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Date:      Wed, 31 Jan 2007 23:48:49 +0100
From:      Erik Trulsson <ertr1013@student.uu.se>
To:        Wesley Shields <wxs@atarininja.org>
Cc:        C?dric Jonas <cedric@decemplex.net>, freebsd-net@freebsd.org, freebsd-current@freebsd.org
Subject:   Re: fxp(4) checksum (offloading?) errors
Message-ID:  <20070131224849.GA86858@owl.midgard.homeip.net>
In-Reply-To: <20070131212534.GC58404@atarininja.org>
References:  <20070131214705.0ee0ba67@ganymed> <20070131212534.GC58404@atarininja.org>

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On Wed, Jan 31, 2007 at 04:25:34PM -0500, Wesley Shields wrote:
> On Wed, Jan 31, 2007 at 09:47:05PM +0100, C?dric Jonas wrote:
> > Hi,
> > 
> > I get TCP/UDP checksum errors with fxp(4). I noticed it after using
> > Wireshark today:
> > 
> > 	Checksum: 0xac18 [incorrect, should be 0xfbc7 (maybe caused by
> > 	checksum offloading?)]
> > 
> > for example.
> > 
> > If I understand it more or less correctly, checksum offloading is
> > performed by or with help of the NIC - only for TCP and UDP, where the
> > checksum fails exactly...
> 
> Yes, the checksums are offloaded to the card.  Maybe I missed it, but
> you were not very clear on where you were seeing the invalid checksums.
> In the case of wireshark running on the machine which is doing the
> offloading then this makes sense as the checksums are computed AFTER
> wireshark sees them.  The best way to know for sure if the checksums
> were computed correctly is to check them on the destination or somewhere
> along the wire to the destination.
> 
> > I didn't found a way to disable it, so I assume it's a bug in the
> > driver... I don't know exactly which infos I must provide additionally
> > to the following, so don't hesitate to ask more :)
> 
> According to ifconfig(8) they can be toggled using -rxcsum and -txcsum,
> but it is dependent upon the NIC/driver in use.  A quick glance through
> fxp(4) indicates that it doesn't do checksum offloading.  I'm sure
> someone will correct me if I am wrong.  :)

Some fxp(4) NICs support checksum offloading, but most don't.
The fairly common 82559 controller chip does not support checksum
offloading.  The 82550 (which can be found on the Intel PRO/100 S cards)
does support checksum offloading.





-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013@student.uu.se



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