Watch dropped IP packets

How to ensure that you don't get too many dropped IP packets.

Watch dropped IP packets
Serdar Yegulalp

Losing too many TCP/IP packets? Could be because you've passed a default limit set in the registry for reassembly headers. If the limit's exceeded, the software may drop out-of-order packets without warning. But you can stop that with a simple registry hack.

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Because of the way TCP/IP works, sometimes packets are deliberately sent out of order (or "fragmented"). They are, of course, reassembled at their destination and defragmented. When the Microsoft TCP/IP stack does this, it looks for a reassembly header to place the fragmented packets into the correct order. If it can't find the header, it may create one.

Since the stack has a limit of 100 such headers, excessive packet fragmentation may result in new fragmented packets simply being dropped without warning. This limit is soft-coded, so if you're finding a lot of packets going missing on you, you may want to raise it.

To change it, fire up your favorite registry editor (or just plain old REGEDIT) and go to HKEY_LOCAL_MACHINESystemCurrentControlSetServicesTcpipParameters. Add the value MaximumReassemblyHeaders (REG_DWORD) data type, and set the data value to a Decimal number between 100 and 65535 (to specify the number of headers). Then reboot.


Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.

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This was first published in September 2001

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