Microsoft KB Archive/244539

= Host Route Can Be Deleted if ICMP Fragmentation Is Needed =

Article ID: 244539

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT 4.0 Service Pack 5, when used with:
 * Microsoft Windows NT 4.0
 * Microsoft Windows NT 4.0 Service Pack 6, when used with:
 * Microsoft Windows NT 4.0
 * Microsoft Windows NT Workstation 4.0
 * Microsoft Windows NT Workstation 4.0 Developer Edition

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This article was previously published under Q244539



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.



Windows 2000
To resolve this problem, obtain the latest service pack for Windows 2000. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

260910 How to Obtain the Latest Windows 2000 Service Pack

Windows NT 4.0
A supported fix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Apply it only to computers that are experiencing this specific problem.

To resolve this problem, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information about support costs, visit the following Microsoft Web site:

http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

NOTE: In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

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

  Date        Time    Size     File name  Platform 02/25/2000 07:47p  174,448  Tcpip.sys  x86 02/25/2000 07:46p  310,704  Tcpip.sys  Alpha



WORKAROUND
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:

120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows NT



Windows 2000
Microsoft has confirmed that this is a problem in Windows 2000. This problem was first corrected in Windows 2000 Service Pack 1.

Windows NT 4.0
Microsoft has confirmed that this is a problem in Windows NT 4.0.

Additional query words: static persistent purge disappear table

Keywords: kbhotfixserver kbqfe kbbug kbfix kbnetwork KB244539

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