Why does the TCP Chimney Offload feature on some ethernet cards fail to pass some network packets to the OS?

Question

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?

Solution

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.

Comments

[ashiso]

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.