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XP SP2 slows down gigabit network performance

Plummeting network throughput rates have been reported on systems with installed Windows XP Service Pack 2. The culprit is a combination of factors.

Gigabit networking hardware is not quite as common as its 10- and 100-megabit counterparts, but it's growing in popularity, and many higher-end workstations ship with gigabit networking as a standard component. (The computer I use, for instance, has an Intel-brand gigabit network port on the motherboard.)

Despite its popularity, some people with gigabit networking hardware have noticed network throughput dropping drastically on systems where they had Windows XP Service Pack 2 installed. The culprit, it turns out, is a combination of factors:

1. Gigabit networking hardware that operates at full speed.

2. Both the network card and its driver support a network function known as Large Send Offload (LSO).

3. High data throughput.

Normally, Large Send Offload is designed to ease some of the burden of high network throughput. It has the network hardware -- i.e., the network card itself -- break the data into network frames rather than having the network stack -- i.e., the computer -- do the job. The data can be sent faster, and the CPU has that much less to do to send more data. Unfortunately, an as-yet-unresolved issue in the Windows Firewall/Internet Connection Sharing service causes LSO to malfunction and work very slowly.

The only solution at this point, pending a fix directly from Microsoft, is to disable the Windows Firewall/Internet Connection Sharing service. On a system where neither the Windows Firewall nor ICS are in use, this service can be disabled without a problem. Note that disabling is not the same as not using the Windows Firewall or ICS; the service itself -- in Control Panel/Administrative Tools/Services -- must be disabled. Reboot after disabling the service to make sure the changes take effect.

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Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check out his Windows 2000 blog for his latest advice and musings on the world of Windows network administrators -- please share your thoughts as well!

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