i've recently updated my rb133c to 3.1 and i noticed that, just after a connection to AP, in -mangle- is automagically created a rule that check for syn packets with mss >= 1433 and set it to 1432 only for forward in pppoe-out interface (mtu 1472).

I've also noticed that other rules like this, are created to set mss for eth1 to 1460, if i don't set this value manually directly on pc-side configuration of ethernet card.

If i try to set the 1432 value directly on pppoe in wlan settings, the "out" rule was always created, differently from the -ethernet- rules that was not created if mtu is set to 1460, but with a diffrent value, always "40" bytes minus from the wlan or ppooe mtu.

I think this is a behaviour for prevent packet fragmentation...

But: there is a way to set a value correctly and manually? How to calculate this value exactly?

How can i control the fragmentation-level on my net?
Is possible to do this with ping/traceroute or making a firewall rule just for mark and count the incoming/outgoing fragmented packets?

Meantime i have updated to 3.2, and set the mtu like this:

pppoe: mtu 1500, mru 1500
wlan1: mtu 1500
eth1: mtu 1500

eth mtu un pc 1440, the same value automatically created in mangle by router.

The "counter" for syn packets with mss > 1440 still zero.

Originally the pppoe mtu was 1472, but at this point, i think it's an unuseful setting...