On Monday 12 January 2009, Walter Dnes wrote: > Seeing discussions about best MTU/MSS for MLPPP on my ISP's forum, I > asked about the optimal settings for a standard DSL connection. > Because of the way ATM cells line up, the optimal MSS is 1408. This > would normally imply MTU 1448, because MTU is normally MSS + 40. The > optimal setting is apparently MTU 1492 and MSS 1408. I tried forcing > MSS to 1408 in my routing config (/etc/conf.d/net), but that also > forces the MTU to 1448. Is there a way to do them separately? > > The only suggestion I've found via Google is iptables "mangle". Does > it manage to change MSS without changing MTU? If so, what is the > invocation in the "mangle" table?
I think it is in the OUTPUT table that you should insert a rule, but I don't have time to search for it now. It would probably be something like: iptables --insert OUTPUT --jump TCPMSS --protocol tcp --set-mss 1408 I think you can also set the advertised (by your machine) MSS for a network using ip route: ip route add 192.168.1.0/24 dev eth0 advmss 1408 PS. I am not sure if the above will break your connection because of dropped packets, or how it will interact with the MTU set at 1492. In my case I have just set my MTU at 1492 to cater for the PPP authentication on my ISP's ADSL network. I leave the MSS to be at what the kernel wants it to be - typically MSS = MTU - 40. -- Regards, Mick
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