA-retrieve@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>; tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

                      /* RTP streams are established between A and VM.  VM
                      determines that the call is likely from UserA, and
                      starts a message retrieval session, prompting for
                      PIN*/

                      /* User A Hangs Up with VM system.  Alternatively, the
                      VM system could initiate the BYE*/

        BYE F8        BYE sip: VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:UserA-retrieve@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        BYE F9        BYE sip: UserA-retrieve@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F10    SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>

    Campbell & Sparks            Informational                     [Page 32]

    RFC 3087                  SIP Service Control                 April 2001

                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F11    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq:  2 BYE
                      Content-Length: 0

    4.3.2 Call to retrieve messages from an authenticated subscriber

       UserA to call the voice mail system to retrieve messages.
       Assumptions: The caller is authenticated using UserA's credentials.
       "sip:UserA-retrieve-auth@vm.wcom.com" maps to UserA's mailbox and the
       behavior of "retrieve messages." The voice mail service trusts the
       proxy not to forward any calls to that URI unless the call is
       authenticated to be from UserA.

       Given these assumptions, The VM service may choose not require a PIN
       for calls to this URI.

    Campbell & Sparks            Informational                     [Page 33]

    RFC 3087                  SIP Service Control                 April 2001

                User A              Proxy                VM Service
                  |                   |                       |
                  |  INVITE F1        |                       |
                  |------------------>|                       |
                  |                   |  INVITE F2            |
                  | (100 Trying) F3   |---------------------->|
                  |<------------------|                       |
                  |                   |  200 OK F4            |
                  |  200 OK F5        |<----------------------|
                  |<------------------|                       |
                  |                   |                       |
                  |  ACK F6           |                       |
                  |------------------>|  ACK F7               |
                  |                   |---------------------->|
                  |                   |                       |
                  |      RTP Both Ways - Deposit Msg for B    |
                  |<-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m-m->|
                  |                   |                       |
                  |  BYE F8           |                       |
                  |------------------>|  BYE F9               |
                  |                   |---------------------->|
                  |                   |                       |
                  |                   |  200 OK F10           |
                  |                   |<----------------------|
                  |  200 OK F11       |                       |
                  |<------------------|                       |
                  |                   |                       |

         Flow Id                            Comments

        INVITE F1     INVITE sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Proxy-Authorization:Digest username="UserA",
                      realm="MCI WorldCom SIP",
                      nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",
                      uri="sip:VoiceMail@wcom.com", response=<appropriately
                      calculated hash goes here>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP

    Campbell & Sparks            Informational                     [Page 34]

    RFC 3087                  SIP Service Control                 April 2001

                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

                      /*Client for A prepares to receive data on port 49170
                      from the network. */

        INVITE F2     INVITE sip:UserA-retrieve-auth@vm.wcom.com SIP/2.0
        Proxy->B1     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: TheBigGuy <sip:UserA@here.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserA 2890844526 2890844526 IN IP4 client.here.com
                      s=Session SDP
                      c=IN IP4 100.101.102.103
                      t=0 0
                      m=audio 49170 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        (100 Trying   SIP/2.0 100 Trying
        F3            Via: SIP/2.0/UDP here.com:5060
        Proxy->A)     From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Content-Length: 0

        200 OK F4     SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060; branch=1
                      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact: VoiceMailSystem <sip:UserA-retrieve-
                      auth@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

    Campbell & Sparks            Informational                     [Page 35]

    RFC 3087                  SIP Service Control                 April 2001

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        200 OK F5     SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      Record-Route: <sip:VoiceMail@wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 INVITE
                      Contact VoiceMailSystem <sip: UserA-retrieve-
                      auth@vm.wcom.com>
                      Content-Type: application/sdp
                      Content-Length: <appropriate value>

                      v=0
                      o=UserB 2890844527 2890844527 IN IP4 vm.wcom.com
                      s=Session SDP
                      c=IN IP4 110.111.112.114
                      t=0 0
                      m=audio 3456 RTP/AVP 0
                      a=rtpmap:0 PCMU/8000

        ACK F6        ACK sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route: <sip:UserA-retrieve-auth@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

        ACK F7        ACK sip:UserA-retrieve-auth@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>; tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 1 ACK
                      Content-Length: 0

    Campbell & Sparks            Informational                     [Page 36]

    RFC 3087                  SIP Service Control                 April 2001

                      /* RTP streams are established between A and VM.  VM
                      determines that the call is likely from UserA, and
                      starts a message retrieval session.  Since the proxy
                      has already authenticated the identity of UserA, the
                      VM does not need to prompt for PIN. */

                      /* User A Hangs Up with VM system.  Alternatively, the
                      VM system could initiate the BYE*/

        BYE F8        BYE sip:VoiceMail@wcom.com SIP/2.0
        A->Proxy      Via: SIP/2.0/UDP here.com:5060
                      Route:<sip:UserA-retrieve-auth@vm.wcom.com>
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        BYE F9        BYE sip: UserA-retrieve-auth@vm.wcom.com SIP/2.0
        Proxy->VM     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F10    SIP/2.0 200 OK
        VM->Proxy     Via: SIP/2.0/UDP wcom.com:5060
                      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq: 2 BYE
                      Content-Length: 0

        200 OK F11    SIP/2.0 200 OK
        Proxy->A      Via: SIP/2.0/UDP here.com:5060
                      From: TheBigGuy <sip:UserA@here.com>
                      To: VoiceMail <sip:VoiceMail@wcom.com>;tag=3145678
                      Call-Id: 12345600@here.com
                      CSeq:  2 BYE
                      Content-Length: 0

    Campbell & Sparks            Informational                     [Page 37]

    RFC 3087                  SIP Service Control                 April 2001

    5. Security Considerations

       This document discusses a usage of SIP/2.0 as defined by RFC 2543[1].
       It introduces no additions, modifications, or restrictions to the
       protocol defined therein.  Any implementation of the concepts in this
       document is subject to the issues discussed there.

    6. Acknowledgments

       The authors would like to thank Chris Cunningham, Steve Donovan, Alan
       Johnston, Henry Sinnreich, Kevin Summers, John Truetken, and Dean
       Willis for their discussion of and contribution to this work.

    References

       [1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
           "SIP: Session Initiation Protocol", RFC 2543, March 1999.

    Authors' Addresses

       Ben Campbell
       dynamicsoft
       5100 Tennyson Parkway
       Suite 1200
       Plano, TX  75024

       EMail: bcampbell@dynamicsoft.com

       Robert J. Sparks
       dynamicsoft
       5100 Tennyson Parkway
       Suite 1200
       Plano, TX  75024

       EMail: rsparks@dynamicsoft.com

    Campbell & Sparks            Informational                     [Page 38]

    RFC 3087                  SIP Service Control                 April 2001

    Full Copyright Statement

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

    Campbell & Sparks            Informational                     [Page 39]