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

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.)

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@ganymed:/usr/obj/usr/src/sys/GANYMED i386





--
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Erik Trulsson
ertr1013@xxxxxxxxxxxxx
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