Date: Fri, 5 Sep 2003 16:47:22 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: freebsd-net@freebsd.org Subject: TCP Segmentation Offload Message-ID: <16216.63066.954104.582195@grasshopper.cs.duke.edu>
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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
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