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rfc:rfc4612

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).

Abstract

 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

Jones & Tamura Historic [Page 1] 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",
 "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 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.

Jones & Tamura Historic [Page 2] 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.

Jones & Tamura Historic [Page 3] 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
    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 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.

Jones & Tamura Historic [Page 4] 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
    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

Jones & Tamura Historic [Page 5] 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
      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.

Jones & Tamura Historic [Page 6] 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

Jones & Tamura Historic [Page 7] RFC 4612 Real-time Facsimile (T.38) - audio /t38 August 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Jones & Tamura Historic [Page 8]

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