KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q244539: Host Route Can Be Deleted if ICMP Fragmentation Is Needed

Article: Q244539
Product(s): Microsoft Windows NT
Version(s): 2000,4.0 SP5,4.0 SP6
Operating System(s): 
Keyword(s): kbnetwork kbtool kbWin2000PreSP1Fix kbWinNT400PreSP7Fixkbfixlist
Last Modified: 08-MAY-2002

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Windows NT Server versions 4.0 SP5, 4.0 SP6, on platform(s):
   - the operating system: Microsoft Windows NT 
- Microsoft Windows NT Workstation versions 4.0 SP5, 4.0 SP6 
- Microsoft Windows 2000 Advanced Server 
- Microsoft Windows 2000 Professional 
- Microsoft Windows 2000 Server 
-------------------------------------------------------------------------------

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
==========

Windows 2000
------------

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

  Q260910 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 described in this article and should be applied only to
systems 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 on support costs, please go to the following address on the World
Wide Web:

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

NOTE: In special cases, charges that are normally incurred for support calls may
be canceled, if a Microsoft Support Professional determines that a specific
update will resolve your problem. Normal 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:

  Q120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows
  NT

STATUS
======

Windows 2000
------------

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

Windows NT 4.0
--------------

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


Additional query words: static persistent purge disappear table

======================================================================
Keywords          : kbnetwork kbtool kbWin2000PreSP1Fix kbWinNT400PreSP7Fix kbfixlist
Technology        : kbWinNTsearch kbWinNTSsearch
Version           : :2000,4.0 SP5,4.0 SP6
Hardware          : ALPHA x86
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.