ISO/IEC 10589:2002
(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)
This International Standard specifies a protocol which is used by Network Layer entities operating the protocol specified in ISO 8473 in Intermediate Systems to maintain routeing information for the purpose of routeing within a single routeing domain. The protocol specified in this International Standard relies upon the provision of a connectionless-mode underlying service.1) This International Standard specifies: a) procedures for the transmission of configuration and routeing information between network entities residing in Intermediate Systems within a single routeing domain; b) the encoding of the protocol data units used for the transmission of the configuration and routeing information; c) procedures for the correct interpretation of protocol control information; and d) the functional requirements for implementations claiming conformance to this International Standard. The procedures are defined in terms of a) the interactions between Intermediate system Network entities through the exchange of protocol data units; b) the interactions between a Network entity and an underlying service provider through the exchange of subnetwork service primitives; and c) the constraints on route determination which must be observed by each Intermediate system when each has a routeing information base which is consistent with the others.
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)
General Information
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Standards Content (Sample)
INTERNATIONAL ISO/IEC
STANDARD 10589
Second edition
2002-11-15
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)
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)
Reference number
©
ISO/IEC 2002
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ii © ISO/IEC 2002 – All rights reserved
Contents
1 Scope ___________________________________________________________________________ 1
2 Normative references ______________________________________________________________ 1
3 Definitions_______________________________________________________________________ 3
3.1 Reference model definitions____________________________________________________________ 3
3.2 Network layer architecture definitions___________________________________________________ 3
3.3 Network layer addressing definitions ____________________________________________________ 3
3.4 Local area network definitions _________________________________________________________ 3
3.5 Routeing framework definitions ________________________________________________________ 3
3.6 Additional definitions _________________________________________________________________ 3
4 Symbols and abbreviations__________________________________________________________ 5
4.1 Data units___________________________________________________________________________ 5
4.2 Protocol data units ___________________________________________________________________ 5
4.3 Addresses___________________________________________________________________________ 5
4.4 Miscellaneous _______________________________________________________________________ 5
5 Typographical conventions _________________________________________________________ 6
6 Overview of the protocol____________________________________________________________ 6
6.1 System types ________________________________________________________________________ 6
6.2 Subnetwork types ____________________________________________________________________ 7
6.3 Topologies __________________________________________________________________________ 7
6.4 Addresses___________________________________________________________________________ 8
6.5 Functional organisation _______________________________________________________________ 8
6.6 Design goals and non-goals ____________________________________________________________ 9
6.7 Environmental requirements__________________________________________________________ 11
6.8 Functional organisation of subnetwork independent components____________________________ 12
7 Subnetwork independent functions __________________________________________________ 14
7.1 Addresses__________________________________________________________________________ 15
7.2 Decision process ____________________________________________________________________ 18
7.3 Update process _____________________________________________________________________ 26
7.4 Forwarding process _________________________________________________________________ 45
7.5 Routeing constants and parameters ____________________________________________________ 48
8 Subnetwork dependent functions____________________________________________________ 49
8.1 Multi-destination circuits on ISs at a domain boundary _________________________________ 49
8.2 Point-to-point subnetworks ___________________________________________________________ 50
8.3 ISO 8208 subnetworks _______________________________________________________________ 54
8.4 Broadcast subnetworks ______________________________________________________________ 59
© ISO/IEC 2002 – All rights reserved iii
9 Structure and encoding of PDUs____________________________________________________ 65
9.1 General encoding rules_______________________________________________________________65
9.2 Encoding of network layer addresses ___________________________________________________65
9.3 Encoding of SNPA addresses__________________________________________________________65
9.4 PDU types _________________________________________________________________________66
9.5 Level 1 LAN IS to IS hello PDU _______________________________________________________66
9.6 Level 2 LAN IS to IS hello PDU _______________________________________________________69
9.7 Point-to-point IS to IS hello PDU ______________________________________________________72
9.8 Level 1 link state PDU _______________________________________________________________75
9.9 Level 2 link state PDU _______________________________________________________________79
9.10 Level 1 complete sequence numbers PDU _________________________________________________84
9.11 Level 2 complete sequence numbers PDU _________________________________________________86
9.12 Level 1 partial sequence numbers PDU___________________________________________________88
9.13 Level 2 partial sequence numbers PDU___________________________________________________90
10 System environment _______________________________________________________________ 91
10.1 Generating jitter on timers _____________________________________________________________91
10.2 Resolution of timers___________________________________________________________________92
10.3 Requirements on the operation of ISO 9542 _______________________________________________93
10.4 Requirements on the operation of ISO 8473 _______________________________________________93
11 System management_______________________________________________________________ 93
11.1 General _____________________________________________________________________________93
--11.2 GDMO definition ______________________________________________________________ 94
--11.2.1 Common GDMO definitions_________________________________________________________94
--11.3 ASN1 modules _____________________________________________________________________127
12 Conformance ___________________________________________________________________ 129
12.1 Conformance for protocol implementation_______________________________________________129
12.1.2 Dynamic conformance ______________________________________________________________131
12.2 Conformance for management information implementation ________________________________133
Annex A__________________________________________________________________________ 135
A.1 Introduction_________________________________________________________________________135
A.2 Abbreviations and special symbols ______________________________________________________135
A.3 Instructions for completing the pics pro formas ___________________________________________135
A.4 Identification ________________________________________________________________________138
A.5 Protocol summary: ISO 10589 general___________________________________________________139
A.6 Protocol summary: ISO 10589 level 1 specific functions_____________________________________143
A.7 Protocol summary: ISO 10589 level 2 specific functions_____________________________________144
iv © ISO/IEC 2002 – All rights reserved
Annex B __________________________________________________________________________ 146
B.1 Addressing and routeing ______________________________________________________________ 146
B.2 Use of the area address field in intra-domain routeing ______________________________________ 148
Annex C __________________________________________________________________________ 150
C.1 Routeing databases ___________________________________________________________________ 150
C.3 Forwarding process __________________________________________________________________ 156
Annex D__________________________________________________________________________ 158
D.1 Congestion control ___________________________________________________________________ 158
D.2 Congestion avoidance _________________________________________________________________ 159
Annex E __________________________________________________________________________ 160
--E.1 Generic managed object class definitions _______________________________________________ 160
--E.2 ASN.1 definitions ___________________________________________________________________ 167
Annex F __________________________________________________________________________ 169
Annex G__________________________________________________________________________ 170
G.1 Introduction ________________________________________________________________________ 170
G.2 Identification of the implementation_____________________________________________________ 171
G.3 Identification of the International Standard in which the management information is defined ____ 171
Annex H__________________________________________________________________________ 175
H.1 Introduction ________________________________________________________________________ 175
H.2 Instructions for completing the MICS proforma to produce a MICS__________________________ 175
H.3 Symbols, abbreviates and terms ________________________________________________________ 175
H.4 Statement of Conformance to the management information _________________________________ 175
Annex I __________________________________________________________________________ 182
I.1 Introduction _________________________________________________________________________ 182
I.2 Adjacency managed object _____________________________________________________________ 182
I.3 Virtual adjacency managed object _______________________________________________________ 185
I.4 Destination system managed object ______________________________________________________ 187
I.5 Destination area managed object ________________________________________________________ 188
I.6 reachableAddress [“ISO/IEC 10589”] ____________________________________________________ 190
Annex J __________________________________________________________________________ 195
J.1 Introduction _________________________________________________________________________ 195
J.2 Instructions for completing the MRCS proforma for name binding to produce a MRCS__________ 195
J.3 Statement of conformance to the name binding ____________________________________________ 195
Index ____________________________________________________________________________ 197
© ISO/IEC 2002 – All rights reserved v
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the
specialized system for worldwide standardization. National bodies that are members of ISO 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. ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work. In
the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of the joint technical committee is to prepare International Standards. 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.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 10589 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 6,
Telecommunications and information exchange between systems.
This second edition cancels and replaces the first edition (ISO/IEC 10589:1992), which has been technically revised. It
incorporates Cor.1:1993, Cor.2:1996, Cor.3:1996, Amd.1:1996 and Amd.2:1999.
Annexes A, E, G, H, I and J form a normative part of this International Standard. Annexes B, C, D and F are for information
only.
Annexes G, H, I and J provide ICS proformas associated with intra-domain routeing protocol management information.
vi © ISO/IEC 2002 – All rights reserved
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
ISO 7498 and by the structure defined in ISO 8648. In particular, 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 ISO 9542 and ISO 8473. ISO 9542 is used to establish
connectivity and reachability between End Systems and Intermediate Systems on individual subnetworks. Data is carried
using the protocol specified in ISO 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 consisting of combinations of many
types of subnetworks. This includes point-to-point links, multipoint links, X.25 subnetworks, and broadcast subnetworks such
as ISO 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 system resides in exactly one area. Routeing within an area is
referred to as Level 1 routeing. Routeing between areas is referred to as Level 2 routeing. 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.
© ISO/IEC 2002 – All rights reserved vii
INTERNATIONAL STANDARD ISO/IEC 10589:2002(E)
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)
1 Scope
This International Standard specifies a protocol which is used by Network Layer entities operating the protocol specified in
ISO 8473 in Intermediate Systems to maintain routeing information for the purpose of routeing within a single routeing
domain. The protocol specified in this International Standard relies upon the provision of a connectionless-mode underlying
1)
service.
This International Standard specifies:
a) procedures for the transmission of configuration and routeing information between network entities residing in
Intermediate Systems within a single routeing domain;
b) the encoding of the protocol data units used for the transmission of the configuration and routeing information;
c) procedures for the correct interpretation of protocol control information; and
d) the functional requirements for implementations claiming conformance to this International Standard.
The procedures are defined in terms of
a) the interactions between Intermediate system Network entities through the exchange of protocol data units;
b) the interactions between a Network entity and an underlying service provider through the exchange of subnetwork service
primitives; and
c) the constraints on route determination which must be observed by each Intermediate system when each has a routeing
information base which is consistent with the others.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this
International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not
apply. However, parties to agreements based on this International Standard are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the
normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International
Standards.
ISO/IEC 7498-1:1994, Information technology – Open Systems Interconnection – Basic Reference Model: The Basic Model
ISO/IEC 7498-3:1997, Information technology – Open Systems Interconnection – Basic Reference Model: Naming and
addressing
ISO/IEC 7498-4:1989, Information processing systems – Open Systems Interconnection – Basic Reference Model – Part 4:
Management framework
ISO/IEC 8208:2000, Information technology – Data communications – X.25 Packet Layer Protocol for Data Terminal
Equipment
1)
See ISO 8473 and its addendum 3 for the mechanisms necessary to realize this service on subnetworks based on ISO/IEC 8208,
ISO 8802, and the OSI Data Link Service.
© ISO/IEC 2002 – All rights reserved 1
ISO/IEC 8348:1996, Information technology – Open Systems Interconnection – Network Service Definition
ISO/IEC 8473-1:1998, Information technology – Protocol for providing the connectionless-mode network service: Protocol
specification
ISO/IEC 8473-4:1995, Information technology – Protocol for providing the connectionless-mode network service: Provision of
the underlying service by a subnetwork that provides the OSI data link service
ISO 8648:1988, Information processing systems – Open Systems Interconnection – Internal organization of the Network Layer
ISO/IEC TR 8802-1:1997, Information technology – Telecommunications and information exchange between systems – Local
and metropolitan area networks – Specific requirements – Part 1: Overview of Local Area Network Standards
ISO/IEC 8802-2:1998, Information technology – Telecommunications and information exchange between systems – Local and
metropolitan area networks – Specific requirements – Part 2: Logical link control
ISO/IEC 8802-3:2000, Information technology – Telecommunications and information exchange between systems – Local and
metropolitan area networks – Specific requirements – Part 3: Carrier sense multiple access with collision detection
(CSMA/CD) access method and physical layer specifications
ISO/IEC 8802-5:1998, Information technology – Telecommunications and information exchange between systems – Local and
metropolitan area networks – Specific requirements – Part 5: Token ring access method and physical layer specifications
ISO/IEC 8802-6:1994, Information technology – Telecommunications and information exchange between systems – Local and
metropolitan area networks – Specific requirements – Part 6: Distributed Queue Dual Bus (DQDB) access method and
physical layer specifications
ISO/IEC 9314 (all parts), Information technology – Fibre Distributed Data Interface (FDDI)
ISO 9542:1988, Information processing systems – Telecommunications and information exchange between systems – End
system to Intermediate system routeing exchange protocol for use in conjunction with the Protocol for providing the
connectionless-mode network service (ISO 8473)
ISO/IEC TR 9575:1995, Information technology – Telecommunications and information exchange between systems – OSI
Routeing Framework
ISO/IEC TR 9577:1999, Information technology – Protocol identification in the network layer
ISO/IEC 15802-1:1995, Information technology – Telecommunications and information exchange between systems – Local
and metropolitan area networks – Common specifications – Part 1: Medium Access Control (MAC) service definition
ISO/IEC 10165-1:1993, Information technology – Open Systems Interconnection – Management Information Services –
Structure of management information: Management Information Model
ISO/IEC 10165-4:1992, Information technology – Open Systems Interconnection – Structure of management information –
Part 4: Guidelines for the definition of managed objects
ISO/IEC 10733:1998, Information technology – Elements of management information related to the OSI Network Layer
ISO/IEC 8824-1:1998, Information technology – Abstract Syntax Notation One (ASN.1): Specification of basic notation
ISO/IEC 8825-1:1998, Information technology – ASN.1 encoding rules: Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)
ISO/IEC 9646-7:1995, Information technology – Open Systems Interconnection – Conformance testing methodology and
framework – Part 7: Implementation Conformance Statements
ISO/IEC 10165-6:1997, Information technology – Open Systems Interconnection – Structure of management information:
Requirements and guidelines for implementation conformance statement proformas associated with OSI management
NOTE 1 – ISO/IEC 9646-1:1994 and ISO/IEC 9646-2:1994 supersede ISO/IEC 9646-1:1991 and ISO/IEC 9646-2:1991 respectively. However, when
this International Standard was under development, the previous editions were valid and this International Standard is therefore based on these editions,
which are listed below.
2 © ISO/IEC 2002 – All rights reserved
ISO/IEC 9646-1:1991, Information technology – Open Systems Interconnection – Conformance testing methodology and
framework – Part 1: General concepts
ISO/IEC 9646-2:1991, Information technology – Open Systems Interconnection – Conformance testing methodology and
framework – Part 2: Abstract test suite specification
3 Definitions
3.1 Reference model definitions
This International Standard makes use of the following terms defined in ISO 7498:
a) Network Layer
b) Network Service access point
c) Network Service access point address
d) Network entity
e) Routeing
f) Network protocol
g) Network relay
h) Network protocol data unit
3.2 Network layer architecture definitions
This International Standard makes use of the following terms defined in ISO 8648:
a) Subnetwork
b) End system
c) Intermediate system
d) Subnetwork service
e) Subnetwork Access Protocol
f) Subnetwork Dependent Convergence Protocol
g) Subnetwork Independent Convergence Protocol
3.3 Network layer addressing definitions
This International Standard makes use of the following terms defined in ISO 8348:
a) Subnetwork address
b) Subnetwork point of attachment
c) Network Entity Title
3.4 Local area network definitions
This International Standard makes use of the following terms defined in ISO 8802:
a) Multi-destination address
b) Media access control
c) Broadcast medium
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
c) Hop
d) Black hole
3.6 Additional definitions
For the purposes of this International Standard, the following definitions apply:
© ISO/IEC 2002 – All rights reserved 3
3.6.1 area: A routeing subdomain which maintains detailed routeing information about its own internal composition, and also
maintains routeing information which allows it to reach other routeing subdomains. It corresponds to the Level 1
subdomain.
3.6.2 neighbour: An adjacent system reachable by traversal of a single subnetwork by a PDU.
3.6.3 adjacency: A portion of the local routeing information which pertains to the reachability of a single neighbour ES or IS
over a single circuit.
Adjacencies are used as input to the Decision Process for forming paths through the routeing domain.
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.
3.6.4 circuit: A subset of the local routeing information base pertinent to a single local SNPA. The system management view
of a circuit is presented in a linkage managed object.
3.6.5 link: The communication path between two neighbours.
A link is “up” when communication is possible between the two SNPAs.
3.6.6 designated IS: The Intermediate system on a LAN, 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.
3.6.7 pseudonode: Where a broadcast subnetwork has n connected Intermediate systems, the broadcast subnetwork itself is
considered to be a pseudonode.
The pseudonode has links to each of the n Intermediate and End systems. Each of the ISs has a single link to the
pseudonode (rather than n-1 links to each of the other Intermediate systems). Link State PDUs are generated on behalf
of the pseudonode by the Designated IS. This is depicted below in figure 1.
3.6.8 broadcast subnetwork: A subnetwork which supports an arbitrary number of End systems and Intermediate systems and
additionally is capable of transmitting a single SNPDU to a subset of these systems in response to a single
SN_UNITDATA request.
3.6.9 general topology subnetwork: A subnetwork which supports an arbitrary number of End systems and Intermediate
systems, but does not support a convenient multi-destination connectionless transmission facility, as does a broadcast
subnetwork.
3.6.10 routeing subdomain: a set of Intermediate systems and End systems located within the same Routeing domain.
3.6.11 level 2 subdomain: the set of all Level 2 Intermediate systems in a Routeing domain.
3.6.12 jitter: a small random variation introduced into the value of a timer to prevent multiple timer expirations in different
systems from becoming synchronised.
IS IS IS IS IS IS
pseudo-
node
ES ES
Figure 1 - Use of a pseudonode to collapse a LAN Topology
4 © ISO/IEC 2002 – All rights reserved
4 Symbols and abbreviations
4.1 Data units
PDU Protocol Data Unit
SNSDU Subnetwork Service Data Unit
NSDU Network Service Data Unit
NPDU Network Protocol Data Unit
SNPDU Subnetwork Protocol Data Unit
4.2 Protocol data units
ESH PDU ISO 9542 End System Hello Protocol Data Unit
ISH PDU ISO 9542 Intermediate System Hello Protocol
Data Unit
RD PDU ISO 9542 Redirect Protocol Data Unit
IIH PDU Intermediate System to Intermediate System
Protocol Data Unit
LSP Link State Protocol Data Unit
SNP Sequence Numbers Protocol Data Unit
CSNP Complete Sequence Numbers Protocol Data
Unit
PSNP Partial Sequence Numbers Protocol Data Unit
4.3 Addresses
AFI Authority and Format Indicator
DSP Domain Specific Part
IDI Initial Domain Identifier
IDP Initial Domain Part
NET Network Entity Title
NPAI Network Protocol Addressing Information
NSAP Network Service Access Point
SNPA Subnetwork Point of Attachment
4.4 Miscellaneous
DA Dynamically Assigned
DED Dynamically Established Data Link
DTE Data Terminal Equipment
ES End System
IS Intermediate System
HDLC High Level Data Link Control
ISDN Integrated Services Digital Network
FDDI Fiber Distributed Data Interface
L1 Level 1
L2 Level 2
LAN Local Area Network
MAC Media Access Control
MAN Metropolitan Area Network
MCS Management conformance summary
MICS Management information conformance statement
MOCS Managed object conformance statement
MRCS Managed relationship conformance statement
NLPID Network Layer Protocol Identifier
PSTN Public Switched Telephone Network
OSIE Open Systems Interconnection Environment
© ISO/IEC 2002 – All rights reserved 5
PCI Protocol Control Information
QoS Quality of Service
SN Subnetwork
SNAcP Subnetwork Access Point
SNDCP Subnetwork Dependent Convergence Protocol
SNICP Subnetwork Independent Convergence Protocol
SRM Send Routeing Message
SSN Send Sequence Numbers
SVC Switched Virtual Circuit
Legend:
Intermediate
ES End System IS
System
ES-IS Protocol Subnetwork Path
Level 1 IS-IS Routeing
IS-IS Area
Level 2 IS-IS Routeing
Routeing Domain
Boundary
Interdomain Routeing
IS
ES
IS
ES
IS
ES
IS
ES
IS
ES IS
IS IS
IS
ES
ES ES
Figure 2 Topologies and Systems supported by Intradomain Routeing
5 Typographical conventions
This International Standard makes use of the following typographical conventions:
a) important terms and concepts appear in italic type when introduced for the first time;
b) protocol constants and management parameters appear in sansSerif type with multiple words run together. The first word
is lower case, with the first character of subsequent words capitalised;
c) protocol field names appear in sansSerif type with each word capitalised; and
d) values of constants, parameters, and protocol fields appear enclosed in “double quotes”.
6 Overview of the protocol
6.1 System types
For the purposes of this International Standard, systems are classified according to the following types:
End Systems: These systems deliver NPDUs to other systems and receive NPDUs from other systems, but do not relay
NPDUs. This International Standard does not specify any additional End system functions beyond those supplied by ISO 8473
and ISO 9542.
Level 1 Intermediate Systems: These systems deliver and receive NPDUs from other systems, and relay NPDUs from other
source systems to other destination systems. They route directly to systems within their own area, and route towards a level 2
Intermediate system when the destination system is in a different area.
6 © ISO/IEC 2002 – All rights reserved
Level 2 Intermediate Systems: These systems act as Level 1 Intermediate systems in addition to acting as a system in the
subdomain consisting of level 2 ISs. Systems in the level 2 subdomain route towards a destination area, or another routeing
domain.
NOTE 2 Operation of a level 2 IS may be restricted on a per circuit basis by setting manualL2OnlyMode to "True". This indicates that
the circuit is to be used only for Level 2 traffic. If all circuits in a Level 2 IS have manualL2OnlyMode set to "True" then the IS does not
operate as a Level 1 IS in the area.
These systems and their topological relationship are illustrated in figure 2.
6.2 Subnetwork types
For the purposes of this International Standard, subnetworks are classified according to the following types:
a) broadcast subnetworks: These are multi-access subnetworks that support the capability of addressing a group of attached
systems with a single NPDU, for instance ISO 8802-3 LANs.
b) general topology subnetworks: These are modelled as a set of point-to-point links each of which connects exactly two
systems.
There are several generic types of general topology sub-networks:
1) multipoint links: These are links between more than two systems, where one system is a primary system, and the
remaining systems are secondary (or slave) systems. The primary is capable of direct communication with any of the
secondaries, but the secondaries cannot communicate directly among themselves.
2) permanent point-to-point links: These are links that stay connected at all times (unless broken, or turned off by system
management), for instance leased lines or private links.
3) dynamically established data links (DEDs): These are links over connection oriented facilities, for instance X.25,
X.21, ISDN, or PSTN networks.
Dynamically established data links can be used in one of two ways:
i) static point-to-point (Static): The call is established upon system management action and cleared only on
system management action (or failure).
ii) dynamically assigned (DA): The call is established upon receipt of traffic, and brought down on timer
expiration when idle. The address to which the call is to be established is determined dynamically from
information in the arriving NPDU(s). No IS–IS routeing PDUs are exchanged between ISs on a DA circuit.
NOTE 3 For the operation of the protocol, a permanent point-to-point link and a static point-to-point DED are equivalent (see 7.3.7,
7.3.9).
All subnetwork types are treated by the Subnetwork Independent functions as though they were connectionless subnetworks,
using the Subnetwork Dependent Convergence functions of ISO 8473 where necessary to provide a connectionless subnetwork
service. The Subnetwork Dependent functions do, however, operate differently on connectionless and connection-oriented
subnetworks.
6.3 Topologies
A single organisation may wish to divide its Administrative Domain into a number of separate Routeing Domains. This has
certain advantages, as described in ISO/IEC TR 9575. Furthermore, it is desirable for an intra-domain routeing protocol to aid
in the operation of an inter-domain routeing protocol, where such a protocol exists for interconnecting multiple routeing
domains.
In order to facilitate the construction of such multi-domain topologies, provision is made for the entering of inter-domain
routeing information. This information is in the form of a set of Reachable Address Prefixes which may be entered either by
System Management, or provided by an inter-domain routeing protocol at the ISs which have links crossing routeing domain
boundaries. The prefix indicates that any NSAPs whose NSAP address matches the prefix may be reachable via the SNPA
with which the prefix is associated. Where this SNPA is connected to a multi-destination subnetwork (e.g., dynamically
assigned DED, broadcast), the prefix also has associated with it the required subnetwork addressing information, or an
© ISO/IEC 2002 – All rights reserved 7
indication that it may be derived from the destination NSAP address (for example, an X.121 DTE address may sometimes be
obtained from the IDI of the NSAP address).
The Address Prefixes are handled by the level 2 routeing algorithm in the same way as information about a level 1 area within
the domain. NPDUs with a destination address matching any of the prefixes present on any Level 2 Intermediate System
within the domain can therefore be relayed (using level 2 routeing) by that IS and delivered out of the domain. (It is assumed
that the routeing functions of the other domain will then be able to deliver the NPDU to its destination.)
Where multiple routeing domains are interconnected using this International Standard, the model used is one in which the
boundaries between routeing domains are on the subnetworks which connect the Intermediate systems. A boundary for a
routeing domain is constructed by marking the linkage managed object associated with a circuit as being externalDomain
rather than internal.
NOTE 4 This model also permits the construction of routeing domains whose scope is not limited by the hierarchical nature of network
layer address assignment. For example, it is possible to construct a routeing domain, or even a single area, whose area addresses are taken
from multiple addressing authorities.
6.4 Addresses
Within a routeing domain that conforms to this International Standard, the Network entity titles of Intermediate systems must
meet the requirements stated in 7.1.4. It is the routeing domain administrative authority’s responsibility to ensure that such is
the case.
All systems shall be able to generate and forward NPDUs containing NSAP addresses in any of the formats specified by ISO
8348. However, the routeing domain’s administrative authority should ascertain that NSAP addresses of End systems meet the
requirements set forth in 7.1.4 in order to take full advantage the routes derived by this protocol.
Within such a domain it is still possible for some End systems to have addresses assigned which do not conform to the rules set
forth in 7.1.4 provided that they meet the more general requirements of ISO 8348, but these End systems may require
additional configuration information to be entered into the Intermediate systems and they may obtain inferior routeing
performance.
NOTE 5 The procedures whereby the routeing domain administrative authority obtains from an appropriate address authority Intermediate
system NETs as required by this International Standard, and End system NSAP addresses as recommended by this International Standard are outside its
scope.
6.5 Functional organisation
The intra-domain IS–IS routeing functions are divided into two groups
- Subnetwork Independent Functions
- Subnetwork Dependent Functions
6.5.1 Subnetwork independent functions
The Subnetwork Independent Functions supply full-duplex NPDU transmission between any pair of neighbour systems. They
are independent of the specific subnetwork or data link service operating below them, except for recognising two generic types
of subnetworks:
- General Topology Subnetworks, which include HDLC point-to-point, HDLC multipoint, and dynamically established
data links (such as X.25, X.21, and PSTN links), and
- Broadcast Subnetworks, which include ISO 8802 LANs.
NOTE 6 This protocol is intended to operate on any broadcast subnetwork which meets the general requirements listed in 6.7. However,
the remainder of this International Standard specifically addresses ISO 8802 LANs. Other LANs, such as FDDI, are believed to be
adequately covered by the specification for ISO 8802 LANs. Other broadcast subnetworks, such as ISO 8802-6 MANs, may not be
adequately covered at this time.
The following Subnetwork Independent Functions are identified:
8 © ISO/IEC 2002 – All rights reserved
- Routeing. The routeing function determines NPDU paths. A path is the sequence of connected systems and links between
a source ES and a destination ES.
The combined knowledge of all the Network Layer entities of all the Intermediate systems within a routeing domain is
used to ascertain the existence of a path, and route the NPDU to its destination. The routeing component at an Intermediate
system has the following specific functions:
• It extracts and interprets the routeing PCI in an NPDU.
• It performs NPDU forwarding based on the destination address.
• It manages the characteristics of the path. If a system or link fails on a path, it finds an alternate route.
• It interfaces with the subnetwork dependent functions to receive reports concerning an SNPA which has become
unavailable, a system that has failed, or the subsequent recovery of an SNPA or system.
• It informs the ISO 8473 error reporting function when the forwarding function cannot relay an NPDU, for instance
when the destination is unreachable or when the NPDU would have needed to be segmented and the NPDU requested
“no segmentation”.
- Congestion control. Congestion control manages the resources used at each Intermediate system.
6.5.2 Subnetwork dependent functions
The subnetwork dependent functions mask the characteristics of the subnetwork or data link service from the subnetwork
independent functions. These include:
- Operation of the Intermediate system functions of ISO 9542 on the particular subnetwork, in order to
• determine neighbour Network entity title(s) and SNPA address(es);
• determine the SNPA address(es) of operational Intermediate systems.
- Operation of the requisite Subnetwork Dependent Convergence Function as defined in ISO 8473, in order to perform
• data link initialisation;
• hop by hop fragmentation over subnetworks with small maximum SNSDU sizes; and
• call establishment and clearing on dynamically established data links.
6.6 Design goals and non-goals
6.6.1 Goals
This International Standard supports the following design requirements. The correspondence with the goals for OSI routeing
stated in ISO/IEC TR 9575 are noted.
- Network Layer Protocol Compatibility: It is compatible with ISO 8473 and ISO 9542. (See 7.5 of ISO/IEC TR 9575),
- Simple End systems: It requires no changes to End systems, nor any functions beyond those supplied by ISO 8473 and
ISO 9542. (See 7.2.1 of ISO/IEC TR 9575),
- Multiple Organisations: It allows for multiple routeing and administrative domains through the provision of static
routeing information at domain boundaries. (See 7.3 of ISO/IEC TR 9575)
- Deliverability: It accepts and delivers NPDUs addressed to reachable destinations and rejects NPDUs addressed to
destinations known to be unreachable,
- Adaptability: It adapts to topological changes within the routeing domain, but not to traffic changes, except potentially as
indicated by local queue lengths. It splits traffic load on multiple equivalent paths. (See 7.7 of ISO/IEC TR 9575),
© ISO/IEC 2002 – All rights reserved 9
- Promptness: The period of adaptation to topological changes in the domain is a reasonable function of the domain
diameter (that is, the maximum logical distance between End Systems within the domain) and Data link speeds. (See 7.4
of ISO/IEC TR 9575),
- Efficiency: It is both processing and memory efficient. It does not create excessive routeing traffic overhead. (See 7.4 of
ISO/IEC TR 9575),
- Robustness: It recovers from transient errors such as lost or temporarily incorrect routeing PDUs. It tolerates imprecise
parameter settings. (See 7.7 of ISO/IEC TR 9575),
- Stability: It stabilises in finite time to “good routes”, provided no continuous topolo
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