Microsoft KB Archive/252366

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INFO: New Registry Key to Limit Number of Reassembly Headers


The information in this article applies to:

  • Microsoft Windows NT Server version 4.0
  • Microsoft Windows NT Workstation version 4.0
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Professional

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.


A new registry parameter, MaximumReassemblyHeaders, has been added to Windows NT 4.0 Service Pack 5 (SP5) and above and Windows 2000 to limit the number of the Internet Protocol (IP) reassembly headers the Transmission Control Protocol (TCP)/IP stack allows.


When reassembling a fragmented IP packet, the Microsoft stack looks for a reassembly header for the fragment in order to fill the fragment into the proper buffer. The stack creates a reassembly header if an existing reassembly header that the fragment belongs to cannot be found.

To guard resource use against IP fragmentation, with many out-of-order fragmented packets occurring in a very short period of time, starting with Windows NT 4.0 SP5, Microsoft has implemented a default limit of 100 reassembly headers. Fragmented packets that arrive when the stack has reached the limit are silently dropped. The limit can be changed by using the following steps to add or modify a registry parameter.

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 or Windows 2000, you should also update your Emergency Repair Disk (ERD).

Follow these steps to change the default setting for the maximum reassembly headers:

  1. Start Registry Editor (Regedt32.exe).
  2. Locate the following registry key:

    HKEY_LOCAL_ MACHINE on Local Machine\System\CurrentControlSet\Services\Tcpip\Parameters\

    NOTE: The registry key is one path.

  3. On the Edit menu, click Add Value, and then type MaximumReassemblyHeaders.
  4. Click REG_DWORD in the Data Type box, and then click OK.
  5. Type a number that represents your limit for maximum reassembly headers in the Data box, and then click OK. You can type any whole number between 100 and 65535, in decimal format.
  6. Quit Registry Editor, and then restart the computer.

Additional query words:

Keywords : kbnetwork kbAPI kbOSWinNT400 kbOSWin2000 kbSDKPlatform kbSDKWin32 kbWinsock _IK kbGrpDSNet
Issue type : kbinfo
Technology : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400xsearch kbwin2000AdvServ kbwin2000AdvServSearch kbwin2000Serv kbWinNTSsearch kbWinNTS400xsearch kbWinNTS400 kbwin2000ServSearch kbwin2000Search kbwin2000ProSearch kbwin2000Pro kbWinAdvServSearch

Last Reviewed: October 23, 2000
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