netdev - Re: UDP implementation and the MSG

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Message-ID: <20210203222054.6zhq47o7rzjstpdq@grml>
Date:   Wed, 3 Feb 2021 23:21:10 +0100
From:   michi1@...haelblizek.twilightparadox.com
To:     kernelnewbies@...nelnewbies.org, netdev@...r.kernel.org,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        jakub@...udflare.com, pabeni@...hat.com
Subject: Re: UDP implementation and the MSG_MORE flag

Hi!

On 15:12 Tue 26 Jan     , Oliver Graute wrote:
> Some UDP packets send via the loopback interface are dropped in the
> kernel on the receive side when using sendto with the MSG_MORE flag.
> Every drop increases the InCsumErrors in /proc/self/net/snmp. Some
> example code to reproduce it is appended below.
> 
> In the code we tracked it down to this code section. ( Even a little
> further but its unclear to me wy the csum() is wrong in the bad case)

I think it is unlikely that the error is on the receiving side. I recommend to
capture the data that is being sent with a sniffer and see if there is
anything wrong. You may also want to try disabling hardware acceleration on
your network devices, because the checksum is sometimes done in hardware.

	-Michi
-- 
programing a layer 3+4 network protocol for mesh networks
see http://michaelblizek.twilightparadox.com