DTJ Number 3 September 1987 - Digital Technical Journals

New Productsmust be able to coexist in the same network.Thus the network architecture must adjust tonew technologies, some of which were not envisionedwhen the architecture was originallydeveloped.Easily ImplementedThe network architecture must be implementedon a range of computer systems, from small personalcomputers to superminicomputer ormainframe systems. The architecture must beimplemented over a range of communicationhardware and be cost effective so that eithersmall or large networks can be constructed. Thisneed implies that the architecture must permitsimple implementations as well as more complexones to conform to the needs of individualcomputer systems and network configurations.Cost EffectiveImplementations of the DNA architecture shouldbe cost effective compared to the alternative ofan application-specific network architecture.This attribute will encourage the use of a commonnetwork architecture, with the resultingeconomies of scale.Design PrinciplesIn addition to the goals described above, thedevelopment of DNA has been guided by anumber of important design principles. Wechose these principles in concert with thegoals and with the benefit of experience inresearch networks. Such networks includethe ARPA, National Physical Laboratory, andCyclades networks. These design principles aredescribed below. Of course, several generaldesign principles, such as simplicity and modularity,also guided the development of the DNAarchitecture.Distribute FunctionsFunctions should be distributed among thecomputer systems in a network to avoidsingle points of failure and encourage paralleloperation.Use a Hierarchically Layered StructureFunctions should be divided into layers to factorarchitecture complexity into easily understoodpieces and to facilitate the architecture's evolution.Lower layers should provide their definedservices without concerning themselves withDigital TecbntcalJournalNo. 3 September 1986upper layers. Upper layers should rely on the servicesprovided by lower laers without havingthe detailed knowledge of how they actuallyprovide those services.Address Computer Systems Un iformlyIt should be possible to communicate betweencomputer systems no matter where they arelocated in the network. This communicationcan be done if nodes are as$igned addresses thatcan be used anywhere in tll'e network to specifythe node as either a soure or destination ofmessages.Implement Functions at the Highest1Practical LevelWhen a function is implemented at a high level,it gains the use of lower-level functions, thussimplifying the implementation of the higherlevelfunction. If a function were implementedat a low level, it might haye to duplicate functionsalready provided at 1some intermediatelevel.·Use Dynamic AdaptaionThe configuration of a computer network willIconstantly change as the network expands and asits components fail and are testored to service. Itis highly desirable for the nFtwork to adapt to itscurrent configuration without manual intervention.This adaptability makes the network easierto install, easier to operate,'and more resistant tofailures.Use Stable, Robust AlgorithmsA computer network is a large, complex system.Like any such system, a etwork may exhibitunanticipated and undesirable behavior. WhenIdesigning critical algorithms, network reliabilityand availability must I not be compromisedin favor of small improements in averageIperformance. ,Evolution of DNA and DECnetProductsIn the ten years since it was first announced, DNAhas had four major phases, each bringing newcapabilities to the DECnet family. These capabilitieshave increased the rapge of computer systemsimplementing the architecture, the numberof applications and communications technologiessupported, and the sie and complexity ofnetwork topologies. In addition to functionalII11I