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@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@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