[Cerowrt-devel] trivial 6in4 fix(?)

Toke Høiland-Jørgensen toke at toke.dk
Mon Jun 17 05:44:42 EDT 2013

Sebastian Moeller <moeller0 at gmx.de> writes:

> Honestly, I think the best thing to do is not so much assume ATM or
> lack of ATM, but simply measure it :)

Right, doing the ping test with payload sizes from 16 to 113 packets
gives me an almost completely flat ping time distribution ranging from
20.3 to 21.3 ms (see attached graphic). So probably I'm on PTM...

> Easy to figure out empirically by hand, by finding the largest ping
> packet size that still passes without fragmentation (see
> http://www.debian.org/doc/manuals/debian-reference/ch05.en.html#_finding_optimal_mtu)

$ ping -c 1 -s $((1500-28)) -M do www.debian.org
PING www.debian.org ( 1472(1500) bytes of data.
1480 bytes from senfl.debian.org ( icmp_seq=1 ttl=45 time=114 ms

--- www.debian.org ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 114.522/114.522/114.522/0.000 ms

$ ping -c 1 -s $((1500-27)) -M do www.debian.org
PING www.debian.org ( 1473(1501) bytes of data.
From icmp_seq=1 Frag needed and DF set (mtu = 1500)

--- www.debian.org ping statistics ---
0 packets transmitted, 0 received, +1 errors

So the MTU seems to be 1500 bytes.

Now, how do I figure out what the PTM overhead is and feed it to HTB? :)

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