Re: fxp(4) checksum (offloading?) errors

On Wed, 31 Jan 2007 22:24:11 +0100
Erik Trulsson <ertr1013_at_student.uu.se> wrote:

> On Wed, Jan 31, 2007 at 09:47:05PM +0100, Cédric 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.
> 
> Is that for incoming or outgoing packets?  If it is for outgoing it is
> perfectly normal and nothing to be worried about (assuming that your
> NIC does checksum offloading.)

Arghs, shame on me. You are fully right... I needed to know it, my
first mail contains quasi the reason of the "bug" :) 
However, thank you!

> 
> If it is for incoming packets it could be a real problem.
> 
> 
> > 
> > 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...
> 
> Checksum offloading means that the calculation of the checksum is
> done by the NIC instead of by the main CPU.  For outgoing packets
> this means that wireshark or tcpdump and all similar programs see the
> packets before they have been handed over to the NIC, which means
> that the correct checksum has not yet been calculated and probably
> contains garbage.
> 
> 
> > 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 :)
> > 
> > Used NIC:
> > 
> > 	fxp0: <Intel 82550 Pro/100 Ethernet> port 0xa400-0xa43f mem
> > 	0xfb030000-0xfb030fff,0xfb000000-0xfb01ffff irq 16 at device
> > 	7.0 on pci2
> > 
> > Used system:
> > 
> > 	FreeBSD ganymed 7.0-CURRENT FreeBSD 7.0-CURRENT #0: Sun Jan
> > 21 16:07:51 CET 2007
> > 	root_at_ganymed:/usr/obj/usr/src/sys/GANYMED  i386
> > 
> 
> 
> 
> 


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Cédric Jonas                                        cedric_at_decemplex.net

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