[asterisk-dev] IAX hardphone

Jeffrey Ollie jeff at ocjtech.us
Tue Feb 15 11:10:50 CST 2011

On Tue, Feb 15, 2011 at 10:58 AM, Mihai Balea <mihai at hates.ms> wrote:
> On Feb 15, 2011, at 11:21 AM, Bill Shaw wrote:
>> Just thought I'd confirm that the UDP checksum being incorrect was my problem (thank you Mihai),  and also that Wireshark incorrectly handles the UDP checksum.  I've corrected my checksum routine and deleted Wireshark - fallen back to Ethereal.
> Glad I could help.
> Weird that your copy of Wireshark did not flag this - I'm using Wireshark on Mac OSX (v 1.4.3) and it pointed me to it. And, after all, Wireshark is the new Ethereal, they renamed the project a while back.
> Maybe it was just a buggy version or maybe just some settings...

Wireshark disables checking checksums because many ethernet cards
offload checksum operations from the CPU.  That means that the kernel
does not compute the checksum on outgoing packets but leaves it up to
the ethernet card to compute the checksum.  Since the checksum is
computed by the hardware just before transmission wireshark can't
capture the packet with the correct checksums and thus produced a lot
of false errors.  It's possible to turn the checksum checking back on
in Wireshark, you just need to look at the preferences for the
protocol you want to check.  You'd probably want to turn off the
checksum offloading on your ethernet card too.

Jeff Ollie

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