find out mtu size and mss

Hi

My MTU size is 1500 🙂 because i’ve got no dial in tunneling of my internet uplink

ping -c 2 -M do -s 1472 gmx.net
PING gmx.net (213.165.65.50) 1472(1500) bytes of data.
1480 bytes from gmx.net (213.165.65.50): icmp_seq=1 ttl=54 time=59.3 ms
1480 bytes from gmx.net (213.165.65.50): icmp_seq=2 ttl=54 time=40.4 ms

when I ping an xdsl uplink of my brother i get this mtu size

:~# ping -c 2 -M do -s 1464 blah.blah.org
PING blah.blah.org (0.0.0.0) 1464(1492) bytes of data.
1472 bytes from blah.blah.org (0.0.0.0): icmp_seq=1 ttl=54 time=63.4 ms
1472 bytes from blah.blah.org (0.0.0.0): icmp_seq=2 ttl=54 time=63.9 ms

28byte is overhead
so when you add 1472 + 28 = 1500
when you add 1464 + 28 = 1492

if you have some tunneling active like pppoe
you need tcp mss clamping

so with pppoe dialin you’ve got mtu 1492
mtu – ip – tcp
1492 – 20 – 20 = 1452

Have fun