What's the deal with hardware checksum and
net.inet.udp.checksum?
Robert Watson
rwatson at FreeBSD.org
Thu Jul 10 10:43:24 UTC 2008
On Wed, 9 Jul 2008, gnn at freebsd.org wrote:
> I would assume that if a card, say the em, has hardware TX checksum that the
> UDP checksum could be calculated by the hardware, but this seems not to be
> the case. The manual pages are unhelpful in this regard.
On the whole, they should be generated in hardware as long as it's not
administratively disabled with ifconfig, and as long as there aren't know bugs
in the hardware for the rev you're using. Just for example, hardware
checksumming is disabled in software for quite a few early 1gbps cards due to
bugs in the hardware causing rather nasty side effects. What specific problem
are you seeing? We do do a software checksum of the pseudo-header, but the
UDP data should be checksummed by hardware.
(The usual test for hardware checksum being enabled on transmit is to tcpdump
the interface and see tcpdump reporting lots of bad checksums, as the BPF
capture happens before hardware checksumming is run -- in principle on the
receive side that shouldn't happen!)
Robert N M Watson
Computer Laboratory
University of Cambridge
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