The pseudonode has links to each of the n Intermediate 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 supports 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 systems and Intermediate systems, but does not support a convenient multi-destination connectionless transmission facility, as does a broadcast subnet work. 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.4 Symbols and Abbreviations 4.1 Data UnitsPDU	Protocol Data UnitSNSDU	Subnetwork Service Data UnitNSDU	Network Service Data UnitNPDU	Network Protocol Data UnitSNPDU	Subnetwork Protocol Data Unit 4.2 Protocol Data UnitsESH PDU	ISO 9542 End System Hello Protocol Data Unit ISH PDU	ISO 9542 Intermediate System Hello Protocol Data UnitRD PDU	ISO 9542 Redirect Protocol Data UnitIIH	Intermediate system to Intermediate system Hello Protocol Data UnitLSP	Link State Protocol Data Unit SNP	Sequence Numbers Protocol Data UnitCSNP	Complete Sequence Numbers Protocol Data UnitPSNP	Partial Sequence Numbers Protocol Data Unit  4.3 AddressesAFI	Authority and Format IndicatorDSP	Domain Specific PartIDI	Initial Domain IdentifierIDP	Initial Domain PartNET	Network Entity TitleNSAP	Network Service Access PointSNPA	Subnetwork Point of Attachment 4.4 MiscellaneousDA	Dynamically AssignedDED	Dynamically Established Data linkDTE	Data Terminal EquipmentES	End SystemIS	Intermediate SystemL1	Level 1L2	Level 2LAN	Local Area NetworkMAC	Media Access ControlNLPID	Network Layer Protocol IdentifierPCI	Protocol Control InformationQoS	Quality of ServiceSN	SubnetworkSNAcP	Subnetwork Access ProtocolSNDCP	Subnetwork Dependent Convergence ProtocolSNICP	Subnetwork Independent Convergence ProtocolSRM	Send Routeing MessageSSN	Send Sequence Numbers MessageSVC	Switched Virtual Circuit5 Typographical ConventionsThis 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 San Serif type with each word capitalised.d)Values of constants, parameters, and protocol fields appear enclosed in double quotes. 6 Overview of the Protocol6.1 System TypesThere are the following types of system: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.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.6.2 Subnetwork TypesThere 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 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 ISIS routeing PDUs are exchanged between ISs on a DA circuit.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 TopologiesA 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 provided 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 associated. Where the subnetwork to which this SNPA is connected is a general topology subnetwork supporting dynamically established data links, the prefix also has associated 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 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.) 6.4 AddressesWithin 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 configuration and be subject to inferior routeing performance.6.5  Functional OrganisationThe intra-domain ISIS routeing functions are divided into two groups-Subnetwork Independent Functions -Subnetwork Dependent Functions6.5.1 Subnetwork Independent FunctionsThe 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.The following Subnetwork Independent Functions are identified-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:7It extracts and interprets the routeing PCI in an NPDU. 7It performs NPDU forwarding based on the destination address. 7It manages the characteristics of the path. If a system or link fails on a path, it finds an alternate route.  7It 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.  7It 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 to7Determine neighbour Network entity title(s) and SNPA address(es) 7Determine the SNPA address(s) of operational Intermediate 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 established data links6.6 Design GoalsThis 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 compatible 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),-Multiple Organisations: It allows for multiple routeing and administrative domains through the provision of static routeing information at domain boundaries. (See clause 7.3 of ISO/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 clause 7.7 of ISO/TR 9575),-Promptness. The period of adaptation to topological changes in the domain is a reasonable function of the