rfc1142.txt

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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 Interme
diate systems and 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 sup
ports an arbitrary number of End systems and In
 
termediate 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 sys
tems and Intermediate systems, but does not sup
port a convenient multi-destination connectionless 
trans

mission facility, as does a broadcast sub

net

 
work. 
3.6.10   
Routeing Subdomain: a set of Intermediate sys
tems and End systems located within the same 
Routeing domain.
3.6.11   
Level 2 Subdomain: the set of all Level 2 Inter
mediate systems in a Routeing domain.
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	Intermediate system to Intermediate system 
Hello 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
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
L1	Level 1
L2	Level 2
LAN	Local Area Network
MAC	Media Access Control
NLPID	Network Layer Protocol Identifier
PCI	Protocol Control Information
QoS	Quality of Service
SN	Subnetwork
SNAcP	Subnetwork Access Protocol
SNDCP	Subnetwork Dependent Convergence Protocol
SNICP	Subnetwork Independent Convergence Proto
col
SRM	Send Routeing Message
SSN	Send Sequence Numbers Message
SVC	Switched Virtual Circuit
5 Typographical Conventions
This International Standard makes use of the following ty
pographical 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 San Serif type with 
each word capitalised.
d)Values of constants, parameters, and protocol fields 
appear enclosed in double quotes.
 
6 Overview of the Protocol
6.1 System Types
There are the following types of system:
End Systems: These systems deliver NPDUs to other sys
tems and receive NPDUs from other systems, but do 
not relay NPDUs. This International Standard does 
not specify any additional End system functions be
yond 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 destina
tion systems. They route directly to systems within 
their own area, and route towards a level 2 Interme
diate system when the destination system is in a dif
ferent area.
Level 2 Intermediate Systems: These systems act as Level 1 
Intermediate systems in addition to acting as a sys
tem in the subdomain consisting of level 2 ISs. Sys
tems in the level 2 subdomain route towards a desti
nation area, or another routeing domain.
6.2 Subnetwork Types
There are two generic types of subnetworks supported.
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 
subnetworks:
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 in
stance 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 estab
lished upon system management action and 
 
cleared only on system management action (or 
failure).
ii)dynamically assigned (DA): The call is estab
lished upon receipt of traffic, and brought 
down on timer expiration when idle. The ad
dress to which the call is to be established is  
determined dynamically from information in 
the arriving NPDU(s). No ISIS routeing 
PDUs are exchanged between ISs on a DA cir
cuit.
All subnetwork types are treated by the Subnetwork Inde
pendent functions as though they were connectionless 
subnetworks, using the Subnetwork Dependent Conver
gence functions of ISO 8473 where necessary to provide a 
connectionless subnetwork service. The  Subnetwork De
pendent 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/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 administrative domains.
In order to facilitate the construction of such multi-domain 
topologies, provision is made for the entering of static 
inter-domain routeing information. This information is pro
vided by a set of Reachable Address Prefixes entered by 
System Management at the ISs which have links which 
cross 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 associ
ated. Where the subnetwork to which this SNPA is con
nected is a general topology subnetwork supporting dy
namically established data links, the prefix also has associ
ated with it the required subnetwork addressing 
information, or an 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 al
gorithm 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 Inter
mediate 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.) 
6.4 Addresses
Within a routeing domain that conforms to this standard, 
the Network entity titles of Intermediate systems shall be 
structured as described in 7.1.1.
All systems shall be able to generate and forward data 
PDUs containing NSAP addresses in any of the formats 
specified by ISO 8348/Add.2. However,  NSAP addresses 
 
of End systems should be structured as described in 7.1.1 in 
order to take full advantage of ISIS routeing. Within such 
a domain it is still possible for some End Systems to have 
addresses assigned which do not conform to 7.1.1, provided 
they meet the more general requirements of 
ISO 8348/Add.2, but they may require additional configura
tion and be subject to inferior routeing performance.
6.5  Functional Organisation
The intra-domain ISIS 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 sys
tems. They are independent of the specific subnetwork or 
data link service operating below them, except for recognis
ing two generic types of subnetworks: 
-General Topology Subnetworks, which include 
HDLC point-to-point, HDLC multipoint, and dynami
cally established data links (such as X.25, X.21, and 
PSTN links), and  
-Broadcast Subnetworks, which include ISO 8802 
LANs.
The following Subnetwork Independent Functions are iden
tified
-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 route
ing 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 follow
ing specific functions:
7It extracts and interprets the routeing PCI in an 
NPDU. 
7It performs NPDU forwarding based on the desti
nation address. 
7It manages the characteristics of the path. If a sys
tem or link fails on a path, it finds an alternate 
route.  
7It interfaces with the subnetwork dependent func
tions to receive reports concerning an SNPA 
which has become unavailable, a system that has 
failed, or the subsequent recovery of an SNPA or 
system.  
7It informs the ISO 8473 error reporting function 
when the forwarding function cannot relay an 
NPDU, for instance when the destination is un
reachable or when the NPDU would have needed 
 
to be segmented and the NPDU requested no seg
mentation.
-Congestion control. Congestion control manages the 
resources used at each Intermediate system. 
6.5.2 Subnetwork Dependent Functions 
The subnetwork dependent functions mask the characteris
tics 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
7Determine neighbour Network entity title(s) and 
SNPA address(es) 
7Determine the SNPA address(s) of operational In
termediate systems 
-Operation of the requisite Subnetwork Dependent 
Convergence Function as defined in ISO 8473 and its 
Addendum 3, in order to perform 
7Data link initialisation 
7Hop by hop fragmentation over subnetworks with 
small maximum SNSDU sizes 
7Call establishment and clearing on dynamically es
tablished data links
6.6 Design Goals
This International Standard supports the following design 
requirements. The correspondence with the goals for OSI 
routeing stated in ISO/TR 9575 are noted.
-Network Layer Protocol Compatibility. It is com
patible with ISO 8473 and ISO 9542. (See clause 7.5 
of ISO/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 clause 7.2.1 of ISO/TR 
9575),