DTJ Number 3 September 1987 - Digital Technical Journals

The DEC net- VA X Product - An Integrated Approach to Networkingnode. Each member node within a cluster canbe directly addressed from any other node inthe network. At times, however, it is alsovery convenient to treat the cluster as a singleDECnet node . Among other advantages , thiscapability makes it possible for mail to be sent tousers with accounts in the VAXcluster systemwithout knowing which member nodes areactive .Associating the cluster with a DECnet addressis accomplished by supplying each node in thecluster with a second address, an "alias," representingthe cluster. Each router in the cluster (atleast one is required) adds the alias address tothe routing vector transmitted to other routersin the network. That makes the routing vectorappear to be the optimal path to the aliasaddress. As a result, the rest of the network cannotdistinguish the cluster alias from the addressof a physical node . This approach has an advantagein that it requires no unique support inother systems and no modifications to the DECnetarchitecture.As it is routed through the network, a messagewith the alias address will eventually arrive at arouter within the VAXcluster system. The routerwill recognize the destination address as its ownalias and select a node within the cluster toreceive the message. The selection process isbased on a weighted, round-robin algorithm. Theend communications layer within the router iscapable of identifying which node is associatedwith each logical link. Therefore, once a connectionhas been established, subsequent messageswill always be routed to the correct node withinthe cluster.Dy namic Asynchronous ConnectionsMany personal computers, ranging from IBM PCsto MicroVAX workstations, are now capable ofrunning the DECnet software over asynchronouslines. Thus has arisen the need for a more secureand easily managed mechanism for setting up terminallines to be used as DECnet communicationslines.Ordinarily, one terminal line must be dedicatedto DECnet use for each asynchronous lineneeded. When those terminal lines are not beingused for DECnet purposes, they cannot be usedas normal terminal lines. To solve this problem,DECnet-VAX introduced, in 1985, dynamic asynchronousconnections which allow an interactiveuser to dynamically convert the terminal linehe is using to a DECnet line. (This conversionrequires that access be from a PC using a terminalemulation package , such as SET HOSTjDTEunder the VMS software, and that the PC can runthe DECnet software.)Mter logging in via the terminal .emulator to anaccount on a routing node, the user directs theVMS system on the routing node to switch the terminalline to DECnet use. The VMS system sendsan escape sequence to the terminal emulator onthe PC. Recognizing the sequence, the emulatorconverts the line to a DECnet link at the localend. Meanwhile, the code in the router convertsthe line at that end to DECnet use. The design ofthe VMS terminal driver makes possible this conversion.The terminal driver separates its functionsbetween class drivers (implementinghigher-level functions) and port drivers (interfacingwith the hardware devices) . The DDCMPasynchronous device support in the DECnet-V AXproduct is implemented as a class driver. Thatmakes it possible to switch dynamically betweenDECnet and terminal use on a particular devicesimply by switching class drivers on the sameport driver.When both ends have switched to DECnet use,the normal routing layer initialization takesplace. Some additional checks happen duringrouting initialization on dynamic lines to ensurethat the node that just switched the line is permittedto do that by the router. These checksgive to the system manager on the router theopportunity to control which nodes should bepermitted to connect to his system.Performance IssuesAs DECnet-VAX has evolved, continuing effo rtshave been made to improve its performance.These efforts have run the gamut from restructuringthe basic modules to including supportaimed at improving specific areas of performance.The remaining sections discuss some ofthe areas that have yielded the greatest performanceincreases.Network Drivers and AncillaryControl ProcessesWherever possible, those functions having thegreatest effect on performance have been implementedin NETDRIVER. There they can be executedat high priority without changing the processcontext, which would be required forfunctions executed in NETACP.96Digital Technical]ournalNo . 3 September 1986