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.
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RFC 4715 ISDN for tel URI November 2006
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.
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RFC 4715 ISDN for tel URI November 2006
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.
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RFC 4715 ISDN for tel URI November 2006
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.
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RFC 4715 ISDN for tel URI November 2006
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).
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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.
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RFC 4715 ISDN for tel URI November 2006
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.
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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).
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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.
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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.
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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)
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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.
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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
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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