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 (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 ....................................................22. Conventions Used in This Document ...............................23. Mechanisms for Transporting T.38 over an IP Network .............24. IANA Considerations .............................................35. SDP Mapping of MIME Parameters ..................................56. Security Considerations .........................................67. Normative References ............................................68. Informative References ..........................................6
ITU-T Recommendation T.38  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  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 .
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  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) .
There are clear advantages in using RTP for T.38 today. For example,
using RTP allows one to take advantage of the redundancy , header
compression , 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-
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  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: email@example.com Subject: Registration of Standard MIME media
MIME media type name: audio
MIME subtype name: t38
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.
T38FaxFillBitRemoval: Indicates the capability to remove and
insert fill bits in Phase C (refer to ), non-ECM data to reduce
bandwidth. This is a boolean parameter (inclusion = true,
exclusion = false). This parameter is defined in ITU-T
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
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
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
The encoding of the IFP RTP packets is defined in ITU-T
Recommendation T.38. This sub-type is not intended for use with
See Section 6 of RFC 4612.
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 , SIP , and H.248  systems.
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)
Magic number(s): File extension(s): Macintosh File Type Code(s):
Person & email address to contact for further information:
Paul E. Jones firstname.lastname@example.org
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 , 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 ; 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
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, ) 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
 ITU-T Recommendation T.38, "Procedures for real-time Group 3
facsimile communication over IP networks", September 2005.
 Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
 Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Casner, S. and P. Hoschka, "MIME Type Registration of RTP
Payload Formats", RFC 3555, July 2003.
 ITU-T Recommendation T.30, "Procedures for document facsimile
transmission in the general switched telephone network", July
8. Informative References
 ITU-T Recommendation H.248, "Gateway Control Protocol", May
 ITU-T Recommendation H.323, "Packet-based multimedia
communications systems", May 2003.
 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.
 Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
 Parsons, G., "Real-time Facsimile (T.38) - image/t38 MIME Sub-
type Registration", RFC 3362, August 2002.
 Perkins, C., et al., "RTP Payload for Redundant Audio Data", RFC
2198, September 1997.
 Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links", RFC 2508, February 1999.
 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.
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709, USA
Phone: +1 919 392 6948
Ricoh Company, LTD.
1-3-6 Nakamagome, Ohta-ku,
Tokyo 143-8555 Japan
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