<pre>Internet Engineering Task Force (IETF)                          J. Stone
Request for Comments: 6498                                      R. Kumar
Category: Informational                                     F. Andreasen
ISSN: 2070-1721                                            Cisco Systems
                                                           February 2012


 <span class="h1">Media Gateway Control Protocol (MGCP) Voiceband Data (VBD) Package and</span>
           <span class="h1">General-Purpose Media Descriptor Parameter Package</span>

Abstract

   This document defines Media Gateway Control Protocol (MGCP) packages
   that enable a Call Agent to authorize and monitor the transition of a
   connection to and from Voiceband Data (VBD) with or without
   redundancy and FEC (forward error correction).  Although the focus is
   on VBD, the General-Purpose Media Descriptor Parameter package can be
   used to authorize other modes of operation, not relevant to VBD, for
   a particular codec.  In addition to defining these new packages, this
   document describes the use of the Media Format Parameter package and
   Fax package with VBD, redundancy, and FEC.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6498">http://www.rfc-editor.org/info/rfc6498</a>.














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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   <a href="#section-1">1</a>. Applicability Statement .........................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-3">3</a>. Terminology .....................................................<a href="#page-5">5</a>
   <a href="#section-4">4</a>. Voiceband Data Package Definition ...............................<a href="#page-5">5</a>
      <a href="#section-4.1">4.1</a>. Events and Signals .........................................<a href="#page-5">5</a>
           <a href="#section-4.1.1">4.1.1</a>. Gateway Controlled Voiceband Data ...................<a href="#page-6">6</a>
           <a href="#section-4.1.2">4.1.2</a>. No Negotiated Procedure for Voiceband Data .........<a href="#page-13">13</a>
   <a href="#section-5">5</a>. General-Purpose Media Descriptor Parameter Package Definition ..16
      <a href="#section-5.1">5.1</a>. LocalConnectionOptions ....................................<a href="#page-16">16</a>
           <a href="#section-5.1.1">5.1.1</a>. General-Purpose Media Descriptor Parameter .........<a href="#page-17">17</a>
   <a href="#section-6">6</a>. Use of Media Format Parameter Package with VBD and Redundancy ..20
   <a href="#section-7">7</a>. Use of Media Format Parameter Package with VBD and FEC .........<a href="#page-22">22</a>
   <a href="#section-8">8</a>. Use of Fax Package with VBD ....................................<a href="#page-23">23</a>
   <a href="#section-9">9</a>. Call Flow Examples .............................................<a href="#page-27">27</a>
      <a href="#section-9.1">9.1</a>. Modem Call with Gateway Controlled VBD ....................<a href="#page-27">27</a>
      9.2. Fax Call with Gateway Controlled VBD and Call
           Agent Controlled T.38 .....................................<a href="#page-33">33</a>
   <a href="#section-10">10</a>. Security Considerations .......................................<a href="#page-42">42</a>
   <a href="#section-11">11</a>. IANA Considerations ...........................................<a href="#page-44">44</a>
   <a href="#section-12">12</a>. Acknowledgements ..............................................<a href="#page-44">44</a>
   <a href="#section-13">13</a>. References ....................................................<a href="#page-44">44</a>
      <a href="#section-13.1">13.1</a>. Normative References .....................................<a href="#page-44">44</a>
      <a href="#section-13.2">13.2</a>. Informative References ...................................<a href="#page-46">46</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Applicability Statement</span>

   This document defines a mechanism that requires media stream
   integrity protection.  The document specifies different alternative
   mechanisms but does not choose one of them as mandatory-to-implement.
   Consequently, the use of this specification is only suitable in
   environments that specify and use at least one of these alternative
   mechanisms.  Please see the Security Considerations section for
   further details.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Introduction</span>

   The term Voiceband Data (or simply VBD) refers to the use of a
   suitable voiceband codec (commonly G.711u or G.711a) for the
   transport of data payloads using RTP as defined in <a href="./rfc3550">RFC 3550</a>
   [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].  This document defines Media Gateway Control Protocol
   (MGCP) [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>] packages that enable a Call Agent to authorize and
   monitor the transition of a connection to and from VBD with or
   without redundancy [<a href="./rfc2198" title="&quot;RTP Payload for Redundant Audio Data&quot;">RFC2198</a>] and FEC (forward error correction)
   [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>].





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   There are a number of different VBD procedures.  These procedures
   vary in terms of how the transition to and from VBD is coordinated
   end to end.  Some coordination techniques are mutually negotiated by
   the two gateways using the Session Description Protocol (SDP)
   [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].  These coordination techniques include

   o  ITU-T Recommendation V.150.1 State Signaling Event (SSE) [<a href="#ref-V1501" title="&quot;Modem-over-IP networks: Procedures for the end-to-end connection of V-series DCEs&quot;">V1501</a>]

   o  ITU-T Recommendation V.152 Payload Type Switching [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>]

   Other coordination techniques are not negotiated.  For example, the
   detection of fax, modem, and text tones in the direction from the IP
   to the General Switched Telephone Network (GSTN) may result in a
   switch to VBD or a change (e.g., disable echo cancellation) to the
   gateway controlled VBD procedure already in place.  The IP-side
   detected tone serves as both a VBD stimulus and a coordination
   technique.

   <a href="./rfc4733">RFC 4733</a> [<a href="./rfc4733" title="&quot;RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals&quot;">RFC4733</a>] and <a href="./rfc4734">RFC 4734</a> [<a href="./rfc4734" title="&quot;Definition of Events for Modem, Fax, and Text Telephony Signals&quot;">RFC4734</a>] can be used to convey fax
   and modem events and tones.  As with IP-side tone detection, the
   telephone event may serve as both a VBD stimulus and a coordination
   technique.  Note that while the use of <a href="./rfc4733">RFC 4733</a> and <a href="./rfc4734">RFC 4734</a> to
   convey fax and modem events and tones is negotiated, the use of
   <a href="./rfc4733">RFC 4733</a> and <a href="./rfc4734">RFC 4734</a> as a gateway VBD coordination technique (at
   present) is not.

   The Voiceband Data (VBD) package is defined to support all VBD
   procedures.  This document does not address the relative merits of
   different procedures nor does it advocate one procedure over another.

   We will use the term VBD to refer to Voiceband Data in general.  In
   referring to VBD in the context of the package, we will use the term
   VBD package.  We use the term "audio" (with double quotes) to refer
   to the IANA media type.  We use the term audio (without double
   quotes) to refer to the use of the "audio" media type for (most
   commonly) voice.

   A package is defined for the General-Purpose Media Descriptor
   Parameter [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>].  In the context of VBD, the General-Purpose Media
   Descriptor Parameter (GPMD) package is used to authorize the
   negotiation of a particular codec for use with VBD.  The General-
   Purpose Media Descriptor Parameter is "general" in nature and may be
   used in applications other than VBD.

   The Media Format Parameter (FM) package [<a href="./rfc3660" title="&quot;Basic Media Gateway Control Protocol (MGCP) Packages&quot;">RFC3660</a>] describes the use
   of the standard audio MIME subtype "RED" in conjunction with the
   "fmtp" LocalConnectionOption in order to authorize the negotiation of
   redundancy [<a href="./rfc2198" title="&quot;RTP Payload for Redundant Audio Data&quot;">RFC2198</a>], to identify the levels of redundancy and the



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   media format associated with each redundancy level.  This document
   will further explore the use of the FM package with VBD and
   redundancy.

   The VBD package is intended to complement the MGCP Fax (FXR) package
   [<a href="./rfc5347" title="&quot;Media Gateway Control Protocol Fax Package&quot;">RFC5347</a>].  This document will explore the use of the FXR package
   with VBD.

   The VBD package definition is provided in <a href="#section-4">Section 4</a>.  The GPMD
   package definition is provided in Section 5.  In <a href="#section-6">Section 6</a>, we
   discuss the use of the FM package with VBD and redundancy.  In
   <a href="#section-7">Section 7</a>, we discuss the use of the FM package with VBD and FEC.  In
   <a href="#section-8">Section 8</a>, we discuss the use of the FXR package with VBD.  In
   <a href="#section-9">Section 9</a>, we provide two call flow examples showing how to use the
   VBD and GPMD packages.  Security considerations are found in
   <a href="#section-10">Section 10</a>, followed by the IANA considerations (<a href="#section-11">Section 11</a>) and
   references.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Terminology</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Voiceband Data Package Definition</span>

   This package is defined for Voiceband Data (VBD).  The package
   defines new events as detailed below.

      Package Name:        VBD

      Package Version:     0

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Events and Signals</span>

   The following events are defined in support of the above:

    -------------------------------------------------------------------
   | Symbol |   Definition                    |  R  |  S  |  Duration  |
   |--------|---------------------------------|-----|-----|------------|
   | gwvbd  | Gateway Controlled VBD          |  x  |     |            |
   | nopvbd | No Negotiated Procedure for VBD |  x  |     |            |
    -------------------------------------------------------------------

   This is standard MGCP package format as defined in <a href="./rfc3435#section-6.6">Section&nbsp;6.6 of
   RFC 3435</a> [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>].  The definitions of the individual events are
   provided in the following subsections.




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<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>.  Gateway Controlled Voiceband Data</span>

   The gwvbd procedure can be used by the gateway to control and decide
   how to handle VBD calls without Call Agent involvement.  The "Gateway
   Controlled Voiceband Data" (or simply "gwvbd") event occurs when a
   gwvbd procedure has been negotiated and VBD stimulus is detected.
   The "gwvbd" event may occur when the gwvbd procedure is updated
   (e.g., upon detecting new stimulus) and when the procedure fails.
   The "gwvbd" event occurs when the gwvbd procedure ends.  The gwvbd
   procedure MUST be negotiated with the other side by passing and
   recognizing relevant parameters via the LocalConnectionDescriptor and
   RemoteConnectionDescriptor.

   The following recommendations from MGCP [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>] apply.

   In this section, we provide a formal description of the protocol
   syntax, using ABNF as defined in "Augmented BNF for Syntax
   Specifications: ABNF" [<a href="./rfc5234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC5234</a>].  The syntax makes use of the core
   rules defined in <a href="./rfc5234#appendix-B.1">Appendix&nbsp;B.1 of [RFC5234]</a>, which are not included
   here.  Furthermore, the syntax follows the case-sensitivity rules of
   [<a href="./rfc5234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC5234</a>], i.e., MGCP is case-insensitive (but SDP is not).  It
   should be noted that ABNF does not provide for implicit specification
   of linear white space, and MGCP messages MUST thus follow the
   explicit linear white space rules provided in the grammar below.
   However, in line with general robustness principles, implementers are
   strongly encouraged to tolerate additional linear white space in
   messages received.

   The RequestedEvent parameter is encoded as

      GwVbdReqEvent = "gwvbd"

   The ObservedEvent parameter is encoded as

      GwVbdObsEvent = GwVbdObsEventStart / GwVbdObsEventUpdate /
                      GwVbdObsEventStop / GwVbdObsEventFailure

      GwVbdObsEventStart   = "gwvbd(start" Rc [Codec] [Coord] [Dir] ")"
      GwVbdObsEventUpdate  = "gwvbd(update" Rc [Codec] [Dir] ")"
      GwVbdObsEventStop    = "gwvbd(stop" [Rc] [Codec] ")"
      GwVbdObsEventFailure = "gwvbd(failure" [Rc] [Codec] ")"










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      Codec = "," *WSP "codec=" CodecString
      CodecString = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-" / "_" /
                     "." / "/")
      Coord = "," *WSP "coord=" CoordinationTechnique
      CoordinationTechnique = "v152ptsw" / "v150fw"
      Rc = "," *WSP "rc=" ReasonCode
      ReasonCode = 1*(ALPHA / DIGIT / "-" / "_" / "." / "/")
                   ; Refer to the values listed in the tables below.
      Dir = "," *WSP "dir=" Direction
      Direction = "GstnToIp" / "IpToGstn"

   ABNF does not provide for position-independent parameters.  The "rc",
   "codec", "coord", and "dir" parameters, if present, MUST appear in
   the relative order shown.

   The "start", "update", "stop", and "failure" ObservedEvent parameters
   are defined as follows:

   1) VBD Start (start)

      The gwvbd procedure was initiated.  The Call Agent SHOULD refrain
      from issuing media handling instructions to the gateway until
      either a "gwvbd(stop)" or "gwvbd(failure)" event is generated.
      One and only one "gwvbd(stop)" or "gwvbd(failure)" event is
      generated corresponding to each "gwvbd(start)" event.

   2) VBD Update (update)

      The gwvbd procedure was updated.  The "gwvbd(update)" event MUST
      only be generated after a "gwvbd(start)" event and before a
      "gwvbd(stop)" or "gwvbd(failure)" event.

   3) VBD Stop (stop)

      The gwvbd procedure ended, and the gateway did not detect any
      errors.  Note that this does not necessarily imply a successful
      fax, modem, or text transmission.  It merely indicates that the
      gwvbd procedure has ended and the procedure itself did not
      encounter any errors.  The "stop" parameter may correspond to a
      change from VBD to a non-VBD "audio" codec or from VBD to another
      media type such as "image" or "text".  This change may be under
      Call Agent or gateway control.  For example, the gateway may
      coordinate the switch from VBD to "image/t38" through the exchange
      of SSEs [<a href="#ref-T38" title="&quot;Procedures for real-time Group 3 facsimile communication over IP networks&quot;">T38</a>] [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>].  For an example involving Call Agent
      control, refer to the "MC" Reason Code.  In both examples, the
      gwvbd procedure ends with the media change.





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   4) VBD Failure (failure)

      The gwvbd procedure ended abnormally.  Some kind of problem was
      encountered in the gwvbd procedure, and the procedure ended.

   When the "gwvbd" event is reported, exactly one of the "start",
   "update", "stop", or "failure" parameters MUST be present and MUST be
   the first parameter supplied.

   The "rc", "codec", "coord", and "dir" ObservedEvent parameters are
   defined as follows:

   1) Reason Code (rc=&lt;ReasonCode&gt;)

      With the "start" and "update" parameters, the reason for
      triggering the switch/change to VBD.  With the "stop" and
      "failure" parameters, the reason for triggering the switch from
      VBD.  The Reason Codes in the following table, which are based on
      the ITU-T Fax/Textphone/Modem Tones Detection package [<a href="#ref-H2482" title="&quot;Gateway control protocol: Facsimile, text conversation and call discrimination packages&quot;">H2482</a>],
      ITU-T V.150.1 Amendment 1 [<a href="#ref-V1501A1" title="&quot;Modem-over-IP networks: Procedures for the end-to-end connection of V-series DCEs, Amendment 1: Modification to SSE reason identifier codes to support voice band data and text relay&quot;">V1501A1</a>], and ITU-T V.152 [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>], may
      be used with the "start" and "update" parameters:






























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       ---------------------------------------------------------------
      | ReasonCode | Description                                      |
      |------------|--------------------------------------------------|
      | CNG        | T.30 fax calling                                 |
      | V21flag    | V.21 tone and flags for fax answering            |
      | CIV18      | V.8 CI with V.18 call function                   |
      | XCI        | V.18 XCI                                         |
      | V18txp     | V.18 txp                                         |
      | Belltone   | Bell 103 carrier, high- or low-frequency channel |
      |            |   (ITU-T Recommendation V.18)                    |
      | Baudot     | Baudot initial tone and character (ITU-T         |
      |            |   Recommendation V.18)                           |
      | Edt        | EDT initial tone and character (ITU-T            |
      |            |   Recommendation V.18)                           |
      | CIdata     | V.8 CI with any data call function               |
      | CT         | V.25 calling tone                                |
      | CIfax      | V.8 CI with fax call function                    |
      | V21tone    | V.21 carrier, high- or low-frequency channel     |
      | V23tone    | V.23 carrier, high- or low-frequency channel     |
      | V8bis      | V.8 bis modem handshaking signal                 |
      | ANS        | V.25 ANS, equivalent to T.30 CED from answering  |
      |            |   terminal                                       |
      | /ANS       | V.25 ANS with periodic phase reversals           |
      | ANSam      | V.8 ANSam                                        |
      | /ANSam     | V.8 ANSam with periodic phase reversals          |
      | CMFax      | V.8 CM sequence indicating fax call function     |
      | JMFax      | V.8 JM sequence indicating fax call function     |
      | CMData     | V.8 CM sequence indicating unspecified data      |
      |            |   call function                                  |
      | JMData     | V.8 JM sequence indicating unspecified data      |
      |            |   call function                                  |
      | CMText     | V.8 CM sequence indicating text call function    |
      | JMText     | V.8 JM sequence indicating text call function    |
      | PTSW       | Payload type switch as defined in V.152          |
       ---------------------------------------------------------------

      For solutions involving textphones using a modulation with
      interspersed text and speech on the same "channel", such as Baudot
      and EDT, the Call Agent SHOULD interpret the ReasonCode parameter
      as part of the "vbd/gwvbd(start)" event in order to differentiate
      between fax, modem, and text.  In the case of interspersed text
      and speech, the Call Agent SHOULD remove the notification request
      for "vbd/gwvbd" upon receiving the "vbd/gwvbd(start)" event in
      order to avoid large numbers of notifications.

      For example,

         vbd/gwvbd(start, rc=Baudot)



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      With a ReasonCode of "PTSW", the Call Agent cannot differentiate
      text from fax/modem.  In this case, the Call Agent SHOULD adopt a
      policy that guards against large numbers of notifications.  We
      consider several such policies.

      The Call Agent MAY remove the notification request for "vbd/gwvbd"
      upon receiving the "vbd/gwvbd(start, rc=PTSW)" event.  With this
      policy, "update", "stop", and "failure" notifications will not be
      generated with text AND fax/modem.

      The Call Agent MAY wait for a subsequent "vbd/gwvbd(update)" event
      that differentiates text from fax/modem.  If the ReasonCode
      indicates interspersed text and speech, the Call Agent SHOULD
      remove the notification request for "vbd/gwvbd".  For example,

         vbd/gwvbd(update, rc=Edt)

      The Call Agent MAY remove the notification request for "vbd/gwvbd"
      upon receiving a "vbd/gwvbd(stop)" event without having
      differentiated between text and fax/modem.

      The Call Agent MAY remove the notification request for "vbd/gwvbd"
      after having received a number of "vbd/gwvbd(start)" events
      without having differentiated between text and fax/modem.  The
      specific number of events after which the notification request is
      removed is considered an implementation detail outside the scope
      of this specification.

      Reason Codes applicable with the "stop" parameter are listed
      below:

             ------------------------------------------------------
            | ReasonCode | Description                             |
            |------------|-----------------------------------------|
            | SIL        | Bidirectional silence                   |
            | Voice      | Voice signals                           |
            | PTSW       | Payload type switch as defined in V.152 |
            | MC         | Media change                            |
             ------------------------------------------------------

      The "MC" Reason Code indicates that the media type has changed
      from "audio" (to "image", "text", ...) or the "audio" media format
      has changed from a VBD codec (for a reason other than "PTSW").
      For example, the gwvbd procedure may be initiated upon detecting
      called terminal identification (CED).  Subsequently, the Call
      Agent controlled T.38 procedure of the MGCP Fax (FXR) package
      [<a href="./rfc5347" title="&quot;Media Gateway Control Protocol Fax Package&quot;">RFC5347</a>] may be initiated upon detecting V.21 flags.  Upon
      receipt of a "t38(start)" event, the Call Agent will instruct the



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      gateway to switch from VBD to T.38 through the use of a
      ModifyConnection command involving a LocalConnectionOption
      encoding method of "L:a:image/t38" and/or a
      RemoteConnectionDescriptor with an "image/t38" media description.
      This stops the gwvbd procedure.  There is no specific
      interdependency between the VBD package and the FXR package (or
      any other package).  The gwvbd procedure is stopped as a
      consequence of the media change, not as a direct consequence of
      the T.38 procedure being initiated.  Note that in this situation
      the "t38(start)" event will be sent before the "gwvbd(stop)"
      event.  The Call Agent MAY choose to infer that the gwvbd
      procedure has ended upon receiving the "t38(start)" event and
      disable the notification of the "gwvbd" event.  Refer to the
      example call flow in <a href="#section-9.2">Section 9.2</a>.

      Reason Codes applicable with the "failure" parameter:

             ----------------------------------------------------
            | ReasonCode | Description                           |
            |------------|---------------------------------------|
            | TO         | Indicates that a timeout has occurred |
             ----------------------------------------------------

      The list of Reason Codes may be extended to include values with
      meaning mutually understood between the gateway and the Call
      Agent.  Obviously, the use of extended values MUST be a
      provisionable option on the gateway in order to ensure
      interoperability with the Call Agent.

   2) Codec String (codec=&lt;CodecString&gt;)

      With the "start" and "update" parameters, the codec parameter
      describes the MIME type associated with the switch/change to VBD
      (e.g., "audio/RED", "audio/PCMU", "audio/PCMA", "audio/G726-32",
      "audio/clearmode", ...).  With the "stop" and "failure"
      parameters, the codec parameter describes the MIME type associated
      with the switch from VBD (e.g., "audio/G729", "image/t38", "text/
      t140", "audio/v150mr", ...).  These strings should be full MIME
      types as listed in <a href="http://www.iana.org/assignments/media-types">http://www.iana.org/assignments/media-types</a>.












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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   3) Coordination Technique (coord=&lt;CoordinationTechnique&gt;)

      The technique used to coordinate the transition to and from VBD
      with the remote endpoint.  The coordination techniques are
      summarized in the following table:

           ------------------------------------------------------
          | CoordinationTechnique | Description                  |
          |-----------------------|------------------------------|
          | v152ptsw              | V.152 Payload Type Switching |
          | v150fw                | V.150.1 SSE                  |
           ------------------------------------------------------

      With the "v152ptsw" coordination technique, payload type switching
      [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>] is used to coordinate the transition to and from VBD.

      With the "v150fw" coordination technique, state signaling events
      [<a href="#ref-V1501" title="&quot;Modem-over-IP networks: Procedures for the end-to-end connection of V-series DCEs&quot;">V1501</a>] are used to coordinate the transition to and from VBD.

      The list of coordination techniques may be extended to include
      values with meaning mutually understood between the gateway and
      the Call Agent.  Obviously, the use of extended values MUST be a
      provisionable option on the gateway in order to ensure
      interoperability with the Call Agent.

   4) Direction of Stimulus (dir=&lt;Direction&gt;)

      With the "start" and "update" parameters, the "dir" parameter
      describes the direction of the stimulus that resulted in the
      switch/change to VBD.

            ---------------------------------------------------
           | Direction | Description                            |
           |-----------|------------------------------------    |
           | GstnToIp  | Stimulus detected in the direction     |
           |           | from the GSTN to IP network,           |
           |           | including fax, modem, and text tones.  |
           | IpToGstn  | Stimulus detected in the direction     |
           |           | from the IP to GSTN network,           |
           |           | including fax, modem, and text tones   |
           |           | (e.g., IP-side tone detection);        |
           |           | RTP packet with VBD payload type       |
           |           | (e.g., V.152 or V.150.1).              |
            ----------------------------------------------------







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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   Call Agents and gateways MUST implement the "start" and "stop"
   parameters and MAY implement the "update" and "failure" parameters.
   Call Agents and gateways MAY implement the "coord", "codec", and
   "dir" parameters.  Call Agents MAY, and gateways MUST, implement the
   "rc" parameter in conjunction with the "start" and "update"
   parameters.  Call Agents and gateways MAY implement the "rc"
   parameter in conjunction with the "stop" and "failure" parameters.  A
   Call Agent MUST ignore all unknown ObservedEvent parameters,
   including parameters that are defined as part of this specification
   and not implemented.

<span class="h5"><a class="selflink" id="section-4.1.1.1" href="#section-4.1.1.1">4.1.1.1</a>.  Gateway Controlled Voiceband Data Examples</span>

   The following examples illustrate the encoding of the "gwvbd(start)"
   event:

           O: vbd/gwvbd(start, rc=ANS)
           O: vbd/gwvbd(start, rc=ANS, codec=audio/PCMU, coord=v152ptsw)
           O: vbd/gwvbd(start, rc=PTSW, codec=audio/RED)

   The following example illustrates the encoding of the "gwvbd(update)"
   event:

           O: vbd/gwvbd(update, rc=/ANSam, dir=IpToGstn)

   The following examples illustrate the encoding of the "gwvbd(stop)"
   event:

           O: vbd/gwvbd(stop)
           O: vbd/gwvbd(stop, rc=SIL, codec=audio/G729)
           O: vbd/gwvbd(stop, rc=MC, codec=image/t38)

   The following examples illustrate the encoding of the
   "gwvbd(failure)" event:

           O: vbd/gwvbd(failure, codec=audio/G729)
           O: vbd/gwvbd(failure, rc=TO, codec=audio/G729)

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>.  No Negotiated Procedure for Voiceband Data</span>

   The "No Negotiated Procedure for Voiceband Data" (or simply "nopvbd")
   event occurs when a VBD procedure has not been negotiated and VBD
   stimulus is detected.  The "nopvbd" event may occur when the
   procedure is updated (e.g., upon detecting new stimulus), when the
   procedure ends, and when the procedure fails.  Even though a
   procedure was not negotiated, a VBD handling procedure MAY still be
   in place locally on the endpoint, as described further below.




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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   The nopvbd procedure MAY involve VBD handling including, but not
   limited to, adjusting gain and jitter, disabling voice activity
   detection, and DC offset filters.  The nopvbd procedure MAY involve
   switching to another codec.  The Call Agent MAY have to issue further
   commands in response to the "nopvbd" event in order to ensure a
   successful VBD call.

   As with the "gwvbd" event, the same recommendations from MGCP
   [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>] regarding ABNF, general robustness principles, and white
   space apply.

      The RequestedEvent parameter is encoded as

        NopVbdReqEvent = "nopvbd"

      The ObservedEvent parameter is encoded as

        NopVbdObsEvent = NopVbdObsEventStart / NopVbdObsEventUpdate /
                         NopVbdObsEventStop / NopVbdObsEventFailure

        NopVbdObsEventStart   = "nopvbd(start" Rc [Codec] [Dir] ")"
        NopVbdObsEventUpdate  = "nopvbd(update" Rc [Codec] [Dir] ")"
        NopVbdObsEventStop    = "nopvbd(stop" [Rc] [Codec] ")"
        NopVbdObsEventFailure = "nopvbd(failure" [Rc] [Codec] ")"

   The following ABNF notation is common with the "gwvbd" ObservedEvent
   parameter:

     Codec = "," *WSP "codec=" CodecString
     CodecString = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-" / "_" /
                    "." / "/")
     Rc = "," *WSP "rc=" ReasonCode
     ReasonCode = 1*(ALPHA / DIGIT / "-" / "_" / "." / "/")
                  ; Refer to the values listed in the tables above.
     Dir = "," *WSP "dir=" Direction
     Direction = "GstnToIp" / "IpToGstn"

   ABNF does not provide for position-independent parameters.  The "rc",
   "codec", and "dir" parameters, if present, MUST appear in the
   relative order shown.











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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   The "start", "update", "stop", and "failure" ObservedEvent parameters
   are defined as follows:

   1) VBD Start(start)

      The nopvbd procedure was initiated.  The Call Agent may have to
      issue further commands in order to ensure a successful VBD call
      (e.g., switch to another codec).  At most one "nopvbd(stop)" or
      "nopvbd(failure)" event MAY be generated corresponding to each
      "nopvbd(start)" event.  The Call Agent MAY need to infer that the
      nopvbd procedure has ended.

   2) VBD Update (update)

      The nopvbd procedure was updated.  The "nopvbd(update)" event MUST
      only be generated after a "nopvbd(start)" event and before a
      "nopvbd(stop)" or "nopvbd(failure)" event.

   3) VBD Stop (stop)

      The nopvbd procedure ended, and the gateway did not detect any
      errors.  Note that this does not necessarily imply a successful
      fax, modem, or text transmission.  It merely indicates that the
      nopvbd procedure has ended and the procedure itself did not
      encounter any errors.  Refer to the definition of the "stop"
      parameter from the "gwvbd" event in <a href="#section-4.1.1">Section 4.1.1</a> for additional
      information.

   4) VBD Failure (failure)

      The nopvbd procedure ended abnormally.  Some kind of problem was
      encountered in the nopvbd procedure, and the procedure ended.

   Call Agents and gateways MUST implement the "start" parameter and MAY
   implement the "update", "stop", and "failure" parameters.  Call
   Agents MAY, and gateways MUST, implement the "rc" parameter in
   conjunction with the "start" and "update" parameters.  Call Agents
   and gateways MAY implement the "rc" parameter in conjunction with the
   "stop" and "failure" parameters.  A Call Agent MUST ignore all
   unknown ObservedEvent parameters including parameters that are
   defined as part of this specification and not implemented.

   The definitions of the "rc", "codec", and "dir" ObservedEvent
   parameters are taken from the "gwvbd" event.

   As with the "gwvbd" event, the same recommendations regarding
   interspersed text and speech apply.




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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


<span class="h5"><a class="selflink" id="section-4.1.2.1" href="#section-4.1.2.1">4.1.2.1</a>.  No Negotiated Procedure for Voiceband Data Examples</span>

   The following examples illustrate the encoding of the "nopvbd(start)"
   event:

           O: vbd/nopvbd(start, rc=ANS)
           O: vbd/nopvbd(start, rc=ANS, codec=audio/PCMU)

   The following example illustrates the encoding of the
   "nopvbd(update)" event:

           O: vbd/nopvbd(update, rc=/ANSam, dir=IpToGstn)

   The following examples illustrate the encoding of the "nopvbd(stop)"
   event:

           O: vbd/nopvbd(stop)
           O: vbd/nopvbd(stop, rc=SIL, codec=audio/G729)
           O: vbd/nopvbd(stop, rc=MC, codec=image/t38)

   The following examples illustrate the encoding of the
   "nopvbd(failure)" event:

           O: vbd/nopvbd(failure, codec=audio/G729)
           O: vbd/nopvbd(failure, rc=TO, codec=audio/G729)

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  General-Purpose Media Descriptor Parameter Package Definition</span>

   This package is defined for the General-Purpose Media Descriptor
   Parameter [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>].  The package defines a new LocalConnectionOption as
   detailed below.

      Package Name:        GPMD
      Package Version:     0

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  LocalConnectionOptions</span>

   The following new LocalConnectionOptions field is defined in support
   of the above:

       ------------------------------------------------------
      | Symbol |   Definition                                |
      |--------|---------------------------------------------|
      | gpmd   | General-Purpose Media Descriptor Parameter  |
       ------------------------------------------------------

   The definition of the LocalConnectionOption is provided in the
   following subsection.



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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  General-Purpose Media Descriptor Parameter</span>

   The General-Purpose Media Descriptor Parameter LocalConnectionOption
   is similar to the "gpmd" SDP [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] attribute defined in ITU-T
   Recommendation V.152 [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>] and is applicable to all of the same
   media formats that the corresponding SDP "gpmd" attribute could be
   used with.

   The General-Purpose Media Descriptor Parameter is encoded as the
   keyword "gpmd" or "o-gpmd", followed by a colon and a quoted string
   beginning with the media format name (MIME subtype only) followed by
   a space, followed by the media format parameters associated with that
   media format:

         gpmd/gpmd:"&lt;format&gt; &lt;parameter list&gt;"

   For simplicity, we will use the terms "codec" and "media format"
   interchangeably in the following.  Multiple media formats may be
   indicated by either repeating the "gpmd" LocalConnectionOption
   multiple times, such as

         L: a:codec1;codec2, gpmd/gpmd:"codec1 parameterX",
                             gpmd/gpmd:"codec2 parameterY"

   or alternatively by having a single "gpmd" keyword followed by a
   colon, and a semicolon-separated list of quoted strings for each
   General-Purpose Media Descriptor Parameter, as in

         L: a:codec1;codec2, gpmd/gpmd:"codec1 parameterX";
                                       "codec2 parameterY"

   The two formats may be mixed:

         L: a:codec1;codec2;codec3, gpmd/gpmd:"codec1 parameterX",
                                    gpmd/gpmd:"codec2 parameterY";
                                              "codec3 parameterZ"

   The carriage returns above are included for formatting reasons only
   and are not permissible in a real implementation.  This holds true
   for all of the examples in this document.

   If it is possible for the same codec to be requested with and without
   the "gpmd" parameter, the following could result:

         L: a:codec1;codec1, gpmd/gpmd:"codec1 parameterX"






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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   However, it would not be clear whether the "gpmd" parameter was to be
   applied to the first or the second occurrence of the codec.  The
   problem is that codec ordering is important (i.e., codecs are listed
   in preferred order), and the above syntax does not provide a way to
   indicate whether "parameterX" is preferred (i.e., associated with the
   first "codec1") or not (i.e., associated with the second "codec1").
   In order to resolve this dilemma, the codec in the "gpmd" media
   format is followed by a colon and an &lt;order&gt;, where &lt;order&gt; is a
   number from one to N for occurrences of the same codec in the codec
   list.  For example,

         L:a:codec1;codec1, gpmd/gpmd:"codec1:2 parameterX"

   indicates that "parameterX" is associated with the second instance of
   "codec1" in the "a:codec1;codec1" list.  If an invalid instance
   number is supplied (e.g., instance 3 where there are only two
   instances), then error code 524 -- inconsistency in local connection
   options -- will be returned.  In the absence of an &lt;order&gt;, the first
   instance is assumed.

   Prepending "gpmd" with the string "o-" (i.e., "o-gpmd") indicates
   that the parameter is optional.  In that case, the gateway may decide
   not to use the "gpmd" parameter specified, or only use it in part.

   If the "gpmd" LocalConnectionOption parameter is not optional (i.e.,
   does not have "o-" in front of it), and the LocalConnectionOption
   parameter value is either not recognized or not supported, then the
   associated codec is considered "not supported".

   When auditing capabilities, the "gpmd" LocalConnectionOption
   parameter MUST be returned with a semicolon-separated list of
   supported formats and/or multiple independent "gpmd" parameters,
   as in

         A: a:codec1;codec2, gpmd/gpmd:"codec1 parameterX";
                                       "codec2 parameterY"

       or

         A: a:codec1;codec1, gpmd/gpmd:"codec1 parameterX"











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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   One example uses the General-Purpose Media Descriptor Parameter
   LocalConnectionOption in conjunction with gateway controlled
   Voiceband Data (or simply VBD) using payload type switching [<a href="#ref-V152" title="&quot;Procedures for supporting Voice- Band Data over IP Networks&quot;">V152</a>].
   In the context of VBD, the &lt;format&gt; must be an RTP/AVP payload type.
   The &lt;parameter list&gt; is a semicolon-separated list of
   "parameter=value" pairs:

     L: a:codec1, gpmd/gpmd:"codec1 parameterX=ValueA;parameterY=ValueB"

   In the example below, G.729 is an audio codec and G.711u is a VBD
   codec:

         L: a:G729;PCMU, gpmd/gpmd:"PCMU vbd=yes"

   The corresponding media description in the SDP as part of the
   connection request acknowledgment might look like

         m=audio 12345 RTP/AVP 18 96
         a=rtpmap:96 PCMU/8000
         a=gpmd:96 vbd=yes

   If a request is made to audit the capabilities of an endpoint, and
   the endpoint supports G.711u as both an audio and VBD codec, then the
   "gpmd" LocalConnectionOption parameter might look like

         A: a:PCMU, p:10-40, e:on, s:on,
            m:sendonly;recvonly;sendrecv;inactive
         A: a:PCMU, p:10-40, e:on, s:off,
            m:sendonly;recvonly;sendrecv;inactive,
            gpmd/gpmd:"PCMU vbd=yes"

   Given that some parameters, e.g., silence suppression, are only
   compatible with G.711u as an audio codec, then the gateway MUST
   return different capability sets corresponding to audio and VBD.

   If we combine V.152 and redundancy [<a href="./rfc2198" title="&quot;RTP Payload for Redundant Audio Data&quot;">RFC2198</a>], an example
   LocalConnectionOption might look like the example below.  In this
   example, G.729 is an audio codec and G.711u is a VBD codec with a
   redundancy level of one:

     L: a:G729;RED;PCMU, gpmd/gpmd:"PCMU vbd=yes", fmtp:"RED PCMU/PCMU"










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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   The corresponding media description in the SDP as part of the
   connection request acknowledgment might look like

         m=audio 12345 RTP/AVP 18 96 97
         a=rtpmap:96 RED/8000
         a=fmtp:96 97/97
         a=rtpmap:97 PCMU/8000
         a=gpmd:97 vbd=yes

   Refer to <a href="#section-6">Section 6</a> for more examples involving V.152 and redundancy.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Use of Media Format Parameter Package with VBD and Redundancy</span>

   The MGCP Media Format Parameter (FM) package [<a href="./rfc3660" title="&quot;Basic Media Gateway Control Protocol (MGCP) Packages&quot;">RFC3660</a>] in conjunction
   with the standard audio MIME subtype "RED" may be used by the Call
   Agent to authorize the negotiation of redundancy [<a href="./rfc2198" title="&quot;RTP Payload for Redundant Audio Data&quot;">RFC2198</a>], to
   identify the levels of redundancy and the media format associated
   with each redundancy level.  An example of this was demonstrated in
   <a href="#section-5">Section 5</a>.

   The FM package states that the "fmtp" LocalConnectionOption MUST be
   returned when auditing capabilities.  Applying this to VBD and
   redundancy might result in

         A: a:PCMU, p:10-40, e:on, s:on,
            m:sendonly;recvonly;sendrecv;inactive
         A: a:RED;PCMU, p:10-40, e:on, s:off,
            m:sendonly;recvonly;sendrecv;inactive,
            gpmd/gpmd:"PCMU vbd=yes",
            fmtp:"RED PCMU/PCMU"

      The FM package defines "instance syntax", in which

        L:a:codec1;codec1, fmtp:"codec1:2 formatX"

   indicates that "formatX" is associated with the second instance of
   "codec1" in the "a:codec1;codec1" list.  The examples in the FM
   package are limited to the use of the instance syntax in conjunction
   with the media format.  We propose the use of the instance syntax in
   conjunction with the media format parameters

        L:a:codec1;codec2;codec3;codec2, fmtp:"codec3 codec2:2/codec2:2"









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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   Let's build on the example of <a href="#section-5">Section 5</a>.  In the example below, G.729
   is an audio codec, and G.711u is both an audio codec and a VBD codec
   with a redundancy level of one:

        L: a:G729;PCMU;RED;PCMU, gpmd/gpmd:"PCMU:2 vbd=yes",
                                 fmtp:"RED PCMU:2/PCMU:2"

   The corresponding media description in the SDP as part of the
   connection request acknowledgment might look like

        m=audio 12345 RTP/AVP 18 0 96 97
        a=rtpmap:96 RED/8000
        a=fmtp:96 97/97
        a=rtpmap:97 PCMU/8000
        a=gpmd:97 vbd=yes

   Note that the relative preference of the LocalConnectionOption
   encoding methods is preserved in the "audio" media formats (i.e.,
   payload types) as part of the media description.  In this example,
   this reflects a preference for V.152 with redundancy versus without.
   No preference is inferred from the relative order of the different
   LocalConnectionOptions, namely "a", "gpmd/gpmd", and "fmtp".

   A Call Agent can authorize the negotiation of audio codecs and VBD
   codecs involving different levels of redundancy.  In the example
   below, G.711u is a VBD codec with a redundancy level of two
   (preferred) or one:

        L: a:G729;RED;RED;PCMU, fmtp:"RED PCMU/PCMU/PCMU",
                                fmtp:"RED:2 PCMU/PCMU",
                                gpmd/gpmd:"PCMU vbd=yes"

   The corresponding media description in the SDP as part of the
   connection request acknowledgment might look like

        m=audio 12345 RTP/AVP 18 96 97 98
        a=rtpmap:96 RED/8000
        a=fmtp:96 98/98/98
        a=rtpmap:97 RED/8000
        a=fmtp:97 98/98
        a=rtpmap:98 PCMU/8000
        a=gpmd:98 vbd=yes









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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   Redundancy can be applied to both audio codecs and VBD codecs.  In
   the example below, G.729 is an audio codec with a redundancy level of
   two and G.711u is a VBD codec with a redundancy level of one:

        L: a:RED;G729;RED;PCMU, fmtp:"RED G729/G729/G729",
                                fmtp:"RED:2 PCMU/PCMU",
                                gpmd/gpmd:"PCMU vbd=yes"

   The corresponding media description in the SDP as part of the
   connection request acknowledgment might look like

        m=audio 12345 RTP/AVP 96 18 97 98
        a=rtpmap:96 RED/8000
        a=fmtp:96 18/18/18
        a=rtpmap:97 RED/8000
        a=fmtp:97 98/98
        a=rtpmap:98 PCMU/8000
        a=gpmd:98 vbd=yes

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Use of Media Format Parameter Package with VBD and FEC</span>

   A Call Agent may authorize the negotiation of forward error
   correction (FEC) [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>] with the standard audio MIME subtype
   "parityfec":

        L: a:PCMU;parityfec

   By default, we assume that FEC packets are to be sent as a separate
   stream.  The corresponding media description in the SDP as part of
   the connection request acknowledgment might look like

         v=0
         c=IN IP4 192.0.2.0
         m=audio 49170 RTP/AVP 0 96
         a=rtpmap:96 parityfec/8000
         a=fmtp:96 49172 IN IP4 192.0.2.0

   If FEC is to be sent as a secondary codec in the redundant codec
   payload format [<a href="./rfc2198" title="&quot;RTP Payload for Redundant Audio Data&quot;">RFC2198</a>], we again leverage the MGCP Media Format
   Parameter (FM) package [<a href="./rfc3660" title="&quot;Basic Media Gateway Control Protocol (MGCP) Packages&quot;">RFC3660</a>] in conjunction with the standard
   audio MIME subtype "RED":

         L: a:G729;RED;PCMU;parityfec, gpmd/gpmd:"PCMU vbd=yes",
                                       fmtp:"RED PCMU/parityfec"







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      The corresponding media description might look like

         v=0
         c=IN IP4 192.0.2.0
         m=audio 49170 RTP/AVP 18 96 97 98
         a=rtpmap:96 RED/8000
         a=fmtp:96 97/98
         a=rtpmap:97 PCMU/8000
         a=gpmd:97 vbd=yes
         a=rtpmap:98 parityfec/8000

   The FM package states that the "fmtp" LocalConnectionOption MUST be
   returned when auditing capabilities.  Applying this to VBD,
   redundancy and FEC might result in

         A: a:PCMU, p:10-40, e:on, s:on,
            m:sendonly;recvonly;sendrecv;inactive
         A: a:RED;PCMU;parityfec, p:10-40, e:on, s:off,
            m:sendonly;recvonly;sendrecv;inactive,
            gpmd/gpmd:"PCMU vbd=yes",
            fmtp:"RED PCMU/parityfec"

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Use of Fax Package with VBD</span>

   The MGCP Fax (FXR) package [<a href="./rfc5347" title="&quot;Media Gateway Control Protocol Fax Package&quot;">RFC5347</a>] is used by a Call Agent to
   authorize fax handling, including Call Agent controlled T.38 and
   gateway procedures such as V.152.  With the FXR package, VBD falls
   into one of two categories: "special fax handling" as part of the
   gateway procedure (resulting in the "gwfax" event), or "no special
   fax handling" as part of the gateway and Off procedures (resulting in
   the "nopfax" event).  In order for a VBD procedure to fall into the
   "special fax handling" category, support for it MUST be negotiated
   with the other side by passing and recognizing relevant parameters
   via the LocalConnectionDescriptor and RemoteConnectionDescriptor.

   A gateway controlled VBD procedure such as V.152 MUST fall into the
   category of gateway controlled mode involving "special fax handling".
   The resulting "gwfax" event is what informs the Call Agent to refrain
   from issuing media handling instructions that could otherwise have a
   negative impact on the gateway procedure.











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   Consider the following example (with shorthand SDP notation):

           CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           C: 1
           M: sendrecv
           L: a:G729;PCMU, gpmd/gpmd:"PCMU vbd=yes", fxr/fx:t38;gw
           X: 1
           R: fxr/t38, fxr/gwfax, fxr/nopfax

           v=0
           c=IN IP4 192.0.2.1
           m=audio 3456 RTP/AVP 18 96
           a=rtpmap:96 PCMU/8000
           a=gpmd:96 vbd=yes

           200 2000 OK
           I: 1

           v=0
           c=IN IP4 192.0.2.2
           m=audio 1296 RTP/AVP 18 96
           a=rtpmap:96 PCMU/8000
           a=gpmd:96 vbd=yes

   The RemoteConnectionDescriptor does not indicate support for "image/
   t38" as a latent capability [<a href="./rfc3407" title="&quot;Session Description Protocol (SDP) Simple Capability Declaration&quot;">RFC3407</a>].  Consequently, the gateway
   will not initiate the T.38 strict fax procedure, "t38", upon
   detecting fax stimulus (i.e., CNG, V.21 flags, etc.).  However, the
   two endpoints did successfully negotiate a gateway controlled VBD
   procedure (e.g., V.152); therefore, a gateway controlled mode
   involving "special fax handling" is used.  The "gwfax(start)" event
   will be generated upon detecting VBD (including fax) stimulus.

   A Call Agent can express a preference for a gateway procedure
   involving "special fax handling" over a T.38 procedure (strict or
   loose).  For example,

         L: fxr/fx:gw;t38

   and

         L: fxr/fx:gw;t38-loose

   However, with the existing syntax of the FXR package, a Call Agent
   cannot express a preference for one gateway procedure over another,
   each with possibly different preferences relative to a T.38
   procedure.




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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   The FXR package allows a gateway to implement additional fax handling
   parameters.  We define just such a parameter by qualifying the
   existing "gw" parameter with a list of one or more MIME types:

        Gateway  = "gw[" mimeType 0*("|" mimeType) "]"
        mimeType = mimeMediaType "/" mimeSubType
        ; mimeMediaType and mimeSubType from
        ;   <a href="http://www.iana.org/assignments/media-types/">http://www.iana.org/assignments/media-types/</a>

   By qualifying the "gw" parameter with a list of MIME types, we narrow
   the scope of the gateway procedure.  Consider the following examples
   in which the Call Agent authorizes the use of a gateway controlled
   fax handling procedure:

       - involving "image/t38" (e.g., T.38oUDPTL, T.38oTCP):

         L: a:G729, fxr/fx:gw[image/t38]

       - involving VBD (e.g., PCMU and V.152):

         L: a:G729;PCMU, gpmd/gpmd:"PCMU vbd=yes", fxr/fx:gw[audio/PCMU]

       - involving VBD with redundancy (e.g., PCMU, V.152,
         and <a href="./rfc2198">RFC 2198</a>):

         L: a:G729;RED;PCMU, fmtp:"RED PCMU/PCMU",
            gpmd/gpmd:"PCMU vbd=yes", fxr/fx:gw[audio/RED|audio/PCMU]

   Only "special fax handling" involving one of the specified MIME types
   is authorized.  Support for "special fax handling" involving one of
   the specified MIME types MUST be negotiated, or this "instance" of
   the gateway procedure is not initiated.  Consider the following
   example in which the Call Agent authorizes the use of a gateway
   controlled fax handling procedure:

       - involving "audio/t38" (e.g., T.38oRTP):

         L: a:G729;t38, fxr/fx:gw[audio/t38]

   In this example, the call will fail if the gateway fails to negotiate
   "audio/t38".

   The "fx" LocalConnectionOption MAY now involve multiple instances of
   the "gw" parameter, each with a different list of MIME types.  In
   order to authorize "no special fax handling", the Call Agent MUST
   include the "gw" parameter without a MIME type, or the "off"





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   parameter.  The instance of the "gw" parameter without a MIME type
   should appear as the last instance of the "gw" parameter.  In the
   following example,

      L: a:G729;PCMU, fxr/fx:gw[image/t38];gw

   the Call Agent authorizes the use of, and expresses a preference for,

   1.  Gateway controlled image/t38 (e.g., T.38oUDPTL)

   2.  Any other gateway procedure with "special fax handling"

   3.  No special fax handling (this is a function of the "fxr/fx:gw"
       parameter as defined in <a href="#section-2.1">Section 2.1</a> of the MGCP Fax (FXR) package
       [<a href="./rfc5347" title="&quot;Media Gateway Control Protocol Fax Package&quot;">RFC5347</a>])

   If present, the "off" parameter should appear as the last parameter.
   In the following example,

      L: a:G729;PCMU;t38, fxr/fx:gw[audio/t38];off

   the Call Agent authorizes the use of, and expresses a preference for,

   1.  Gateway controlled audio/t38 (e.g., T.38oRTP)

   2.  No special fax handling

   We can express relative preferences for different gateway controlled
   fax handling procedures, not only with respect to one another, but
   with respect to T.38 procedures.  Consider the following preferential
   list of fax handling procedures:

   1.  Gateway controlled audio/t38 (e.g., T.38oRTP)

   2.  Gateway controlled image/t38 (e.g., T.38oUDPTL)

   3.  Call Agent controlled image/t38

   4.  Gateway controlled VBD with redundancy (e.g., PCMU, V.152, and
       <a href="./rfc2198">RFC 2198</a>)

   5.  Gateway controlled VBD without redundancy (e.g., PCMU and V.152)

   6.  Any other gateway procedure with "special fax handling"

   7.  No special fax handling (this is a function of the "fxr/fx:gw"
       parameter as defined in <a href="#section-2.1">Section 2.1</a> of the MGCP Fax (FXR) package
       [<a href="./rfc5347" title="&quot;Media Gateway Control Protocol Fax Package&quot;">RFC5347</a>])



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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   This would be expressed as

     L: a:G729;PCMU;t38;RED;PCMU,
        gpmd/gpmd:"PCMU:2 vbd=yes",
        fmtp:"RED PCMU:2/PCMU:2",
        fxr/fx:gw[audio/t38|image/t38];t38;gw[audio/RED|audio/PCMU:2];gw

   Note that the bracketed form of the "gw" parameter is NOT defined as
   part of the VBD package.  The bracketed form of the "gw" parameter is
   defined as an extension to the FXR package.  Gateways that implement
   the bracketed form of the "gw" parameter MUST return this form of the
   parameter when capabilities are audited as illustrated by the
   following example:

        A: fxr/fx:t38;t38-loose;gw[audio/t38|image/t38];gw;off

   Support for the bracketed "gw" parameter MAY be spread across
   multiple capability lines:

        A: a:RED;PCMU, p:10-40, e:on, s:off,
           m:sendonly;recvonly;sendrecv;inactive,
           gpmd/gpmd:"PCMU vbd=yes",
             fmtp:"RED PCMU/PCMU",
             fxr/fx:gw[audio/RED|audio/PCMU]
        A: a:t38, fxr/fx:gw[audio/t38]
        A: a:image/t38, fxr/fx:t38;t38-loose;gw[image/t38]

   A Call Agent SHOULD only attempt to leverage the bracketed form of
   the "gw" parameter in conjunction with an endpoint that indicates
   support for the bracketed syntax as part of its capabilities.

   Call Agents and gateways that do not support this form of the "gw"
   parameter MUST ignore the bracketed MIME type information consistent
   with the MGCP grammar [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>].

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Call Flow Examples</span>

   In this section, we provide two call flow examples.  The first one
   illustrates a modem call under gateway control using V.152.  The
   second one illustrates a fax call under gateway control using V.152
   and Call Agent controlled T.38.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Modem Call with Gateway Controlled VBD</span>

   In this example, both sides support gateway controlled VBD using
   V.152 with redundancy.  We assume that the originating and
   terminating Call Agents communicate via the Session Initiation
   Protocol (SIP) [<a href="./rfc3261" title="&quot;SIP: Session Initiation Protocol&quot;">RFC3261</a>]:



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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


    ------------------------------------------------------------------
   | #|     GW-o      |     CA-o      |      CA-t     |      GW-t     |
   |==|===============|===============|===============|===============|
   | 1|             &lt;-|CRCX           |               |               |
   | 2|     200(sdp-o)|-&gt;             |               |               |
   | 3|               |  INVITE(sdp-o)|-&gt;             |               |
   | 4|               |               |    CRCX(sdp-o)|-&gt;             |
   | 5|               |               |             &lt;-|200 (sdp-t)    |
   | 6|               |             &lt;-|200(sdp-t)     |               |
   | 7|             &lt;-|MDCX(sdp-t)    |               |               |
   | 8|            200|-&gt;             |               |               |
   |--|---------------|---------------|---------------|---------------|
   | 9|               |               |               |&lt;- ANS/T.30 CED|
   |10|               |               |           &lt;- NTFY(gwvbd start)|
   |11|               |               |            200|-&gt;             |
   |12|NTFY(gwvbd start) -&gt;           |               |               |
   |13|             &lt;-|200            |               |               |
   |--|---------------|---------------|---------------|---------------|
   |14|               |               |               | (modem ends)  |
   |15|               |               |           &lt;- NTFY(gwvbd stop) |
   |16|               |               |            200|-&gt;             |
   |17|NTFY(gwvbd stop) -&gt;            |               |               |
   |18|             &lt;-|200            |               |               |
    ------------------------------------------------------------------

   Step 1:

      The Call Agent issues a CreateConnection command to the gateway,
      instructing it to use G.729 media encoding and to notify it of the
      "gwvbd" and "nopvbd" events.  The Call Agent authorizes the
      negotiation of G.711u as a VBD codec with a redundancy level
      of one:

      CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
      C: 1
      L: a:G729;RED;PCMU, gpmd/gpmd:"PCMU vbd=yes", fmtp:"RED PCMU/PCMU"
      M: recvonly
      R: vbd/gwvbd, vbd/nopvbd
      X: 1
      Q: process, loop











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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   Step 2:

      The gateway acknowledges the command and includes SDP with codec
      information as well as V.152 and redundancy information:

           200 1000 OK
           I:1

           v=0
           o=- 25678 753849 IN IP4 192.0.2.1
           s=-
           c=IN IP4 192.0.2.1
           t=0 0
           m=audio 3456 RTP/AVP 18 96 97
           a=rtpmap:96 RED/8000
           a=fmtp:96 97/97
           a=rtpmap:97 PCMU/8000
           a=gpmd:97 vbd=yes

   Step 3:

      The originating Call Agent sends a SIP INVITE message with the SDP
      to the terminating Call Agent.

   Step 4:

      The terminating Call Agent issues a CreateConnection command to
      the terminating gateway, instructing it to use G.729 media
      encoding and to notify it of the "gwvbd" and "nopvbd" events.
      Again, the Call Agent authorizes the negotiation of G.711u as a
      VBD codec with a redundancy level of one:

      CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
      C: 2
      L: a:G729;RED;PCMU, gpmd/gpmd:"PCMU vbd=yes", fmtp:"RED PCMU/PCMU"
      M: sendrecv
      R: vbd/gwvbd, vbd/nopvbd
      X: 20
      Q: process, loop












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      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=-
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 3456 RTP/AVP 18 96 97
      a=rtpmap:96 RED/8000
      a=fmtp:96 97/97
      a=rtpmap:97 PCMU/8000
      a=gpmd:97 vbd=yes

   Step 5:

      The terminating gateway supports V.152 and redundancy, and the
      RemoteConnectionDescriptor included indicates that the other side
      supports V.152 and redundancy.  The terminating gateway sends back
      a success response with its SDP, which also includes V.152 and
      redundancy information:

           200 2000 OK
           I:2

           v=0
           o=- 25678 753849 IN IP4 192.0.2.2
           s=-
           c=IN IP4 192.0.2.2
           t=0 0
           m=audio 1296 RTP/AVP 18 96 97
           a=rtpmap:96 RED/8000
           a=fmtp:96 97/97
           a=rtpmap:97 PCMU/8000
           a=gpmd:97 vbd=yes

   Step 6:

      The terminating Call Agent sends back a SIP 200 OK response to the
      originating Call Agent, which in turn sends a SIP ACK (not shown).














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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   Step 7:

      The originating Call Agent in turn sends a ModifyConnection
      command to the originating gateway:

           MDCX 1001 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
           C: 1
           I: 1
           M: sendrecv

           v=0
           o=- 25678 753849 IN IP4 192.0.2.2
           s=-
           c=IN IP4 192.0.2.2
           t=0 0
           m=audio 1296 RTP/AVP 18 96 97
           a=rtpmap:96 RED/8000
           a=fmtp:96 97/97
           a=rtpmap:97 PCMU/8000
           a=gpmd:97 vbd=yes

      Since the RemoteConnectionDescriptor indicates that the other side
      supports V.152 and redundancy, the gateway will in fact be able to
      use the gateway controlled VBD procedure with redundancy.  Had
      there not been any support for V.152 in the
      RemoteConnectionDescriptor, then this command would still have
      succeeded; however, there would be no negotiated procedure for VBD
      handling.

   Step 8:

      The gateway acknowledges the command.  At this point, a call is
      established using G.729 encoding, and if a VBD call is detected,
      the gateway controlled VBD procedure will be initiated.

   Steps 9-10:

      A modem call now occurs.  The terminating gateway detects a T.30
      CED tone (a.k.a. V.25 ANS) in the GSTN-to-IP direction and begins
      transmitting RTP packets with the negotiated redundant VBD payload
      type (96).

      The "gwvbd(start)" event occurs, and a Notify command is sent to
      the Call Agent:

           NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           O: vbd/gwvbd(start, rc=ANS, codec=audio/RED, coord=v152ptsw)
           X: 20



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   Step 11:

      The Call Agent acknowledges the Notify command:

           200 2500 OK

   Step 12:

      Upon receiving an RTP packet with the redundant VBD payload type
      (96), the originating gateway begins transmitting RTP packets with
      the redundant VBD payload type.

      The "gwvbd(start)" event occurs, and a Notify command is sent to
      the Call Agent:

           NTFY 1500 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
           O: vbd/gwvbd(start, rc=PTSW, codec=audio/RED)
           X: 1

   Step 13:

      The Call Agent acknowledges the Notify command:

           200 1500 OK

   Steps 14-15:

      The modem call ends.  The terminating gateway detects
      bidirectional silence and begins transmitting RTP packets with the
      negotiated audio payload type (18).

      The "gwvbd(stop)" event occurs, and a Notify command is sent to
      the Call Agent:

           NTFY 2501 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           O: vbd/gwvbd(stop, rc=SIL, codec=audio/G729)
           X: 20

   Step 16:

      The Call Agent acknowledges the Notify command:

           200 2501 OK








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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   Step 17:

      Upon receiving an RTP packet with the audio payload type (18), the
      originating gateway begins transmitting RTP packets with the audio
      payload type.

      The "gwvbd(stop)" event occurs, and a Notify command is sent to
      the Call Agent:

           NTFY 1501 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
           O: vbd/gwvbd(stop, rc=PTSW, codec=audio/G729)
           X: 1

   Step 18:

      The Call Agent acknowledges the Notify command:

           200 1501 OK

   The modem call is now over.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Fax Call with Gateway Controlled VBD and Call Agent Controlled</span>
<span class="h3">      T.38</span>

   In this example, both sides support gateway controlled VBD using
   V.152 with redundancy and Call Agent controlled T.38.  We assume that
   the originating and terminating Call Agent communicate via the
   Session Initiation Protocol (SIP) [<a href="./rfc3261" title="&quot;SIP: Session Initiation Protocol&quot;">RFC3261</a>]:























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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


    ------------------------------------------------------------------
   | #|     GW-o      |     CA-o      |      CA-t     |      GW-t     |
   |==|===============|===============|===============|===============|
   | 1|             &lt;-|CRCX           |               |               |
   | 2|     200(sdp-o)|-&gt;             |               |               |
   | 3|               |  INVITE(sdp-o)|-&gt;             |               |
   | 4|               |               |    CRCX(sdp-o)|-&gt;             |
   | 5|               |               |             &lt;-|200 (sdp-t)    |
   | 6|               |             &lt;-|200(sdp-t)     |               |
   | 7|             &lt;-|MDCX(sdp-t)    |               |               |
   | 8|            200|-&gt;             |               |               |
   |--|---------------|---------------|---------------|---------------|
   | 9|               |               |               |&lt;- ANS/T.30 CED|
   |10|               |               |           &lt;- NTFY(gwvbd start)|
   |11|               |               |            200|-&gt;             |
   |12|NTFY(gwvbd start) -&gt;           |               |               |
   |13|             &lt;-|200            |               |               |
   |14|               |               |               &lt;- V.21 Preamble|
   |15|               |               |             &lt;- NTFY(t38 start)|
   |16|               |               |            200|-&gt;             |
   |17|               |               |      MDCX(t38)|-&gt;             |
   |18|               |               |             &lt;-|200(sdp-t2)    |
   |19|               |             &lt;-|INVITE(sdp-t2) |               |
   |20|             &lt;-|MDCX(sdp-t2)   |               |               |
   |21|    200(sdp-o2)|-&gt;             |               |               |
   |22|               |    200(sdp-o2)|-&gt;             |               |
   |23|               |               |   MDCX(sdp-o2)|-&gt;             |
   |24|               |               |             &lt;-|200            |
   |25| V.21 Preamble |-&gt;             |               |               |
   |26|NTFY(t38 start)|-&gt;             |               |               |
   |27|             &lt;-|200            |               |               |
   |--|---------------|---------------|---------------|---------------|
   |28|               |               |               |   (fax ends)  |
   |29|               |               |             &lt;-|NTFY(t38 stop) |
   |30|               |               |            200|-&gt;             |
   |31|NTFY(t38 stop) |-&gt;             |               |               |
   |32|             &lt;-|200            |               |               |
    ------------------------------------------------------------------

   Step 1:

      The Call Agent issues a CreateConnection command to the gateway,
      instructing it to use G.729 media encoding and to use either the
      strict T.38 procedure or the gateway procedure.  Consequently, the
      Call Agent requests notification of the "t38", "gwfax", "gwvbd",
      and "nopvbd" events.  The Call Agent authorizes the negotiation of
      G.711u as a VBD codec with a redundancy level of one:




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     CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
     C: 1
     L: a:G729;RED;PCMU, gpmd/gpmd:"PCMU vbd=yes", fmtp:"RED PCMU/PCMU",
        fxr/fx:t38;gw
     M: recvonly
     R: fxr/t38, fxr/gwfax, vbd/gwvbd, vbd/nopvbd
     X: 1
     Q: process, loop

   Step 2:

      The gateway acknowledges the command and includes SDP with codec
      information as well as capability, V.152, and redundancy
      information:

           200 1000 OK
           I:1

           v=0
           o=- 25678 753849 IN IP4 192.0.2.1
           s=-
           c=IN IP4 192.0.2.1
           t=0 0
           a=pmft: T38
           m=audio 3456 RTP/AVP 18 96 97
           a=rtpmap:96 RED/8000
           a=fmtp:96 97/97
           a=rtpmap:97 PCMU/8000
           a=gpmd:97 vbd=yes
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cdsc: 4 image udptl t38

      Note that V.152 requires the use of the session-level "a=pmft" SDP
      attribute in order to express a preference for T.38 over V.152 for
      fax handling.

   Step 3:

      The originating Call Agent sends a SIP INVITE message with the SDP
      to the terminating Call Agent.

   Step 4:

      The terminating Call Agent issues a CreateConnection command to
      the terminating gateway, instructing it to use G.729 media
      encoding and to use either the strict T.38 procedure or the
      gateway procedure.  Consequently, the Call Agent requests



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      notification of the "t38", "gwfax", "gwvbd", and "nopvbd" events.
      Again, the Call Agent authorizes the negotiation of G.711u as a
      VBD codec with a redundancy level of one:

     CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
     C: 2
     L: a:G729;RED;PCMU, gpmd/gpmd:"PCMU vbd=yes", fmtp:"RED PCMU/PCMU",
        fxr/fx:t38;gw
     M: sendrecv
     R: fxr/t38, fxr/gwfax, vbd/gwvbd, vbd/nopvbd
     X: 20
     Q: process, loop

     v=0
     o=- 25678 753849 IN IP4 192.0.2.1
     s=-
     c=IN IP4 192.0.2.1
     t=0 0
     a=pmft: T38
     m=audio 3456 RTP/AVP 18 96 97
     a=rtpmap:96 RED/8000
     a=fmtp:96 97/97
     a=rtpmap:97 PCMU/8000
     a=gpmd:97 vbd=yes
     a=sqn: 0
     a=cdsc: 1 audio RTP/AVP 18 96 97
     a=cdsc: 4 image udptl t38

   Step 5:

      The terminating gateway supports T.38, and the
      RemoteConnectionDescriptor included indicates that the other side
      supports T.38 as well, so the strict T.38 Call Agent controlled
      procedure requested can be used.  The terminating gateway supports
      V.152 and redundancy, and the RemoteConnectionDescriptor included
      indicates that the other side supports V.152 and redundancy, so
      gateway controlled VBD using V.152 and redundancy can be used for
      modem and text transmissions.  The terminating gateway sends back
      a success response with its SDP, which also includes capability,
      V.152, and redundancy information:

           200 2000 OK
           I:2

           v=0
           o=- 25678 753849 IN IP4 192.0.2.2
           s=-
           c=IN IP4 192.0.2.2



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           t=0 0
           a=pmft: T38
           m=audio 1296 RTP/AVP 18 96 97
           a=rtpmap:96 RED/8000
           a=fmtp:96 97/97
           a=rtpmap:97 PCMU/8000
           a=gpmd:97 vbd=yes
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cdsc: 4 image udptl t38

   Step 6:

      The terminating Call Agent sends back a SIP 200 OK response to the
      originating Call Agent, which in turn sends a SIP ACK (not shown).

   Step 7:

      The originating Call Agent in turn sends a ModifyConnection
      command to the originating gateway:

           MDCX 1001 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
           C: 1
           I: 1
           M: sendrecv

           v=0
           o=- 25678 753849 IN IP4 192.0.2.2
           s=-
           c=IN IP4 192.0.2.2
           t=0 0
           a=pmft: T38
           m=audio 1296 RTP/AVP 18 96 97
           a=rtpmap:96 RED/8000
           a=fmtp:96 97/97
           a=rtpmap:97 PCMU/8000
           a=gpmd:97 vbd=yes
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cdsc: 4 image udptl t38

      The ModifyConnection command does not repeat the
      LocalConnectionOptions sent previously.  As far as fax handling is
      concerned, the gateway therefore attempts to continue using the
      current fax handling procedure, i.e., strict Call Agent controlled
      T.38.  Since the capability information indicates that the other
      side supports T.38, the gateway will in fact be able to use the
      strict Call Agent controlled T.38 procedure.  Since the



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      RemoteConnectionDescriptor indicates that the other side supports
      V.152 and redundancy, the gateway will in fact be able to use the
      V.152 VBD procedure with redundancy.

   Step 8:

      The gateway acknowledges the command.  At this point, a call is
      established using G.729 encoding, and if a fax call is detected,
      the Call Agent controlled T.38 procedure will be initiated.  If a
      modem or text call is detected, the V.152 VBD procedure will be
      initiated.

   Steps 9-10:

      The terminating gateway detects the T.30 CED tone (a.k.a. V.25
      ANS).  Since both fax and modem calls can start with this
      sequence, it is not possible to determine that this is a fax call
      until step 14, where the V.21 fax preamble is detected.  The
      terminating gateway begins transmitting RTP packets with the
      negotiated redundant VBD payload type (96).

      The "gwvbd(start)" event occurs, and a Notify command is sent to
      the Call Agent:

           NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           O: vbd/gwvbd(start, rc=ANS, codec=audio/RED, coord=v152ptsw)
           X: 20

   Step 11:

      The Call Agent acknowledges the Notify command:

           200 2500 OK

   Step 12:

      Upon receiving an RTP packet with the redundant VBD payload type
      (96), the originating gateway begins transmitting RTP packets with
      the redundant VBD payload type.

      The "gwvbd(start)" event occurs, and a Notify command is sent to
      the Call Agent:

           NTFY 1500 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
           O: vbd/gwvbd(start, rc=PTSW, codec=audio/RED)
           X: 1





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   Step 13:

      The Call Agent acknowledges the Notify command:

           200 1500 OK

   Steps 14-15:

      The terminating gateway detects the V.21 fax preamble.

      The terminating gateway is using the Call Agent controlled T.38
      strict procedure for fax calls, so the "t38(start)" event occurs,
      and a Notify command is sent to the Call Agent:

           NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           O: fxr/t38(start)
           X: 20

   Step 16:

      The Call Agent acknowledges the Notify command:

           200 2500 OK

   Step 17:

      The Call Agent then instructs the terminating gateway to change to
      using the "image/t38" MIME type instead:

           MDCX 2002 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           C: 2
           I: 2
           L: a:image/t38
           R: fxr/t38
           X: 21

      Note that the Call Agent is no longer requesting notification of
      the "gwvbd" event.













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   Step 18:

      The terminating gateway sends back a success response with its
      SDP, which also includes the "image/t38" media description:

           200 2002 OK

           v=0
           o=- 25678 753850 IN IP4 192.0.2.2
           s=-
           c=IN IP4 192.0.2.2
           t=0 0
           m=image 1296 udptl t38
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cpar: a=rtpmap:96 RED/8000
           a=cpar: a=fmtp:96 97/97
           a=cpar: a=rtpmap:97 PCMU/8000
           a=cpar: a=gpmd:97 vbd=yes
           a=cdsc: 4 image udptl t38

      The gwvbd procedure ends due to the media type change.  The
      "gwvbd(stop)" event notification would normally be sent at this
      point; however, the Call Agent is no longer requesting
      notification of the "gwvbd" event.  The Call Agent would have
      inferred from the "t38(start)" event that the gwvbd procedure
      ended.

   Step 19:

      The terminating Call Agent sends a re-INVITE to the originating
      Call Agent with the updated SDP.

   Step 20:

      The originating Call Agent then sends a ModifyConnection command
      to the originating gateway:

           MDCX 1003 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
           C: 1
           I: 1
           R: fxr/t38
           X: 2

           v=0
           o=- 25678 753850 IN IP4 192.0.2.2
           s=-
           c=IN IP4 192.0.2.2



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           t=0 0
           m=image 1296 udptl t38
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cpar: a=rtpmap:96 RED/8000
           a=cpar: a=fmtp:96 97/97
           a=cpar: a=rtpmap:97 PCMU/8000
           a=cpar: a=gpmd:97 vbd=yes
           a=cdsc: 4 image udptl t38

   Step 21:

      The originating gateway changes to T.38 and sends back a success
      response with the updated SDP:

           200 1003 OK

           v=0
           o=- 25678 753850 IN IP4 192.0.2.1
           s=-
           c=IN IP4 192.0.2.1
           t=0 0
           m=image 3456 udptl t38
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cpar: a=rtpmap:96 RED/8000
           a=cpar: a=fmtp:96 97/97
           a=cpar: a=rtpmap:97 PCMU/8000
           a=cpar: a=gpmd:97 vbd=yes
           a=cdsc: 4 image udptl t38

      Again, the gwvbd procedure ends due to the media type change.  The
      "gwvbd(stop)" event notification would normally be sent at this
      point; however, the Call Agent is no longer requesting
      notification of the "gwvbd" event.

   Step 22:

      The originating Call Agent sends a SIP 200 OK response with the
      updated SDP to the terminating Call Agent, which in turn sends a
      SIP ACK (not shown).










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   Step 23:

      The terminating Call Agent sends a ModifyConnection with the
      updated SDP to the terminating gateway:

           MDCX 2002 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
           C: 2
           I: 2

           v=0
           o=- 25678 753850 IN IP4 192.0.2.1
           s=-
           c=IN IP4 192.0.2.1
           t=0 0
           m=image 3456 udptl t38
           a=sqn: 0
           a=cdsc: 1 audio RTP/AVP 18 96 97
           a=cpar: a=rtpmap:96 RED/8000
           a=cpar: a=fmtp:96 97/97
           a=cpar: a=rtpmap:97 PCMU/8000
           a=cpar: a=gpmd:97 vbd=yes
           a=cdsc: 4 image udptl t38

   Steps 24-32:

      These steps correspond to the Call Agent controlled T.38 strict
      procedure as defined in the MGCP Fax (FXR) package [<a href="./rfc5347" title="&quot;Media Gateway Control Protocol Fax Package&quot;">RFC5347</a>].

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Security Considerations</span>

   This document defines two new packages, both of which have security
   considerations in two areas:

   1.  MGCP signaling message security

   2.  Media stream security

   From an MGCP signaling security point of view, the MGCP VBD and GPMD
   packages define extensions to the basic MGCP signaling specification
   in accordance with the procedures specified in MGCP [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>], and
   hence the MGCP signaling security considerations and recommendations
   provided in <a href="./rfc3435#section-5">Section&nbsp;5 of [RFC3435]</a> (namely the use of IPsec) apply
   here as well.  Lack of MGCP signaling integrity protection can in
   general be detrimental to any use of MGCP, and the two packages
   defined here do not change that.  From a confidentiality point of
   view, the VBD package is not believed to convey any vulnerable or
   privacy-sensitive information.  The GPMD package is slightly
   different inasmuch as it does not define any specific parameters that



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   are believed to require confidentiality; however, it is a generic
   parameter that can carry any codec parameter information, and hence
   it is possible that confidential information is conveyed through this
   parameter.  If confidentiality of any such potential information is a
   concern, confidentiality protection of the MGCP signaling MUST be
   provided as well.  It should be noted that <a href="./rfc5406#section-8">Section&nbsp;8 of [RFC5406]</a>
   provides considerations for specifying the use of IPsec that are
   above and beyond those provided in [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>]; however, given that the
   use of IPsec for MGCP applies to all of MGCP, and not just the MGCP
   VBD and GPMD packages, we do not specify such additional detail here.

   From a media stream security point of view, the MGCP VBD and GPMD
   packages again define extensions that rely on the general use of
   media streams defined in MGCP [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>], and hence the MGCP media
   stream security considerations and recommendations provided in
   <a href="./rfc3435#section-5.1">Section&nbsp;5.1 of [RFC3435]</a> apply here as well.  Lack of media stream
   security can in general be detrimental to any media stream
   established via MGCP, and the two packages defined here do not change
   that.  Confidentiality concerns apply as for any other media stream.
   Integrity concerns are further compounded by the GPMD package's use
   of payload type switching, state signaling events, and media stream
   in-band triggers to drive overall Voiceband Data operation: Integrity
   protection with replay protection MUST be used to counter these
   threats.

   Ideally, there would be a single mandatory-to-implement media stream
   security mechanism to provide this integrity protection, and in
   theory there is, since MGCP [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>] defines a media stream security
   mechanism.  However, the standard MGCP media stream security
   mechanism defined in [<a href="./rfc3435" title="&quot;Media Gateway Control Protocol (MGCP) Version 1.0&quot;">RFC3435</a>] relies on the encryption key ("k=")
   field defined in the original SDP specification [<a href="./rfc2327" title="&quot;SDP: Session Description Protocol&quot;">RFC2327</a>], the use of
   which is no longer recommended in the current SDP specification
   [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].  In practice, this mechanism has also seen very limited
   implementation, and hence there is not much value in relying on it.
   Still, the integrity protection requirement remains, and there are
   several different ways this can be achieved:

   Secure RTP:   For RTP-based media streams, the use of Secure RTP
      [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>] with an associated key management mechanism is generally
      preferred at the time of this writing; however, such a mechanism
      has currently not been defined for MGCP.

   PacketCable Security:   The PacketCable Network-Based Call Signaling
      Protocol [<a href="#ref-NCS" title="&quot;PacketCable(TM) 1.5 Specifications: Network-Based Call Signaling Protocol, PKT-SP-NCS1.5-I03- 070412&quot;">NCS</a>] defines another media stream security mechanism
      that is generally supported by PacketCable-compliant
      implementations.  Implementations targeted for those environments
      SHOULD implement this security mechanism.




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   Lower-Level Security:   In the absence of a common media stream
      security mechanism supported by both endpoints, a lower-level
      security mechanism, e.g., IPsec, MUST be used.  Note that since
      there is no inherent MGCP signaling support for such a lower-level
      security mechanism, it MUST be configured by other means.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  IANA Considerations</span>

   The IANA has registered the following MGCP packages:

      Package Title                               Name     Version
      -------------                               ----     -------
      Voiceband Data                              VBD      0
      General-Purpose Media Descriptor Parameter  GPMD     0

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Acknowledgements</span>

   Several people have contributed to the development of the MGCP VBD
   and GPMD packages and the use of the MIME subtypes "RED" and
   "parityfec" with the FM package for VBD with redundancy and FEC.  In
   particular, the authors would like to thank Flemming Andreasen, John
   Atkinson, Bill Foster, and the CableLabs PacketCable TGCP/NCS focus
   team for their contributions.  Many thanks to Billy Hare for doing a
   thorough review of this document.

   Joe Stone and Rajesh Kumar are the main authors of this document;
   security considerations and final editor role were provided by
   Flemming Andreasen.  Sandeep Sharma was editor on earlier versions of
   the document.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  References</span>

<span class="h3"><a class="selflink" id="section-13.1" href="#section-13.1">13.1</a>.  Normative References</span>

   [<a id="ref-H2482">H2482</a>]    International Telecommunication Union - Telecommunication
              Standardization Sector, "Gateway control protocol:
              Facsimile, text conversation and call discrimination
              packages", ITU-T Recommendation H.248.2, November 2000.

   [<a id="ref-NCS">NCS</a>]      CableLabs(R), "PacketCable(TM) 1.5 Specifications:
              Network-Based Call Signaling Protocol, PKT-SP-NCS1.5-I03-
              070412", April 2007.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.






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   [<a id="ref-RFC2198">RFC2198</a>]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
              Parisis, "RTP Payload for Redundant Audio Data", <a href="./rfc2198">RFC 2198</a>,
              September 1997.

   [<a id="ref-RFC3407">RFC3407</a>]  Andreasen, F., "Session Description Protocol (SDP) Simple
              Capability Declaration", <a href="./rfc3407">RFC 3407</a>, October 2002.

   [<a id="ref-RFC3435">RFC3435</a>]  Andreasen, F. and B. Foster, "Media Gateway Control
              Protocol (MGCP) Version 1.0", <a href="./rfc3435">RFC 3435</a>, January 2003.

   [<a id="ref-RFC3550">RFC3550</a>]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, <a href="./rfc3550">RFC 3550</a>, July 2003.

   [<a id="ref-RFC3660">RFC3660</a>]  Foster, B. and F. Andreasen, "Basic Media Gateway Control
              Protocol (MGCP) Packages", <a href="./rfc3660">RFC 3660</a>, December 2003.

   [<a id="ref-RFC4566">RFC4566</a>]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", <a href="./rfc4566">RFC 4566</a>, July 2006.

   [<a id="ref-RFC4733">RFC4733</a>]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", <a href="./rfc4733">RFC 4733</a>,
              December 2006.

   [<a id="ref-RFC4734">RFC4734</a>]  Schulzrinne, H. and T. Taylor, "Definition of Events for
              Modem, Fax, and Text Telephony Signals", <a href="./rfc4734">RFC 4734</a>,
              December 2006.

   [<a id="ref-RFC5109">RFC5109</a>]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", <a href="./rfc5109">RFC 5109</a>, December 2007.

   [<a id="ref-RFC5234">RFC5234</a>]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>,
              January 2008.

   [<a id="ref-RFC5347">RFC5347</a>]  Andreasen, F. and D. Hancock, "Media Gateway Control
              Protocol Fax Package", <a href="./rfc5347">RFC 5347</a>, October 2008.

   [<a id="ref-V1501">V1501</a>]    International Telecommunication Union - Telecommunication
              Standardization Sector, "Modem-over-IP networks:
              Procedures for the end-to-end connection of V-series
              DCEs", ITU-T Recommendation V.150.1, January 2003.








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<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


   [<a id="ref-V1501A1">V1501A1</a>]  International Telecommunication Union - Telecommunication
              Standardization Sector, "Modem-over-IP networks:
              Procedures for the end-to-end connection of V-series DCEs,
              Amendment 1: Modification to SSE reason identifier codes
              to support voice band data and text relay",
              ITU-T Recommendation V.150.1 Amendment 1, January 2005.

   [<a id="ref-V152">V152</a>]     International Telecommunication Union - Telecommunication
              Standardization Sector, "Procedures for supporting Voice-
              Band Data over IP Networks", ITU-T Recommendation V.152,
              January 2005.

<span class="h3"><a class="selflink" id="section-13.2" href="#section-13.2">13.2</a>.  Informative References</span>

   [<a id="ref-RFC2327">RFC2327</a>]  Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", <a href="./rfc2327">RFC 2327</a>, April 1998.

   [<a id="ref-RFC3261">RFC3261</a>]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>,
              June 2002.

   [<a id="ref-RFC3711">RFC3711</a>]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              <a href="./rfc3711">RFC 3711</a>, March 2004.

   [<a id="ref-RFC5406">RFC5406</a>]  Bellovin, S., "Guidelines for Specifying the Use of IPsec
              Version 2", <a href="https://www.rfc-editor.org/bcp/bcp146">BCP 146</a>, <a href="./rfc5406">RFC 5406</a>, February 2009.

   [<a id="ref-T38">T38</a>]      International Telecommunication Union - Telecommunication
              Standardization Sector, "Procedures for real-time Group 3
              facsimile communication over IP networks",
              ITU-T Recommendation T.38, April 2004.


















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-47" ></span>
<span class="grey"><a href="./rfc6498">RFC 6498</a>                    MGCP VBD Package               February 2012</span>


Authors' Addresses

   Joe Stone
   Cisco Systems
   2200 East President George Bush Highway
   Richardson, TX  75082
   USA

   EMail: joestone@cisco.com
   URI:   <a href="http://www.cisco.com/">http://www.cisco.com/</a>


   Rajesh Kumar
   Cisco Systems
   Mail Stop SJCE/1/1
   190 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: rkumar@cisco.com
   URI:   <a href="http://www.cisco.com/">http://www.cisco.com/</a>


   Flemming Andreasen
   Cisco Systems
   Iselin, NJ  08830
   USA

   EMail: fandreas@cisco.com
   URI:   <a href="http://www.cisco.com/">http://www.cisco.com/</a>





















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