INFO: New Registry Key to Limit Number of Reassembly Headers

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: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

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\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.