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Internet Engineering Task Force                 Eric Zimmerer
Internet Draft                         Level 3 Communications
draft-ietf-mmusic-sip-isup-mime-00.txt          Aparna Vemuri
July 1999                              Level 3 Communications
Expires: January 2000

                       The SIP ISUP/MIME type

Status of this Memo

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.  Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups.  Note that other groups may also
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Internet-Drafts are draft documents valid for a maximum of six months
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1. Abstract

This document  proposes the definition of an application/ISUP media
type, according to the rules defined in RFC 2048 [1].

2. Introduction

ISUP (ISDN User part) defined in the ITU-T recommendations Q.761-4 is
a signaling protocol used between telephony switches. There exists a
need to transport ISUP messages between SoftSwitches as being part of
the payload of SIP [2] messages. The following discussion is specific
to this usage and would not apply to the transportation of ISUP
messages in other applications.

3. The application/ISUP media type

The ISUP messages are composed of arbitrary binary data. The best way
to encode these would be to use binary encoding. This is in
conformance with the restrictions imposed on the use of binary data

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Internet Draft     The SIP ISUP/MIME type            July 1999

conformance with the restrictions imposed on the use of binary data
for MIME (RFC 2045 [3]). It should be noted that the rules mentioned
in the RFC 2045 apply to Internet mail messages and not to SIP
messages. Binary has been preferred over Base64 encoding because
the latter would only result in adding bulk to the encoded messages
as well as prove costly in terms of processing power.
This media type is defined by the following information:

Media type name: application
Media subtype name: ISUP
Required parameters: none
Optional parameters: version
Encoding scheme: binary
Security considerations: See section 5.

Note: It is mandatory for SoftSwitches to specify the 'version' of
the ISUP message. Proxies, redirect servers, etc., have no need to
process/specify this information.

The use of the 'version' parameter allows differentiation between
different ISUP variants. This enables the terminating SoftSwitch (also
known as media gateway) to recognize and parse the message correctly,
or (possibly) to reject the message if the  particular ISUP variant is
not supported. The idea here is to allow to specify a preference of
version, so that the following scenarios are possible: "I only like
application/isup;version=lcd" or "I accept application/isup (but don't
really know the details; I just pass them on to some other tool that
displays/munges them)".
The following is how a typical header would look:-

        Content-Type: application/ISUP
        Version: ETSIv1
        Content-Transfer-Encoding: binary

Table 1 is a partial list of protocol versions supported by the
'application/ISUP' media type.

        Version         Protocol
        -------         --------
        ANSI-ISUP       ANSI ISUP
        ETSI-ISUP       ETSI ISUP
        GR-317          Bellcore ISUP GR-317
        BTNUP           BT NUP
        R2              R2

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Internet Draft     The SIP ISUP/MIME type            July 1999

4. Illustrative example

SIP message format requires a Request line followed by Header lines
followed by a CRLF separator followed by the message body. To
illustrate the use of the 'application/ISUP' media type, below is
an INVITE message which has the originating SDP information and
an encapsulated ISUP IAM.

Note that the two payloads are demarcated by the boundary parameter
(specified in RFC 2046 [4]) which in the example has the value
"unique-boundary-1". This is part of the specification of MIME
multipart and is not related to the 'application/ISUP' media type.

        INVITE sip:13039263142@Den1.level3.com SIP/2.0
        From: sip:13034513355@den3.level3.com
        To: sip:13039263142@Den1.level3.com
        Call-ID: DEN1231999021712095500999@Den1.level3.com
        Content-Type: Application/ Multipart
        Content-Length: 327

        MIME-Version: 1.0
        Content-Type: multipart/mixed; boundary=unique-boundary-1

        Content-Type: application/SDP; charset=ISO-10646

        o=ezimmerer 2890844526 2890842807 IN IP4
        s=SDP seminar
        c=IN IP4 MG122.level3.com
        t= 2873397496   2873404696
        m=audio 9092 RTP/AVP 0 3 4

        Content-Transfer-Encoding: binary

        89 8b 0e 95 1e 1e 1e 06 26 05 0d f5 01 06 10 04 00


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5. Security considerations

The security mechanisms described in RFC 2543 (SIP - Session
Initiation Protocol) should suffice. No new security considerations
are thought necessary.

6. Authors

Eric Zimmerer
Level 3 Communications
Louisville, CO, USA
Phone: 303-926-3142
EMail: eric.zimmerer@level3.com

Aparna Vemuri
Level 3 Communications
Louisville, CO, USA
Phone: 303-926-3768
EMail: aparna.vemuri@level3.com

7. References

[1] Freed, Klensin, Postel, "Multipart Internet Mail Extensions (MIME)
Part Four: Registration Procedures" RFC 2048, Internet Engineering
Task Force, November 1996.

[2] Handley, Schulzrinne, Schooler and Rosenberg, "Session Initiation
Protocol (SIP)" RFC 2543, Internet Engineering Task Force, March 1999.

[3] Freed, Borenstein, "Multipart Internet Mail Extensions (MIME) Part
One: Format of Internet Message Bodies" RFC 2045, Internet Engineering
Task Force, November 1996.

[4] Freed, Borenstein, "Multipart Internet Mail Extensions (MIME) Part
Two: Media Types" RFC 2046, Internet Engineering Task Force, November

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