IPTEL M. Munakata
Internet-Draft S. Schubert
Expires: January 02, 2007 T. Ohba
NTT
Jul 01, 2006
ISDN subaddress encoding type for tel URI
draft-munakata-iptel-isub-type-04.txt
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Copyright Notice
Copyright (C) The Internet Society (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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3.1. SIP-ISDN Interconnection . . . . . . . . . . . . . . . . . 4
3.2. ISDN-SIP-ISDN Interconnection . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Parameter Definition . . . . . . . . . . . . . . . . . . . . . 7
6. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Gateway Behavior . . . . . . . . . . . . . . . . . . . . . 9
6.2. SIP Entity Behavior . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Structure of ISDN Subaddress Information Element . . 10
Appendix B. Structure of NSAP Addresses . . . . . . . . . . . . . 11
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative Reference . . . . . . . . . . . . . . . . . . . 13
10.2. Informative Reference . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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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
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]
doesn't define a solution to carry information of a subaddress type
(whether the subaddress is NSAP or User Specific) nor an identifier
for the encoding type used.
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 enough to describe
ISDN subaddresses, and additional information is needed.
Lack of information describing the encoding type of ISDN
subaddress will make it difficult, for ISDN terminal receiving
ISDN subaddress from 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 "isub" parameter is an NSAP address as well as its encoding type.
In addition, this document specifies the accommodating values to be
carried in "isub" parameter for each encoding type used.
2. Terminology
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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, problems described in 3.1. and 3.2. would be observed.
3.1. SIP-ISDN Interconnection
The following diagrams 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 "isub" parameter
to address an ISDN terminal behind Private Branch Exchange (PBX), the
encoding type of the ISDN subaddress currently can not be specified.
Therefore gateway sitting between SIP network and ISDN network can
not translate the value of "isub" into an ISUP Initial Address
Message (IAM) properly as 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
The following diagrams 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 ISDN subaddress along with its encoding
type information, Gateway 1 translates the subaddress into "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 "isub", it can not
translate the value of "isub" parameter back 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 an ISDN
subaddress along with information of subaddress encoding type.
Req 1: When "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 "isub" parameters in tel URIs, the encoding SHOULD
be specified by using the optional "isub-encoding" parameter
unless the encoding of ISDN subaddress is IA5 (AFI=0x50).
5. Parameter Definition
The parameter defined in this draft 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 the ISUP message.
nsap: Indication that the "isub" parameter value needs to be
encoded using encoding type defined in ISO8348[6]
other than IA5 (AFI=0x50) or BCD (AFI=0x48).
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Note: Q.931 [7] defines an "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 networks 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 3-octet header followed by either an NSAP address or a
user specified address. The NSAP address consists of an IDP (AFI and
conditionally IDI) that identifies an encoding type of the
subaddress, and a 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 reference the 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 ISUP message was an
NSAP address with an encoding type of IA5 (AFI=0x50), of which DSP
value was translated and set to "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 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 "isub"
parameter in hexadecimal represented as US-ASCII characters 0-9 and
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A-F.
If the ISDN subaddress is not an NSAP address, the entity
translating the message SHOULD treat the message as if neither
"isub-encoding" nor "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
Gateway compliant to this specification that receives message/signal
from ISDN network containing 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 "isub" parameter using characters permitted for
"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 "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 neither encoded
using IA5 (AFI=0x50) nor 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 "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 "isub"
parameter as specified in RFC 3966 [2]
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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 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).
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
John Elwell, James Rafferty, Steve Norreys, Michael Hammer, Ray
Forbes, Martin Dolly, Cullen Jennings and Henning Schulzrinne for
providing extensive and constructive reviews and feedbacks.
Appendix A. Structure of ISDN Subaddress Information Element
The structure of ISDN subaddress information element in ISUP messages
is defined in Q.931 [7] as follows.
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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 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 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 identifies 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 4bits of subaddress information) when the numbers of
digits composing 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
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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 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 two 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 it
depends on the value of AFI.
Typical encoding type of the ISDN subaddress, IA5, is identified as
AFI=0x50. When AFI value is 0x50, the length of IDI is zero,
therefore 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. The length of IDI is also zero when the encoding type is BCD
(AFI=0x48). The NSAP address for when AFI's value set to either 0x50
or 0x48 is shown below. As shown, DSP starts from the 2nd octet of
the NSAP address.
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+--------------------------------------------------+
| IDP | |
|-----| D S P |
| AFI | |
+--------------------------------------------------+
0 1 ... Octets ... max. 20
Figure 5 Structure of NSAP Addresses (AFI=0x50 or AFI=0x48)
Appendix C. Change Log
Changes from 00 to 01. - Many editorial fixes and grammatical
correction. - Added a text on "User-Specific" address type. - Added a
text on "odd/even" indicator. - Changed the requirements to clearly
address the requirements this draft is trying to meet.
Changes from 01 to 02. - GEN-ART provided some constructive feedbacks
and we reflected some of the recommended changes.
1. Some of the SHOULD were changed to MUST to hold a better
compatibility with the pre-existing implementation.
2. Fixed numerical grammatical errors.
3. Added an appendix to explain the ISDN subaddress information
element.
4. Expanded the appendix on NSAP address structure to clarify some
of the ambiguity that was addressed.
5. Clarified some of the text on entity behaviors.(Section 6)
Changes from 02 to 03. - Reflected comments provided by IESG.
1. parameter defined in this draft was renamed from "isub-type" to
"isub-encoding".
2. Added a reference to a draft handling the IANA registration.
10. References
10.1. 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.
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10.2. 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.
[7] INTERNATIONAL Telecommunications 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 Number
Authority (IANA) tel Uniform Resource Identifier (URI) Parameter
Registry", draft-ietf-iptel-tel-reg-02.txt, May 2006.
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Authors' Addresses
Mayumi Munakata
NTT Corporation
Phone: +81 422 36 7565
Email: munakata.mayumi at lab.ntt.co.jp
Shida Schubert
NTT Corporation
Phone: +1 604 762 5606
Email: shida at 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 at lab.ntt.co.jp
URI: http://www.ntt.co.jp
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