Date: Tue, 7 Feb 2006 09:34:11 +0900 From: Pyun YongHyeon <pyunyh@gmail.com> To: Nate Lawson <nate@root.org> Cc: Alan Cox <alc@cs.rice.edu>, src-committers@FreeBSD.org, Oleg Bulyzhin <oleg@FreeBSD.org>, cvs-all@FreeBSD.org, cvs-src@FreeBSD.org Subject: Re: cvs commit: src/sys/dev/bge if_bge.c Message-ID: <20060207003411.GA11752@rndsoft.co.kr> In-Reply-To: <43E7DAC2.8060101@root.org> References: <200602020958.k129wWtc066930@repoman.freebsd.org> <20060202100637.GB24350@lath.rinet.ru> <20060205235817.GQ5499@cs.rice.edu> <20060206221141.GA57775@lath.rinet.ru> <43E7CBD5.5090203@root.org> <20060206223436.GB57775@lath.rinet.ru> <43E7D662.6000608@root.org> <43E7DAC2.8060101@root.org>
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On Mon, Feb 06, 2006 at 03:24:50PM -0800, Nate Lawson wrote: > Nate Lawson wrote: > >Oleg Bulyzhin wrote: > > > >>On Mon, Feb 06, 2006 at 02:21:09PM -0800, Nate Lawson wrote: > >> > >>>Oleg Bulyzhin wrote: > >>> > >>>> nq = q->m_nextpkt; > >>>> q->m_nextpkt = NULL; > >>>> m->m_pkthdr.csum_flags &= q->m_pkthdr.csum_flags; > >>>>- m->m_pkthdr.csum_data += q->m_pkthdr.csum_data; > >>>>+ sum = m->m_pkthdr.csum_data + q->m_pkthdr.csum_data; > >>>>+ m->m_pkthdr.csum_data = (sum & 0xffff) + (sum >> 16); > >>>> m_cat(m, q); > >>>> } > >>>>#ifdef MAC > >>> > >>> > >>>I'm not familiar with this code. So m->m_pkthdr.csum_data is 32 > >>>bits? Couldn't the same thing be achieved with making it 16 bits > >>>since the add will wrap normally? > >> > >> > >>It will not work cause it's not just a trivial sum it's so called > >>"1's complement sum" (refer rfc1071 for details). > > > > > >You're right, but aren't you missing the NOT step? > >1. 2's complement sum > >2. Add in carry > >3. 1's complement of result (not) > > Sam Leffler mentioned this comment from NetBSD would be helpful. Might > you add it to clear up misunderstandings like I had? > > sys/mbuf.h has useful comments not found in the freebsd file: > > /* > * record/packet header in first mbuf of chain; valid if M_PKTHDR set > * > * A note about csum_data: For the out-bound direction, the low 16 bits > * indicates the offset after the L4 header where the final L4 checksum > value > * is to be stored and the high 16 bits is the length of the L3 header (the > * start of the data to be checksumed). For the in-bound direction, it AFAIK FreeBSD doesn't embed the length of the L3 header in csum_data. If we do that it would help much for hardwares that lacks hardware pseudo checksum capability.(e.g. hme(4), sk(4) etc) ATM these drivers should find out L3 header length by traversing the mbuf chain. > is only > * valid if the M_CSUM_DATA flag is set. In this case, an L4 checksum > has been > * calculated by hardware, but it is up to software to perform final > * verification. > * > * Note for in-bound TCP/UDP checksums, we expect the csum_data to NOT > * be bit-wise inverted (the final step in the calculation of an IP > * checksum) -- this is so we can accumulate the checksum for fragmented > * packets during reassembly. > */ > > Thanks, > -- > Nate -- Regards, Pyun YongHyeon
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