Disabling Keep Alive Connections in IIS 1.0

• Microsoft Internet Information Server version 1.0

SYMPTOMS

Microsoft Internet Information Server (IIS) allows "keep alive connections" (that is, persistent connections) by default. If clients do not close keep alive connections, this may result in a large number of TCP connections in the ESTABLISHED state on the IIS server. These connections can be identified by running "netstat -n" to see whether a large number of connections exist in the ESTABLISHED state.

Because IIS allows keep alive connections by default, any HTTP requests from a client can result in connections to the server that are not closed until they are either reset by the client, or timed out by the server. The default timeout for established connections in IIS is 900 seconds (15 minutes).

CAUSE

IIS allows keep alive connections by default.

MORE INFORMATION

IIS allows keep alive connections as an optimization to reduce the total number of TCP connections a client needs to download all the elements (Images, Audio, etc.) defined on a page. Keep alive connections allow clients to re-use established TCP connections to the HTTP server to download several elements on a page. Without keep alive connections a separate TCP connection is established to get each element on a page, increasing the load on the HTTP server.

NOTE: A description of the TCP state machine can be found in Internet RFC 793. A description of the HTTP/1.0, and HTTP/1.1 protocol can be found on http://www.w3.org/pub/WWW/Protocols

RESOLUTION

Obtain the fix described below in the STATUS section, and then modify the registry according the following steps. W3svc.dll was modified to accept a registry entry that disables support for keep alive connections.

The new registry entry is AllowKeepAlives.

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

1. Run "Net Stop w3svc"

2. Run Registry Editor (REGEDT32.EXE).

3. From the HKEY_LOCAL_MACHINE subtree, go to the following key:

         \SYSTEM\CurrentControlSet\Services\W3svc\Parameters
   

   The parameters described below are located under this Registry subkey.

   The following describes a new value for the W3SVC service parameters key that can only be set by adding an entry in Registry Editor. This value entry does not appear by default in the Registry, so you must add an entry if you want to change its default value. The W3SVC service must be restarted for changes to take effect.

   AllowKeepAlives

         Data Type = REG_DWORD
         Value:
                  0 - Do not allow keep alive connections.
        (Default) 1 - Allow keep alive connections.
   
   

4. Run "Net Start w3svc"

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 3.51. A fix to this problem is in development, but has not been regression-tested and may be destabilizing in production environments. Microsoft does not recommend implementing this fix at this time. Contact Microsoft Product Support Services for more information on the availability of this fix.