change mtu

[Markus Schönhaber]

Eberhard Roloff wrote:
> While imho this is technically correct, the mtu surely
> influences your actual network speed negatively, when e.g. the
> frame size is to large and in consequence, many packets are
> fragmented and need to be reassembled. With german dsl, it is
> known that often dsl-connections work better with a slighly
> reduced mtu of say 1452.

No. Setting the MTU of DSL-Connections to a value of 1452 will keep the 
overall transmission speed below the possible optimum.
One can safely assume that today the technology with the smallest payload size 
used on the path a packet takes (to whereever that may be) is ethernet. 
Ethernet allows a payload size of 1500 bytes, meaning that the maximum size 
of an IP packet that can be transmitted without fragmentation (MTU) is 1500. 
Since DSL uses PPPoE which adds an 8-byte header to the packet, this leaves 
1492 bytes for the IP packet and thus a MTU of 1492.
In the past, it has been the case that some brain-dead admins of some web 
servers filtered those "very very evil" ICMP packets and thereby broke PMTU 
detection (detection of the smallest MTU on the path a packet takes). As a 
result, computers on a LAN, using the normal MTU on ethernet of 1500, that 
were connected to the Internet via a DSL router couldn't detect that the 
biggest IP package that could be sent without need for fragmentation was 1492 
bytes which made reliable data transmission from those servers impossible. 
One could workaround this issue by creating an IPtables rule on the router 
clamping the MSS to the PMTU of 1492. I don't use such a rule anymore and 
didn't encounter problems though.

Regards
  mks