MIME media types for ISUP and QSIG Objects

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2001). All Rights Reserved.

Abstract

This document describes MIME types for application/ISUP and application/QSIG objects for use in SIP applications, according to the rules defined in RFC 2048. These types can be used to identify ISUP and QSIG objects within a SIP message such as INVITE or INFO, as might be implemented when using SIP in an environment where part of the call involves interworking to the PSTN.

1. Introduction

ISUP (ISDN User part) defined in the ITU-T recommendations Q.761-4 is a signaling protocol used between telephony switches. There are configurations in which ISUP-encoded signaling information needs to be transported between SIP entities as part of the payload of SIP messages, where the preservation of ISUP data is necessary for the proper operation of PSTN features.

QSIG is the analogous signaling protocol used between private branch exchanges to support calls within private telephony networks. There is a similar need to transport QSIG-encoded signaling information between SIP entities in some environments.

This document is specific to this usage and would not apply to the transportation of ISUP or QSIG messages in other applications. These media types are intended for ISUP or QSIG application information that is used within the context of a SIP session, and not as general purpose transport of SCN signaling.

The definition of media types for ISUP and QSIG application information does not address fully how the non-SIP and SIP entities exchanging messages determine or negotiate compatibility. It is assumed that this is addressed by alternative means such as the configuration of the interworking functions.

This is intended to be an IETF approved MIME type, and to be defined through an RFC. NOTE: usage of Q.SIG within SIP is neither endorsed nor recommended as a result of this MIME registration.

3. Proposed new media types

   ISUP and QSIG messages are composed of arbitrary binary data that is
   transparent to SIP processing. The best way to encode these is to use
   binary encoding. This is in 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 and possibly be more costly in terms of
   processing power.

3.1 ISUP Media Type

This media type is defined by the following information:

Media type name: application
Media subtype name: ISUP
Required parameters: version
Optional parameters: base
Encoding scheme: binary
Security considerations: See section 5.

The ISUP message is encapsulated beginning with the Message Type Code (i.e., omitting Routing Label and Circuit ID Code).

   The use of the 'version' parameter allows network administrators to
   identify specific versions of ISUP that will be exchanged on a
   bilateral basis. This enables a particular client such as a
   SoftSwitch/Media Gateway Controller to recognize and parse the
   message correctly,  or (possibly) to reject the message if the
   specified ISUP version is not supported. This specification places no
   constraints on the values that may be used in 'version'; these are
   left to the discretion of the network administrator.

This 'version' could, for example, be used to identify a network- specific implementation of ISUP, e.g., X-NetxProprietaryISUPv3, or to identify a well-known standard version of ISUP, e.g., itu-t or ansi.

A 'base' parameter can optionally be included in some cases (e.g., if the receiver may not recognize the 'version' string) to specify that the encapsulated ISUP can also be processed using the identified 'base' specification. Table 1 provides a list of 'base' values supported by the 'application/ISUP' media type, including whether or not the forward compatibility mechanism defined in ITU-T 1992 ISUP is supported.

Table 1: ISUP 'base' values

      base             protocol                 compatibility
      
      itu-t88          ITU-T Q.761-4 (1988)     no
      itu-t92+         ITU-T Q.761-4 (1992)     yes
      ansi88           ANSI T1.113-1988         no
      ansi00           ANSI T1.113-2000         yes
      etsi121          ETS 300 121              no
      etsi356          ES 300 356               yes
      gr317            BELLCORE GR-317          no
      ttc87            JT-Q761-4(1987-1992)     no
      ttc93+           JT-Q761-4(1993-)         yes

The Content-Disposition header [5] may be included to describe how the encapsulated ISUP is to be processed, and in particular what the handling should be if the received Content-Type is not recognized. The default disposition-type for an ISUP message body is "signal". This type indicates that the body part contains signaling information associated with the session, but does not describe the session.

Supplementing the description of the Content-Disposition header in [5], as well as any characterization of the Content-Disposition header in the SIP standard, is the following BNF describing disposition-types and disposition-params that may be used in the header of ISUP and QSIG MIME bodies.

      Content-Disposition   =  "Content-Disposition" ":"
                           disposition-type *( ";" disposition-param )
      disposition-type      =  "signal" |  disp-extension-token
      disposition-param     =  "handling" "="
                           ( "optional" | "required" | other-handling )
                           |   generic-param
      other-handling        =  token
      disp-extension-token  =  token

A full definition of the use of the "handling" parameter is given in the IANA Considerations section below. The following is how a typical header would look ('base' may be omitted):

      Content-Type: application/ISUP; version=nxv3; base=etsi121
      Content-Disposition: signal; handling=optional

3.2 QSIG Media Type

The application/QSIG media type is defined by the following information:

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

The use of the 'version' parameter allows identification of different QSIG variants. This enables the terminating Connection Server to recognize and parse the message correctly, or (possibly) to reject the message if the particular QSIG variant is not supported.

Table 2 is a list of protocol versions supported by the 'application/QSIG' media type.

Table 2: QSIG versions

         version         protocol
         -------         --------
         iso             ISO/IEC 11572 (Basic Call) and
                         ISO/IEC 11582 (Generic Functional Protocol)

The following is how a typical header would look (Content-Disposition not included in this instance):

      Content-Type: application/QSIG; version=iso

The default Content-Disposition disposition-type is "signal" as in an ISUP body part. The "handling" parameter described above can also be used for QSIG bodies.

4. Illustrative examples

4.1 ISUP

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

         INVITE sip:13039263142@Den1.level3.com SIP/2.0
         Via: SIP/2.0/UDP den3.level3.com
         From: sip:13034513355@den3.level3.com
         To: sip:13039263142@Den1.level3.com
         Call-ID: DEN1231999021712095500999@Den1.level3.com
         CSeq: 8348 INVITE
         Contact: <sip:jpeterson@level3.com>
         Content-Length: 436
         Content-Type: multipart/mixed; boundary=unique-boundary-1
         MIME-Version: 1.0
         
         --unique-boundary-1
         Content-Type: application/SDP; charset=ISO-10646
         
         v=0
         o=jpeterson 2890844526 2890842807 IN IP4 126.16.64.4
         s=SDP seminar
         c=IN IP4 MG122.level3.com
         t= 2873397496  2873404696
         m=audio 9092 RTP/AVP 0 3 4
         --unique-boundary-1
         Content-Type: application/ISUP; version=nxv3;
         base=etsi121
         Content-Disposition: signal; handling=optional
         01 00 49 00 00 03 02 00 07 04 10 00 33 63 21
         43 00 00 03 06 0d 03 80 90 a2 07 03 10 03 63
         53 00 10 0a 07 03 10 27 80 88 03 00 00 89 8b
         0e 95 1e 1e 1e 06 26 05 0d f5 01 06 10 04 00
         --unique-boundary-1--
   
   Note: Since binary encoding is used for the ISUP payload, each byte
   is encoded as a byte, and not as a  two-character hex representation.
   Hex digits were used in the document because a literal encoding of
   those bytes would have been confusing and unreadable.

4.2 QSIG

To illustrate the use of the 'application/QSIG' media type, below is an INVITE message which has the originating SDP information and an encapsulated QSIG SETUP message.

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/QSIG' media type.

         INVITE sip:14084955072@sc1.nortelnetworks.com SIP/2.0
         Via: SIP/2.0/UDP sc10.nortelnetworks.com
         From: sip:14085655675@sc10.nortelnetworks.com
         To: sip:14084955072@sc1.nortelnetworks.com
         Call-ID: 1231999021712095500999@sc12.nortelnetworks.com
         CSeq: 1234 INVITE
         Contact: <sip:14085655675@sc10.nortelnetworks.com>
         Content-Length: 358
         Content-Type: multipart/mixed; boundary=unique-boundary-1
         MIME-Version: 1.0
         
         --unique-boundary-1
         Content-Type: application/SDP; charset=ISO-10646

v=0
o=audet 2890844526 2890842807 5 IN IP4 134.177.64.4 s=SDP seminar
c=IN IP4 MG141.nortelnetworks.com
t= 2873397496 2873404696
m=audio 9092 RTP/AVP 0 3 4

         --unique-boundary-1
         Content-type:application/QSIG; version=iso
         
         08 02 55 55 05 04 02 90 90 18 03 a1 83 01
         70 0a 89 31 34 30 38 34 39 35 35 30 37 32
         --unique-boundary-1--

5. Security considerations

Information contained in ISUP and QSIG bodies may include sensitive customer information, potentially requiring use of encryption.

Security mechanisms are provided in RFC 2543 (SIP - Session Initiation Protocol) and should be used as appropriate for both the SIP message and the encapsulated ISUP or QSIG body.

6. IANA considerations

This document registers the "application/ISUP" and "application/QSIG" MIME media types.

Registrations for the 'version' symbols used within the ISUP and QSIG MIME types must specify a definitive specification reference, identifying a particular issue of the specification, to which the new symbol shall refer. Identifying a definite specification reference requires a review process; the authors recommend that a subject matter expert be designated as described in RFC 2434 [6] under Expert Review.

Note that where a specification is fully peer-to-peer backwards compatible with a previous issue (i.e., the compatibility mechanism is supported by both), then there is no need for separate symbols to be registered. The symbol for the original specification should be used to identify backwards-compatible upgrades of that specification as well.

Symbols beginning with the characters 'X-' are reserved for non- standard usage (e.g., cases in which a token other than a string representing an issue of an ISUP specification is appropriate for characterizing ISUP within an administrative domain). Such non- standard version can only be transmitted between administrative domains in accordance with a bilateral agreement. These symbols should be administered under the Private Use policy described in RFC 2434.

This document registers a new disposition-type for the Content- Disposition header, 'signal', to be used when a MIME body contains supplemental signaling information (ISUP and QSIG as MIME bodies being examples of this).

This document also defines a Content Disposition parameter, "handling". The handling parameter, handling-parm, describes how the UAS should react if it receives a message body whose content type or disposition type it does not understand. If the parameter has the value "optional", the UAS MUST ignore the message body; if it has the value "required", the UAS MUST return 415 (Unsupported Media Type). If the handling parameter is missing, the value "required" is to be assumed.

7. Authors Addresses

Eric Zimmerer
Rankom Inc.
19500 Pruneridge Ave Suite #4303
Cupertino, CA 95014, USA
EMail: eric_zimmerer@yahoo.com

   Aparna Vemuri
   Qwest Communications
   6000 Parkwood Pl
   Dublin, OH 43016, USA
   EMail: Aparna.Vemuri@Qwest.com
   
   Jon Peterson
   NeuStar, Inc
   1800 Sutter Street, Suite 570
   Concord, CA 94520, USA
   EMail: jon.peterson@neustar.com
   
   Lyndon Ong
   Ciena
   Cupertino, CA, USA
   EMail: lyndon_ong@yahoo.com

Francois Audet
Nortel Networks
4301 Great America Parkway
Santa Clara, CA 95054, USA
EMail: mzonoun@nortelnetworks.com

Mo Zonoun
Nortel Networks
4301 Great America Parkway
Santa Clara, CA 95054, USA
EMail: audet@nortelnetworks.com

   M. Watson
   Nortel Networks
   Maidenhead, UK
   EMail: mwatson@nortelnetworks.com

8. References

   [1] Freed, N., Klensin, J. and J. Postel, "Multipart Internet Mail
       Extensions (MIME) Part Four: Registration Procedures", BCP 13,
       RFC 2048, November 1996.
   
   [2] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
       "Session Initiation Protocol (SIP)", RFC 2543, March 1999.
   
   [3] Freed, N. and N. Borenstein, "Multipart Internet Mail Extensions
       (MIME) Part One: Format of Internet Message Bodies", RFC 2045,
       November 1996.
   
   [4] Freed, N. and N. Borenstein, "Multipart Internet Mail Extensions
       (MIME) Part Two: Media Types", RFC 2046, November 1996.
   
   [5] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
       Information in Internet Messages: The Content-Disposition Header
       Field", RFC 2183, August 1997.
   
   [6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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