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Versions: 00 01 02 03 04 RFC 4715

Network Working Group                                        M. Munakata
Internet-Draft                                               S. Schubert
Expires: August 30, 2006                                         T. Ohba
                                                                     NTT
                                                       February 26, 2006


               ISDN subaddress encoding type for tel URI
                 draft-munakata-iptel-isub-type-00.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Without a tel URI parameter to carry encoding type of Integrated
   Services Digital Network(ISDN) subaddress, interworking between ISDN
   User Part(ISUP) network and Session Initiation Protocol(SIP) network
   is impossible in some cases.  To solve the problem, this document
   specifies a new tel URI parameter to carry encoding type of ISDN
   subaddress.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  SIP-ISDN Interconnection . . . . . . . . . . . . . . . . .  5
     3.2.  ISDN-SIP-ISDN Interconnection  . . . . . . . . . . . . . .  7
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Parameter Definition . . . . . . . . . . . . . . . . . . . . .  9
   6.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Gateway behavior . . . . . . . . . . . . . . . . . . . . . 10
     6.2.  Length and format of isub parameter  . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Structure of NSAP Addresses  . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative Reference  . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative Reference  . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16































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1.  Introduction

   RFC3966[2] defines the tel URI parameter "isub" that is designed to
   carry Integrated Services Digital Network(ISDN) subaddresses.

   In a ISDN User Part(ISUP) message, NSAP address[6] or user specified
   address can be carried as ISDN subaddress.  Network Service Access
   Point(NSAP) address accommodates various types of address information
   along with an identifier that shows the address type and its encoding
   type.

   "isub" parameter can also carry any type of address but RFC3966
   doesn't define a solution to carry information of subaddress type
   (whether the subaddress is NSAP address or not) and an identifier of
   an encoding type.

   Most commonly used encoding type for ISDN subaddress is International
   Alphabet 5(IA5)[5].  RFC 3966 says "ISDN subaddresses typically
   contain IA5 characters but may contain any octet value".
   Nevertheless, IA5 is just one of the encoding types among various
   types of encoding used in NSAP address.  Therefore parameter to carry
   isub value alone is not enough to describe ISDN subaddresses, and
   additional information is needed.

      Lack of information describing the encoding type of "isub"
      parameter will most likely make it difficult for ISDN terminal
      receiving ISDN subaddress from SIP network(SIP-ISDN
      Interconnection) to interpret the "isub" parameter value as
      gateway may translate it using the encoding type not anticipated.
      It will also make it difficult to recover the original ISDN
      subaddress value when ISUP message is translated to SIP message
      and back to ISUP message(ISDN-SIP-ISDN Interconnection), as there
      is no place holder to carry the encoding type currently in SIP
      message, the encoding type that was present in the original ISUP
      message will be lost.

   To solve the issue, this specification defines "isub-type" parameter
   to carry information whether the value of "isub" parameter is NSAP
   address or not and its encoding type.  In addition, this document
   specifies the meaning of the value in "isub" parameter.











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2.  Terminology

   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[1].














































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3.  Problem Statement

   Without a tel URI parameter to carry encoding type of ISDN
   subaddress, such problems shown in 3.1 and 3.2 would happen.

3.1.  SIP-ISDN Interconnection

   The following diagram shows an issue that will be observed when
   interworking between SIP network and ISDN network addressing an ISDN
   subaddress.  When a SIP equipment sends a request with "isub"
   parameter to address an ISDN terminal behind Private Branch
   Exchange(PBX), the encoding type of ISDN subaddress currently can not
   be specified.  Therefore gateway between SIP network and ISDN network
   can not translate the value of "isub" into ISUP Initial Address
   Message(IAM) properly because the encoding type information of the
   ISDN subaddress is missing.



































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                                                   ISDN Terminal
                                                         +-----+
                                                    |--->| Bob |
                  SIP Network <---|---> ISDN        |    |12345|
                                                    |    +-----+
     SIP Equipment                                  |
      +-----+      +-----+      +----+      +-----+ |    +-----+
      |Alice|----->|Proxy|----->| GW |----->| PBX |----->|Carol|
      +-----+      +-----+      +----+      +-----+ |    +-----+
                                                    |
                                                    |    +-----+
                                                    |--->|David|
                                                         +-----+


       Alice       Proxy          GW   Switch  PBX         Bob
         |           |             |      |     |           |
         |  INVITE   |             |      |     |           |
         |---------->|   INVITE    |      |     |           |
         |           |------------>| IAM  |     |           |
         |           |             |----->|SETUP|           |
         |           |             |      |---->|   SETUP   |
         |           |             |      |     |---------->|
         |           |             |      |     |           |

                Figure 1: SIP-ISDN Interconnection



         INVITE tel:+17005554141;isub=12345 SIP/2.0


         Note: SETUP is an ISDN message used between ISDN switch and
               ISDN end terminal.

















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3.2.  ISDN-SIP-ISDN Interconnection

   Following diagram shows the issue that will be observed when
   interworking message with ISDN subaddress between ISDN network that
   traverses through SIP network.  When an ISDN terminal sends a message
   that contains ISDN subaddress with its encoding type information,
   Gateway 1 translates the subaddress into "isub" parameter in SIP
   message.  However, its encoding type is dropped because there is no
   room for encoding type in SIP message.  When Gateway 2 receives
   "isub", it can not translate "isub" value into IAM message properly
   because the encoding type information of the ISDN subaddress is
   missing.



                                                           ISDN Terminal
                                                                 +-----+
                                                            |--->| Bob |
         ISDN  <---|--->  SIP Network  <---|---> ISDN       |    |12345|
                                                            |    +-----+
   ISDN Terminal                                            |
   +-----+      +-----+     +-----+     +-----+     +-----+ |    +-----+
   |Alice|----->| GW1 |---->|Proxy|---->| GW2 |---->| PBX |----->|Carol|
   +-----+      +-----+     +-----+     +-----+     +-----+ |    +-----+
                                                            |
                                                            |    +-----+
                                                            |--->|David|
                                                                 +-----+

    Alice  Switch  GW1        Proxy        GW2  Switch  PBX         Bob
      |       |     |           |           |     |     |            |
      | SETUP |     |           |           |     |     |            |
      |------>| IAM |           |           |     |     |            |
      |       |---->|  INVITE   |           |     |     |            |
      |       |     |---------->|  INVITE   |     |     |            |
      |       |     |           |---------->| IAM |     |            |
      |       |     |           |           |---->|SETUP|            |
      |       |     |           |           |     |---->|   SETUP    |
      |       |     |           |           |     |     |----------->|
      |       |     |           |           |     |     |            |

                Figure 2: ISDN-SIP-ISDN Interconnection



             INVITE tel:+17005554141;isub=12345 SIP/2.0





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4.  Requirements

   The followings are requirements for a solution to carry ISDN
   subaddress along with information of subaddress encoding type.

   Req 1:  When "isub" parameter is present in a SIP message, the type
           of ISDN subaddress (whether it is an NSAP address or not) and
           its encoding type MUST be identifiable.

   Req 2:  It MUST be possible for entity setting the "isub" parameter
           in a SIP message, to specify the type of ISDN
           subaddress(whether it is an NSAP address or not) and the
           encoding type used.






































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5.  Parameter Definition

   The parameter defined in this draft is represented as a tel URI,
   which describes the encoding type information of the ISDN
   subaddress(ISUB Type).  It accommodates some of the information that
   is lacking in the "isub" parameter defined in RFC3966[2].  It is an
   optional parameter to tel URI.  The ABNF[3] syntax is as follows.



   isub-type           = isub-type-tag "=" isub-type-value
   isub-type-tag       = "isub-type"
   isub-type-value     = "nsap-ia5" / "nsap-bcd" / "nsap" / token


   The semantics of these "isub-type" values are described below:

   nsap-ia5: Indication that the "isub" parameter value needs to be
             encoded using IA5(AFI=0x50) when translated to ISUP
             message.

   nsap-bcd: Indication that the "isub" parameter value needs to be
             encoded using Binary Coded Decimal(BCD)(AFI=0x48) when
             translated to ISUP message.

   nsap:     Indication that the "isub" parameter value needs to be
             encoded using encoding type defined in ISO8348 other than
             IA5 (AFI=0x50) or BCD(AFI=0x48).


   An example of the syntax of the "isub-type" parameter (in a small
   fragment of a SIP[4] message) is given below:

      INVITE tel:+17005554141;isub=12345;isub-type=nsap-ia5 SIP/2.0
      To: <tel:+17005554141;isub=12345;isub-type=nsap-ia5>
      From: "Bob" <sip:bob@biloxi.example.com>;tag=1928301774















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6.  Usage

   It is anticipated that this URI parameter will be used along with the
   "isub" parameter defined in RFC3966[2] when interworking between ISUP
   networks and SIP network.  The URI parameter defined here is an
   optional parameter to the tel URI.  To find out more about the NSAP
   address, please reference the Appendix in Section 8.

   If the "isub-type" is absent, and the message is interpreted by an
   entity on the SIP network, the entity SHOULD assume that the original
   ISDN subaddress from ISUP message, which was translated to the "isub"
   parameter, was decoded using IA5(AFI=0x50) and handle the message
   accordingly.

   If the "isub-type" is absent, and the message is handled by a gateway
   translating the SIP message to ISUP message, the gateway SHOULD
   encode the value in the "isub" parameter using IA5(AFI=0x50) when
   translating the message into ISUP message.

   If the "isub-type" value is "nsap", the encoding type is assumed to
   be in the first two characters of "isub" parameter in hexadecimal
   format(% HEXDIG HEXDIG).

6.1.  Gateway behavior

   Gateway receiving message/signal from ISDN network containing ISDN
   subaddress SHOULD check the encoding used for the subaddress, and set
   the equivalent "isub" parameter and "isub-type" parameter to the SIP
   message in a form of tel URI parameter.  Following text is a
   recommended guideline when translating ISUP message containing ISDN
   subaddress.

      If the ISDN subaddress is encoded using IA5(AFI=0x50), the entity
      translating the message SHOULD set "nsap-ia5" to the "isub-type"
      and value after 5th octet of the subaddress information element in
      original ISUP message as "isub" value in ASCII characters.

      If the ISDN subaddress is encoded using BCD(AFI=0x48), the entity
      translating the message SHOULD set "nsap-bcd" to the "isub-type"
      and decode the value after 5th octet of the subaddress information
      element in original ISUP message using BCD and set it as the
      "isub" value in ASCII characters(numbers).

      If the ISDN subaddress is an NSAP address but neither encoded by
      IA5(AFI=0x50) nor BCD(AFI=0x48), the entity translating the
      message SHOULD set "nsap" to the "isub-type" and value after 4th
      octet of the subaddress information element in original ISUP
      message as "isub" value in hexadecimal format(% HEXDIG HEXDIG).



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      If the ISDN subaddress is not an NSAP address, the entity
      translating the message MAY set any value as a token.

6.2.  Length and format of isub parameter

   An entity setting an "isub" parameter is RECOMMENDED to set the
   "isub-type" value with the encoding type used.  Following text is a
   recommended guideline when setting the "isub" parameter.

      If the "isub-type" is "nsap-ia5", the "isub" parameter value
      SHOULD NOT exceed 19 ASCII characters.

      If the "isub-type" is "nsap-bcd", the "isub" parameter value
      SHOULD NOT exceed 38 ASCII characters(numbers).

      If the "isub-type" is "nsap", the "isub" parameter value SHOULD
      NOT exceed 40 ASCII characters and it SHOULD be in hexadecimal
      format(% HEXDIG HEXDIG).

      Entity MAY define other type of "isub-type" as a token.































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7.  Security Considerations

   The parameter defined here adds no new security considerations to
   those discussed in [2]















































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8.  Appendix

8.1.  Structure of NSAP Addresses

   In ISUP messages, ISDN subaddress is represented as NSAP address.
   NSAP address is defined as follows in ISO8348.

   NSAP address consists of Initial Domain Part(IDP) and Domain Specific
   Part(DSP).  Then IDP consists of two fields, Authority and Format
   Identifier(AFI) and Initial Domain Identifier(IDI).  The maximum
   length of NSAP address is 20 octets.


        <----------------- NSAP Address ----------------->

        +------------------------------------------------+
        |   I D P   |                                    |
        |-----------|             D S P                  |
        | AFI | IDI |                                    |
        +------------------------------------------------+

                          Figure 3



   AFI value is two hexadecimal digits(00-FF), and it identifies IDI
   format and DSP syntax.

   IDI value is decimal digits, and it identifies a network addressing
   domain or authority responsible for allocating values of the DSP.

   Typical encoding type of ISDN subaddress, IA5, is identified with
   AFI=0x50.  When AFI value is 0x50, IDI is null, and the length of IDP
   is 2 digits(1 octet).  In this case, DSP value is a subaddress
   encoded by IA5 and its maximum length is 19 octets.
















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9.  References

9.1.  Normative Reference

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
        December 2004.

   [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

9.2.  Informative Reference

   [4]  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.

   [5]  International Telecommunications Union, "International Reference
        Alphabet (IRA) (Formerly International Alphabet No. 5 or IA5) -
        Information technology - 7-bit coded character set for
        information interchange", Recommendation T.50, 1992.

   [6]  INTERNATIONAL STANDARD, "Information technology - Open Systems
        Interconnection - Network service definition", ISO/IEC 8348.

























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Authors' Addresses

   Mayumi Munakata
   NTT Corporation

   Phone: +81 422 36 7565
   Email: munakata.mayumi@lab.ntt.co.jp


   Shida Schubert
   NTT Corporation

   Phone: +1 604 762 5606


   Takumi Ohba
   NTT Corporation
   9-11, Midori-cho 3-Chome
   Musashino-shi, Tokyo  180-8585
   Japan

   Phone: +81 422 59 7748
   Email: ohba.takumi@lab.ntt.co.jp
   URI:   http://www.ntt.co.jp

























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