Strict "No Response" Protocol Check Causes TN3270 Session Hang (195997)

IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows Registry

SYMPTOMS

When using the TN3270 Service to communicate with a certain host application, the TN3270E client emulator would fail to receive the UNBIND indication while exiting the application, causing the client session to stop responding (hang).

When this occurs, SNA Server may log the following event to the Windows NT application event log:

Event ID: 13 Source: SNA Server Service Description: (1141) Message Sequence Error (SENSE = 400A) EXPLANATION The SNA Server (node) received an invalid or unexpected message from a remote node during an SNA Session....

According to the IBM SNA Formats guide (GA27-3136), sense code 400A indicates "No-Response Not Allowed: No-response was specified on a request when not permitted. (Used only on EXR)."

CAUSE

The host application was incorrectly sending an RQN (Request No Response) message to the client emulator when only RQD (Request Definite Response) and RQE (Request Exception Response) protocol was negotiated on the BIND command for the 3270 session. This was causing SNA Server to detect an SNA protocol violation, reporting a 400A sense code to the TN3270 Service. However, the subsequent host UNBIND indication was not passed to the TN3270 client emulator.

RESOLUTION

SNA Server 3.0

To resolve this problem, obtain the latest service pack for SNA Server version 3.0. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

184307 How to Obtain the Latest SNA Server Version 3.0 Service Pack

SNA Server 4.0

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

WORKAROUND

If possible, correct the host application so that the invalid RQN message is not sent on the session. Or, modify the DLOGMOD entry such that RQN messages are allowed on the session.

STATUS

Microsoft has confirmed this to be a problem in SNA Server versions 3.0, 3.0 Service Pack 1, 3.0 Service Pack 2, 3.0 Service Pack 3, 4.0, and 4.0 Service Pack 1. This problem was first corrected in SNA Server 3.0 Service Pack 4.

MORE INFORMATION

Once the SNA Server update has been applied, the NO400A registry setting must be specified in order to relax protocol checking for invalid RQN messages, as described below.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

Start Registry Editor (Regedt32.exe).
Locate the following key in the registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\ SnaServr\Parameters
On the Edit menu, click Add Value, and then add the following registry value: Key Name: NO400A Data Type: REG_SZ Value: <any value>
Quit Registry Editor.

The presence of the "NO400A" registry key disables the SNA Server check for invalid RQN messages. This will cause SNA Server to never report the 400A sense code.