network protocols handbook.pdf

72Protocols GuideTCP/IP - Network Layer ProtocolsProtocol NameNARP: NBMA Address ResolutionProtocolProtocol DescriptionThe NBMA Address Resolution Protocol (NARP) allows a sourceterminal (a host or router), wishing to communicate over a Non-Broadcast, Multi-Access (NBMA) link layer network, to find outthe NBMA addresses of a destination terminal if the destinationterminal is connected to the same NBMA network as thesource.A conventional address resolution protocol, such as ARP for IP,may not be sufficient to resolve the NBMA address of the destinationterminal, since it only applies to terminals belonging tothe same IP subnetwork, whereas an NBMA network can consistof multiple logically independent IP subnets.Once the NBMA address of the destination terminal is resolved,the source may either start sending IP packets to the destination(in a connectionless NBMA network such as SMDS) or may firstestablish a connection to the destination with the desired bandwidthand QOS characteristics (in a connection oriented NBMAnetwork such as ATM).An NBMA network can be non-broadcast either because it technicallydoesn’t support broadcasting (e.g., an X.25 network) orbecause broadcasting is not feasible for one reason or another(e.g., an SMDS broadcast group or an extended Ethernet wouldbe too large).Protocol Structure8 16 32bitVersion Hop Count ChecksumType Code UnusedDestination IP addressSource IP addressNBMA Len.NBMA address (variable length)these are the IP addresses of the NARP requestorand the target terminal for which the NBMA addressis destined.• NBMA Length and NBMA Address - The NBMA lengthfield is the length of the NBMA address of the sourceterminal in bits.Related protocolsARPSponsor SourceNARP is defined by IETF (http://www.ietf.org) in RFC 1735.Referencehttp://www.javvin.com/protocol/rfc1735.pdfNBMA Address Resolution Protocol (NARP)• Version - NARP version number. Currently this valueis 1.• Hop Count - Indicates the maximum number of NASsthat a request or reply is allowed to traverse beforebeing discarded.• Checksum - Standard IP checksum over the entireNARP packet (starting with the fixed header).• Type - NARP packet type. The NARP Request has atype code 1; NARP Reply has a type code 2.• Code - A response to an NARP request may containcached information. If an authoritative answer is desired,then code 2.• Source and Destination IP Address - Respectively,