network protocols handbook.pdf

264Protocols GuideISO Protocols - Network LayerProtocol NameES-IS: End System to IntermediateSystem Routing ExchangeProtocolES-IS Header:1byte 1byte 1byte 1byte 1byteNLPIDLength0 0 0 Type2bytesH-Time2 bytesVersionReservedChecksumProtocol DescriptionEnd System to Intermediate System Routing Exchange Protocol(ES-IS), developed by ISO, permits End Systems and IntermediateSystems to exchange configuration and routing informationto facilitate the operation of the routing and relaying functions ofthe Network Layer in the ISO network environment. In an ISOnetwork, there are End Systems, Intermediate Systems, Areasand Domains. End systems are user devices. Intermediate systemsare routers. Routers are organized into local groups called‘areas’, and several areas are grouped together into a ‘domain’.ES-IS, working in conjunction with CLNP, IS-IS, and IDRP, providescomplete routing over the entire network.ES-IS provides solutions for the following practical problems:1. For end systems to discover the existence and reachabilityof intermediate systems that can route NPDUsto destinations on subnetworks other than the one(s) towhich the end system is directly connected.2. For end systems to discover the existence and reachabilityof other end systems on the same subnetwork.3. For intermediate systems to discover the existence andreachability of end systems on each of the subnetworksto which they are directly connected.4. For end systems to decide which intermediate system touse to forward NPDUs to a particular destination whenmore than one intermediate system is accessible.• NLPID – Network Layer Protocol Identification. Thevalue of this field shall be 1000 0010• Length – Length Indicator is the length of the entirePDU• Version – Protocol ID extension. This identifies astandard version of ISO xxxx, End System to IntermediateSystem Routing Exchange Protocol for usein conjunction with ISO 8473.• Reserved – Must be zero.• Type –The Type code field identifies the type of theprotocol data unit.• H-Time – Holding time field specifies for how long thereceiving Network entity should retain the configuration/routinginformation contained in this PDU.• Checksum – Error checking which is computed onthe entire PDU header.Related protocolsIS-IS, CLNP, IDRP, CONPSponsor SourceES-IS is defined in ISO (www.iso.org).Referencehttp://www.javvin.com/protocol/rfc955.pdfEnd System to Intermediate System Routing Exchange Protocolfor use in conjunction with ISO 8473ES-IS provides two types of information to Network entitieswhich support its operation: a) Configuration Information, whichpermits End Systems to discover the existence and reachabilityof Intermediate Systems and permits Intermediate Systems todiscover the existence and reachability of End Systems; and b)Route Redirection Information which allows Intermediate Systemsto inform End Systems of (potentially) better paths to usewhen forwarding NPDUs to a particular destination. A NetworkEntity may choose to support either the Configuration Information,the Route Redirection Information, neither, or both.Protocol StructureES-IS Protocol Data Unit contains the following:ES-IS HeaderNetworkaddressSubnetworkaddressOption