Host Route Can Be Deleted if ICMP Fragmentation Is Needed

ID: Q244539

The information in this article applies to:

Microsoft Windows NT Server versions 4.0 SP5, 4.0 SP6
on the following platforms: NT
Microsoft Windows NT Workstation versions 4.0 SP5, 4.0 SP6

SYMPTOMS

When a user has defined manual host routes using the Route.exe tool, certain routes to remote hosts may be removed from the route table after 10-15 minutes after usage.

CAUSE

This problem can occur when an ICMP Destination Unreachable Packet is sent with the DF Flag set. When this occurs, it sends along the Next Hop MTU, and this causes the Host route to be overwritten by the ICMP source route. ICMP host routes are subject to deletion if their MTU changes.

Windows NT uses a Path MTU detection algorithm that sets the DoNotFragment bit on TCP packets. This behavior will cause intermediate routers with a smaller MTU than the packet's size to reject the packet and send an ICMP Destination Unreachable:Fragmentation Needed. When Windows NT receives this packet, it updates the internal routing table with the new MTU. This update would also overwrite the route's source field to ICMP. This could cause a user-entered host route to disappear when Windows NT periodically removes aged ICMP host routes.

RESOLUTION

A supported fix that corrects this problem is now available from Microsoft, but it has not been fully regression tested and should be applied only to systems experiencing this specific problem. If you are not severely affected by this specific problem, Microsoft recommends that you wait for the next Windows NT 4.0 service pack that contains this fix.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:

http://www.microsoft.com/support/supportnet/overview/overview.asp

The English version of this fix should have the following file attributes or later:


   Date      Time                 Size    File name     Platform
   -------------------------------------------------------------
10/25/1999   5:00PM              168kb     TCPIP.SYS    Intel<BR/>
10/25/1999   4:58PM              299kb     TCPIP.SYS    Alpha<BR/>

To work around this problem, use either of the following methods:

Adjust the MTU on the router.

You can disable PMTU Discovery which will limit the MTU to 576 bytes for all non-local destination addresses. You can do this by setting this registry entry to a value of zero: Value name: EnablePMTUDiscovery
Value type: REG_DWORD - Boolean
Valid range: 0,1 (False, True)
Default: 1 (True)

Location:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

For additional information explanations of TCP registry entries, click the article number below to view the article in the Microsoft Knowledge Base:

Q120642 TCP/IP and NBT Configuration Parameters for Windows NT

STATUS

Microsoft has confirmed this to be a problem in Windows NT 4.0.

Additional query words: static persistent purge disappear table

Keywords : kbnetwork kbtool kbbug4.00 kbfix4.00
Version : winnt:4.0 SP5,4.0 SP6
Platform : winnt
Issue type : kbprb