DTJ Number 3 September 1987 - Digital Technical Journals

A Digital Network Architecture OverviewData flow on a logical link can be modeled bya pair of message queues in each direction. Onequeue in each direction handles the transmissionof "normal data" between higher-layer protoc olmodules; that pair is used by all higher-level protocols.The other queue pair handles transmissionof occasional shon "interrupt" messages;this pair is used by some higher-level protocols.For example, the virtual terminal protocol usesinterrupt messages to transmit interrupt commands,suc h as those generated when a userenters the command CTRL-Y to a VMS system. Oneach queue, data flows on each queue independentlyof the other queue. Data is transferredwhen the requesting session control module providesa message to be transmitted and the receivingsession control module indicates its willingnessto receive, by providing a buffer forexample. NSP uses protocol messages and flowcontrol algorithms to ensure an orderly data flowon logical links. An orderly flow takes place evenif limited by the ability of the sources to providedata, the network to transmit data, or the destinationto receive data.In providing the reliable logical link service;NSP must exist in a hostile environment. In particular,NSP must operate correctly when therouting layer occasionally loses, reorders, orduplicates messages. Moreover, NSP must dealwith potential confusion created by computers'crashing at one or both ends of the logical link;this is the problem of "half-open connections." 10· 11 NSP deals with these problems byassigning logical link identifiers to each logicallink and by assigning sequence numbers toeach data or flow-control message sent on eachlink. Timers are used to detect errors and initiateretries of operations: Should excessive retriesappear to be required, NSP will repon this problemto the session control layer, whic hcan decide to break the connection or continueretrying.9NSP has evolved with eac h phase of DNA. In·· Phase II, the NSP protocol was revised to allowdynamic sharing of message buffers between logical links. This capability, called optimistic flowcontrol, must deal with the delay between the. time that data is requested and the time that dataarrives. For example, during this delay on onelogical link, data on other links might arrive,thus consuming all the available NSP buffers.The NSP protocol was designed to handle thiscase correctly without deadlock.The Phase II version of NSP ran right on top ofa data link protocol, the Digital Data CommunicationsMessage Protocol (DDC MP). The DDC MPprotocol provided a reliable point-to-point communications service, rendering unnecessaryNSP's use of timers to detect lower layer failures.The Phase III version of NSP was designed to runon top of the routing layer. This version includeda timer capability to detect and recover fromrouting layer failures, such as the loss of a messagewhen an alternate route must be selectedfollowing a node or link failure.Only minor changes were made to NSP inPhase IV. Two changes improved the prot oc olperformance by reducing the number of controlmessages exchanged to perform flow-control anderror-recovery functions. First, the protocol messageformats and procedures were allowed tocombine control messages with each other andwith data messages. Second, provision was madefor selectively delaying acknowledgement messages,making it possible to send many data messagesfor eac h acknowledgement. In a typicalimplementation of NSP, these changes make itpossible for more than 90 percent of the messagestransmitted by NSP to be data messages.Reducing the number of messages exchangedimproves the throuhput of DECnet implementationson Ethernet LANs by reducing the CPU timeneeded to generate, transmit, rec eive, anddecode control messages. Reducing the numberof messages decreases the common-c arriercharges when running NSP over X. 25 public datanetworks, which charge for each packet.Routing LayerThe routing layer provides a network-wide messagedelivery service. 12 This layer accepts messagesfrom the end communic ations layer in asource node and forwards the packets, possiblythrough intermediate nodes, to a destinationnode. The routing layer implements a datagramservice, which delivers packetS on a best-effonbasis.n The routing layer makes no absoluteguarantees against packets being lost, duplicated,or delivered out of order. Such guaranteesare made by the end communications layer. Toprovide this network-wide service, the routinglayer calculates routes, using them to forwardpackets. In the forwarding process, the routinglayer must attempt to avoid or at least control anycongestion that results from overloading the networkwith excessive traffic. The routing layer18Digital TecbnicalJournalNo. 3 September 1986