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Versions: 00 01 02 03 04 05 06 07 08 RFC 5347

Internet Engineering Task Force                            F. Andreasen
Internet Draft                                            Cisco Systems
Document: draft-andreasen-mgcp-fax-04.txt                March 23, 2005
Category: Informational


               Media Gateway Control Protocol Fax Package

Status of this Memo

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   This Internet-Draft will expire on September, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines a Media Gateway Control Protocol (MGCP)
   package to support fax calls.  The package allows for fax calls to
   be supported in two different ways.  The first one utilizes ITU-T
   Recommendation T.38 for fax relay under the control of the Call
   Agent.  The second one lets the gateway decide upon a method for fax
   transmission as well as handle the details of the fax call without
   Call Agent involvement.

Conventions used in this document

   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 BCP 14, RFC-2119
   [RFC2119].


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Table of Contents

1. Introduction......................................................3
2. Fax Package Definition............................................3
 2.1 LocalConnectionOptions..........................................3
  2.1.1 T.38 Procedure (Strict or Loose)............................5
  2.1.2 Gateway Procedure...........................................7
  2.1.3 Off Procedure...............................................7
  2.1.4 Mode Operation..............................................8
  2.1.5 Detecting a Fax Call........................................9
  2.1.6 Considerations for Which Procedures to Request.............10
 2.2 Events and Signals.............................................11
  2.2.1 Gateway Controlled Fax (gwfax).............................12
  2.2.2 No Special Fax Handling (nopfax)...........................12
  2.2.3 T.38 fax relay(t38):.......................................13
 2.3 Connection Parameters..........................................14
 2.4 Negotiation of T.38 Parameters.................................15
 2.5 Implementation Considerations..................................16
  2.5.1 Media IP Address and Port for T.38.........................16
  2.5.2 Case Sensitivity...........................................16
  2.5.3 Boolean Indicator after T.38 Parameters....................17
3. Call Flow Examples...............................................17
 3.1 Call Agent Controlled T.38 Strict..............................17
 3.2 Multiple and Different Options.................................23
 3.3 Interaction with SIP Endpoints.................................30
4. Security Considerations..........................................34
5. IANA Considerations..............................................35
6. Summary of Changes...............................................35
7. Normative References.............................................36
8. Informative References...........................................36
9. Acknowledgements.................................................36
10. Author's Address................................................36



















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1.
  Introduction

   This document defines a Media Gateway Control Protocol (MGCP)
   [RFC3435] package that enables MGCP controlled gateways to support
   fax calls.  The package enables fax calls to be supported in two
   different ways.  The first one utilizes ITU-T Recommendation T.38
   using either UDPTL or TCP (see [T38]) for fax relay under the
   control of the Call Agent.  The second one lets the gateway decide
   upon a method for fax transmission as well as handle the details of
   the fax call without Call Agent involvement.

   The fax package definition is provided in Section 2 and in Section 3
   we provide two call flow examples showing how to use it.  Security
   considerations are found in Section 4, followed by the IANA
   considerations and references.

2.
  Fax Package Definition

   A package is defined for fax.  The package defines new
   localconnectionoptions, events, and connection parameters as
   detailed below.

   Package Name:        FXR
   Package Version:     0

2.1
   LocalConnectionOptions

   A new Fax LocalConnectionOptions (LCO) parameter is defined for fax
   handling.  The Call Agent supplies this fax LCO to indicate the
   desired fax handling procedure to the Media Gateway.  The fax LCO
   contains a list of desired fax handling procedures ordered by
   preference with the most desired procedure listed first.  When the
   parameter is explicitly included in a command, the gateway MUST be
   able to use at least one of the listed procedures for the command to
   succeed.  The list can currently indicate one or more of the
   following procedures (see Section 2.1.1 to 2.1.4 for further details
   on these):

   * T.38 Strict        Use T.38 [T38] with either UDPTL or TCP for fax
     relay and have the Call Agent control it.  Assuming the procedure
     can be used (see Section 2.1.1), a switch to T.38 procedures will
     be initiated upon fax detection and a "t38(start)" event will be
     generated (see Section 2.2).  This mode requires an indication of
     T.38 support from the remote side in order to be used, as
     described further in Section 2.1.1.

   * T.38 Loose         Identical to T.38 Strict procedure, except that
     an indication of T.38 support from the remote side is not required
     for the procedure to be used.



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   * Off                Do not invoke any special procedure for fax,
     except for echo cancellation adjustment and possibly switching to
     another codec.

   * Gateway            Let the gateway control and decide how to
     handle fax calls without Call Agent involvement.  This includes
     the case where the gateway does not do anything special for fax,
     hence by definition this procedure can always be supported.  If
     the gateway does invoke a special procedure upon detection of fax,
     it will generate a "gwfax(start)" event so the Call Agent can be
     notified about it (see Section 2.2).  The Call Agent SHOULD then
     refrain from issuing potentially conflicting commands to the
     gateway until the gateway ends its special fax handling procedure.

   A gateway that ends up not being able to invoke any special
   procedure for fax will generate a "nopfax(start)" event (see Section
   2.2) upon detection of fax.

   The set of possible values (i.e. procedures) for the fax LCO is
   extensible.  The prefix "x-", which indicates an optional extension,
   and the prefix "x+", which indicates a mandatory extension, are
   reserved for vendor-specific use.

   In CreateConnection commands, the fax LCO value defaults to
   "gateway".  In ModifyConnection commands, the fax LCO value defaults
   to its current value on the connection.  Thus, if
   LocalConnectionOptions are either omitted or the fax LCO is not
   included in a ModifyConnection command, the previous fax LCO value
   for the connection is retained, but without affecting the outcome of
   the command; consequently, the gateway may now not apply any special
   procedure to fax.  If the Call Agent wants to ensure that a command
   succeeds only when a fax procedure is applied, the command needs to
   include the fax LCO explicitly.

        As an example of this, assume that the CreateConnection command
        successfully specified the use of "T.38 Strict", and a
        ModifyConnection command is now received without the fax LCO,
        but with a RemoteConnectionDescriptor indicating no support for
        T.38; in that case, the ModifyConnection command will succeed,
        however T.38 procedures will no longer be invoked upon fax
        detection.  Had the Call Agent instead included the fax LCO set
        to "T.38 Strict", the command would have failed.

   If multiple fax parameter values are provided, the gateway MUST
   choose one of the procedures specified according to the order in
   which they are supplied, except as follows:

   1. If "gateway" would have been selected and it would have resulted
      in no special procedure being applied, and

   2. there are other procedures than "off" which are specified after
      "gateway" (e.g., "t38")



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   then the gateway MUST use the most preferred of those subsequent
   procedures that can be supported.  If none of those subsequent
   procedures can be supported, the gateway reverts to not invoking any
   special procedure for fax.  Please refer to Section 2.1.4 for
   further details on determining which procedures can be supported.

   The fax LCO parameter is encoded as the keyword "fx" (prefixed with
   the package name per [RFC3435]), followed by a colon and a semicolon
   separated list of values where T.38 Strict is encoded as "t38", T.38
   Loose is encoded as "t38-loose", gateway is encoded as "gw", and off
   is encoded as "off".

   The following example illustrates use of PCMU or G.729 for audio
   encoding and T.38 Strict fax relay (preferred) or gateway control
   for fax:

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

   It should be noted, that MGCP allows the CreateConnection command to
   omit both LocalConnectionOptions and a RemoteConnectionDescriptor
   thereby letting the gateway decide upon the media parameters to use.
   When the T.38 fax package is supported, the gateway thus could
   choose to do either audio or T.38 fax relay in such cases.  Most
   likely, the Call Agent requires one or the other to be used, and
   hence it SHOULD NOT omit both LocalConnectionOptions and a
   RemoteConnectionDescriptor in CreateConnection commands.

   When auditing capabilities, the fax LCO may be returned with a semi-
   colon separated list of supported fax handling parameters.  The
   values "t38", "t38-loose", "off" and "gw" MAY be omitted from such a
   list as they are always implied.  Gateways that implement additional
   parameters SHOULD return these additional parameters when
   capabilities are audited as illustrated by the following example:

        A: a:image/t38, fxr/fx:mypar, ...

   In the following subsections we provide additional detail on the
   above defined fax procedures.

2.1.1  T.38 Procedure (Strict or Loose)

   When a gateway is instructed to use one of the T.38 procedures
   (strict or loose), also known as Call Agent controlled T.38 mode,
   the "m=" line in the SDP returned will not indicate use of UDPTL-
   based or TCP-based T.38 (unless the gateway was also instructed to
   use "image/t38" for the media stream).  Any other entity seeing this
   SDP will not know whether T.38 is supported or not and hence whether
   it is safe to attempt a switch to T.38 upon fax detection.  To
   remedy this dilemma, capability information for T.38 (if supported)
   using the SDP Simple Capability Declaration extensions [RFC3407]
   MUST be included.  Other capability information is included as well,
   regardless of whether the Call Agent authorized use of those in the
   connection handling command or not.  A subsequent attempt to

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   actually use these may of course not succeed, e.g., because the Call
   Agent LCOs does not allow them to be used.  The following example
   illustrates the RFC 3407 capability descriptor û note the inclusion
   of both current (audio) and latent (T.38) capabilities as specified
   in RFC 3407:

        m=audio 3456 RTP/AVP 18
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 18
        a=cdsc: 2 image udptl t38

   For a list of T.38 related parameters to be included in the SDP,
   please refer to T.38 Annex D [T38].

   Upon fax detection, a gateway which has successfully been instructed
   to use one of the T.38 procedures will:

   1. Initiate the T.38 fax relay procedure and mute the media channel
      in both the send and receive direction (unless the media channel
      is already using T.38).

   2. Generate a "t38(start)" event.

   3. Await further instructions from the Call Agent in order to
      initiate the actual media change (unless the media channel is
      already using T.38).

   The Call Agent instructs the gateway to perform the media change by
   sending it a ModifyConnection command with "image/t38" listed as the
   encoding method in the LocalConnectionOptions (receipt of a
   ModifyConnection command without LocalConnectionOptions but with a
   RemoteConnectionDescriptor containing an "m=" line with the MIME
   type "image/t38" would achieve the same).  Per the normal MGCP codec
   negotiation procedures (see [RFC3435] Section 2.6), if a
   RemoteConnectionDescriptor was included as well, it needs to include
   an "m=" line with "image/t38" as an acceptable media format in order
   for the command to succeed.  The gateway may choose between the
   UDPTL and TCP transport protocols at its own discretion subject to
   the normal MGCP codec negotiation procedures (in practice, TCP-based
   implementations are currently rare).

   If a RemoteConnectionDescriptor was not included with the
   ModifyConnection command sent to a gateway that initiated the T.38
   procedure, it is possible (in fact likely), that the last received
   RemoteConnectionDescriptor did not include an "m=" line listing
   "image/t38" as an acceptable media format.  In that case, the
   endpoint cannot send T.38 media to the other side.  The endpoint
   MUST instead wait for an updated RemoteConnectionDescriptor with
   "image/t38" as an acceptable media format and a supported transport
   protocol (UDPTL or TCP).  The T.38 fax procedure continues when an
   acceptable RemoteConnectionDescriptor is received.  An acceptable
   RemoteConnectionDescriptor contains an "m=" line with the
   "image/t38" MIME type (using the normal SDP syntax) and a supported
   transport protocol (UDPTL or TCP).  If the fax call fails, e.g., due

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   to a fax timeout, while either waiting for the Call Agent to
   instruct the gateway to switch to "image/t38" or waiting for an
   acceptable RemoteConnectionDescriptor, a "t38(stop)" or a
   "t38(failure)" event MUST be generated.  When the T.38 procedure
   ends, a "t38(stop)" or "t38(failure)" event MUST be generated.

   Finally, the Call Agent may need to abort a T.38 procedure that is
   in progress.  This can for example be done when the remote side was
   unable to switch to T.38, and a fallback to fax passthrough using an
   audio codec is attempted.  The Call Agent instructs the endpoint to
   abort an in-progress T.38 procedure by use of the "off" Fax LCO as
   illustrated below:

        L: fxr/fx:off

   We now define "time t38init" as the point in time where the T.38
   procedure was initiated, and "time t38abort" as the point in time
   where the Call Agent aborts an in-progress T.38 procedure.  If the
   Call Agent at time t38abort instructs or enables the endpoint to
   revert to one or more codecs that were in use just prior to time
   t38init, the endpoint SHOULD use media stream parameters that mimic
   the most recent LocalConnectionDescriptor issued before time
   t38init.  For example, IP-address and UDP port, payload formats used
   and their payload type mapping, should all be the same as before
   time t38init.  This will enable the fallback to be as rapid as
   possible. A LocalConnectionDescriptor is returned as usual, i.e.,
   only if one or more parameters changed since the last
   LocalConnectionDescriptor issue (e.g. if a T.38 LCD was issued or a
   transport address in the audio LCD was changed).

2.1.2  Gateway Procedure

   A gateway using the gateway procedure, also known as Gateway
   controlled mode, may initiate special fax handling upon detecting a
   fax call.  The details of this special fax handling are outside the
   scope of this document.  However, in order to use any special fax
   handling, support for it MUST be negotiated with the other side by
   passing and recognizing relevant parameters via the
   LocalConnectionDescriptor and RemoteConnectionDescriptor.  If the
   other side has not indicated support for the special fax handling
   desired, the gateway MUST NOT attempt to initiate it.  When special
   fax handling is initiated, a "gwfax(start)" event MUST be generated
   thereby enabling the Call Agent to differ between the Call Agent and
   gateway controlled mode while still being informed about the actual
   change to fax.  When the special gateway handling of fax ends, a
   "gwfax(stop)" or "gwfax(failure)" event MUST be generated.

2.1.3  Off Procedure

   A gateway using the "off" procedure will not invoke any special fax
   procedures, e.g. T.38, when detecting a fax.  However, the gateway
   may still adjust local echo cancellation and/or switch to an

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   alternative codec as needed (in particular, this does not preclude
   the use of RTP-based T.38).  Also, a "nopfax(start)" event MUST be
   generated; a corresponding "stop" event however will not.

     Generating a "stop" event would imply that the gateway had to
     infer when the fax call ends, which involves processing of the
     media stream.  However, when using the "off" mode, such processing
     is not expected to occur.

2.1.4  Mode Operation

   For each of the above modes, the RemoteConnectionDescriptor provides
   information on what procedure(s) the other side supports.  The
   following rules are used to determine which procedure to use:

   1. Whatever the Call Agent specified in the Fax
      LocalConnectionOptions for the current command MUST be adhered
      to.  If the gateway cannot satisfy any of the options, the
      command fails (error code 532 - unsupported value(s) in
      LocalConnectionOptions is RECOMMENDED).

   2. If both Fax LocalConnectionOptions and a
      RemoteConnectionDescriptor are provided, the procedure selected
      MUST be supported by both sides - this is currently only an issue
      for "T.38 Strict".  A procedure can be satisfied by the remote
      side if:

      * the relevant MIME media type, e.g. "image/t38", is included in
        the "m=" line in the RemoteConnectionDescriptor, or

      * the relevant MIME media type is included as a capability (see
        [RFC3407]) in the RemoteConnectionDescriptor.

      If the gateway cannot select any of the procedures in the Fax
      LocalConnectionOptions, the command fails (error code 532 is
      RECOMMENDED).  Note that "T.38 Loose", "gateway", and "off" by
      definition can always be supported by an implementation that
      supports this package, irrespective of what the
      RemoteConnectionDescriptor indicates.

   3. If the Call Agent did not include any Fax LocalConnectionOptions
      or a RemoteConnectionDescriptor with the command, the gateway
      MUST continue using whichever procedure it is currently using.

   4. If the Call Agent did not include any Fax LocalConnectionOptions,
      but a RemoteConnectionDescriptor was included, the gateway MUST
      follow rule 2 in selecting a procedure.  In so doing, the default
      Fax LocalConnectionOptions, i.e. "gateway" in CreateConnection,
      or the current value in ModifyConnection, MUST be used.  In the
      case of ModifyConnection, the outcome of the command does not
      depend on the gateway being able to select one of these "default"
      procedures (as described in Section 2.1).  Note that this is not


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      an issue for the CreateConnection command, since the default
      value can always be supported by definition.

   5. A previously received RemoteConnectionDescriptor does not affect
      what procedure can be selected.  Only a
      RemoteConnectionDescriptor supplied with the current command
      affects the procedure selection.  However, in order to send media
      of a given type (e.g. "image/t38"), the most recently received
      RemoteConnectionDescriptor MUST include a corresponding media
      line.

   The following examples illustrate the use of the above rules:

   Per rule 1, a gateway that only supports standard T.38 fax relay
   will fail a command that only contains the fax option "mypar"
   whereas it will succeed a command that contains, "t38-loose", "gw",
   "off" or no Fax LCO.  A command that only contained "t38", i.e. use
   of T.38 in "strict" mode, may or may not succeed (depending on the
   RemoteConnectionDescriptor).

   A gateway supporting T.38 that receives a CreateConnection command
   with the fax handling LCO set to "t38" and a
   RemoteConnectionDescriptor with neither a T.38 capability nor a T.38
   media stream will fail per rule 2.  Had the fax handling LCO
   included either "t38-loose", "gw" or "off", the command would have
   succeeded and any of the procedures included could have been
   selected.

   Assume a gateway supporting T.38 has successfully executed a
   CreateConnection command with fax handling set to "t38" (i.e.
   strict).  If the gateway now receives a ModifyConnection command
   without a fax handling LCO but with a RemoteConnectionDescriptor
   that has neither a T.38 capability nor a media stream with
   "image/t38", the command will succeed (since rule 1 has no effect in
   that case).  However, per rule 2 and 4, there will not be any T.38
   procedure in place.  Had the CA instead included a fax handling LCO
   set to "t38" again, the command would have failed per rule 2.

   Finally, it should be noted that a switch to T.38 can be initiated
   by either one or both of the originating and terminating gateways
   and hence implementations MUST be prepared to handle this.  This
   includes the case where both sides initiate the switch, which for
   example can occur when the originating fax generates Calling Tone
   (CNG) and the terminating fax detects V.21 fax preamble (see [T30])
   before the switch to T.38 has been performed on the terminating
   side.

2.1.5  Detecting a Fax Call

   A fax call can be detected by several different means, e.g. V.21 fax
   preamble, T.30 CNG tone, or V.8 signals, depending on the fax
   transmission method being used. Implementations of this package MUST
   at a minimum detect a fax call based on V.21 fax preamble.

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   Triggering based on T.30 CNG tone MAY be done; this is generally
   considered acceptable for G3 and lower fax speeds.  However, when
   used with T.38 version 2 or earlier, it will impact V.34 high-speed
   fax.  The reason is, that T.38 version 2 (and earlier) does not
   support the V.8 ANSam and CM signals used with V.34 fax, and hence
   the V.34 faxes will downspeed to G3 (14.400 bps) or lower when using
   T.38 version 2 (or earlier).  Also, a few rare cases of modems
   generating T.30 CNG tones for non-fax call have been reported; such
   modems would generate a false trigger for fax.  As a consequence of
   the above, it is RECOMMENDED that implementations of this package
   which support T.30 CNG based fax detection provide a configuration
   option to disable it for T.38 version 2 (or earlier).

2.1.6  Considerations for Which Procedures to Request

   It is important to understand the implications of using any one of
   the above defined procedures.  Furthermore, multiple alternative
   procedures can be requested, however not all combinations make
   sense.  In this section, we elaborate on both of these issues.

   Use of the T.38 strict mode is ideal in an environment where it is
   known, that other endpoints generate RFC 3407 capability
   descriptions with T.38 fax relay information.  If a
   RemoteConnectionDescriptor without T.38 fax relay capabilities is
   received in such an environment, it is known that the other side
   does not support T.38, and hence an unsuccessful attempt to switch
   to T.38 (which in turn may lead to a failed fax call) can be
   avoided.  If it is not known whether other endpoints support the RFC
   3407 capability descriptors or not, the tradeoff is less clear.  The
   advantage is, that a switch to T.38 will only be attempted if it is
   known that the other side supports it, however endpoints that do not
   indicate support for T.38 may still support it; T.38 however will
   not be used with these which in turn may lead to unnecessary fax
   failures with low-bandwidth codecs or lossy networks.

   Use of the T.38 loose mode involves the same considerations as for
   T.38 strict, however the pros and cons are reversed.  If a peer
   endpoint does not support T.38, the T.38 loose mode will still
   attempt to switch to T.38 (and fail), which in turn may lead to a
   failed fax call.  On the other hand, if the peer endpoint does not
   support the RFC 3407 capability descriptors, but the peer endpoint
   does in fact support T.38, T.38 would  still be used with this mode.

   In summary, there is no single good answer to the use of either T.38
   strict or T.38 loose mode; it depends on the capabilities of the
   endpoints involved as well as the trade-off between potentially
   letting fax calls fail due to lack of capability indications (where
   T.38 otherwise is supported) versus potentially letting fax calls
   fail due to an unsuccessful switch to T.38 (because T.38 in fact was
   not supported).  It should be noted, that Call Agents may have means
   beyond RFC 3407 capability descriptors to determine if a peer
   endpoint supports T.38 or not.  For example, when SIP is used as the

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   signaling protocol with other peers (e.g. Call Agents or other SIP
   devices), the SIP OPTIONS method can be used to learn whether T.38
   is supported.  Also, if the Call Agent allows use of high-bandwidth
   codecs with redundancy when support for T.38 is not indicated, fax
   calls may still succeed without the use of T.38, even in networks
   with non-negligible packet loss.

   When the gateway controlled mode is selected, there will only be
   special fax handling if the two peer endpoints support the same fax
   handling method; note that the details of the actual method is
   entirely up to the vendor.  Also note, that if the two peer
   endpoints either do not support the same method for fax handling, or
   the method is not indicated in the SDP exchanged, there will be no
   special fax handling in place.  Furthermore, the Call Agent will not
   be aware that this is the case until the fax transmission starts and
   a "nopfax(start)" event is generated.

   The off mode is straightforward; there will be no special procedure
   in place for fax handling, except for the usual handling of echo
   cancellation and possibly a change to a higher bandwidth codec.

   Having looked at the individual procedures in more detail, we now
   elaborate on some of the combinations of procedures that may be
   requested:

   * T.38 strict:       If the T.38 strict procedure is placed after
     the T.38 loose or the off procedure (both of which can always be
     supported), it will not be selected.  Apart from this, it makes
     little sense to request both T.38 strict and T.38 loose.

   * T.38 loose:        The T.38 loose procedure can always be
     supported, so any procedure specified after T.38 loose will not be
     selected.

   * Gateway:           The gateway controlled procedure can always be
     supported.  If the gateway controlled procedure would have
     resulted in no special fax procedure and further options (except
     off) are provided, those procedures will be attempted.  If neither
     of those procedures can be supported, there will be no special fax
     procedure in place.

   * Off:               The off procedure can always be supported.  Any
     procedure specified after this one will not be selected.

2.2
   Events and Signals

   The following events are defined in support of the above:







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    ------------------------------------------------------------------
   | Symbol  |   Definition               |  R  |   S     Duration    |
   |---------|----------------------------|-----|---------------------|
   |  gwfax  | Gateway controlled fax     |  x  |                     |
   |  nopfax | No special fax handling    |  x  |                     |
   |  t38    | T.38 fax relay             |  x  |                     |
    ------------------------------------------------------------------

   The definitions of the individual events are provided in the
   following subsections.

2.2.1  Gateway Controlled Fax (gwfax)

   The "gateway controlled fax" event occurs when the gateway handled
   fax procedure either starts, stops or fails. The event is encoded as
   "gwfax" and the following event parameters, which apply to
   ObservedEvents only, are defined:

     * start           Gateway controlled fax procedure was initiated.
       The Call Agent SHOULD refrain from issuing media handling
       instructions to the gateway until either a "gwfax(stop)" or
       "gwfax(failure)" event is generated.

     * stop            Gateway controlled fax procedure ended and the
       gateway did not detect any errors.  Note that this does not
       necessarily imply a successfully transmitted fax.  It merely
       indicates that the gateway controlled fax procedure has ended
       and the procedure itself did not encounter any errors.  Media
       parameters for the connection are as before the gateway handled
       fax procedure started.

     * failure         The gateway controlled fax procedure ended
       abnormally.  Some kind of problem was encountered in the gateway
       controlled fax procedure and the procedure ended.  Media
       parameters are as before the gateway handled fax procedure
       started.

   One of the above parameters will be present when the event is
   reported.  The "gwfax" event MAY be parameterized with additional
   parameters in ObservedEvents, however it is RECOMMENDED that one of
   the above parameters is the first parameter supplied.  Unknown
   parameters MUST be ignored.

     The following example illustrates the encoding of the "gwfax"
     event:

        O: fxr/gwfax(start)
        O: fxr/gwfax(stop, foobar)

2.2.2  No Special Fax Handling (nopfax)



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   The "no special fax handling" event occurs when there is no special
   fax handling procedure in place and a fax call is detected.  This
   can happen either due to no special fax handling procedure being
   requested (including "off"), or negotiation leading to no special
   fax handling procedure being possible.  The event is encoded as
   "nopfax" and the following event parameter, which applies to
   ObservedEvents only, is defined:

     * start           No special fax handling procedure is in place,
       however a fax call is now detected.  The Call Agent may have to
       issue further commands in order to ensure a successful fax call
       (e.g., switch to another codec).

   The above parameter will be present when the event is reported.  The
   "nopfax" event MAY be parameterized with additional parameters on
   ObservedEvents, however it is RECOMMENDED that the above parameter
   is the first parameter supplied.  Unknown parameters MUST be
   ignored.  Note, that this event currently cannot be parameterized
   with "stop" or "failure" as it only detects the beginning of a fax
   call.

   The following example illustrates the encoding of the "nopfax"
   event:

        O: fxr/nopfax(start)

2.2.3  T.38 fax relay(t38):

   The "T.38 fax relay" event occurs when one of the T.38 fax relay
   procedures (strict or loose) either starts, stops or fails. The
   event is encoded as "t38" and the following event parameters, which
   apply to ObservedEvents only, are defined:

     * start           A fax call was detected on the endpoint and the
       Call Agent controlled T.38 fax relay procedure was initiated.
       The Call Agent SHOULD modify each side of the connection to
       start using the "image/t38" media format, unless they already
       do. Note that, as long as use of the Call Agent controlled T.38
       relay procedure is in effect, the event will be generated upon
       fax call detection irrespective of the current encoding method
       on any connections on the endpoint (incl. "image/t38"). The
       T38(start) event MUST be generated at most once by the endpoint
       per fax call regardless of whether or not it is requested again
       in a subsequent requested events list.

     * stop            Call Agent controlled T.38 fax relay procedure
       ended and the gateway did not detect any errors.  Note that this
       does not necessarily imply a successfully transmitted fax.  It
       merely indicates that the Call Agent controlled T.38 fax relay
       procedure has ended and the procedure itself did not encounter
       any errors.  The Call Agent may want to modify the media
       parameters for each side of the connection.  Note that, in
       contrast to the gateway controlled fax procedure case, media

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       parameters such as codecs do not automatically revert to their
       values before the start of the fax call; echo cancellation and
       silence suppression however do per the procedures in [RFC3435]
       Section 2.3.5. The "t38(stop)" event MUST NOT be generated
       unless a corresponding "t38(start)" event for the fax call in
       question was generated earlier.

     * failure         Call Agent controlled T.38 fax relay procedure
       ended abnormally.  Some kind of problem in the Call Agent
       controlled T.38 fax relay procedure was encountered and the
       procedure ended.  The Call Agent may want to modify the media
       parameters for each side of the connection.  Note that, in
       contrast to the gateway controlled fax procedure case, media
       parameters such as codecs do not automatically revert to their
       state before the start of the fax call; echo cancellation and
       silence suppression however do per the procedures in [RFC3435]
       Section 2.3.5. The "t38(failure)" event MUST NOT be generated
       unless a corresponding "t38(start)" event for the fax call in
       question was generated earlier.

   One of the above parameters will be present when the event is
   reported.  The "t38" event MAY be parameterized with additional
   parameters, however it is RECOMMENDED that one of the above
   parameters is the first parameter supplied.  Unknown parameters MUST
   be ignored.

   The following example illustrates the encoding of the "t38" event:

        O: fxr/t38(start)
        O: fxr/t38(foobar, stop)

2.3
   Connection Parameters

   The connection parameters for the connection, that measures packets
   and octets sent and received, MUST include packets and octets for
   fax handling as well.  Interarrival jitter and average transmission
   delay calculation however MAY not be performed while fax is in
   progress, e.g., if T.38 is used.  In such cases, the interarrival
   jitter and average transmission delay calculations are simply
   suspended until calculations can resume, e.g., by changing back to
   an RTP-based media stream again.

   In addition to these connection parameters, the fax package defines
   the following connection parameters, which gateways MAY support:

   Number of fax pages sent (PGS):

     The cumulative number of fax pages sent by the endpoint for the
     life of the connection. The parameter is encoded as "PGS" and the
     value supplied is a string of up to nine decimal digits.

   Number of fax pages received (PGR):


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     The cumulative number of fax pages received by the endpoint for
     the life of the connection. The parameter is encoded as "PGR" and
     the value supplied is a string of up to nine decimal digits.

   The following example illustrates the use of these parameters:

        P: FXR/PGS=3, FXR/PGS=0, PS=1245, OS=62345, ...

2.4
    Negotiation of T.38 Parameters

   T.38 Annex D defines a number of T.38 parameters that can be
   negotiated in the SDP.  Currently, T.38 does not specify procedures
   for how each of these parameters is negotiated, and in particular
   whether each side has to use the same value or not.  In the absence
   of that, it was considered to add such definitions and procedures
   here. However, it is expected that T.38 will rectify the above,
   which could lead to conflicting definitions and procedures. To avoid
   that, we instead assume the existence of an offer/answer section for
   T.38 where T.38 Annex D parameters are classified as either
   declarative or negotiated, and we then provide guidelines for how to
   map such definitions and procedures to the MGCP fax package defined
   here.

   MGCP does not specify use of the offer/answer model, but instead
   operates with the concept of connection handling commands (e.g.
   CreateConnection and ModifyConnection) that may include a
   RemoteConnectionDescriptor (SDP) and in turn may generate a
   LocalConnectionDescriptor (SDP) in their response.

   When an MGCP endpoint receives a CreateConnection command without a
   RemoteConnectionDescriptor, it should follow the corresponding T.38
   procedures for generating an initial offer and return the resulting
   the SDP in its LocalConnectionDescriptor.

   When an MGCP endpoint receives a CreateConnection command with a
   RemoteConnectionDescriptor, it should follow the corresponding T.38
   procedures for receiving an initial offer and generating an answer
   to it. The resulting SDP is returned in the
   LocalConnectionDescriptor.

   When an MGCP endpoint receives a ModifyConnection command with a
   RemoteConnectionDescriptor, it cannot determine whether this
   corresponds to an answer to an initial offer or a new offer. This is
   not an issue for declarative parameters since they can be specified
   independently in either direction. Negotiated parameters however
   require some consideration:

   When an offerer receives an answer to a previous offer, the
   negotiation has completed and the parameters negotiated can no
   longer be changed with this offer/answer exchange. The negotiated
   parameters may be subject to certain validation checks. Conversely,
   when an answerer receives an offer, the negotiation is open and the
   answerer may change some of the offered negotiated parameters. Since

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   the MGCP endpoint does not know which situation it is in, it can not
   perform the "offerer" validation checks. Likewise, in order to
   ensure that any required negotiation actually takes place, it needs
   to process incoming SDP as an offer. If the SDP in fact does
   correspond to an offer then this is obviously correct behavior. If
   the SDP however corresponds to an answer, and one or more negotiated
   parameters did change, then this will result in a new SDP. The Call
   Agent may or may not contain sufficient intelligence to determine
   whether this new SDP needs to result in another offer/answer
   exchange or not.

     For example, if the initial offer (from CreateConnection without
     SDP) contained fax version 2, and the answer (from
     CreateConnection with SDP) contained fax version 0, then the
     corresponding ModifyConnection command (with SDP) will result in
     an updated SDP with fax version also set to zero. If this was the
     only change in the updated SDP, a new offer/answer exchange would
     not be needed. Note that this example does not imply that it is
     generally considered a good idea for Call Agents to parse SDP in
     order to determine whether new offer/answer exchanges are needed
     or not.

   Finally, a ModifyConnection without SDP that generates an SDP needs
   to be considered. The SDP generated may either correspond to an
   initial offer/answer exchange or a subsequent offer/answer exchange.
   The endpoint should generate SDP as if it was part of a subsequent
   offer/answer exchange. If the Call Agent does not desire such
   semantics, it can simply create a new connection instead.

2.5
   Implementation Considerations

2.5.1  Media IP Address and Port for T.38

   When an endpoint is instructed to change to or from T.38 for a media
   stream, it SHOULD continue using the same IP address and port as the
   media stream is currently using, since this will minimize any
   Quality of Service, Network Address Translator (NAT) and Firewall
   interactions from the change.  However, if an endpoint has a good
   reason, it MAY choose not to follow this recommendation.

   When an endpoint uses the same port for RTP audio and T.38 with
   either UDPTL or TCP, packets of one type (e.g. T.38) may be received
   while expecting packets of another type (RTP audio).  Since there is
   explicit signaling to indicate which type is expected at any given
   point in time, this does not introduce any new problems.  In other
   words, the receiver does not operate as a demultiplexer with a need
   to determine if a given packet received is an RTP audio packet or a
   T.38 UDPTL/TCP packet.  The receiver simply processes incoming
   packets as usual.  If T.38 packets are expected, then incoming
   packets are validated against T.38, and if RTP audio packets are
   expected, then incoming packets are validated against RTP.
2.5.2  Case Sensitivity


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   The IANA has registered the uppercase string "UDPTL" as the
   transport protocol identifier to be used for UDP-based T.38.
   However, the examples provided in Recommendation T.38 as well as
   most (if not all) current implementations use the lowercase string
   "udptl" instead.  Implementations conforming to this package SHOULD
   generate the lowercase string "udptl" and accept the lowercase,
   uppercase, and mixed upper/lowercase strings as being equivalent.

   The attribute "T38MaxBitRate" is incorrectly registered with IANA as
   "T38maxBitRate" (lower-case "m").  In accordance with T.38 examples
   and common implementation practice, the form "T38MaxBitRate" SHOULD
   be generated by implementations conforming to this package.

   In general, it is RECOMMENDED that implementations of this package
   accept both lowercase, uppercase, and mixed upper/lowercase
   encodings of all the T.38 attributes.
2.5.3  Boolean Indicator after T.38 Parameters

   Some implementations incorrectly use a colon (':') followed by a
   number (zero or one) after the attributes T38FaxFillBitRemoval,
   T38FaxTranscodingMMR and T38FaxTranscodingJBIG.  Implementations
   that receive such erroneous encodings MAY interpret the value ":0"
   as lack of support for the option and all other values as indicating
   support of the option in question.


3.
  Call Flow Examples
   In this section, we provide two example call flows. The first one
   illustrates a T.38 fax call under Call Agent control on both the
   originating and terminating side.  The second one illustrates the
   use of multiple and different options on the two sides.

3.1
   Call Agent Controlled T.38 Strict

   In this example, both sides are under strict T.38 Call Agent
   control. We assume the originating and terminating Call Agent
   communicate via the Session Initiation Protocol (SIP) [RFC3261]
   (also, see [SIPfax]):















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    ------------------------------------------------------------------
   | #|     GW-o      |     CA-o      |      CA-t     |      GW-t     |
   |==|===============|===============|===============|===============|
   | 1|             <-|CRCX           |               |               |
   | 2|     200(sdp-o)|->             |               |               |
   | 3|               |  INVITE(sdp-o)|->             |               |
   | 4|               |               |    CRCX(sdp-o)|->             |
   | 5|               |               |             <-|200 (sdp-t)    |
   | 6|               |             <-|200(sdp-t)     |               |
   | 7|             <-|MDCX(sdp-t)    |               |               |
   | 8|            200|->             |               |               |
   |--|---------------|---------------|---------------|---------------|
   | 9|               |               |               |  <- ANS/      |
   |  |               |               |               |      T.30 CED |
   |10|               |               |               |  <- T.30 fax  |
   |  |               |               |               |     preamble  |
   |11|               |               |             <-|NTFY(t38 start)|
   |12|               |               |            200|->             |
   |13|               |               |      MDCX(t38)|->             |
   |14|               |               |             <-|200(sdp-t2)    |
   |15|               |             <-|INVITE(sdp-t2) |               |
   |16|             <-|MDCX(sdp-t2)   |               |               |
   |17|    200(sdp-o2)|->             |               |               |
   |18|               |    200(sdp-o2)|->             |               |
   |19|               |               |   MDCX(sdp-o2)|->             |
   |20|               |               |             <-|200            |
   |--|---------------|---------------|---------------|---------------|
   |21|               |               |               |   (fax ends)  |
   |22|               |               |             <-|NTFY(t38 stop) |
   |24|               |               |            200|->             |
    ------------------------------------------------------------------

   Step 1:

   The Call Agent issues a CreateConnection command to the gateway
   instructing it to use PCMU media encoding and to use the strict Call
   Agent controlled T.38 procedure.  Consequently, the Call Agent asks
   the gateway to notify it of the t38 event:

        CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
        C: 1
        L: a:PCMU, fxr/fx:t38
        M: recvonly
        R: fxr/t38
        X: 1

   Step 2:

   The gateway acknowledges the command and includes SDP with codec
   information as well as RFC 3407 capability information:





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        200 1000 OK
        I:1

        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 3456 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   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 PCMU media encoding and to
   use the strict Call Agent controlled T.38 procedure.  Consequently,
   the Call Agent asks the gateway to notify it of the t38 event:

        CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
        C: 2
        L: a:PCMU, fxr/fx:t38
        M: sendrecv
        R: fxr/t38
        X: 20

        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 3456 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 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 sends back
   a success response with its SDP which also includes capability
   information:





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        200 2000 OK
        I:2

        v=0
        o=- 25678 753849 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=audio 1296 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 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 turns 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 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=audio 1296 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 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 the other side
   supports T.38, the gateway will in fact be able to use the strict
   Call Agent controlled T.38 procedure.  Had there not been any
   support for T.38 in the RemoteConnectionDescriptor, then this
   command would still have succeeded, however there would be no
   special fax handling procedure (since strict mode could not be
   supported).

   Step 8:




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   The gateway acknowledges the command.  At this point, a call is
   established using PCMU encoding, and if a fax call is detected, the
   Call Agent controlled T.38 procedure will be initiated.

   Step 9-11:

   A fax call now occurs.  First, the T.30 CED tone (a.k.a. V.25 ANS)
   is sent which in this case is simply passed through the current PCMU
   encoding.  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 10, where the V.21 fax preamble is detected.

   The gateway was instructed to apply the Call Agent controlled T.38
   procedure for fax calls, so the "t38(start)" event occurs and is
   notified to the Call Agent:

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

   Step 12:

   The Call Agent acknowledges the Notify command:

        200 2500 OK

   Step 13:

   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

   Step 14:

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

        200 2002 OK

        v=0
        o=- 25678 753850 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=image 1296 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

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   Note, that since the gatewayÆs current RemoteConnectionDescriptor
   (as opposed to the LocalConnectionDescriptor returned here) does not
   list "image/t38" as a valid encoding method, the terminating gateway
   is still muting the media and is now waiting for an updated
   RemoteConnectionDescriptor with "image/t38".

   Step 15:

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

   Step 16:

   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

        v=0
        o=- 25678 753850 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=image 1296 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 17:

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

        200 1003 OK

        v=0
        o=- 25678 753850 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=image 3456 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 18:

   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 19:

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

        MDCX 2003 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
        C: 2
        I: 2

        v=0
        o=- 25678 753850 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=image 3456 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 20:

   The terminating gateway sends back a success response:

        200 2003 OK

   Since the terminating gateway now has a RemoteConnectionDescriptor
   with "image/t38" as valid media, it can start exchanging T.38 with
   the originating gateway.

   Step 21, 22:

   When the fax ends, a "t38(stop)" event is generated, which is
   notified to the Call Agent:

        NTFY 2501 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
        O: t38(stop)
        X: 3

   Step 23:

   The Call Agent acknowledges the Notify command:

        200 2501 OK

   The fax call is now over. The Call Agent may now decide to change
   back to a voice codec, delete the connection, or something
   different.

3.2
   Multiple and Different Options

   In this example, the originating gateway is instructed to use the
   gateway procedure whereas the terminating gateway is given a choice

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   between gateway procedure and strict t38 procedure. Furthermore, the
   originating fax machine is generating CNG tone.

    ------------------------------------------------------------------
   | #|     GW-o      |     CA-o      |      CA-t     |      GW-t     |
   |==|===============|===============|===============|===============|
   | 1|             <-|CRCX           |               |               |
   | 2|     200(sdp-o)|->             |               |               |
   | 3|               |  INVITE(sdp-o)|->             |               |
   | 4|               |               |    CRCX(sdp-o)|->             |
   | 5|               |               |             <-|200 (sdp-t)    |
   | 6|               |             <-|200(sdp-t)     |               |
   | 7|             <-|MDCX(sdp-t)    |               |               |
   | 8|            200|->             |               |               |
   |--|---------------|---------------|---------------|---------------|
   | 9|         CNG ->|               |               |               |
   |10|               |               |               |<- ANS/T.30 CED|
   |11|               |               |               |<- T.30 fax p. |
   |12|               |               |             <-|NTFY(t38 start)|
   |13|               |               |            200|->             |
   |14|               |               |      MDCX(t38)|->             |
   |15|               |               |             <-|200(sdp-t2)    |
   |16|               |             <-|INVITE(sdp-t2) |               |
   |17|             <-|MDCX(sdp-t2)   |               |               |
   |18|    200(sdp-o2)|->             |               |               |
   |19|               |    200(sdp-o2)|->             |               |
   |20|               |               |   MDCX(sdp-o2)|->             |
   |21|               |               |             <-|200            |
   |--|---------------|---------------|---------------|---------------|
   |22|               |               |               |   (fax ends)  |
   |23|               |               |             <-|NTFY(t38 stop) |
   |24|               |               |            200|->             |
    ------------------------------------------------------------------

   Step 1:

   The Call Agent issues a CreateConnection command to the gateway
   instructing it to use PCMU media encoding and to use the gateway
   procedure.  Consequently, the Call Agent asks the gateway to notify
   it of the gwfax event:

        CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0
        C: 1
        L: a:PCMU, fxr/fx:gw
        M: recvonly
        R: fxr/gwfax
        X: 1

   Step 2:

   The gateway acknowledges the command and includes SDP with codec
   information as well as capability information:



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        200 1000 OK
        I:1

        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 3456 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38
        a=X-FaxScheme123

   We assume the gateway supports some other fax scheme and it
   indicates this by including an attribute "FaxScheme123"

   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 PCMU media encoding and to
   use either the gateway procedure or strict Call Agent controlled
   T.38 procedure.  Consequently, the Call Agent asks the gateway to
   notify it of both the gwfax and t38 events:

        CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
        C: 2
        L: a:PCMU, fxr/fx:gw;t38
        M: sendrecv
        R: fxr/t38, fxr/gwfax
        X: 20


        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 3456 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38
        a=X-FaxScheme123

   Step 5:

   The terminating gateway does not support any special gateway fax
   handling, however it does support T.38, and the
   RemoteConnectionDescriptor included indicates that the other side

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   supports T.38 as well, so the strict T.38 Call Agent controlled
   procedure requested can be honored.  The terminating gateway sends
   back a success response with its SDP which also includes capability
   information:

        200 2000 OK
        I:2

        v=0
        o=- 25678 753849 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=audio 1296 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 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 turns 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 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=audio 1296 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 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, i.e. the gateway procedure.  The SDP
   information returned however does not indicate support for the
   "FaxScheme123", and hence the originating gateway will not invoke
   any special fax handling procedure for this call.





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   Step 8:

   The gateway acknowledges the command. At this point, a call is
   established using PCMU encoding and if a fax call is detected, no
   special fax handling procedure will occur.

   Step 9-12:

   First, a CNG tone is generated by the originating fax thereby
   indicating a fax call.  If the gateway was using either of the T.38
   modes, or it had negotiated support for special gateway handling
   procedure with the other side, a "t38(start)" or "gwfax(start)"
   event would now have been generated and the switch to T.38 (or
   special gateway handling) could start.  However, since the
   negotiation with the terminating gateway resulted in the originating
   gateway not doing anything special for fax, no such event is
   generated.  Instead, the "nopfax(start)" event is now generated, but
   since the Call Agent has not requested this event, it is not
   detected and hence not reported to the Call Agent.  Consequently,
   the CNG tone is simply passed through the current PCMU encoding
   without the (originating) Call Agent being aware of the fax call.

   Subsequently, the T.30 CED tone (a.k.a. V.25 ANS) occurs which in
   this case is also simply passed through the current PCMU encoding.
   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 11,
   where the V.21 fax preamble is detected.

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

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

   Step 13:

   The Call Agent acknowledges the Notify command:

        200 2500 OK

   Step 14:

   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


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   Step 15:

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

        200 2002 OK

        v=0
        o=- 25678 753850 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=image 1296 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38


   Note, that since the terminating gatewayÆs last received
   RemoteConnectionDescriptor (as opposed to the
   LocalConnectionDescriptor returned here) did not list "image/t38" as
   a valid encoding method, the terminating gateway is still muting the
   media and is now waiting for an updated RemoteConnectionDescriptor
   with "image/t38".

   Step 16:

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

   Step 17:

   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

        v=0
        o=- 25678 753850 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=image 1296 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 18:

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


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                           MGCP Fax Package                 March 2005

        200 1003 OK

        v=0
        o=- 25678 753850 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=image 3456 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 19:

   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).

   Step 20:

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

        MDCX 2003 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
        C: 2
        I: 2

        v=0
        o=- 25678 753850 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=image 3456 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 21:

   The terminating gateway sends back a success response:

        200 2003 OK

   Since the terminating gateway now has a RemoteConnectionDescriptor
   with "image/t38" as valid media, it can start exchanging T.38 with
   the originating gateway.

   Step 22, 23:

   When the fax ends, a "t38(stop)" event is generated, which is
   notified to the Call Agent:

        NTFY 2501 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0
        O: t38(stop)

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                           MGCP Fax Package                 March 2005

        X: 3

   Step 24:

   The Call Agent acknowledges the Notify command:

        200 2501 OK

   The fax call is now over.  The Call Agent may now decide to change
   back to a voice codec, delete the connection, or something
   different.

3.3
    Interaction with SIP Endpoints

   In this example, we show interaction with a SIP endpoint that does
   not support the RFC 3407 capability descriptors.  To accommodate
   such endpoints, the T.38 loose mode is being used (at the risk of
   initiating T.38 to an endpoint that does not support it).

    ------------------------------------------------------------------
   | #|     GW-o      |     CA-o      |    SIP-UA-t   |      fax      |
   |==|===============|===============|===============|===============|
   | 1|             <-|CRCX           |               |               |
   | 2|     200(sdp-o)|->             |               |               |
   | 3|               |  INVITE(sdp-o)|->             |               |
   | 4|               |             <-|200(sdp-t)     |               |
   | 5|               |            ACK|->             |               |
   | 6|             <-|MDCX(sdp-t)    |               |               |
   | 7|            200|->             |               |               |
   |--|---------------|---------------|---------------|---------------|
   | 8|               |               |               |  <- ANS/      |
   |  |               |               |               |      T.30 CED |
   | 9|               |               |               |  <- T.30 fax  |
   |  |               |               |               |     preamble  |
   |10|               |             <-|INVITE(sdp-t2) |               |
   |11|             <-|MDCX(sdp-t2)   |               |               |
   |12|    200(sdp-o2)|->             |               |               |
   |13|               |    200(sdp-o2)|->             |               |
   |14|               |             <-|ACK            |               |
   |--|---------------|---------------|---------------|---------------|
   |15|               |               |               |   (fax ends)  |
   |16|               |             <-|BYE            |               |
   |17|               |            200|->             |               |
    ------------------------------------------------------------------

   Step 1:

   The Call Agent issues a CreateConnection command to the gateway
   instructing it to use PCMU media encoding and to use the loose Call
   Agent controlled T.38 procedure.  Consequently, the Call Agent asks
   the gateway to notify it of the t38 event:

        CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0

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        C: 1
        L: a:PCMU, fxr/fx:t38-loose
        M: recvonly
        R: fxr/t38
        X: 1

   Step 2:

   The gateway acknowledges the command and includes SDP with codec
   information as well as RFC 3407 capability information:

        200 1000 OK
        I:1

        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 3456 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 3:

   The originating Call Agent sends a SIP INVITE message with the SDP
   to the terminating Call Agent (not all SIP details shown here):

        INVITE sip:bob@biloxi.example.com SIP/2.0
        ...
        Content-Type: application/sdp
        Content-Length: 167

        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 3456 RTP/AVP 0
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 4:

   The terminating SIP User Agent sends back a SIP 200 OK response (not
   all SIP details shown) to the originating Call Agent:

        SIP/2.0 200 OK
        ...
        Content-Type: application/sdp
        Content-Length: 100


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                           MGCP Fax Package                 March 2005

        v=0
        o=- 25678 753849 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=audio 1296 RTP/AVP 0

   Note that the terminating SIP User Agent does not use the RFC 3407
   capability descriptor to indicate support for (or lack of support
   for) T.38.

   Step 5:

   The originating Call Agent receives the SIP 200 response and sends a
   SIP ACK message to the terminating SIP UA.

   Note that the Call Agent does not know whether the peer entity
   supports T.38 or not.  In order to figure this out, the Call Agent
   could send a SIP OPTIONS request to the terminating SIP UA
   requesting it to return its capabilities (not shown).  Note that
   this can of course be done towards any SIP peer, e.g. if the other
   side was a Call Agent speaking SIP it could be done there too.

   Step 6:

   The originating Call Agent in turns 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 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=audio 1296 RTP/AVP 0

   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. loose Call Agent controlled
   T.38.  The T.38 loose procedure can always be supported, and hence a
   switch to T.38 will be attempted if the originating gateway detects
   a fax call.

   Step 7:

   The gateway acknowledges the command.  At this point, a call is
   established using PCMU encoding, and if a fax call is detected, the
   Call Agent controlled T.38 procedure will be initiated.


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   Step 8-9:

   A fax call now occurs.  First, the T.30 CED tone (a.k.a. V.25 ANS)
   is sent which in this case is simply passed through the current PCMU
   encoding.  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 9, where the V.21 fax preamble is detected.

   Step 10:

   The terminating SIP UA in fact does support T.38, and upon detecting
   the fax call, it attempts to change to T.38. Consequently, it sends
   a re-INVITE to the originating Call Agent with an updated SDP
   indicating a switch to T.38.

        INVITE sip:ca@ca-o.whatever.net SIP/2.0
        ...
        Content-Type: application/sdp
        Content-Length: 100

        v=0
        o=- 25678 753850 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=image 1296 udptl t38


   Step 11:

   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

        v=0
        o=- 25678 753850 IN IP4 128.96.41.2
        s=-
        c=IN IP4 128.96.41.2
        t=0 0
        m=image 1296 udptl t38

   Step 12:

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

        200 1003 OK

        v=0
        o=- 25678 753850 IN IP4 128.96.41.1
        s=-

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                           MGCP Fax Package                 March 2005

        c=IN IP4 128.96.41.1
        t=0 0
        m=image 3456 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38

   Step 13:

   The originating Call Agent sends a SIP 200 OK response with the
   updated SDP to the terminating SIP User Agent:

        SIP/2.0 200 OK
        ...
        Content-Type: application/sdp
        Content-Length: 167

        v=0
        o=- 25678 753850 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=image 3456 udptl t38
        a=sqn: 0
        a=cdsc: 1 audio RTP/AVP 0 18
        a=cdsc: 3 image udptl t38


   Step 14:

   The terminating SIP User Agent receives the SIP 200 and sends a SIP
   ACK.

   Since the terminating SIP User Agent now has a
   RemoteConnectionDescriptor with "image/t38" as valid media, it can
   start exchanging T.38 with the originating gateway (and vice versa).

   Step 15-17:

   When the fax ends, the terminating SIP UA decides to tear down the
   call and hence sends a SIP BYE message, which the Call Agent
   responds to with a SIP 200.

   The Call Agent may now decide to change back to a voice codec,
   delete the connection, or something different.

4.
  Security Considerations

   The MGCP fax package itself is not known to introduce any new
   security concerns.  However, implementers should note, that T.38
   media is currently transported over UDP (UDPTL) or TCP in clear and
   without any integrity protection.  If for example security services
   are in place to protect RTP media streams, these will thus not be in

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                           MGCP Fax Package                 March 2005

   effect for the T.38 media stream.  If such lack of security is a
   concern, the fax LocalConnectionOptions allowing T.38 in this
   package SHOULD NOT be used, i.e. the "off" (or a new secure
   extension) fax LocalConnectionOption should be used.

5.
  IANA Considerations

   The IANA is hereby requested to register the following MGCP package:

     Package Title         Name     Version
     -------------         ----     -------
     Fax                   FXR        0

6.
  Summary of Changes

   Changes since -03:

   * Boiler plate updated per RFC 3978 and RFC 3979.

   * Clarified that Fax LCO values are ordered by preference with
     caveats around the "gateway" procedure (Section 2.1) .

   * Added cautionary note around omitting both LCOs and a
     RemoteConnectionDescriptor in CreateConnection (Section 2.1).

   * Added "t38-loose" to list of values that may be omitted when
     auditing capabilities (Section 2.1).

   * Upgraded use of RFC 3407 capability declarations to MUST (Section
     2.1.1)

   * Clarified that muting of media applies to both the send and
     receive direction (Section 2.1.1).

   * Clarified that timeouts may also occur while waiting for the call
     agent to switch to "image/t38" (Section 2.11).

   * Defined how a call agent can abort a T.38 procedure (Section
     2.1.1).

   * Added new Section 2.1.5 on detecting a fax call.

   * Added new Section 2.1.6 on considerations for which procedure to
     request.

   * Clarified when and how the T38(start), T38(stop), and T38(failure)
     events are generated (Section 2.2.3).

   * Added new Section 2.4 on Negotiation of T.38 Parameters.

   * Added new Implementation Considerations in Section 2.5.



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                           MGCP Fax Package                 March 2005

   * Added new example call flow showing interaction with SIP endpoints
     (Section 3.3.)

7.
  Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3435]    F. Andreasen, B. Foster, "Media Gateway Control
   Protocol (MGCP) Version 1.0", RFC 3435, January 2003.

   [T38]        ITU-T Recommendation T.38, "Procedures for real-time
   Group 3 facsimile communication over IP networks", 03/2002.

   [RFC3407]    F. Andreasen, "Session Description Protocol (SDP)
   Simple Capability Declaration", RFC 3407, October 2002.

8.
  Informative References

   [T30]        ITU-T Recommendation T.30, "Procedures for document
   facsimile transmission in the general switched telephone network",
   07/03.

   [RFC3261]    J. Rosenberg, H. Schulzrinne, G. Camarillo, A.
   Johnston, J. Peterson, R. Sparks, M. Handley, E. Schooler, "SIP:
   Session Initiation Protocol", RFC 3261, June 2002.

   [SIPfax]     Mule, J., and J. Li, "SIP Support for Real-time Fax:
   Call Flow Examples and Best Current Practices", work in progress.

9.
  Acknowledgements

   Several people have contributed to the development of the MGCP fax
   package.  In particular, the author would like to thank Bill Foster,
   Paul Jones, Gary Kelly, Rajesh Kumar, Dave Horwitz, Hiroshi Tamura,
   Rob Thompson and the CableLabs PacketCable NCS focus team for their
   contributions.

10.
   Author's Address

   Flemming Andreasen
   Cisco Systems
   499 Thornall Street, 8th Floor
   Edison, NJ 08837

   Email: fandreas@cisco.com








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                           MGCP Fax Package                 March 2005


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Acknowledgment

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