Date: Thu, 27 Sep 2001 12:03:30 -0400 From: Sandeep Joshi <sandeepj@research.bell-labs.com> To: freebsd-hackers@freebsd.org Subject: Re: TCP&IP cksum offload on FreeBSD 4.2 Message-ID: <3BB34DD2.FC65196A@research.bell-labs.com>
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Ron, This may be of interest... http://citeseer.nj.nec.com/stone00when.html When The CRC and TCP Checksum Disagree Jonathan Stone, Craig Partridge SIGCOMM -Sandeep On Thu, 27 Sep 2001, Ronald G Minnich wrote: > > I have a question on the checksum offloading. Has anyone measured any > incidence of data corruption between the PCI card and memory. In other > words, when you offload checksums the end-to-end checking becomes > card-to-card checking, and the possibility exists that what goes in memory > at the destination end is not what was sent at the source. Very remote > possibility, of course, but ... > > It's not that the data gets corrupted (usually). It's that > once-in-a-100-trillion errors could result in the occasional dropped > half-packet or missed word (i.e. overflow). The missed word problem is > usual a miscommunication between card and PCI chipset about how a PCI > ABORT is supposed to work ... which we've seen on some very recent > just-released chipset/network card combinations,. > > Does this happen? Yes. We've seen it on, to name just two, HIPPI800 and > Myrinet cards. In each case it was not actual data corruption, it was > "can't happen" DMA scenarios that once in a very long while (1 in 10^14 or > so) resulted in bits of packets getting corrupted. Each of these cards > has a very high-quality end-end CRC for the data, and Myrinet has flow > control. We're not the only place that has seen this problem, and I've > been told that many commerical Myrinet clients run IP over Myrinet because > of these types of problems (of course FreeBSD has the fastest IP over > Myrinet anyway, so it's not like that's a huge problem). > > Is it likely? Well, on one cluster here, with 48 machines and 12 > interfaces per machine, it's not only likely, it's a given. Without > software checksums you are going to get data corruption. > > What I don't know is whether offloaded checksums on commodity ethernet > cards have seen anything similar. > > I assume that checksums across all the frags are done by the kernel (i.e. > NFS would checksum the full UDP packet)? Has anyone measured to see if > there is corruption occuring on the frags, ever? Of course it would > probably take a while ... > > Thanks in advance for any information you might have. > > ron > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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