Multiple NVRunCmd Commands May Cause Unpredictable Results (193813)

SYMPTOMS

An SNA Server may experience unpredictable behavior if it receives multiple NVRunCmd commands from the host over one of its configured host connections. This will only occur if the SNA Server receives NVRunCmd commands while it is currently processing a previous NVRunCmd.

The following is a sequence that caused the PU (Physical Unit) defined in VTAM for a SNA Server host connection to hang:

   Host                                    SNA Server
   ----                                    ----------

   (1) NMVT (DIR D:\SNA\SYSTEM\*.*)  ->
   (2) NMVT (DIR C:\*.*)             ->
                                     <-     (2) NMVT -RSP (Sense 084B0003)
                                     <-     (1) Data (1st portion of data)
   (3) NMVT (DIR C:\*.*)             ->
                                     <-     (1) (Final data for NMVT #1)

In this sequence, the host sends three NVRunCmds over the connection defined in SNA Server. The first NVRunCmd command (designated as number 1) is accepted, and eventually completes, returning the contents of the D:\Sna\System directory. The second NVRunCmd command (designated as number 2), is rejected by SNA Server. This request is rejected with an IBM Sense Code of 084B0003, which indicates that the requested resources are not available. SNA Server does not respond to the third NVRunCmd command (designated as number 3) at all. The lack of response is what causes the PU defined for this host connection to hang.

It is possible that other symptoms may also occur since SNA Server is not correctly handling multiple concurrent NVRunCmd commands.

CAUSE

The NVRunCmd service is designed to process only one command at a time. If subsequent commands are received while a previous command is being processed, the subsequent NVRunCmd commands will be rejected by SNA Server.

The problem is that the SNA Server service doesn't properly handle the case when multiple NVRunCmd commands are received at the same time. When this occurs, the SNA Server may correctly reject one or more of the commands it receives while processing the initial request. However, it may also attempt to process one of the NVRunCmd commands while the initial request is still processing, and this can lead to unpredictable results such as the PU hanging.

RESOLUTION

SNA Server 3.0

SNA Server 4.0

Microsoft has confirmed this to be a problem in SNA Server version 4.0 and 4.0 SP1. This problem was corrected in the latest SNA Server version 4.0 U.S. Service Pack. For information on obtaining this Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K

MORE INFORMATION

After applying the hotfix for this problem, SNA Server will reject all subsequent NVRunCmd commands if a NVRunCmd command is being processed.

For more information on the NVRunCmd service, please refer to one of the following:

- SNA Server Administration Guide - SNA Server Online Help