New ProductsCommon Network LayerThe common network layer provides routing andmultiplexing functions between the da ta linklayer below and the SNA interface layer above.Data units received are routed to the entity (inthe SNA interface layer) that owns the SNA sessionto which the data units belong. Data unitssent are routed to the appropria te data link layer.This layer implements the SNA path-controlprotocols and some - but not all - of thetransmission-control protocols, including thecommon session control. The PU and LU servicesmanagement functions are also pa rt of thislayer.Data Link LayerThe data link layer provides error-free transmissionof data units over a physical link. Currentimplementations of the data link layer supportonly the SDLC secondary station mode. However,the structure could allow the addition of otherdata link protocols (such as X. 25) in the future.This la yer corresponds exactly to the da ta linklayer in both the DNA and SNA architectures.Physica! LayerThe physical la yer provides the means to controland use the physical connections that transmitdata between systems. This la yer can support awide variety of device types and interface standards,and can be implemented in various hardwarejsoftware mixtures. This layer correspondsexactly to the physical la yer in both the DNA andSNA architectures.<strong>Digital</strong>'s SNA Gateway AccessArchitectureThe SNA gateway access architecture prescribes ·the transport mechanism that allows SNA inter·fa ce layer modules in a DECnet host node to gainaccess to common network layer modules in anSNA gateway node. This mechanism is at theheart of the gateway access model for productdesign. Hence it is used implicitly by most of thecurrent DECnetjSNA product set.Overview of the ArchitecturalStructureTwo modules are needed to implement the SNAgateway service: the SNA access module and theSNA gateway module. The two modules communicateby means of a protocol that operates overa DECnet logical link. The protocol is termed theSNA gateway access protocol, or GAP, and isdepicted in Figure 8. GAP is a fairly straightforwardDNA application layer protocol. It makesuse of the features provided by the DNA sessioncontrol and lower la yers, in , particu1ar, flow control,error control, and message segmentationand reassembly. Hence GAPineed not contain anysuch mechanisms itself.The SNA access module is! part of the SNA interface layer, implementing what was referred toabove as the ba sic level of interface. The SNAgateway module runs as a separate process in thega teway system, in which the module uses servicesthat provide it with the functions of pathcontrol and.common session control.A brief example is presen ted in the followingsection in lieu of listing the specific operationsof GAP in detail. This example illustrates some ofthe message flows that take place between theSNA access and gateway modules.Example of Message !flowsThis example describes the( exchanges needed toestablish a session and to transfer data, with thesession being terminated by the IBM application.Figure 9 illustrates the actions tha t take place. Inthe following description, the "user" is a higherlevelentity that utilizes the SNA gateway service.In the context of the SNA product architecture,such a user will in fact be part of the SNA interface layer.To initiate the connection, the user programissues a connect call. Included in the parametersof this call are the name of the ga teway node, thesecondary LU (SLU) address to be used, the nameof the primary LU (PLU) to be the session pa rtner,and sundry other SNA parameters requiredfor the connection. 1The SNA access module ihen allocates internalIresources and establishes a DECnet logical linkto the SNA gateway module in the specified gatewaynode. In turn, the SNA gateway allocatesresources for the session and waits.The SNA access module then transmits a GAPconnect message to the SNA gateway module.The SNA gateway module allocates the requestedSLU address and transmits an SNA initiate-selfmessage to the SSCP, which informs the PLU viaan SNA control-initiate message.Eventually, the PLU transmits a bind to thegateway. The bind is forwarded to the SNA access ·module as a bind-data mes1>age and ultimately tothe user program in resporyse to a read-bind call.I<strong>Digital</strong> Tecbnical]ournalNo. 3 <strong>September</strong> 198643
The DECnetjSNA Gateway ProductThe user program then agrees to the session byissuing an accept message, which causes the SNAaccess module to send a bind-accept message tothe SNA gateway module. The gateway modulethen acknowledges the bind (that is, transmits apositive response), and the LUs are now consideredto be in session.The user program can now exchange datamessages with the IBM application. To effectan exchange, the program uses the transmit-callsand receive-calls functions of the SNA accessmodule. Note that higher-level protocol initializationmay well be needed before true enduserdata exchange can begin. Such details,however, are not known to the SNA access andSNA gateway modules. During this data transferphase, the SNA gateway module operatesas a simple message switch, passing data toand from the SNA access module withoutinterpretation.At some point, the PLU will terminate the sessionby sending an unbind message to the gateway.At that point the SNA gateway module disconnectsthe logical link with the SNA accessmodule, supplying an appropriate reason code inthe disconnect message. The user program willread this reason code and issue a close-call tocause the SNA access module to deallocate itsresources.Relationship between Productsand ArchitectureThe SNA product architecture allows considerablefreedom with respect to the distribution offunctions when a particular product is beingdesigned. Various amounts of a product can belocated in the DECnetjSNA Gateway, whatever isappropriate for the design. <strong>Digital</strong>' s currentproducts conform to either the gateway accessmodel or the server model.DEC netSYS TEMI(USERMODULESNAACCESSMODULEIDEC netNETWORK)SNA GATEWAY ACCESSPROTOCOL (OPERATEDOVER A DNA LOGICALLINK)DECnetSYSTEMCONTAININGGATEWAYSNAGATEWAYMODULEISNAPROTOCOL EMULATIONMODULESNA/SDLCDATA LINK CONTROLMODULE(SNANETWORK)Figure 8Structure of <strong>Digital</strong>'s SNA Gateway44 <strong>Digital</strong> TecbnicaljournalNo. 3 <strong>September</strong> 1986
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