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Network Working Group                                           P. Jones
Request for Comments: 4612                           Cisco Systems, Inc.
Category: Historic                                             H. Tamura
                                                     Ricoh Company, LTD.
                                                             August 2006

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

Status of This Memo

   This memo defines a Historic Document for the Internet community.  It
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


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

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   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 .........................................6
   7. Normative References ............................................6
   8. Informative References ..........................................6

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

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",
   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 UDP Transport Layer (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 RTP Control Protocol (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
   understand RTP.  This includes applications such as firewalls,
   Network Address Translation (NAT) devices, and gateways that bridge
   two IP networks, which generally support RTP before most other real-
   time media.

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

   Lastly, since today most T.38 data is generated by gateways that
   bridge two Public Switched Telephone Network (PSTN) networks, it is
   quite natural to expect that 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 has been
   registered, by the IANA 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

   Required parameters:

      rate:  The RTP timestamp clock rate, which SHOULD be 8000Hz.  The
      clock frequency MAY be set to any value, but it SHOULD be set to
      the same value as that 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.

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

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

   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

   Security considerations:

      See Section 6 of RFC 4612.

   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.

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

   Published specification:

      ITU-T Recommendation T.38, "Procedures for real-time Group 3
      facsimile communication over IP networks", September 2005

   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

   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

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

6.  Security Considerations

   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.  For example,
   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", September 2005.

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

   [3]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", STD 64,
        RFC 3550, July 2003.

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

   [5]  Casner, S. and P. Hoschka, "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

8.  Informative References

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

   [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.

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

   [10] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
        Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
        3711, March 2004.

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

   [14] Koren, T., Casner, S., Geevarghese, J., Thompson, B., and P.
        Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High
        Delay, Packet Loss and Reordering", RFC 3545, July 2003.

Authors' Addresses

   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@cs.ricoh.co.jp

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RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006

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