ISO/IEC 8473-3:1995

Information technology — Protocol for providing the connectionless-mode network service: Provision of the underlying service by an X.25 subnetwork

Specifies the way in which the underlying service assumed by the protocol defined by ITU-T Rec. X.233 ISO/IEC 8473-1 is provided by a subnetwork that conforms to ITU-T Recommendation X.25 through the operation of a Subnetwork Dependent Convergence Function (SNDCF) as described in ISO/IEC 8648. Also provides the PICS proforma for this protocol, in compliance with the relevant requirements and in accordance with the relevant guidance.

Technologies de l'information — Protocole de fourniture du service de réseau en mode sans connexion: Fourniture du service sous-jacent par un sous-réseau X.25

General Information

• Status: Published
• Publication Date: 01-Mar-1995
• ICS: 35.100.30 - Network layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6/WG 7 - Network, transport and future network
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 28-Jun-2001

Standards Content (Sample)

INTERNATIONAL
ISO/IEC
STANDARD
8473-3
First edition
1995-03-I 5
Information technology - Protocol for
providing the connectionless-mode
network service: Provision of the
underlying service by an X.25 subnetwork
Technologies de I’information - Protocole de foumiture du service de
r&eau en mode sans connexion: Fourniture du service sous-jacent par un
sous-rkseau X. 25
Reference number
lSO/lEC 8473-3:1995(E)

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ISO/IEC 8473-3: 1995(E)
Contents
Page
1
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.
..~..................................................................... 1
2.1 Identical Recommendations 1 International Standards
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2.2 Paired Recommendations I International Standards identical in technical content
1
2.3 Additional references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.
2
3.1 Reference model definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*.~.
2
3.2 Network layer architecture definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*.
Network layer addressing definitions
3.4 X.25 definitions .,.~.”.~.~.,. 2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.~. 2
4 Abbreviations
3
Subnetwork dependent convergence function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .“.~.~.~.~.~. 3
5.1 General model
. . . . . . . . . . . . . . . . . . .“. 3
5.2 Subnetwork user data
. . . . . . . . . . . . . . .a. 3
5.3 Subnetwork dependent convergence functions used with X.25 subnetworks
........................................................................................................................................ 7
Annex A - PICS proforma
7
A. 1 Introduction .
..................................................................................................... 7
A.2 Abbreviations and special symbols
.................................................................................. 7
A.3 Instructions for completing the PICS proforma
9
A.4 Identification .
................................. 10
A.5 Subnetwork dependent convergence functions for use with X.25 subnetworks
0 ISO/IEC 1995
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and microfilm, without permission in writing from the publisher.
ISO/IEC Copyright Office l Case postale 56 l CH- 1211 Geneve 20 l Switzerland
Printed in Switzerland
ii

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ISO/IEC 8473-3: 1995(E)
0 ISO/IEC
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are members of IS0 or IEC participate in the
development of International Standards through technical committees established
by the respective organization to deal with particular fields of technical activity.
IS0 and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with
IS0 and IEC, also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the
joint technical committee are circulated to national bodies for voting. Publication
as an International Standard requires approval by at least 75 % of the national
bodies casting a vote.
International Standard ISO/IEC 8473-3 was prepared by Joint Technical Com-
mittee ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecom-
munications and information exchange between systems, in collaboration with
ITU-T. The identical text is published as ITU-T Recommendation X.622.
ISO/IEC 8473 consists of the following parts, under the general title Information
technology - Protocol for providing the connectionless-mode network service:
- Part I: Protocol specification
- Part 2: Provision of the underlying service by an ISOAEC 8802
subnetwork
- Part 3: Provision of the underlying service by an X.25 subnetwork
- Part 4: Provision of the underlying service by a subnetwork that
provides the OSI data link service
Annex A forms an integral part of this part of ISO/IEC 8473.
. . .
Ill

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ISO/IEC 8473-3: 1995(E)
0 ISO/IEC
Introduction
This is one of a set of Recommendations and International Standards produced to facilitate the interconnection of open
systems. The set covers the services and protocols required to achieve such interconnection.
This Recommendation I International Standard is positioned with respect to other related Recommendations and
International Standards by the layers defined in ITU-T Rec. X.200 1 ISO/IEC 7498-l a In particular, it defines the way in
which an X.25 subnetwork may be used within the Network layer to provide the abstract underlying service with respect
to which the protocol defined by ITU-T Rec. X.233 1 ISO/IEC 8473-1 is specified.
In order to evaluate the conformance of a particular implementation of this protocol, it is necessary to have a statement
of which of the protocol’s capabilities and options have been implemented. Such a statement is called a Protocol
Implementation Conformance Statement (PICS), as defined in CCITT Rec. X.290 I ISOLIEC 9646- 1. A PICS proforma,
from which a PICS may be prepared for a specific implementation, is included in this Recommendation I International
Standard as normative Annex A.
iv

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ISO/IEC 8473-3 : 1995 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
INFORMATION TECHNOLOGY - PROTOCOL FOR PROVIDING
THE CONNECTIONLESS-MODE NETWORK SERVICE:
PROVISION OF THE UNDERLYING SERVICE BY AN X.25 SUBNETWORK
1 Scope
This Recommendation I International Standard specifies the way in which the underlying service assumed by the
protocol defined by ITU-T Rec. X.233 I ISO/IEC 8473-l is provided by a subnetwork that conforms to ITU-T
Recommendation X.25 through the operation of a Subnetwork Dependent Convergence Function (SNDCF) as described
in ISOAEC 8648.
This Recommendation I International Standard also provides the PICS proforma for this protocol, in compliance with the
relevant requirements, and in accordance with the relevant guidance, given in CCITT Rec. X.290 I ISO/IEC 9646-l.
2 Normative references
The following Recommendations and International Standards contain provisions which, through reference in this text,
constitute provisions of this Recommendation I International Standard. At the time of publication, the editions indicated
were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this
Recommendation I International Standard are encouraged to investigate the possibility of applying the most recent
editions of the Recommendations and Standards listed below. Members of IEC and IS0 maintain registers of currently
valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently
valid ITU-T Recommendations.
21 . Identical Recommendations I International Standards
-
ITU-T Recommendation X.200 (1994) I ISOLIEC 7498- 1: 1994, Information technology - Open Systems
interconnection - Basic Reference Model.
-
CCITT Recommendation X.21 3 (1992) I ISO/IEC 8348: 1993, Information technology - Network service
definition for Open Systems Interconnection.
22 . Paired Recommendations I International Standards identical in technical content
-
CCITT Recommendation X.290 (1992), OSI conformance testing methodology and framework for
protocol Recommendations for CCITT applications - General concepts.
ISO/IEC 9646- 1: 199 1, Information technology - Open Systems Interconnection - Conformance testing
methodology and framework - Part 1: General concepts.
23 . Additional references
-
ITU-T Recommendation X.25 (1993), Inter$ace between data terminal equipment (DTE) and data circuit-
terminating equipment (DCE) for terminals operating in the packet mode and connected to public data
networks by dedicated circuit.
-
CCITT Recommendation X. 12 1 ( 1992), International numbering plan for public data networks.
-
ISO/IEC 8208: 1990, Information technology - Data communications - X.25 Packet Layer Protocol for
Data Terminal Equipment.
-
ISO/IEC 8648: 1988, Information processing systems - Open Systems Znterconnection - Internal
organization of the network layer.
ITU-T Rec. X.622 (1994 E)
1

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ISO/IEC 8473-3 : 1995 (E)
3 Definitions
Reference model definitions
31 .
This Recommendation I International Standard makes use of the following terms defined in ITU-T Rec. X.200 I
ISO/IEC 7498- 1:
network entity;
a>
b) Network layer;
service;
C>
d) service data unit;
e) protocol control information.
32 .
Network layer architecture definitions
This Recommendation I International Standard makes use of the following terms defined in ISO/IEC 8648:
subnetwork;
a>
b) subnetwork dependent convergence protocol;
subnetwork dependent convergence function;
C>
d) subnetwork access protocol.
33 . Network layer addressing definitions
This Recommendation I International Standard makes use of the following term defined in CCITT Rec. X.213 I
ISOIIEC 8348:
-
subnetwork point of attachment.
34 . X.25 definitions
This Recommendation I International Standard makes use of the following terms defined in ITU-T Rec. X.25 and
ISO/IEC 8208:
a) data circuit-terminating equipment;
b) data terminal equipment;
c) logical channel;
d) permanent virtual circuit;
virtual circuit.
e)
4 Abbreviations
CLNP Connectionless-mode network protocol
DCE Data circuit-terminating equipment
DTE Data terminal equipment
PDU Protocol data unit
PVC Permanent virtual circuit
Quality of service
0
QS
SDU Service data unit
Subnetwork
SN
SNDCF Subnetwork dependent convergence function
SNDCP Subnetwork dependent convergence protocol
SNICP Subnetwork independent convergence protocol
SNAcP Subnetwork access protocol
SNPA Subnetwork point of attachment
2 ITU-T Rec. X.622 (1994 E)

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ISOLIEC 8473-3 : 1995 (E)
Subnetwork connection reference
SNCR
SNSDU Subnetwork service data unit
Subnetwork dependent convergence function
5
51 l General model
The general model for providing the underlying service assumed by the protocol in conjunction with a real subnetwork
that uses a connectionless subnetwork access protocol is as follows. The generation of an SN-UNITDATA Request by
the CLNP results in the generation of a corresponding subnetwork-specific UNITDATA request by the subnetwork
dependent convergence function. The receipt of a subnetwork-specific UNITDATA indication associated with delivery
of a connectionless data unit to its destination causes the SNDCF to generate an SN-UNITDATA Indication to the
CLNP.
The general model for providing the underlying service assumed by the CLNP in conjunction with a real subnetwork
that uses a connection-mode subnetwork access protocol is as follows. The generation of an SN-UNITDATA Request by
the CLNP causes a connection (logical channel, logical link, or the equivalent) to be made available for the transmission
of SN-User-data. If a connection cannot be made available, the SN-UNITDATA Request is discarded. The receipt of
subnetwork-specific PDUs containing SN-User-data causes the SNDCF to generate an SN-UNITDATA Indication to the
CLNP.
Where a real subnetwork is designed to use either a connectionless-mode or a connection-mode subnetwork access
protocol, the provision of the underlying service assumed by the CLNP is achieved by using the connectionless-mode
alternative.
52 . Subnetwork user data
The SN-Userdata is an ordered multiple of octets, and is transferred transparently between the specified subnetwork
points of attachment.
The underlying service assumed by the CLNP is required to support a service data unit size of at least 5 12 octets.
If the minimum service data unit sizes supported by all of the subnetworks involved in the transmission of a particular
PDU are known to be large enough that segmentation is not required, then either the full protocol or the non-segmenting
protocol subset may be used.
Data received from a subnetwork with protocol identification specifying this protocol (see ITU-T Rec. X.233 I
ISO/IEC 8473-l) shall be processed according to this Recommendation I International Standard.
NOTE - Data with
other protocol identification should be ignored, since it may have been sent by an implementation
supporting additional protocols
intended for use with this protocol.
53 . Subnetwork dependent convergence functions used with X.25 subnetworks
The connection-mode service offered by subnetworks that use the X.25 Packet Level Protocol defined in ISO/IEC 8208
or ITU-T Rec. X.25 is manipulated by the subnetwork dependent convergence function so that a virtual circuit is made
available for the transmission of SN-User-data following the generation of an SN-UNITDATA Request by the CLNP. In
general, no explicit subnetwork dependent convergence protocol control information is exchanged between peer network
entities during the data phase of operation in order to provide this mapping of service.
The SN-Destination-Address and SN-Source-Address parameters in the SN-UNITDATA request and indication are the
CCITT Rec. X. 12 1 DTE addresses used by the X.25 subnetwork.
If the X.25 subnetwork does not provide calling DTE information, a null SN-Source-Address parameter is supplied in
the SN-UNITDATA Indication. The SNDCF shall include its own DTE address in the “calling DTE” field of the X.25
Call Request packet, in the case that the subnetwork does not include this parameter but permits its inclusion by DTEs.
NOTE - Some subnetworks which use the X.25 PLP employ addressing schemes other than CCITT’ Rec. X. 121. The use of
addressing schemes other than CCITT’ Rec. X. 121 (e.g. CCITI’ Recommendations E. 163 and E. 164) is not precluded.
The SN-User-data parameter carries user data up to a maximum size specified by the subnetwork authority. The
underlying service assumed by ITU-T Rec. X.233 I ISO/IEC 8473-l requires that a subnetwork be capable of supporting
a minimum service data unit size of 5 12 octets.
NOTE - The M-bit may be used in cases where an X.25 subnetwork cannot directly support a minimum packet size of 512
octets as well as in situations where a service data unit size greater than the minimum is required, e.g. where the non-segmenting
protocol subset is used.
IT&T Rec. X.622 (1994 E)
3

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ISOKEC 8473-3 : 1995 (E)
5.3.1 Call setup considerations
The mechanism and timing for opening a virtual circuit prior to the transmission of SN-User-data are a local matter. The
opening of a virtual circuit may be initiated by:
the arrival of an SNSDU to be transmitted over an X.25 subnetwork at a time when no suitable virtual
a)
circuit is available;
which an
the local queue of requests waiting for an existing virtual circuit reaching a threshold size at
b)
additional virtual circuit shall be made available (if possible) to mai ntain the requested QoS; or
the explicit intervention of system management.
C)
When it has been determined that a (new) virtual circuit must be made available, the calling SNDCF performs all
functions associated with establishing a virtual circuit. The called SNDCF performs those operations associated with
accepting a call, but generates no SN-UNITDATA indication until the call setup is completed, at which time X.25 Data
packet(s) may be exchanged. In general, the receipt of X.25 Data packets containing SN-User-data causes the SNDCF to
generate an SN-UNITDATA indication to the CLNP. X.25 Reset packets, if received, have no effect on the operation of
the SNDCF. The necessary procedures for the correct operation of the X.25 PLP are followed.
5.3.2 Call clearing considerations
The mechanisms for determining when a virtual circuit is to be cleared following the transmission of SN-User-data by
the SNDCF are local matters. Examples of circumstances which would cause the SNDCF to clear a virtual circuit are:
the expiration of a timeout period following the transmission of one or more PDUs (see 5.3.4);
a>
the need to use a specific interface to open an alternate virtual circuit from the local network entity to a
W
different remote network entity;
the explicit intervention of system management; or
C)
a provider-initiated clear of a virtual circuit.
d)
When it has been determined that a virtual circuit shall be cleared, the SNDCF performs all functions associated with
clearing a call. All packets other than a Clear Confirmation or Clear Indication are ignored. The same actions apply to
receipt of a Clear Indication. In these circumstances, the SNDCF will retain user data submitted via SN-UNITDATA
requests while attempting to establish a new circuit; however, the SNDCF shall discard the user data if the transit delay
indicated to the CLNP is likely to be exceeded.
NOTE - It is not a requirement that virtual circuits be dynamically opened or closed for the correct operation of the
SNDCF herein described. The use of permanent virtual circuits (PVCs) or the maintenance of virtual circuits in a open state from
system initialization is not precluded.
5.3.3 Protocol discrimination
The first octet of the call user data field of the Call Request packet shall be set to the value that indicates that the virtual
circuit is to be used to provide the underlying service assumed by ITU-T Rec. X.233 I ISO/IEC 8473-l. The value is
defined in ITU-T Rec. X.233 I ISO/IEC 8473-l.
5.3.4 Timeout periods
Timeout periods may be used to determine when a virtual circuit should be cleared (for example, when a virtual circuit
has been idle for a long period of time) or when additional virtual circuits should be opened (for example, when there is
an excessively long queue of data units waiting for the initial logical channel).
Implementations may choose to clear a virtual circuit after it has been idle for some period of time. If a timer is selected
for this purpose, it is used in the following manner. When a virtual circuit is made available for the transmission of
SNSDUs, a timer is initiated with a value representing the maximum period of time this virtual circuit may remain idle.
Each time a data unit is transmitted by the underlying service, the timer is reset to this initial value. If no data units are
queued for processing and this timer expires, the virtual circuit is cleared.
The selection of timeout values is a local matter.
NOTES
Additional virtual circuits may be opened when there is an excessively long queue of data units waiting for the initial
1
logical channel. The timeout periods for determining when such additional virtual circuits are to be cleared may be shorter than the
timeout period for the initial virtual circuit. (The timeout period may also be a fixed period of time.) Implementations may choose to
close all additional virtual circuits if the queue of data units to be transmitted reaches some threshold (possibly zero).
2 Timeout periods are selected on the basis of economic and implementation-spe
...