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

On Wed, 31 Jan 2007 22:24:11 +0100
Erik Trulsson <ertr1013@xxxxxxxxxxxxx> 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@ganymed:/usr/obj/usr/src/sys/GANYMED i386







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Cédric Jonas cedric@xxxxxxxxxxxxx

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