DTJ Number 3 September 1987 - Digital Technical Journals

The DECnetjSNA Gateway ProductSegmentation allows user code to send andreceive buffers larger than the buffers used at thedata link level.Philosophy behind the GatewayIn building the gateway, we wanted a system thatcould be developed completely within Digital'sengineering groups. We also wanted a systemthat would allow the values chosen by our engineersto be overwritten by customers with theirown rietwork values. This capability requiredthat data structures be allocated dynamically duringthe initial gateway startup.User Level ComponentsThe user level components are as follows:• The initial load program (SNAIL) , which managesSNA configuration with only a DECnetinterface• The gateway access server (GAS) , whichswitches messages from one network to theother• The remote job server (RJSRV) , which providesIBM remote job submission functions• The host command facility server (HCFSRV) ,which provides IBM terminals with a methodof reaching DECnet-VAX hostsSNAIL, the Initial Load ProgramWe chose to accommodate customers' networkvalues to the gateway during initialization bymeans of a plain text file (configuration file) .Such files are common; to all of Digital's operatingsystems, and network routines are provided ··that can read these files.The text in the configuration files for the SNAnetwork is divided into three areas based on thethree SET commands used to configure the gateway.(A SET command in the SNA gateway is similarto the one used for DECnet NCP.)• The first SET command is used to determinethe type of modem signal control being used.This SET LINE command allows two choices:full duplex, meaning the. modem leads areheld "high"; and half duplex, meaning themodem leads are varied according to the sendingand receiving rules.• The second SET command defines the circuitparameters. This command sets up the datalink station address, the number of SNA logicalunits that the circuit can handle, an identificationvalue used in dial-up configurations, andother items in the SNA network.• The third SET command defines a shorthandname, SET ACCESS, for a number of SNA networkconnection items. This command definesa list of LUs, a circuit name, and an IBM application. The user can specify a single accessname rather than all the needed parameters.SNAIL is an RSX-privileged task that deciphersthe RSXjSNA PE data structures and reads theconfiguration file on the DECnet host node.SNAIL parses the commands from the configurationfi le, traces the RSXjSNA PE data structures,and then places the configuration information inthe correct location in the data structures. In thecase of LU databases and access names, eachstructure is allocated and linked into the existingdatabase.Part of the SNAIL code detects errors duringthe command parsing of the configuration filerecords . Not having a console, the gatewayneeds a means of reporting errors, and DECnetevent messages supply that means. The numberand contents of each line in error are mergedinto a line of text that is sent to the DECnet hostnode.After loading the gateway, the DECnet networkmanager must check that the software and informationfrom the configuration fi le have been. loaded correctly. This checking is done by moni·toring the DECnet event messages that appear onthe host. These messages provide status informationon successful steps and error information forfailed steps.Gateway Access Server (GAS)GAS provides support for the gateway access protocols(GAP). GAS is basically a messageswitcher that receives messages from the IBMSNA network session and sends them along thecorrect DECnet logical links. GAS also takes messagesreceived from DECnet logical links andsends them to the correct IBM SNA sessions. ADECnet logical link and an IBM SNA session areassociated with one another at connection time.Connections into the IBM applications are alwaysinitiated from the DECnet side of the gateway.GAS concerns itself only with the SNA bindmessage because it determines the buffe r sizethat the gateway will receive from and transmitto the IBM SNA network. These sizes are allo-50Digital TecbnicalJournalNo. 3 September 1986