ISO/IEC 10589:1992
(Main)Information technology — Telecommunications and information exchange between systems — Intermediate system to Intermediate system intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473)
Information technology — Telecommunications and information exchange between systems — Intermediate system to Intermediate system intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473)
Specifies procedures for the transmission of configuration and routeing information between network entities, the encoding of the protocol data units used for the transmission of the configuration and routeing information, procedures for the correct interpretation of protocol control information, and the functional requirements for implementations claiming conformance to this standard. Procedures are defined in terms of the interactions between Intermediate System Network entities through the exchange of protocol data units, between a Network entity and an underlying service provider, and the constraints on route determination.
Technologies de l'information — Communication de données et échange d'informations entre systèmes — Protocole intra-domaine de routage d'un système intermédiaire à un système intermédiaire à utiliser conjointement avec le protocole fournissant le service de réseau en mode sans connexion (ISO 8473)
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INTERNATIONAL
STANDARD
10589
First edition
1992-06-I 5
Information technology - Telecommunications
and information exchange between systems -
Intermediate system to Intermediate system
intra-domain routeing information exchange
protocol for use in conjunction with the protocol
for providing the connectionless-mode Network
Service (IS0 8473)
Technologies de I’informafion .-- Communication de donn&es et &change
d’informations entre systhes .-- Protocole M-a-domaine de routage
d’un systhe intermediair-e 3 cm systhne intermediaire ZI utiliser
conjointement avec le protocole fownissant Ie service de r&eau en
mode sans connexion (IS0 8473)
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ISO/I EC 10589: 1992(E)
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ISO/IEC 10589:1992 (E)
Contents
1
1 Scope .
1
...........................................................
2 Normative references
2
3 Definitions .
3
.................................................
4 Symbols and abbreviations
4
..................................................
Typographical conventions
5
4
Overview of the protocol .
6
.................................... 10
Subnetwork independent functions
7
35
......................................
8 Subnetwork dependent functions
....................................... 48
9 Structure and encoding of PDUs
66
..........................................................
' 10 System environment
67
........................................................
11 System management
100
...................................................................
12 Conformance
105
.............................................................
Annex A PICS pro forma
....................................... 117
Annex B Supporting technical material
.................... 121
Annex C Implementation guidelines and examples
............................... 127
Annex D Congestion control and avoidance
............ 129
Annex E Syntax imported from IS0 101655 (SC6 GMI)
141
..................................................................
Annex F Bibliography
143
Index .
0 ISO/lEC 1992
All rights reserved. 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.
ISOAEC Copyright Office l Case Postale 56 * Cti-1211 Gekve 20 * Switzerland
Printed in Switzerland
ii
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ISOllEC 105893 992 (E)
Foreword
IS0 (the international Organization for Standardization) and IEC (the
international 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 com-
mittees collaborate in fields of mutual interest. Other international or-
ganizations, 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 I. Draft International Standards
adopted by the joint technical committee are circulated to national bod-
ies for voting. Publication as an International Standard requires ap-
proval by at least 75 % of the national bodies casting a vote.
International Standard ISO/IEC 10589 was prepared by Joint Technical
Committee ISO/IEC JTC 1, information technology.
Annexes A and E form an integral part of this International Standard.
Annexes B, C, D and F are for information only.
. . .
III
---------------------- Page: 3 ----------------------
ISOAEC 10589:1992 (E)
Introduction
This International Standard is one of a set of International Standards produced to facilitate the
interconnection of open systems. The set of standards covers the services and protocols required
to achieve such interconnection.
The protocol defined in this International Standard is positioned with respect to other related
standards by the layers defined in IS0 7498 and by the structure defined in IS0 8648. In par-
ticular, it is a protocol of the Network Layer. This protocol permits Intermediate Systems within
a routeing domain to exchange configuration and routeing information to facilitate the operation
of the routeing and relaying functions of the Network Layer.
The protocol is designed to operate in close conjunction with IS0 9542 and IS0 8473. IS0 9542
is used to establish connectivity and reachability between End Systems and Intermediate sys-
tems on individual subnetworks, Data is carried using the protocol specified in IS0 8473. The
related algorithms for route calculation and maintenance are also described.
The intra-domain IS-IS routeing protocol is intended to support large routeing domains consist-
ing of combinations of many types of subnetworks. This includes point-to-point links,
multipoint links, X.25 subnetworks, and broadcast subnetworks such as IS0 8802 LANs.
In order to support large routeing domains, provision is made for Intra-domain routeing to be
organised hierarchically. A large domain may be administratively divided into areas. Each sys-
tem resides in exactly one area. Routeing within an area is referred to as Level I rouleing.
Routeing between areas is referred to as Level 2 route@. Level 2 Intermediate systems keep
track of the paths to destination areas. Level 1 Intermediate systems keep track of the routeing
within their own area. For an NPDU destined to another area, a Level 1 Intermediate system
sends the NPDU to the nearest level 2 IS in its own area, regardless of what the destination area
is. Then the NPDU travels via level 2 routeing to the destination area, where it again travels via
level 1 routeing to the destination End system.
iV
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ISOllEC 10589:1992 (E)
INTERNATIONAL STANDARD
Information technology - Telecommunications and
information exchange between systems - Intermediate
system to Intermediate system intra-domain routeing
information exchange protocol for use in conjunction with the
protocol for providing the connectionless-mode Network
Service (IS0 8473)
of applying the most recent editions of the International Stan-
1 Scope
dards listed below. Members of IEC and IS0 maintain regis-
ters of currently valid International Standards.
This International Standard specifies a protocol which is used
by Network Layer entities operating the protocol specified in
IS0 7498:1984, Information processing systems - Open
IS0 8473 in Intermediate Systems to maintain routeing infor-
Systems Interconnection - Basic Reference Model.
mation for the purpose of routeing within a single routeing do-
main. The protocol specified in this International Standard re-
IS0 7498fAdd. 1: 1987, Information processing systems -
lies upon the provision of a connectionless-mode underlying
Open Systems Interconnection - Basic Reference Model -
1)
service.
Addendum I : Connectionless-mode Transmission.
This International Standard specifies:
IS0 749%3:1989, Information processing systems - Open
Systems Interconnection - Basic Reference Model -
a) procedures for the transmission of configuration and
Part 3: Naming and addressing.
routeing information between network entities residing in
Intermediate Systems within a single routeing domain;
IS0 749%4:1989, Information processing systems - Open
b) the encoding of the protocol data units used for the trans- Systems Interconnection - Basic Reference Model -
mission of the configuration and routeing information;
Part 4: Management framework.
c) procedures for the correct interpretation of protocol con-
ISO/IEC 8208:1990, Information technology - Data com-
trol information; and
munications -X.25 packet Layer Protocol for Data Termi-
nal Equipment.
d) the functional requirements for implementations claiming
conformance to this International Standard.
IS0 8348~1987, Information processing systems - Data
The procedures are defined in terms of communications -Network service definition.
e) the interactions between Intermediate system Network
IS0 8348/Add. 1: 1987, Information processing systems -
entities through the exchange of protocol data units;
Data communications -Network Service Definition - Ad-
dendum I : Connectionless-mode transmission.
f) the interactions between a Network entity and an under-
lying service provider through the exchange of subnet-
IS0 8348fAdd.2: 1988, Information processing systems -
work service primitives; and
Data communications -Network Service Definition - Ad-
dendum 2: Network layer addressing.
g) the constraints on route determination which must be ob-
served by each Intermediate system when each has a
IS0 8473~1988, Information processing systems - Data
routeing information base which is consistent with the
- Protocol for providing
communications the
others.
connectionless-mode network service.
ISOJIEC 8473lAdd.3: 1989, Information processing systems
2 Normative references
- Data Communications - Protocol for providing the
connectionless-mode network service - Addendum 3: Pro-
vision of the underlying service assumed by IS0 8473 over
The following International Standards contain provisions
subnetworks which provide the OSI data link service.
which, through reference in this text, constitute provisions of
this International Standard. At the time of publication, the edi-
IS0 8648:1990, Information processing systems - Open
tions indicated were valid. All International Standards are sub-
Systems Interconnection - Internal organisation of the
ject to revision, and parties to agreements based on this Inter-
national Standard are encouraged to investigate the possibility Network Layer.
1) See IS0 8473 and its addendum 3 for the mechanisms necessary to realise this service on subnetworks based on ISO/IEC 8208, IS0 8802, and the OS1
Data Link Service.
1
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ISOAEC 10589:1992 (E)
ISO/IEC 8802-l. l ‘I, Informatio n technology - Telecommu-
3 Definitions
nications and information exchange between systems - Lo-
cal area networks - Part 1: General Introduction.
3.1 Reference model definitions
IS0 8802-2: 1989, Information processing systems - Local
area networks - Part 2: Logical link control.
This International Standard makes use of the following terms
defined in IS0 7498:
ISO/IEC 8802-3: 1990, Information processing systems -
Local area networks - Part 3: Carrier sense multiple access
Network Layer
a>
with collision detection (CSMAICD) access method and physi-
Network Service access point
b)
cal layer specifications.
Network Service access point address
4
Network entity
d)
ISO/IEC 8802-S:‘), Information technology - Local area
Routeing
e>
networks - Part 5: Token ring access method and physical
Network protocol
f)
layer specifications.
Network relay
s>
Network protocol data unit
h)
ISO/IEC 8802-6. l ‘I, Informatio n technology - Local area
networks - Part 6: Distributed Queue Dual Bus (DQDB) ac-
32 0 Network layer architecture
cess method and physical layer spec@ations.
definitions
ISO/IEC 93 14: 1989, Information processing systems - Fiber
Distributed Data Interface (FDDI}.
This International Standard makes use of the following terms
defined in IS0 8648:
IS0 9542: 1988, Information processing systems - Telecom-
munications and information exchange between systems -
Subnetwork
End system to Intermediate system Routeing exchange pro- a>
End system
tocol for use in conjunction with the protocol for providing b)
Intermediate system
the connectionless -mode network service (IS0 8473). 4
Subnetwork service
d>
Subnetwork Access Protocol
e)
ISO/IEC TR 9575: 1990, Information technology - Telecom-
Subnetwork Dependent Convergence Protocol
0
munications and information exchange between systems -
Subnetwork Independent Convergence Protocol
Ia
OSI Routeing Framework.
ISO/IEC TR 9577: 1990, Information technology - Telecom-
33 l Network layer addressing
munications and information exchange between systems -
definitions
Protocol identification in the network layer.
ISO/IEC 10039: 1991, Information technology - Open Sys-
This International Standard makes use of the following terms
defined in IS0 8348/Add.2:
tems Interconnection - Local area networks - Medium
Access Control (MAC) service definition.
a) Subnetwork address
ISO/IEC 10165-l?, rnformation technology - Open Sys- b) Subnetwork point of attachment
c) Network Entity Title
tems Interconnection - Structure of Management Informa-
tion - Part I: Management Information Model.
3.4 Local area network definitions
ISOIIEC 10165-4:‘), Information technology - Open Systems
Interconnection - Structure of management information -
This International Standard makes use of the following terms
Part 4: Guidelines for the definition of managed objects.
defined in IS0 8802:
ISO/IEC 10733: ‘I, Information technology - Telecommuni-
a) Multi-destination address
cations and information exchange between systems - Ele-
b) Media access control
ments of management information relating to OSI Network
c) Broadcast medium
Layer standards.
3.5 Routeing framework definitions
This International Standard makes use of the following terms
defined in ISO/IEC TR 9575:
a) Administrative Domain
b) Routeing Domain
cl Hop
d) Black hole
1) To be published
-
2
---------------------- Page: 6 ----------------------
ISOllEC 10589:1992 (E)
broadcast subnetwork: A subnetwork which sup-
3.6.8
3.6 Additional definitions
ports an arbitrary number of End systems and Inter-
mediate systems and additionally is capable of trans-
For the purposes of this International Standard, the following
mitting a single SNPDU to a subset of these systems
definitions apply:
in response to a single SN-UNITDATA request.
3.6.1 area: A routeing subdomain which maintains de- general topology subnetwork: A subnetwork which
3.6.9
tailed routeing information about its own internal supports an arbitrary number of End systems and In-
composition, and also maintains routeing information
termediate systems, but does not support a convenient
which allows it to reach other routeing subdomains. It multi-destination connectionless transmission facility,
corresponds to the Level 1 subdomain.
as does a broadcast subnetwork.
by tra-
3.6.2 neighbour : An adjacent system reachable 3.6.10 routeing subdomain: a set of Intermediate systems
and End systems located within the same Routeing
versa1 of a single subnetwork by a PDU.
domain.
3.6.3 adjacency: A portion of the local routeing informa-
level 2 subdomain: the set of all Level 2 Intermedi-
3.6.11
tion which pertains to the reachability of a single
ate systems in a Routeing domain.
neighbour ES or IS over a single circuit.
to the Decision Process 3.6.12 jitter: a small random variation introduced into the
Adjacencies are used as input
routeing domain. value of a timer to prevent multiple timer expirations
for forming paths through the
in different systems from becoming synchronised.
A separate adjacency is created for each neighbour on
a circuit, and for each level of routeing (i.e. level 1
and level 2) on a broadcast circuit.
4 Symbols and abbreviations
3.6.4 circuit: A subset of the local routeing information
base pertinent to a single local SNPA. The system
4.1 Data units
management view of a circuit is presented in a link-
age managed object.
PDU Protocol Data Unit
SNSDU Subnetwork Service Data Unit
3.6.5 link: The communication path between two
bours.
NSDU Network Service Data Unit
NPDU Network Protocol Data Unit
A link is “up” when communication is possible be-
SNPDU Subnetwork Protocol Data Unit
tween the two SNPAs.
3.6.6 designated IS: The Intermediate system on a LAN
4.2 Protocol data units
which is designated to perform additional duties. In
particular it generates Link State PDUs on behalf of
the LAN, treating the LAN as a pseudonode.
ESH PDU IS0 9542 End System Hello Protocol Data Unit
ISH PDU IS0 9542 Intermediate System Hello Protocol
3.6.7 pseudonode: Where a broadcast subnetwork has n Data Unit
connected Intermediate systems, the broadcast sub-
RD PDU IS0 9542 Redirect Protocol Data Unit
network itself is considered to be a pseudonode.
IIH PDU Intermediate system to Intermediate system
Hello Protocol Data Unit
The pseudonode has links to each of the n Intermedi-
LSP
Link State Protocol Data Unit
ate and End systems. Each of the ISs has a single link
SNP Sequence Numbers Protocol Data Unit
n-1 links to each of
to the pseudonode (rather than
CSNP Complete Sequence Numbers Protocol Data
the other Intermediate systems). Link State PDUs are
Unit
generated on behalf of the pseudonode by the Desig-
PSNP Partial Sequence Numbers Protocol Data Unit
nated IS. This is depicted below in figure 1.
.-m.
l /-
me.
.
l
. .
.
l s.*. l
.
\ . *
.
.
IS .--m. IS
IS
. .
.
.
. .’ . n - = . . . .
.
. . .
.
l rmmmmam* .* -
- . .
l
.
.
l *. .
l l
ES
ES . l
Figure 1 - Use of a pseudonode to collapse a LAN Topology
---------------------- Page: 7 ----------------------
ISOAEC 10589:1992 (E)
SSN Send Sequence Numbers
svc Switched Virtual Circuit
4.3 Addresses
AFI Authority and Format Indicator
5 Typographical conventions
DSP Domain Specific Part
Initial Domain Identifier
IDI
This International Standard makes use of the following typo-
IDP Initial Domain Part
graphical conventions:
NET Network Entity Title
NPAI Network Protocol Addressing Information
a) important terms and concepts appear in italic type when
NSAP Network Service Access Point introduced for the first time;
Subnetwork Point of Attachment
SNPA
b) protocol constants and management parameters appear in
sansserif type with multiple words run together. The
4.4 Miscellaneous
first word is lower case, with the first character of subse-
quent words capita&d;
DA Dynamically Assigned
c) protocol field names appear in San Serif type with each
DED Dynamically Established Data link
word capitalised; and
DTE Data Terminal Equipment
ES
End System
d) values of constants, parameters, and protocol fields ap-
IS Intermediate System
pear enclosed in “double Quotes”.
1
HDLC High Level Data Link Control
ISDN Integrated Services Digital Network
FDDI Fiber Distributed Data Interface
6 Overview of the protocol
Ll Level 1
L2 Level 2
LAN Local Area Network
6.1 System types
MAC Media Access Control
MAN Metropolitan Area Network
For the purposes of this International Standard, systems are
NLPID Network Layer Protocol Identifier classified according to the following types:
PSTN Public Switched Telephone Network
End Systems: These systems deliver NPDUs to other sys-
OSIE Open Systems Interconnection Environment
tems and receive NPDUs from other systems, but do
PCI Protocol Control Information
not relay NPDUs. This International Standard does
QoS Quality of Service
not specify any additional End system functions be-
SN Subnetwork
yond those supplied by IS0 8473 and IS0 9542.
SNAcP Subnetwork Access Protocol
SNDCP Subnetwork Dependent Convergence Protocol
Level 1 Intermediate Systems: These systems deliver and
SNICP Subnetwork Independent Convergence Protocol receive NPDUs from other systems, and relay
SRM NPDUs from other source systems to other destina-
Send Routeing Message
legend :
PI End System
m Intermediate System
v =a-r.r- ES-IS Protocol - Subnelwork Path
A
m
?&% IS-IS Arec I
--o Level 1 IS-IS Rout&g
/.p:“.~.~~~ \
...........
.............
................
.
.................... ...................................
...................... \
....................... Domain Boundary
...................... . 5~ Routeing
........................ \ - Level 2 IS-IS Routeing
........................ .
. .
...........................
........................... \
............................ \
1 .
...............................
.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~. w n Interdomain Routeina
....... . . . . . . ...... 5 .
A
Figure 2 - Topologies and Systems supported by Intradomain Routeing
4
---------------------- Page: 8 ----------------------
ISOAEC 10589:1992 (E)
work service. The Subnetwork Dependent functions do, how-
tion systems. They route directly to systems within
ever, operate differently on connectionless and connection-
their own area, and route towards a level 2 Interme-
oriented subnetworks.
diate system when the destination system is in a dif-
ferent area.
6.3 Topologies
Level 2 Intermediate Systems: These systems act as Level 1
Intermediate systems in addition to acting as a sys-
A single organisation may wish to divide its Administrative
tem in the subdomain consisting of level 2 ISs. Sys-
Domain into a number of separate Routeing Domains. This has
tems in the level 2 subdomain route towards a desti-
certain advantages, as described in ISO/IEC TR 9575. Further-
nation area, or another routeing domain.
more, it is desirable for an intra-domain routeing protocol to
aid in the operation of an inter-domain routeing protocol,
These systems and their topological relationship are illustrated
where such a protocol exists for interconnecting multiple
in figure 2.
routeing domains.
In order to facilitate the construction of such multi-domain to-
6.2 Subnetwork types
pologies, provision is made for the entering of inter-domain
routeing information. This information is in the form of a set
subnetworks
For the purposes of this International Standard,
of Reachable Address prefixes which may be entered either by
are classified according to the following types:
System Management, or provided by an inter-domain routeing
protocol at the ISs which have links crossing routeing domain
broadcast subnehvorks: These are multi-access subnet-
a>
boundaries. The prefix indicates that any NSAPs whose NSAP
works that support the capability of addressing a group of
address matches the prefix may be reachable via the SNPA
attached systems with a single NPDU, for instance
with which the prefix is associated. Where this SNPA is con-
IS0 8802-3 LANs.
nected to a multi-destination subnetwork (e.g., dynamically as-
signed DED, broadcast), the prefix also has associated with it
general topology subnetworks: These are modelled as a
b)
the required subnetwork addressing information, or an indica-
set of point-to-point links each of which connects exactly
tion that it may be derived from the destination NSAP address
two systems.
(for example, an X. 121 DTE address may sometimes be ob-
tained from the ID1 of the NSAP address).
There are several generic types of general topology sub-
networks:
The Address Prefixes are handled by the level 2 routeing algo-
rithm in the same way as information about a level 1 area
multipoint links: These are links between more than
1)
within the domain. NPDUs with a destination address match-
two systems, where one system is a primary system,
ing any of the prefixes present on any Level 2 Intermediate
and the remaining systems are secondary (or slave)
System within the domain can therefore be relayed (using
systems. The primary is capable of direct communi-
level 2 routeing) by that IS and delivered out of the domain. (It
cation with any of the secondaries, but the secondar-
is assumed that the routeing functions of the other domain will
ies cannot communicate directly among themselves.
then be able to deliver the NPDU to its destination.)
permanent point-to-point links: These are links that
2)
Where multiple routeing domains are interconnected using this
stay connected at all times (unless broken, or turned
International Standard, the model used is one in which the
off by system management), for instance leased lines
boundaries between routeing domains are on the subnetworks
or private links.
which connect the Intermediate systems. A boundary for a
routeing domain is constructed by marking the linkage man-
dynamically established data links (DEDs): These are
3)
aged object associated with a circuit as being externalDomain
links over connection oriented facilities, for instance
rather than internal.
X.25, X.21, ISDN, or PSTN networks.
in one of
Dynamically established data links can be used NOTE 1 This model also permits the construction of route-
ing domains whose scope is not limited by the hierarchical
two ways:
nature of network layer address assignment. For example, it
static point-to-point (Static): The call is estab- is possible to construct a routeing domain, or even a single
9
area, whose area addresses are taken from multiple address-
lished upon system management action and
ing authorities.
cleared only on system management action (or
failure).
6.4 Addresses
ii) dynamically assigned (DA): The call is estab-
lished upon receipt of traffic, and brought down
Within a routeing domain that conforms to this International
on timer expiration when idle. The address to
Standard, the Network entity titles of Intermediate systems
which the call is to be established is determined
must meet the requirements stated in 7.1.4. It is the routeing
dynamically from information in the arriving
NPDU(s). No IS-IS routeing PDUs are ex- domain administrative authority’s responsibility to ensure that
changed between ISs on a DA circuit. such is the case.
All systems shall be able to generate and forward NPDUs con-
All subnetwork types are treated by the Subnetwork Independ-
taining NSAP addresses in any of the formats specified by
ent functions as though they were connectionless subnetworks,
IS0 8348/Add.2. However, the routeing domain’s administra-
using the Subnetwork Dependent Convergence functions of
tive authority should ascertain that NSAP addresses of End
IS0 8473 where necessary to provide a connectionless subnet-
5
---------------------- Page: 9 ----------------------
ISOAEC 10589:1992 (E)
l It performs NPDU forwarding based on the destina-
systems meet the requirements set forth in 7.1.4 in order to
take full advantage the routes derived by this protocol. Within tion address.
such a domain it is still possible for some End systems to have
0 If a system
It manages the characteristics of the path.
addresses assigned which do not conform to the rules set forth
alternate route.
or link fails on a path, it finds an
in 7.1.4 provided that they meet the more general requirements
of IS0 8348/Add.2, but these End systems may require addi-
l It interfaces with the subnetwork dependent functions
tional configuration information to be entered into the Interme-
to receive reports concerning an SNPA which has be-
diate systems and they may obtain inferior routeing perform-
come unavailable, a system that has failed, or the
ance.
subsequent recovery of an SNPA or system.
NOTE 2 The procedures whereby the routeing domain ad-
l It informs the IS0 8473 error reporting function
ministrative authority obtains from an appropriate address
when the forwarding function cannot relay an NPDU,
authority Intermediate system NETS as required by this In-
for instance when the destination is unreachable or
ternational Standard, and End system NSAP addresses as
when the NPDU would have needed to be segmented
recommended by this International Standard are outside its
and the NPDU requested “no segmentation”.
scope.
- Congestion control. Congestion control manages the re-
6.5 Functional organisation
sources used at each Intermediate system.
The intra-domain IS-IS routeing functions are divided into two
6.52 Subnetwork dependent functions
groups
The subnetwork dependent functions mask the characteristics
Subnetwork Independent Functions
of the subnetwork or data link service from the subnetwork in-
dependent functions. These include:
Subnetwork Dependent Functions
- Operation of the Intermediate system functions of
6.5.1 Subnetwork independent functions IS0 9542 on the particular subnetwork, in order to
l determine neighbour Network entity title(s) and
The Subnetwork Independent Functions supply full-duplex
SNPA address(
NPDU transmission between any pair of neighbour systems.
They are independent of the specific subnetwork or data link
0
determine the SNPA address of operational Inter-
service operating below them, except for recognising two ge-
mediate systems.
neric types of subnetworks:
- Operation of the requisite Subnetwork Dependent Con-
- General Topology Subnetworks, which include HDLC
vergence Function as defined in IS0 8473 and its adden-
point-to-point, HDLC multipoint, and dynamically estab-
dum 3, in order to perform
lished data links (such as X.25, X.21, and PSTN links),
and
0
data link initialisation;
- Broadcast Subnetworks, which include IS0 8802
l hop by hop fragmentation over subnetworks with
LANS.
small maximum SNSDU sizes; and
0
call establishment and clearing on dynamically estab-
NOTE 3 This protocol is intended to operate on any broad-
lished data links.
cast subnetwork which meets the general requirements listed
in 6.7. However, the re
...
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