DTJ Number 3 September 1987 - Digital Technical Journals

New Productswill access the appropriate file using normalRMS calls. For operations on the volatile database,NML will issue a QIO to NETDRIVER,which in turn forwards the request to NETACP,where the request is honored.Data Link DriversThere is a separate device driver for each communicationsdevice that can be used by DECnet­VAX . In most cases these communicationsdevices can be used by non-DECnet applicationsas we ll. The same device driver is used toprovide the data link interface to NETDRIVER, aswell as the QIO interface for user-written applicationsnot using the DECnet software.The data link drivers supply the needed support for the variety of lower level protocols providedin the DECnet-VAX product. These protocolsinclude the synchronous and asynchronousDigital Data Communications Message Protocol(DDCMP) , Ethernet, IEEE 802, and SystemsCommunications Architecture (SCA) for communicatingacross a VAXcluster communicationsinterface (CI) .File Access ListenerThe File Access Listener is the component of RMSthat is activated to service a request for access tothe local file system by a remote node . As such,FAL is an extension to RMS on the remote system.FAL uses RMS services to access local files andDAP to send data back to the requesting node .VAX/VMS Environment in theNeh.IJork KernelThe underlying structure of DECnet-V AX wasdesigned around the special environment providedby the VMS system. The manner in whichnetwork programs are created and the environment·in which they run are governed more bythe design of the surrounding operating systemthan by the network architecture. Some importantaspects of this design are the way networkobjects are identified and activated and the use ofVMS command procedures. This use provides asimple, transparent mechanism for creating anetwork task.The DECnet architecture defines two classes ofexecution entities that have addresses within anetwork and with which network communicationscan be established. The first are called network"objects," identified by node address andobject number. The second ilre network "tasks,"addressed by node and task name. Network tasksare actually a special case of network objects,with object number zero reserved for identifyingnetwork tasks. If a logical link specifies objectnumber zero, it also supplies a task name identifyinga particular network task on the targetnode.In the VMS system, execuion of each programimage takes place within the context of a process.One process will tyically run multipleIimages serially during its lifetime. A flexibleImechanism was needed to associate a request fora network object or task ith the right process'running the right image .DECnet-VAX has three mechanisms to identifythe execution entity that will be associated witha network object or task. In the fi rst, an imageregisters itself in the network database as thespecified object or task. The image then waits forI(or initiates) connections with other programs inthe network.The second mechanism I involves creating anentry in a local database tliat identifies networkobjects. The database information specifieseither a command proced h re or an executableimage to be run in response to a request to connectto the specified object,. Upon receiving sucha connect request, NETACP will create a processin a specified account that executes the commandprocedure or image . Setting up an entry inthe object database provipes the flexibility tospecify account informatibn and privileges forthe object when it is activ£ted.The third mechanism isl a catchall. This acti-'vates a command procedure to serve as a networktask in the absence of ether an entry in theobject database or a nontransparent declarationof the network task by a running image . Uponreceiving a connect request for a network tasknot identified by either of the first two methods,DECnet-VAX assumes that a command procedureresides in the default dirctory of the accountspecified with the reque;st. DECnet-VAX thencreates a process to execute this command procedure.For example, upn receiving a connectrequest for a task called NETISK, undeclared byany running image , DECne-VAX will assume thata command procedure tailed NETTSK.COMresides in the default direttory.The DECnet-VAX software also causes a logicalname, SYS $NET, to be created for this process.SYS $NET translates to a data structure containingconnection information .for this logical link.Digital TecbntcaiJournalNo. 3 September 198693