Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration
RFC 4612

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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).

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