Re: Wondering about TCP checksum errors in AFP-over-TCP

[Martin Visser <martinvisser99 () gmail com>]

There really is a small risk in deselecting "Validate TCP checksums if
possible". If your TCP headers were really corrupted then most likely
this would be because of physical layer errors. However, as with most
media there is built-in physical layer error detection (or
correction), corrupted packets are going to either be discarded or
corrected before Wireshark gets to see it. The only time you really
need to validate TCP checksums would be if you are  developing your
own TCP stack or developing devices that try to manipulate the TCP
headers in interesting ways - not something most network engineers or
sysadmins are doing.


Regards, Martin

MartinVisser99 () gmail com



On 29 March 2011 04:17, Kok-Yong Tan <ktan () realityartisans com> wrote:
> Got it.  Thanks.  I found the preference for "Validate TCP checksums if
> possible" and deselected it.  I guess this leaves me kind of "flying blind"
> if there are legitimate TCP checksum errors as I was hoping the "if
> possible" in the above preference would somehow take into account TCP
> offloading but I suppose it can't.  Oh well...
> On Sun, Mar 27, 2011, at 12:52, Anders Broman wrote:
>
> Kok-Yong Tan skrev 2011-03-27 16:51:
>
> It says that "Wireshark 1.2 and above disable IP, TCP and UDP checksum by
> default."  Does this mean that it disables checksum [validation] by default?
>  If so, note that I'm running Wireshark 1.4.3 with default settings on a
> MacOS X 10.5.8 system and I'm still seeing the TCP checksum errors.
>
> If you upgraded from an earlier version your (old)preferences will be
> retained...
> /Anders
>
> On Sun, Mar 27, 2011, at 01:19, Martin Visser wrote:
>
> In pretty much all case where you are seeing TCP checksum errors on a
> server, it will because of the various TCP offload features of the NIC /
> driver. If can capture on the wire (that is using port-mirroring on a
> switch) this will confirm this, or alternatively turn off those features on
> your server temporarily while testing.
> See http://wiki.wireshark.org/CaptureSetup/Offloading for details
> Regards, Martin
>
> MartinVisser99 () gmail com
>
>
> On 27 March 2011 10:07, Kok-Yong Tan <ktan () realityartisans com> wrote:
> > On Sat, Mar 26, 2011, at 18:53, Kok-Yong Tan wrote:
> >
> > > Just for kicks, I decided to do a wireshark trace of AFP-over-TCP
> > > conversations between my Apple MacOS X 10.4.11 Tiger server and my Apple
> > > MacOS X 10.5.8 Leopard (PPC) client.  Surprisingly, I'm seeing lots of TCP
> > > checksum errors (no ssh going on here in the connection since it's all
> > > protected on my internal LAN) on packets going in both directions.  Now, if
> > > the TCP stack were damaged either on the client or the server or both, I
> > > would expect connection issues and all packets going through to exhibit the
> > > checksum errors.  But I don't and not all packets are exhibiting checksum
> > > errors between the two machines.  Only some.  Of course, this is manifesting
> > > itself in slower than expected throughput between the server and client
> > > since I assume that TCP checksum errors result in retransmits.  The server
> > > is connected to a ZyXEL GS2024 switch via LACP 802.3ad with 1 IP address in
> > > use in the two-NIC bonded pipe.  Could this be causing the TCP checksum
> > > errors?
> >
> > More info on this:  I'm beginning to think that the LACP/IEEE802.3ad
> > bonding of the server with the switch has nothing to do with it as I'm
> > seeing the same checksum errors between the client (which only has one NIC
> > and doesn't use LACP/IEEE802.3ad) and public servers hosted at akamai.net,
> > doubleclick.com, etc., and even my externally hosted mail server.
>
>
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