[Cerowrt-devel] deployed some cero this weekend, chasing checksums

[Robert Bradley]

It looks more like data corruption of various forms as opposed to a fault
in checksumming:

- Truncation of some layer-4 data including headers to 75 octets
- Some bad TCP packets have stored header lengths of 0 octets
- I often see lines of incrementing bytes (30 31 32 etc.).  For example,
packet 962 has a train of values from 0x10 to 0x2f, starting at position
0x003a (the TCP timestamps).  I think these are meant to be fragments from
the ping packets (which contain 8 octets then values 0x10 to 0x37), but
these are straying into non-ICMP packets.
- There are pieces of HTTP in non-HTTP protocols.  For example, packet 1394
is supposed to be UDP, but looks like it is really TCP traffic with the
wrong protocol number.  The checksum is still invalid in either case.
- It is possible to corrupt layer-4 checksums only, leaving the IP layer
untouched.


On 28 January 2013 07:52, Dave Taht <dave.taht at gmail.com> wrote:

> Put up a pic http://snapon.lab.bufferbloat.net/~d/yurt
>
> they aren't bad all the time, but when they go bad, bad things happen.
>
>
> On Sun, Jan 27, 2013 at 11:41 PM, Dave Taht <dave.taht at gmail.com> wrote:
>
>>
>> I have been debugging some weirdness for a while. You might want to do
>> some captures on the latest cero and look at checksums.
>>
>> An unreasonably high number of checksum issues seem to be happening, but
>> there doesn't appear to be a whole lot of pattern to it, as yet.
>>
>> I will simplify. I pinged locally and 8.8.8.8 and surfed the web, and a
>> symptom is that some other routers can't ping sometimes nor access much of
>> the internet beyond the gateway. They can always reach the gateway.
>>
>> in the interim, the topology on this capture are
>>
>> 172.30.102.17 - laptop via ethernet to
>> 172.20.102.1 - cerowrt 3.7.4-4 via ethernet to
>> 172.20.6.1 - ubnt 3.3.8-26 via mesh to
>> 172.20.142.11 - ubnt 3.7.4-4 via ethernet to
>> * 192.168.100.1 - cerowrt 3.7.2 capture point (yes, updating that)
>> 10.0.10.1 - comcast box (yes, double nat, fixing that)
>>
>> I took a capture on the se00 interface
>>
>> tcpdump -i se00 -w/tmp/yurt.cap host 172.20.102.17
>>
>> and stuck that capture there:
>>
>> http://snapon.lab.bufferbloat.net/~d/yurt/yurt.cap
>>
>> and then looked at it with wireshark with this filter
>>
>> ip.checksum_bad == 1
>>
>> and scratched my head at the error rate (about 1%) and the pattern (lack
>> thereof)
>>
>> I will simplify in the mroning
>>
>> --
>> Dave Täht
>>
>> Fixing bufferbloat with cerowrt:
>> http://www.teklibre.com/cerowrt/subscribe.html
>
>
>
>
> --
> Dave Täht
>
> Fixing bufferbloat with cerowrt:
> http://www.teklibre.com/cerowrt/subscribe.html
>



-- 
Robert Bradley
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