DTJ Number 3 September 1987 - Digital Technical Journals

A Digital Network Architecture OverviewLOCAL NODEREMOTE NODEUSER LAYERTHE ACCESSING PROGRAM:•NF"i' UTILITY•A VAX/VMS COMMAND• A USER-WRITTEN PROGRAMNETWORK APPLICATION LAYER.----t DAP - SPEAKING ROUTINES:• NFARs, OR ROUTINES IN THELOCAL FILE SYSTEM (E.G.,RMS)SESSION CONTROL LAYEREND COMMUNICATION LAYERROUTING LAYERDATA LINK LAYERNETWORK APPLICATION LAYER·- ------ -·DAP - SPEAKING SERVER:• FILE ACCESS LISTENERSESSION CONTROL LAYEREND COMMUNICATION LAYERROUTING LA YEADATA LINK LAYERDAP - DATA ACCESS PROTOCOLFCS - FILE CONTROL SYSTEMRMS - RECORD MANAGEMENT SERVICESNFARs - NETWORK FILE ACCESS ROUTINESNFT - NETWORK FILE TRANSFER UTILITYFigure 3File Transfer across a NetworkDAP's principal design problem was accommodatingthe needs of diverse file systems. It wasnecessary to define a mapping between the featuresand functions of each different system. Thisdefinition was not always easy to make. Some systemshad differing capabilities (for example,some supported index files); others had differingmeans of providing similar capabilities (forexample, stream or record structures for textfiles). Moreover, it was very important for filetransfers between like systems to operate at maximumefficiency and to be completely transparent.For example, it should be possible to copy afile from one VMS system to another and stillretain exactly the same bit patterns in the copy.Two design approaches were studied to achievethese capabilities: using a common, or canonical,file format in protocol messages; and performingneeded translations. The canonical formatwas rejected because it was not transparentor efficient enough in the homogeneous case.The second approach, in which translation isperformed at the client DAP protocol module,was adopted.Session Control LayerThe session control layer resides directly abovethe end communications layer.8 The session controllayer provides system-dependent, process-toprocesscommunication functions for processesresiding in the user, network management, andnetwork application layers. These functionsbridge the gap between the pure communicationfunctions provided by the end communicationslayer and the functions required by processesrunning under an operating system. The communicationservice provided by the session controllayer is connection oriented: an initiating processrequests a connection to a destination process.The session control layer manages theseconnections. Once a connection is established,data flows between the processes without furtherintervention by the session control layer, usingthe facilities provided by the end communicationslayer.When establishing a connection, the higherlayer specifies th e destination process in twoparts: first, by destination node, then, by processwithin destination node. Destination nodes are16Digital TecbnicalJournalNo. 3 September 1986