Slow Virtual PC network with Windows 7

Ok yet again this is one of those notes to myself blog entries that might help someone else with a problem of network performance with a virtual PC.

Unlike my friend Liam Westley who is now a MVP for virtualization after moving to windows 7 recently I decided that I'd just use the Virtual PC that comes with windows 7 rather than fighting with either Hyper-V or Installing the older Virtual Server family.

I've spent some time looking into a problem where when I uploaded to Windows 2003 server I had running in a VM the upload speed was terrifyingly slow (Using BITS as it happens but UNC connections weren't any better) from my laptop, downloads seem to be OK. Worse still other people who uploaded to the server got the kind of performance figures I was expecting to see.

Anyway turns out the problem is TCP task offloading this kb article 888750 at Microsoft covers how to turn off task offloading and somewhat explains the problem.

Basically you create a DWORD key called DisableTaskOffload in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters and set it to 1 then reboot your PC.