Improvements for EIS/3270 and APPC Resource Location (138834)

SYMPTOMS

This article describes how the SNA Server client server interface locates an available LU for both APPC and EIS/3270 applications, both with and without Service Pack 1 installed.

Microsoft has updated the following files. Service Pack 1 is planned to be released in December 1995. To obtain these files before Service Pack 1 is released, contact Microsoft Product Support Services. Service Pack 1 includes the following modified files:

   Operating System     Modified Files
   ---------------------------------------------------------------
   Windows NT           SNABASE.EXE, SNACFG.DLL, SNADMOD.DLL, WAPPC32.DLL
   Windows 95           SNADMOD.DLL, WAPPC32.DLL
   Windows 3.x          WDMOD.DLL, WINAPPC.DLL

MORE INFORMATION

EIS/3270, Without Service Pack 1 Installed

Over the sponsor connection, the EIS/3270 application makes a sepdcrec() call to acquire a list of the LUs/pools that are assigned to the client's user record. On a connect request to open a session, the application issues an OPEN SSCP request on the chosen LU/Pool name. The underlying DMOD (part of the client server interface) attempts to locate the server that can house this available LU by connecting to each server in the client's local service table one by one until the specified LU/Pool is satisfied. It must do this because the local service table does not contain any LU/Pool information; it contains only server names.

APPC, Without Service Pack 1 Installed

During the TP_START request and ALLOCATE verb processing that a TP does to get a conversation, the underlying DMOD (part of the client server interface) attempts to locate the server that houses this available LU by connecting to each server in the client's local service table one by one until the specified LU-LU pair is satisfied. It must do this because the local service table does not contain any APPC LU information; it contains only server names.

EIS/3270, With Service Pack 1 Installed

The client still makes the normal sepdcrec() call to acquire a list of the LUs/pools that are assigned to the client's user record. As part of the OPEN SSCP message processing, the SNA Server client software sends a new RPC request (RPC_LU_SERVER_LIST) to the SNA Server that is supporting the client computer's sponsor connection. The requested LU name from the OPEN SSCP is part of the RPC request. The sponsor server returns a list of the servers who can possibly support the requested LU. If the LU is a dedicated LU, the list has only one server name. If it is a POOL the list may have up to 50 server names. After this RPC response, the resource locating continues normally except that the client tries (in random order) only the servers whose names are in the LUs server list. In general, the client tries only the servers that can actually support the LU resource.

APPC, With Service Pack 1 Installed

The RPC request is sent as part of the TP_START request and ALLOCATE verb processing. In both cases, the server returns the list of servers where the local or remote LU Alias is configured. The rest is similar to 3270 resource location.

In both the EIS/3270 and the APPC, the RPC request is sent only once per LU. The SNA Server client interface cashes the LU/Server information internally. In Windows NT and Windows 95, the cache is flushed when the application exits. In Windows 3.x, the cache is flushed when the client server piece (Wnap.exe) terminates.

NOTE: If the sponsor server is not running SNA Server 2.11 with Service Pack 1 installed, the client DMOD sends only one RPC request. After it gets the "not supported" error it does not try again.

If the RPC request fails for any reason, for example if the sponsor does not support the new RPC request, the DMOD uses the old resource locating algorithm.

No changes are required to existing emulators, including the 3270 application that ships with SNA Server.