Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration
draft-jones-avt-audio-t38-05

Network Working Group
Internet Draft                                                 P. Jones
<draft-jones-avt-audio-t38-05.txt>                  Cisco Systems, Inc.
Expires: December 2004                                        H. Tamura
                                                    Ricoh Company, Ltd.
                                                              June 2004

                  Real-Time Facsimile (T.38) - audio/t38
                        MIME Sub-type Registration

Status of this Memo

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   [Note to RFC Editor: All references to RFC XXXX are to be replaced by
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Abstract

   This document defines the MIME sub-type audio/t38.  The usage of this
   MIME type, which is intended for use within SDP, is specified within
   ITU-T Recommendation T.38.

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................2

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   3. Mechanisms for Transporting T.38 over an IP Network............2
   4. IANA Considerations............................................3
   5. SDP Mapping of MIME Parameters.................................5
   6. Security Considerations........................................5
   7. Normative References...........................................6
   8. Informative References.........................................6
   9. Author's Address...............................................7
   10. Full Copyright Statement......................................7

1.   Introduction

   ITU-T Recommendation T.38 [1] defines the Internet Facsimile Protocol
   (IFP) for carriage of facsimile data over IP networks.  As one
   option, IFP packets may be carried within an RTP [3] stream, either
   as the only content within the media stream or switched with other
   audio payload types.

   This memo provides rationale for using RTP as a transport for fax
   signaling and specifies the MIME type associated with said signaling.

2.   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 RFC 2119 [4].

3.   Mechanisms for Transporting T.38 over an IP Network

   When T.38 was first approved in 1998, it allowed for the transport of
   T.38 via UDP (using UDPTL, rather than RTP) or TCP.  As of the time
   of this publication, UDPTL is the predominant means for transporting
   T.38 data over an IP network.  In support of that, RFC 3362 [11] was
   published in order to allow devices to signal their desire to use
   UDPTL to transport T.38.

   A number of issues were raised with respect to the usage of UDPTL for
   the long-term, though.  Specifically, there were concerns over the
   fact that UDPTL does not provide the same kind of statistics
   reporting as RTCP.  Further, there are no procedures in place for
   encrypting and protecting the integrity of the UDPTL stream.  While
   the latter could be addressed in UDPTL, doing so would require a lot
   of effort and would largely be a duplication of the security work
   already completed within the IETF, e.g. Secure RTP (SRTP) [10].

   There are clear advantages in using RTP for T.38 today.  For example,
   using RTP allows one to take advantage of the redundancy [12], header
   compression [13][14], and other RTP-related work within the IETF.
   Using RTP, as opposed to UDPTL, for transport provides better
   interoperability with a wider range of devices that know and

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   understand RTP.  This includes applications such as firewalls, NAT
   devices, and gateways that bridge two IP networks, which are
   generally support RTP before most other real-time media.

   Lastly, since most T.38 data today is generated by gateways that
   bridge two PSTN networks, it is quite natural to expect the
   transition from audio to fax should happen within the same media
   stream.  The reason is that the T.38 data is simply an alternative
   representation of information received on the PSTN circuit.  If the
   T.38 data is encapsulated in RTP, the gateways can easily transition
   from audio to fax and back again and can simply use the payload type
   to indicate the type of media that it is currently transmitting.

   With these considerations in mind, the ITU-T amended T.38 [1] to
   allow RTP to be used to transport T.38.  With that, a new MIME
   registration (audio/t38) is needed to allow for T.38 to be switched
   along with audio within the same RTP session.

4.   IANA Considerations

   One new MIME type and associated RTP payload format is to be
   registered, as described below.

   To: ietf-types@iana.org
   Subject: Registration of Standard MIME media type audio/t38

   MIME media type name: audio

   MIME subtype name: t38

   Require parameters:
      rate: The RTP timestamp clock rate, which SHOULD be 8000Hz. The
      clock frequency MAY be set to any value, but SHOULD be set to the
      same value as for any audio packets in the same RTP stream in
      order to avoid RTP timestamp rate switching.

      T38FaxRateManagement: Indicates the fax rate management model
      as defined in T.38.  Values may be "localTCF" or "transferredTCF".
      This parameter is defined in ITU-T Recommendation T.38.

   Optional parameters:
      T38FaxFillBitRemoval: Indicates the capability to remove and
      insert fill bits in Phase C (refer to [6]), non-ECM data to
      reduce bandwidth.  This is a boolean parameter (inclusion = true,
      exclusion = false).  This parameter is defined in ITU-T
      Recommendation T.38.

      T38FaxTranscodingMMR: Indicates the ability to convert to/from MMR
      from/to the line format for increasing the compression of the data

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      and reducing the bandwidth in the packet network.  This is a
      boolean parameter (inclusion = true, exclusion = false).
      This parameter is defined in ITU-T Recommendation T.38.

      T38FaxTranscodingJBIG: Indicates the ability to convert to/from
      JBIG to reduce bandwidth.  This is a boolean parameter
      (inclusion = true, exclusion = false).  This parameter is 
defined
      in ITU-T Recommendation T.38.

      T38FaxVersion: This is the version number of ITU-T Rec. T.38.
      New versions shall be compatible with previous versions.  Absence
      of this parameter indicates version 0.  The version is expressed
      as an integer value.  This parameter is defined in ITU-T
      Recommendation T.38.

      T38FaxMaxBuffer: Indicates the maximum number of octets that can
      be stored on the remote device before an overflow condition
      occurs. It is the responsibility of the transmitting application
      to limit the transfer rate to prevent an overflow. The negotiated
      data rate should be used to determine the rate at which data is
      being removed from the buffer.  Value is an integer.  This
      parameter is defined in ITU-T Recommendation T.38.

      T38FaxMaxDatagram: The maximum size of the payload within an RTP
      packet that can be accepted by the remote device.  This is an
      integer value.  This parameter is defined in ITU-T Recommendation
      T.38.

   Encoding considerations:
      The encoding of the IFP RTP packets is defined in ITU-T
      Recommendation T.38.  This sub-type is not intended for
      use with e-mail.

   Security considerations:

      See Section 6 of RFC XXXX.

   Interoperability considerations:

      ITU-T Recommendation T.38 defines the procedures, syntax, and
      parameters for the carriage of T.38 over RTP within the context
      of H.323 [8], SIP [9], and H.248 [7] systems.

   Published specification:

      ITU-T Recommendation T.38, "Procedures for real-time Group 3
      facsimile communication over IP networks", (2002) with Amendment
      2, January 2004.

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   Applications which use this media type:

      Real-time facsimile (fax)

   Additional information:

       Magic number(s):
       File extension(s):
       Macintosh File Type Code(s):

   Person & email address to contact for further information:

       Paul E. Jones
       paulej@packetizer.com

   Intended usage: COMMON

   Author/Change controller: Paul E. Jones

5.   SDP Mapping of MIME Parameters

   The MIME information described in section 4 is utilized in SDP in
   order to establish T.38 media streams.  Specifically:

      o  The MIME type ("audio") goes in SDP "m=" as the media name.

      o  The MIME subtype ("t38") goes in SDP "a=rtpmap" as the
         encoding name.

      o  The parameter "rate" also goes in "a=rtpmap" as clock
         rate.

   The MIME type defines several required and optional parameters to
   qualify the operation of T.38; these are to be used as defined in RFC
   3555 [5], Section 2.  The parameters are provided as a semi-colon
   separated list of "parameter" or "parameter=value" pairs using the
   "a=fmtp" parameter defined in SDP [2]; the "parameter" form is used
   for boolean values, where presence equals "true" and absence "false".

   Consider the following example, which describes a media stream that
   allows the transport of G.711 audio and T.38 fax information:

      m=audio 6800 RTP/AVP 0 98
      a=rtpmap:98 t38/8000
      a=fmtp:98 T38FaxVersion=2;T38FaxRateManagement=transferredTCF

6.   Security Considerations

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   T.38 is vulnerable to attacks that are common to other types of RTP
   and SRTP payloads.  However, unlike audio, T.38 data may be
   manipulated in ways that are more obtrusive than audio.  As examples,
   rogue packets may cause transmission failure and manipulated packets
   may alter terminal identity.

   The security considerations discussed in the RTP specification and
   any applicable RTP profile (for example, [10]) are applicable to
   T.38.  Regarding SRTP configuration, fax payloads SHOULD NOT use an
   HMAC-SHA1 authentication tag that is shorter than 80 bits.

7.   Normative References

   [1]  ITU-T Recommendation T.38, "Procedures for real-time Group 3
        facsimile communication over IP networks", 2002 with Amendment
        2, January 2004.

   [2]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [3]  Schulzrinne, et al., "RTP: A Transport Protocol for Real-Time
        Applications", RFC 3550, July 2003.

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

   [5]  Casner, S., Hoschka, P., "MIME Type Registration of RTP Payload
        Formats", RFC 3555, July 2003.

   [6]  ITU-T Recommendation T.30, "Procedures for document facsimile
        transmission in the general switched telephone network", July
        2003.

8.   Informative References

   [7]  ITU-T Recommendation H.248, "Gateway Control Protocol", May
        2002.

   [8]  ITU-T Recommendation H.323, "Packet-based multimedia
        communications systems", May 2003.

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

   [10] Baugher, et al., "The Secure Real-time Transport Protocol
        (SRTP)", RFC 3711, March 2004.

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   [11] Parsons, G., "Real-time Facsimile (T.38) - image/t38 MIME Sub-
        type Registration", RFC 3362, August 2002.

   [12] Perkins, C., et al., "RTP Payload for Redundant Audio Data", RFC
        2198, September 1997.

   [13] Casner, S., Jacobson, V., "Compressing IP/UDP/RTP Headers for
        Low-Speed Serial Links", RFC 2508, February 1999.

   [14] Koren, T., et al, "Enhanced Compressed RTP (CRTP) for Links with
        High Delay, Packet Loss and Reordering", RFC 3545, July 2003.

9.   Author's Address

   Paul E. Jones
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709, USA
   Phone: +1 919 392 6948
   Email: paulej@packetizer.com

   Hiroshi Tamura
   Ricoh Company, Ltd.
   1-3-6 Nakamagome, Ohta-ku,
   Tokyo 143-8555 Japan
   Phone: +81-3-3777-8124
   Fax: +81-3-5742-8859
   Email: tamura@toda.ricoh.co.jp

10.    Full Copyright Statement

   Copyright (C) The Internet Society (2004).

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   retain all their rights.

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