Adjusting PPTP KeepAlive Frequency

Some ISDN Connected Clients/Networks that use PPTP may notice high costs associated with continuos ISDN link caused by the idle inactivity timer never expiring. This PPTP keepalive packet can be controlled to reduce the frequency of this test so that it is greater than the idle inactivity timeout value. PPTP, by default, sends "keepalive" packet every 60 seconds to verify the tunnel. This frequency can be adjusted by altering the registry.

The keepalive traffic consists of an echo request and an echo reply. When there is no traffic through the tunnel for 60 seconds, the client sends an echo request to confirm the validity of the tunnel. To confirm that these keepalive packets are being sent, the user can use Network Monitor to parse the traffic.

The packets look like the following:

Please note the release version of Network Monitor has to be altered to correctly display the PPTP packets. For additional information, please see the following article in the Microsoft Knowledge Base:



This must be set at both ends of the connection to limit the "tunnel test"

The InactivityIdleSeconds registry setting starts a timer that measures the number of seconds between packets. When the timer reaches the default of 60 seconds, it instructs the system to send a control channel packet to the server. The EchoReplyTimeoutSeconds registry setting defines how long the client will wait for a response to the echo request.

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 alter the default, the user must add the following registry parameters to the system.
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Raspptpe\Parameters \Configuration\
InactivityIdleSeconds: REG_DWORD: default: 60 Number of seconds PPTP will go with no activity on the control channel.
EchoReplyTimeoutSeconds: REG_DWORD: default: 60 Number of seconds PPTP will wait for a reply from an echo request. If reply is not received, PPTP drops the connection.

For additional information, please see the following article in the Microsoft Knowledge Base: