I've been reading a little about TCP Segmentation Offload (aka TSO). We don't appear to support it, but at least 2 of our supported nics (e1000 and bge) apparently could support it.
The gist is that TCP pretends the nic has a large mtu, and passes a large (> the mtu on the link layer) packet down to driver and then the nic. The nic then fragments the large packet into smaller (<=mtu) packets. It uses the initial TCP header as a template to construct the headers for the "fragments.". The people who implemented it on linux claim a 50% CPU savings for an Intel 1Gb/s adaptor with a 1500 byte mtu. It seems like it could be implemented rather easily by adding an if_hwassist flag (CSUM_TSO). If this flag is set on the interface found by tcp_mss(), then the mss is set to 56k. This causes TCP to generate huge packets. We then add a check in ip_output() after the (near the existing CSUM_FRAGMENT check) which checks to see if its both a TCP packet, and if CSUM_TSO is set in the if_hwassist flags. If so, the huge packet is passed on down to the driver. Does this sound reasonable? The only other thing I can think of is that some nics might not be able to handle such a large mss, and we might want to stuff the maximum mss value into the ifnet struct. I don't have a bge or an e1000, so I'm not ready to actually implement this. I'm more considering firmware optimizations for our product, and would implement it in a few months, after making the firmware changes. Drew _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "[EMAIL PROTECTED]"
