TN3270 Server Keeps SNA Session Open When TN3270 Client is Restarted

ID: Q183786

The information in this article applies to:

Microsoft SNA Server, versions 3.0, 3.0 SP1, 3.0 SP2, 4.0

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SYMPTOMS

If a TN3270 client is not shut down gracefully (for example, powered off and on or restarted) while having an open session through the SNA Server TN3270 Service, the TN3270 Server continues to keep the session open for 2 hours.

If the TN3270 client is configured to open a specific TN3270 device (LUA LU), then the TN3270 client will receive the following error when attempting to open the session while the server operates as if it's still in use:

[TN3270-31] ERROR - Telnet device <devicename> in use.

CAUSE

The Windows NT TCP/IP driver does not report TCP/IP session errors until its default keep alive interval expires. This keep alive timeout defaults to 2 hours. If a TN3270 client is powered off or restarted, the SNA Server TN3270 Service is not notified until 2 hours elapse.

WORKAROUND

The optimal solution is to modify the Windows NT TCP/IP driver's KeepAliveTime registry parameter where the SNA Server TN3270 Service is running, as described below, and then restart Windows NT.

NOTE: The SNA Server TN3270 Service also has an "Idle Timeout" parameter that will automatically end a TN3270 client session that is idle for the timeout interval (which defaults to 120 minutes). However, this is a less optimal solution although this feature may be useful in some customer circumstances.

RESOLUTION

The TN3270 Service sets the SO_KEEPALIVE socket option, so it's possible to tune the following Windows NT TCP/IP values to control the time interval before the TN3270 Server is notified of a client outage. Tuning this parameter will affect Windows NT TCP/IP keepalive behavior for all TCP/IP socket applications that set the SO_KEEPALIVE socket option.

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

To change these parameters, perform the following procedure:

Start Registry Editor (REGEDT32.EXE), and to the following subkey:
```
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services 
```

Add a value to the key described in the appropriate entry below, by Add Value from the Edit menu, typing in the value, and using the "Data Type" check box to set the value type.

Click OK.

Quit Registry Editor and restart the system for the changes to take effect.

All of the TCP/IP parameters are registry values located under one of two different subkeys of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services:

Tcpip\Parameters

<Adapter Name>\Parameters\Tcpip

where <Adapter Name> refers to the subkey for a network adapter that TCP/IP is bound to, such as Lance01. Values under the latter key(s) are specific to each adapter. Parameters for which there may be both a DHCP and statically-configured value may or may not exist depending on whether the system/adapter is DHCP configured and/or static override values have been specified. A restart of the system is required for a change in any of these parameters to take effect.


KeepAliveTime
   Key: Tcpip\Parameters
   Value Type: REG_DWORD - Time in milliseconds
   Valid Range: 1 - 0xFFFFFFFF
   Default: 7,200,000 (two hours)
   Description: The parameter controls how often TCP attempts to verify
   that an idle connection is still intact by sending a keep alive packet.
   If the remote system is still reachable and functioning, it will
   acknowledge the keep alive transmission. Keep alive packets are not sent
   by default. This feature may be enabled on a connection by an
   application.

NOTE: The following KeepAliveInterval does not need to be changed, though it is provided below for reference purposes.


KeepAliveInterval
   Key: Tcpip\Parameters
   Value Type: REG_DWORD - Time in milliseconds
   Valid Range: 1 - 0xFFFFFFFF
   Default: 1000 (one second)
   Description: This parameter determines the interval separating keep
   alive retransmissions until a response is received. After a response is
   received, the delay until the next keep alive transmission is again
   controlled by the value of KeepAliveTime. The connection will be aborted
   after the number of retransmissions specified by
   TcpMaxDataRetransmissions has gone unanswered.

MORE INFORMATION

The TN3270E specification (RFC 1647) section 13.3 suggests two possible methods for TN3270 client and server to implement the keepalive mechanism: the Telnet IAC NOP command, or the Telnet DO TIMING-MARK option. However, RFC 1647 does not require that any form of keep-alive mechanism be employed by either clients or servers. The SNA Server TN3270 Service chooses not to implement its own keep-alive mechanism, leaving it up to the administrator to tune the Windows NT TCP/IP KeepAliveTime as described above.

Additional query words:

Keywords : snatn3270
Version : WINDOWS:3.0,3.0 SP1,3.0 SP2,4.0
Platform : WINDOWS
Issue type : kbhowto kbprb