Date: Sat, 21 Aug 2004 01:55:10 +0300 From: Ruslan Ermilov <ru@FreeBSD.org> To: Don Lewis <truckman@FreeBSD.org> Cc: current@FreeBSD.org Subject: Re: on amd64 tcp4 cksums are bad (FYI) Message-ID: <20040820225510.GB8714@ip.net.ua> In-Reply-To: <200408202249.i7KMnX2h003904@gw.catspoiler.org> References: <20040820222026.GC73564@ip.net.ua> <200408202249.i7KMnX2h003904@gw.catspoiler.org>
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[-- Attachment #1 --] On Fri, Aug 20, 2004 at 03:49:33PM -0700, Don Lewis wrote: > On 21 Aug, Ruslan Ermilov wrote: > > On Fri, Aug 20, 2004 at 11:07:34PM +0300, Maxim Sobolev wrote: > >> Andrew Gallatin wrote: > > >> >You're almost certainly using a driver which offloads transmit > >> >checksums. (both fxp and em do) Since BPF sniffs the packet before it > >> >leaves the host, the checksum has not yet been calculated, so it looks > >> >bad. > >> > >> Is it possible to detect this situation and flag tcpdump somehow, so > >> that it don't trust checksum? With the widespread adoption of GigE > >> cards, this "problem" is likely to be more and more common. > >> > > It's easy to detect using the m_pkthdr.csum_flags. It shouldn't > > be impossible to make a writable mbuf chain copy, and call > > in_delayed_cksum() on a copy, before calling bpf_mtap(). > > >From a performance point of view, you'd probably want defer calculating > the checksum until after the packet has passed the BPF filter, otherwise > you'd consume an excessive amount of CPU time when sniffing for > infrequently occurring packets on a high bandwidth network interface. > Note that this is only for outgoing packets originated on this host. Also, at least with ng_bpf(4) you have an opportunity to watch both matching and non-matching packets, i.e., all of them. Yes, performance will degrade if we do this, and I don't think it's a good idea to commit this, but if someone wants it, they now know what to do. ;) Cheers, -- Ruslan Ermilov ru@FreeBSD.org FreeBSD committer [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (FreeBSD) iD8DBQFBJoFOqRfpzJluFF4RAmUHAKCGQNA+U+0yAeot+/90uoqTyCEFqQCfYQS3 gr0wgU00TUL7zFeXFGfbl70= =+JLS -----END PGP SIGNATURE-----help
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