Re: Strange TCP problem with awge0

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

On 2 Jan, 2015, at 11:23 , Martin Husemann <martin%duskware.de@localhost> wrote:
> On Fri, Jan 02, 2015 at 11:03:20AM -0800, Dennis Ferguson wrote:
>>> 		9 discarded for bad checksums
>> 
>> This one is suspicious.  Does this increment when the ftp connection hangs?
> 
> Yes, it does. Wouldn't those dropped packets be in the local capture though?
> When I ask wireshark to validate the tcp checksums, there are no failures
> in the local capture.

tcpdump thinks those packets have correct checksums too, so the
next guess might be that the software checksum function being used
in the packet processing path to check the checksum has a bug.  It
might also be related to how the packet ends up being stored in its
mbuf.

A decade ago I would never have guessed that software checksum bugs
could happen without being quickly noticed, but now the prevalence
of hardware checksum checks means the software doesn't get exercised
as much as it once did so I'm now always a bit suspicious of it (as
well as the hardware checksums, since there's a big variety of hardware).

Dennis Ferguson