How does FreeBSD handle tcp checksum offloading

[Chuck Swiger]

On May 24, 2007, at 12:10 PM, Andrew Falanga wrote:
> I'm curious as to how FreeBSD handles checksum offloading for TCP
> packets.  Is this on by default?

If a particular NIC supports checksum offloading, it is typically  
enabled by default.

> It seems that it's only relevant for specific drivers rather than  
> something that's just assumed.

That's correct.  Most of the gigabit NICs support it, but few of the  
older NICs do.

> How can I determine if this is supported for the hardware I'm  
> running?  Is it
> common for tcp checksum offloading to be in the fxp driver?  Can one
> turn it off (though, it honestly sounds like no one would wish to do
> so)?

You can use ifconfig to see whether the RXCSUM & TXCSUM options are  
listed, and you can use ifconfig to enable or disable it:

      rxcsum, txcsum
              If the driver supports user-configurable checksum  
offloading,
              enable receive (or transmit) checksum offloading on the  
inter-
              face.  Some drivers may not be able to enable these  
flags inde-
              pendently of each other, so setting one may also set  
the other.
              The driver will offload as much checksum work as it can  
reliably
              support, the exact level of offloading varies between  
drivers.

      -rxcsum, -txcsum
              If the driver supports user-configurable checksum  
offloading,
              disable receive (or transmit) checksum offloading on  
the inter-
              face.  These settings may not always be independent of  
each
              other.

I don't believe that the fxp NICs support checksum offloading.

-- 
-Chuck