AVT Working Group
Internet Draft P. Jones
<draft-jones-avt-audio-t38-02.txt> Cisco Systems, Inc.
Expires: February 2004 November 2003
Real-Time Facsimile (T.38) - audio/t38
MIME Sub-type Registration
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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[Note to RFC Editor: All references to RFC XXXX are to be replaced by
references to the RFC number of this memo, when published. Also, the
reference to ITU-T Recommendation T.38 Amendment YYYY must be
replaced with the appropriate amendment number.]
Abstract
This document defines the MIME sub-type audio/t38. The packetization
and 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
3. Mechanisms for Transporting T.38 over an IP Network............2
4. IANA Considerations............................................3
5. SDP Mapping of MIME Parameters.................................4
6. Security Considerations........................................4
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7. Normative References...........................................5
8. Informative References.........................................5
9. Author's Address...............................................6
10. Full Copyright Statement......................................6
1. Introduction
ITU-T Recommendation T.38 [1] defines the Internet Facsimile Protocol
(IFP) and packetization of that protocol for carriage 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 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 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 on-going within the IETF.
There are clear advantages in using RTP for T.38 today. For example,
using RTP allows one to take advantage of the redundancy [11], header
compression [11][12], 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 firewall and NAT
devices, multipoint control units (MCUs), and media gateways that
bridge two IP networks.
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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 is equal to the sampling
rate. If the T.38 stream is associated with other audio streams
in an RTP session, the chosen rate SHOULD match that of the other
streams. Otherwise, a rate of 8000 Hz SHOULD be used.
Optional parameters: none
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 5 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 [7], SIP [8], and H.248 [6] systems.
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Published specification:
ITU-T Recommendation T.38, "Procedures for real-time Group 3
facsimile communication over IP networks", with Amendment YYYY,
January 2004.
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 is described in section 5 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.
Consider the following example, which describes a media stream that
allows the transport of G.711 audio and T.38 fax information:
m=audio 6808 RTP/AVP 0 98
a=rtpmap:98 t38/8000
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
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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 [3] and SRTP [5]
specification and any applicable RTP profile (for example, [9]) 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",Amendment YYYY,
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] Baugher, et al., "The Secure Real-time Transport Protocol",
draft-ietf-avt-srtp-09.txt, July 2003.
8. Informative References
[6] ITU-T Recommendation H.248, "Gateway Control Protocol", May
2002.
[7] ITU-T Recommendation H.323, "Packet-based multimedia
communications systems", May 2003.
[8] 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.
[9] Schulzrinne, H. and Casner, S. "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551, July 2003.
[10] Parsons, G., " Real-time Facsimile (T.38) - image/t38 MIME Sub-
type Registration", August 2002.
[11] Perkins, C., et al., "RTP Payload for Redundant Audio Data",
September 1997.
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[11] Casner, S., Jacobson, V., "Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links", February 1999.
[12] Koren, T., et al, "Enhanced Compressed RTP (CRTP) for Links with
High Delay, Packet Loss and Reordering", July 2003.
9. Author's Address
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
Phone: +1 919 392 6948
Email: paulej@packetizer.com
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11. Full Copyright Statement
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