Attachmate Extra! 3270 V4.x Causes Trap in SNA Server Service

• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, and 2.11

SYMPTOMS

An application exception (trap) occurs in the SNA Server service (SNASERVR.EXE) when used with the Attachmate Extra! 3270 version 4.x emulator.

If you have Dr. Watson (DRWTSN32.EXE) configured as the default debugger, the DRWTSN32.LOG shows an exception similar to the following:

   Application exception occurred:
      App: snaservr.DBG (pid=147)
      When: 8/1/1995 @ 16:39:1.590
      Exception number: c0000005 (access violation)

   Creating dump file C:\SNA\TRACES\SNADUMP.LOG for SNASERVR.EXE

   EXPLANATION
   A process (SNASERVR.EXE) ended abnormally due to a protection
   violation or similar condition and a dump file SNADUMP.LOG has been
   created that contains information about the state of the process at
   the time of the exception. Register information is contained in your
   Windows NT root directory in the file DRWTSN32.LOG.

CAUSE

A 3270 emulator has two connections to the SNA Server service. One connection is for SSCP-LU data and the other one is for PLU-SLU (Primary Logical Unit - Secondary Logical Unit) data. The SNA Server service trap is caused because the 3270 emulator is sending a PLU-SLU message on the connection that is only meant to handle SSCP-LU data. This is behavior which the SNA Server service is not expecting, and it traps when it accesses its session control block for the SSCP-LU session and treats it as a control block for the PLU-SLU session.

RESOLUTION

Microsoft has modified the file, SNASERVR.EXE, to handle this error.

A 3270 emulator communicates with the SNA Server service over the Emulator Interface Specification (EIS) interface. For performance reasons, this is a trusted interface - that is, the SNA Server service assumes that messages across this interface are correctly addressed and formatted. The fix for this trap involves producing a SNA Server service which actually checks that messages received from the emulator on the connection for SSCP-LU data are in fact valid messages for that session, and discards them if they are not. This fix allows the client to continue to function as if nothing had happened and it prevents the SNA Server service from trapping.

A reproduction scenario hasn't been found to reliably cause this condition, so the problem in the emulation is still under investigation. However, with this update, the SNA Server now checks for this condition and no longer leads to an application exception.

STATUS

Microsoft has confirmed this to be a problem in SNA Server for Windows NT. This problem was corrected in the latest SNA Server for Windows NT, 2.11 U.S. Service Pack. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K