network protocols handbook.pdf

64Protocols GuideTCP/IP - Network Layer ProtocolsProtocol NameEGP: Exterior Gateway ProtocolProtocol DescriptionExterior Gateway Protocol (EGP) is for exchanging routing informationbetween two neighbor gateway hosts in a network ofautonomous systems. EGP is commonly used between hostson the Internet to exchange routing table information. The protocolis based on periodic polling using Hello/I-Heard-You (I-H-U)message exchanges to monitor neighbor reachability and Pollcommands to solicit Update responses. The routing table containsa list of known routers, the addresses they can reach, anda cost metric associated with the path to each router so thatthe best available route is chosen. Each router polls its neighborat intervals between 120 to 480 seconds and the neighborresponds by sending its complete routing table. EGP-2 is thelatest version of EGP.A more recent exterior gateway protocol, the Border GatewayProtocol (BGP), provides additional capabilities.Protocol StructureHere are the EGP message types:Name FunctionRequest request acquisition of neighbor and/or initializepolling variablesConfirm confirm acquisition of neighbor and/or initializepolling variablesRefuse refuse acquisition of neighborCease request de-acquisition of neighborCease-ack confirm de-acquisition of neighborHello request neighbor reachabilityI-H-U confirm neighbor reachabilityPoll request net-reachability updateUpdate net-reachability updateError errorThe common portion of the message format:complement of the one’s complement sum of theEGP message starting with the EGP version numberfield. When computing the checksum the checksumfield itself should be zero.• Autonomous System Number -- Assigned numberidentifying the particular autonomous system.• Sequence Number -- Send state variable (commands)or receive state variable (responses and indications).Related protocolsIP, TCP, BGP, IGPSponsor SourceEGP is defined by IETF (http://www.ietf.org) RFC904.Referencehttp://www.javvin.com/protocol/rfc904.pdfExterior Gateway Protocol formal specification8 16 24 32bitVersion Type Code StatusChecksumSequence numberAutonomous System number(Different for different messages)• Version -- The version number. This version is version2.• Type -- Identifies the message type.• Code -- Identifies the message code.• Status -- Contains message-dependent status information.• Checksum -- The EGP checksum is the 16-bit one’s