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RFC 4715 - The Integrated Services Digital Network (ISDN) Subadd


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Network Working Group                                        M. Munakata
Request for Comments: 4715                                   S. Schubert
Category: Informational                                          T. Ohba
                                                                     NTT
                                                           November 2006

            The Integrated Services Digital Network (ISDN)
                  Subaddress Encoding Type for tel URI

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

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

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Problem Statement ...............................................3
      3.1. SIP-ISDN Interconnection ...................................3
      3.2. ISDN-SIP-ISDN Interconnection ..............................4
   4. Requirements ....................................................5
   5. Parameter Definition ............................................6
   6. Usage ...........................................................6
      6.1. Gateway Behavior ...........................................7
      6.2. SIP Entity Behavior ........................................8
   7. Security Considerations .........................................9
   8. IANA Considerations .............................................9
   9. Acknowledgements ................................................9
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................12

1.  Introduction

   RFC 3966 [2] defines a tel URI parameter "isub" that is designed to
   carry Integrated Services Digital Network (ISDN) subaddresses.

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

   The "isub" parameter can carry any type of address, but RFC 3966 [2]
   does not define a solution to carry information on a subaddress type
   (whether the subaddress is NSAP or user specific) or an identifier
   for the encoding type used.

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

      Lack of information describing the encoding type of ISDN
      subaddress will make it difficult for an ISDN terminal receiving
      the ISDN subaddress from the SIP network (SIP-ISDN
      Interconnection) to interpret the "isub" parameter value, as a
      gateway may translate it using a wrong encoding type and end up
      with a wrong subaddress value due to inconsistency in the encoding
      type used.  It will also make it difficult to recover the original
      ISDN subaddress value when an ISUP message is translated to a SIP
      message and translated back to the ISUP message (ISDN-SIP-ISDN
      Interconnection).  As there is no placeholder to carry the
      encoding type in the SIP message, the encoding type information
      that was present in the original ISUP message will be lost, and
      reconstructing the intended ISDN subaddress value is nearly
      impossible.

   To solve the issues presented, this specification defines an "isub-
   encoding" parameter to carry information describing whether the value
   of the "isub" parameter is an NSAP address as well as its encoding
   type.  In addition, this document specifies the accommodating values
   to be carried in the "isub" parameter for each encoding type used.

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].

3.  Problem Statement

   Without a tel URI parameter to carry an encoding type of ISDN
   subaddress, the problems described in Sections 3.1. and 3.2. might be
   observed.

3.1.  SIP-ISDN Interconnection

   The diagrams in Figure 1 show an issue that will be observed when
   interworking between SIP network and ISDN network with an ISDN
   subaddress.  When SIP equipment sends a request with an "isub"
   parameter to address an ISDN terminal behind Private Branch Exchange
   (PBX), the encoding type of the ISDN subaddress currently cannot be
   specified.  Therefore, gateway sitting between the SIP network and
   ISDN network cannot translate the value of "isub" into an ISUP
   Initial Address Message (IAM) properly as the encoding type
   information of the ISDN subaddress is missing.

                                                     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.

3.2.  ISDN-SIP-ISDN Interconnection

   The diagrams in Figure 2 show an issue that will be observed when
   interworking messages with an ISDN subaddress between two ISDN
   networks that traverses through SIP networks.  When an ISDN terminal
   sends a message that contains an ISDN subaddress along with its
   encoding type information, Gateway 1 translates the subaddress into
   an "isub" parameter in a SIP message.  However, its encoding type
   information is dropped because there is no placeholder for the
   encoding type in the SIP message.  When Gateway 2 receives the
   "isub", it cannot translate the value of the "isub" parameter back
   into the 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

4.  Requirements

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

   Req 1:   When the "isub" parameter is present but no "isub-encoding"
            parameter is present in a tel URI, the encoding of the ISDN
            subaddress in the original message MUST be assumed to be IA5
            (AFI=0x50).

   Req 2:   When using the "isub" parameters in tel URIs, the encoding
            SHOULD be specified by using the optional "isub-encoding"
            parameter unless the encoding of the ISDN subaddress is IA5
            (AFI=0x50).

5.  Parameter Definition

   The parameter defined in this document is represented as a tel URI
   parameter, which describes the encoding type information of the ISDN
   subaddress.  It is an optional parameter to tel URI to accommodate
   some of the information lacking in the "isub" parameter defined in
   RFC 3966 [2].  The ABNF [3] syntax is as follows.

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

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

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

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

   nsap:     Indication that the "isub" parameter value needs to be
             encoded using the encoding type defined in ISO 8348 [6]
             other than IA5 (AFI=0x50) or BCD (AFI=0x48).

      Note: Q.931 [7] defines a "user specified" subaddress type, but
            this document does not specify any behavior or value for
            "user specified" subaddress type.  Therefore, the "user
            specified" subaddress is beyond the scope of this document.

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

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

6.  Usage

   It is anticipated that a tel URI parameter defined in this document
   will be used along with an "isub" parameter defined in RFC 3966 [2]
   when interworking between an ISUP network and a SIP network.  The URI
   parameter defined here is an optional parameter to the tel URI and is
   useful only when it's accompanying the "isub" parameter.

   An ISDN subaddress information element carried in the ISUP message
   consists of a 3-octet header followed by either an NSAP address or a
   user-specified address.  The NSAP address consists of an Initial
   Domain Part (IDP) (Authority and Format Identifier (AFI) and
   conditionally Initial Domain Identifier (IDI)) that identifies an
   encoding type of the subaddress, and a Domain Specific Part (DSP)
   that represents the subaddress value itself.

   To find out more about the ISDN subaddress information element and
   the NSAP address including definition of AFI, IDI, IDP, and DSP,
   please refer to Appendices A and B.

   If the "isub-encoding" is absent, and a message is interpreted by an
   entity on the SIP network, the entity compliant to this specification
   MUST assume that the original ISDN subaddress in an ISUP message was
   an NSAP address with an encoding type of IA5 (AFI=0x50), of which the
   DSP value was translated and set to the "isub" parameter value, and
   MUST handle the message accordingly.

   If the "isub-encoding" is absent, and the message is handled by a
   gateway translating the SIP message to ISUP message, the gateway
   compliant to this specification MUST encode the value in the "isub"
   parameter using IA5 (AFI=0x50) and set the encoded value into the DSP
   part of the NSAP address when translating the message into an ISUP
   message.

   If the value of "isub-encoding" is set to "nsap", the encoding type
   (AFI) is assumed to be in the first two characters of the "isub"
   parameter in hexadecimal represented as US-ASCII characters 0-9 and
   A-F.

   If the ISDN subaddress is not an NSAP address, the entity translating
   the message SHOULD treat the message as if neither the "isub-
   encoding" nor the "isub" parameters existed, unless it has a prior
   knowledge of the encoding method used.

   When an entity that is not compliant to this specification handles
   the message with the "isub-encoding" parameter, it would simply
   ignore the parameter and its value.

6.1.  Gateway Behavior

   A gateway compliant to this specification that receives a message/
   signal from an ISDN network containing an ISDN subaddress MUST check
   the encoding used for the subaddress and MUST follow the procedures
   given below.

      If the ISDN subaddress is an NSAP address encoded using IA5
      (AFI=0x50), the entity MAY set the "isub-encoding" parameter to
      the value "nsap-ia5" and set the DSP value of the NSAP address as
      the value for the "isub" parameter using characters permitted for
      the "isub" parameter as specified in RFC 3966 [2] or omit the
      "isub-encoding" parameter.

      If the ISDN subaddress is an NSAP address encoded using BCD
      (AFI=0x48), the entity MUST set the "isub-encoding" parameter to
      the value "nsap-bcd" and set the decoded DSP value of the NSAP
      address as the value for the "isub" parameter in US-ASCII
      characters using numbers.

         Note: Each semi-octet should be translated into numbers (e.g.
               01011001 would be translated as 5 and 9).

      If the ISDN subaddress is an NSAP address but is not encoded using
      either IA5 (AFI=0x50) or BCD (AFI=0x48), the entity translating
      the message MUST set the "isub-encoding" parameter to the value
      "nsap" and the entire NSAP address as the value for the "isub"
      parameter in hexadecimal represented as US-ASCII characters (0-9
      and A-F).

      If the ISDN subaddress is not an NSAP address, the entity
      translating the message SHOULD NOT generate any "isub-encoding" or
      "isub" parameters, unless it has a private agreement with the
      recipient about what to do in this case.

6.2.  SIP Entity Behavior

   An entity compliant to this specification setting an "isub" parameter
   MUST follow the procedures given below.

      If the ISDN subaddress is an NSAP address encoded using IA5
      (AFI=0x50), the entity MAY set the "isub-encoding" to "nsap-ia5".
      The "isub" parameter value MUST NOT exceed 19 characters.  The
      characters used MUST follow the syntax defined for the "isub"
      parameter as specified in RFC 3966 [2].

      If the ISDN subaddress is an NSAP address encoded using BCD
      (AFI=0x48), the entity MUST set the "isub-encoding" to "nsap-bcd".
      The "isub" parameter value MUST NOT exceed 38 US-ASCII characters
      (numbers).

      If the ISDN subaddress is an NSAP address encoded using an
      encoding type other than IA5 (AFI=0x50) or BCD (AFI=0x48), the
      entity MUST set the "isub-encoding" to "nsap".  The "isub"
      parameter value MUST NOT exceed 40 US-ASCII characters and it MUST
      be in hexadecimal represented as US-ASCII characters (0-9 and A-
      F).  The first two characters of the "isub" parameter MUST be the
      encoding type (AFI) in this case.

7.  Security Considerations

   The parameter defined here adds no new security considerations to
   those discussed in RFC 3966 [2].

8.  IANA Considerations

   This document requires no action by IANA.

   Further information on a registry for tel parameters is covered in
   [8].

9.  Acknowledgements

   The authors thank John Elwell, James Rafferty, Steve Norreys, Michael
   Hammer, Ray Forbes, Martin Dolly, Cullen Jennings, and Henning
   Schulzrinne for providing extensive and constructive reviews and
   feedback.

Appendix A.  Structure of an ISDN Subaddress Information Element

   The structure of an ISDN subaddress information element in ISUP
   messages is defined in Q.931 [7] as follows.

                                Bits
            8     7     6     5     4     3     2     1     Octets
         +-----+-----------------------------------------+
         |  0  |  1     1     1     0     0     0     0  |   1
         +-----+-----------------------------------------+
         |  Length of called party subaddress contents   |   2
         +-----+-----------------------------------------+
         |  1  | Subaddress type | o/e |  0     0     0  |   3
         +-----+-----------------------------------------+
         |                                               |   4
         |            Subaddress information             |
         |                                               |
         |                                               |
         |                                               |
         +-----------------------------------------------+ max. 23

        Figure 3:  Structure of an ISDN Subaddress Information Element

   Although the length varies, the maximum length of an ISDN subaddress
   information element shown in the figure above is 23 octets.  The
   first 3 octets are the header.  The rest of the octets comprise the
   subaddress information that is either an NSAP address or a "user
   specified" address.

   The 1st octet is a called party subaddress information element
   identifier that identifies that this information element is a called
   party subaddress.  The 2nd octet represents the length of called
   party subaddress contents.

   The 5th to 7th bits of the 3rd octet identify the type of subaddress.
   This field is set to 0 0 0 when the subaddress is an NSAP address.
   It is set to 0 1 0 when the subaddress is "user specified".

   The 4th bit of the 3rd octet is an odd/even indicator.  The odd/even
   indicator is used when the type of subaddress is "user specified"
   with the encoding type of BCD, to enable an entity to pad the missing
   bits (last 4 bits of the subaddress information) when the number of
   digits composing the subaddress is odd.

      Note: When interworking with SIP, it is recommended not to
            translate the padding bits to "isub" parameter.

Appendix B.  Structure of NSAP Addresses

   In ISUP messages, the ISDN subaddress is generally represented as an
   NSAP address.  The NSAP address is defined as follows in ISO 8348
   [6].

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

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

           +--------------------------------------------------+
           |    I D P    |                                    |
           |-------------|             D S P                  |
           | AFI |  IDI  |                                    |
           +--------------------------------------------------+
           0     1       k     ...  Octets  ...         max. 20

                 Figure 4:  Structure of NSAP Addresses

   The AFI value is 2 hexadecimal digits (00-FF), and it identifies the
   IDI format and the DSP syntax.

   The IDI value when present is represented as decimal digits, and it
   identifies a network addressing domain or authority responsible for
   allocating values of the DSP.  The length of IDI varies and depends
   on the value of AFI.

   The typical encoding type of the ISDN subaddress, IA5, is identified
   as AFI=0x50.  When the AFI value is 0x50, the length of IDI is zero;
   therefore, the length of IDP is 2 digits (1 octet).  In this case,
   the DSP value is a subaddress encoded by IA5, and its maximum length
   is 19 octets.  The length of IDI is also zero when the encoding type
   is BCD (AFI=0x48).  The NSAP address for when the AFI value is set to
   either 0x50 or 0x48 is shown below.  As shown, DSP starts from the
   2nd octet of the NSAP address.

           +--------------------------------------------------+
           | IDP |                                            |
           |-----|                 D S P                      |
           | AFI |                                            |
           +--------------------------------------------------+
           0     1            ...  Octets  ...          max. 20

        Figure 5  Structure of NSAP Addresses (AFI=0x50 or AFI=0x48)

10.  References

10.1.  Normative References

   [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.

10.2.  Informative References

   [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 Telecommunication 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,
        2002.

   [7]  International Telecommunication Union, "ISDN User-Network
        Interface Layer 3 Specification for Basic Call Control",
        Recommendation Q.931, 1998.

   [8]  Jennings, C. and V. Gurbani, "The Internet Assigned Numbers
        Authority (IANA) tel Uniform Resource Identifier (URI) Parameter
        Registry", Work in Progress, May 2006.

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
   EMail: shida@ntt-at.com

   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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