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Internet Engineering Task Force MMUSIC WG
Internet Draft Wedlund/Jiang/Schulzrinne
ietf-mmusic-sdp-t38-00.txt Ericsson/Columbia U./Columbia U.
December 15, 1998
Expires: June 1999
SDP Extensions for Fax over IP Using T.38
STATUS OF THIS MEMO
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited.
ABSTRACT
Fax over IP is currently using SMTP, i.e. the fax is sent
as an e-mail. It would be desireable to support a fax
delivery that can return status of the transmission in
real time, such as whether the phone number or address
was correct, whether the remote side was busy, etc. The
ITU is standardizing a protocol for transferring real
time fax over IP, T.38. This standard is meant to be
used with the H.323 standard, but it is also possible to
use it together with SIP, provided that SDP is extended
to support the necessary parameters. This document
defines extensions to SDP to support the use of T.38 for
real-time fax.
1 Introduction
Fax is a popular means for transferring documents between locations.
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Internet Draft T38 December 15, 1998
Traditionally, this has been done over the telephone network, as
defined in the ITU specification T.30 [1]. Some of the reasons for
the populatity of fax is that the sender of a fax gets a notification
that the fax has been successfully sent, and that the receiver gets
information on the senders telephone number and the time the fax was
received. Another reason is that fax transmission over a telephone
connection can not easily be eavesdropped. When introducing fax over
IP, these benefits should be preserved. The ITU standard T.38 [2]
defines how to transport the fax signals over IP. It is expected that
the fax channel is set up by some other means, e.g., through H.323 or
SIP. Annex D of H.323 [3] describes how H.323 supports fax over IP.
In this document we will describe how SIP and SDP can do the same. It
would probably be possible to even support the T.38 scope with SIP
and SDP, but that is for future work.
2 Introduction to T.38
The ITU T.38 recommendation defines how to transfer fax in realtime
between fax gateways and/or IP fax machines in an IP network.
Transport of the fax signals is done either by TCP or UDP, and
reliability with UDP is achieved through error control mechanisms in
T.38, which can be either parity FEC or packet redundancy. The
recommendation assumes that a network connection has already been
established by the two (or more) peers. This is similar to
traditional fax, where a phone line is allocated before the actual
fax signalling starts. The issues that are left to be handled by
other mechanisms are addressing, identification, authentication, and
creation of the fax connection. The fax machines must also have
agreed on whether to use UDP or TCP for transport, and in case of
UDP, the error control scheme to use.
3 Session Initiation Protocol
The Session Initiation Protocol (SIP) [4] already provides mechanisms
for user (fax machine) location, caller identification, call
establishment, and authentication. No additions are needed to support
the use of T.38.
4 Extensions to SDP
The Session Description Protocol (SDP) (RFC 2327 [5]) provides
mechanisms for describing sessions. The information that needs to be
represented in SDP for T.38 is
o the fact that T.38 is to be used,
o whether to use TCP or UDP for transport, and
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Internet Draft T38 December 15, 1998
o which type of error control to be used by T.38.
Thus, the SDP message could be the following:
v=0
o=faxgw1 2890844526 2890842807 IN IP4 128.59.19.68
s=FAX message
e=faxsupport@company.com
t=2873397496 0
c=IN IP4 128.59.19.68
m=application 49170 udp t38
a=t38errctl:parFEC
In order to do this, "application/t38" needs to be registered as a
MIME type according to the recommendations in [5]. The choice of TCP
or UDP can already be represented in SDP, and the error control
scheme should be represented as an attribute.
5 Security Considerations
SIP provides security mechanisms for authentication of caller, and
encryption of SIP messages including the SDP payload. For the T.38
flow, IP security mechanisms, as defined in RFC 1825 [6], can be
used.
6 Authors' Addresses
Elin Wedlund
Switchlab
Ericsson Telecom AB
S-126 25 Stockholm
SWEDEN
electronic mail: elin.wedlund@etx.ericsson.se
Wenyu Jiang
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: wenyu@cs.columbia.edu
Henning Schulzrinne
Dept. of Computer Science
Columbia University
Wedlund/Jiang/Schulzrinne [Page 3]
Internet Draft T38 December 15, 1998
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
7 Bibliography
[1] International Telecommunication Union, "Procedures for document
facsimile transmission in the general switched telephone network,"
Recommendation T.30, Telecommunication Standardization Sector of ITU,
Geneva, Switzerland, July 1996.
[2] International Telecommunication Union, "Procedures for real-time
group 3 facsimile communication over IP networks," Recommendation
T.38, Telecommunication Standardization Sector of ITU, Geneva,
Switzerland, June 1998.
[3] International Telecommunication Union, "Visual telephone systems
and equipment for local area networks which provide a non-guaranteed
quality of service," Recommendation H.323, Telecommunication
Standardization Sector of ITU, Geneva, Switzerland, May 1996.
[4] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Internet Draft, Internet Engineering
Task Force, Nov. 1998. Work in progress.
[5] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[6] R. Atkinson, "Security architecture for the internet protocol,"
RFC 1825, Internet Engineering Task Force, Aug. 1995.
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