In http://support.microsoft.com/kb/942861 Microsoft reports that in some situations Microsoft SQL Server experiences network issues due to the network card having a TCP Chimney Offload turned on. I have other server software that is experiencing similar issues. The result is that same as with SQL Server, turn off the TCP Chimney Offload and everything goes back to normal.

What does this TCP Chimney Offload feature do? What does it prevent certain network packets from reaching the final application?


TCP Chimney is one of many features which help offload much of the network processing from the CPU and into the network card. The problem with it is that some network cards don't do it correctly. When this happens, checksums get messed up or packets get mangled in mysterious ways. Mangled packets are as good as dropped packets and therefore don't make it to their destination.

Btw, Broadcom cards are notorious for this bug.

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TCP Chimney seems to be a form of TCP Offload Engine that works with Windows to optimize TCP traffic. Normally TCP offload is a lot more monolithic, whereas Chimney seems to allow a partial-offload method that streamlines some things. It's a proprietary Window thing. Microsoft's Exchange Blog has some information about Chimney.

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  • It's actually a hardware feature and not proprietary to Windows. Linux has also been adding support for many of these features. It's likely Linux will suffer many of the same bugs as well. – Jesse Weigert Jun 4 '09 at 8:32

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