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This article describes a software update that enables a keepalive mechanism for the TN3270 service for the detection of active TCP/IP connections between a TN3270 client and the TN3270 service. You can engage the keepalive mechanism by enabling the KeepAlive interval option on the Settings tab of the TN3270 properties dialog box in the SNA Manager MMC snap-in.

Software update information

A supported feature that changes the product's default behavior is now available from Microsoft Support. However, this feature is intended to change only the behavior that this article describes. Apply it only to systems that specifically require it.

This update is included in Cumulative Update 2 for Host Integration Server 2013.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

More Information

The TN3270 service that is included with Host Integration Server includes an Idle timeout option that enables the TN3270 service to disconnect idle TN3270 client sessions.

Before this update, the TN3270 service did not include a keepalive mechanism that checked for the presence of active TCP/IP connections between TN3270 clients and the TN3270 service. The lack of a keepalive mechanism created situations in which the TN3270 service maintained active sessions to an IBM mainframe application until the Idle timeout period expired. This was true even if the TCP/IP connection between the TN3270 client and TN3270 service ended previously. In this situation, TN3270 clients could not reconnect to the IBM mainframe application until the Idle timeout period expired.

After you apply the update, the TN3270 service can be configured to send a "DO TIMING-MARK" message to a TN3270 client when the configured keepalive interval is reached. If the TN3270 service does not receive a "WONT TIMING-MARK" message in response, the TN3270 service ends the session to the IBM mainframe because the TN3270 client is no longer connected to the TN3270 service. This frees the session for use by the next TN3270 client that requests the session. 

The KeepAlive interval is specified in minutes and has a valid range of values from 1 to 1440. A value of 0 indicates that the keepalive mechanism is disabled. This is the same as disabling the KeepAlive interval option.

When the KeepAlive interval option is enabled, the following registry data is updated:

Key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Sna Server\CurrentVersion

DWORD: KeepAlive

Value: 0 – 1440 (decimal) where 0 = disabled and 1 – 1440 is the keepalive interval in minutes.


  • The Idle timeout and KeepAlive interval options can be used at the same time to enable TN3270 sessions to be cleaned up because of idle (inactive) TN3270 clients and also abnormally disconnected TN3270 clients (for example, network connection outages).

  • The KeepAlive registry key and value that are created when you enable the KeepAlive interval option are not written to the registry until you save the configuration change in SNA Manager. The registry key and value are not written when you click Apply or OK in the TN3270 properties dialog box.

  • The change does not take effect dynamically if the TN3270 service is active. The TN3270 service reads its registry values only at startup. So, the service will have to be restarted when you make this change.

  • Clearing the Idle timeout and KeepAlive interval check boxes sets the values to zero and disables the text box. When the check boxes are selected, they are populated by the default values (Idle timeout = 120, KeepAlive interval = 10).

  • You have to enable the KeepAlive interval option by running SNA Manager on each HIS server that you want to update. This is because the option adds a registry entry, and this cannot be done on remote HIS servers from one instance of SNA Manager.

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