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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


   This document is an Internet-Draft. Internet-Drafts are working
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         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.

Wedlund/Jiang/Schulzrinne                                     [Page 1]

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

Wedlund/Jiang/Schulzrinne                                     [Page 2]

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:

     o=faxgw1 2890844526 2890842807 IN IP4
     s=FAX message
     t=2873397496 0
     c=IN IP4
     m=application 49170 udp t38

   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

6 Authors' Addresses

   Elin Wedlund
   Ericsson Telecom AB
   S-126 25 Stockholm
   electronic mail:  elin.wedlund@etx.ericsson.se

   Wenyu Jiang
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   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
   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.

Wedlund/Jiang/Schulzrinne                                     [Page 4]

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