depending on your hardware, you may be seeing bogus checksums in outbound
packets, but the packets on the wire have valid checksums because the hardware
computes the checksums and sets them.
I know I've run into this one before when doing load testing.
David Lang
On Mon, 28 Jan 2013, Robert Bradley wrote:
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 <[email protected]> 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 <[email protected]> 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 T?ht
Fixing bufferbloat with cerowrt:
http://www.teklibre.com/cerowrt/subscribe.html
--
Dave T?ht
Fixing bufferbloat with cerowrt:
http://www.teklibre.com/cerowrt/subscribe.html
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