2009-03-03 I'm typing this while sitting in the Tha ...
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I'm typing this while sitting in the Thalys high-speed train between Antwerp and Brussels (so no 300 km/h yet). The first class has free wi-fi Internet access. I had to lower the mtu of the wireless interface to be able to use ssh and screen to access my normal home environment, so path mtu discovery is probably b0rken somewhere. I appear from a Belgian IP, no idea how they do the uplink from the train.
Update 2009-03-06: Found on the Thalys website: WiFi - Internet access for everyone! with an explanation how satellite Internet and UMTS/GPRS are used to tunnel the Internet to the train. The whole tunneling thing is probably what is causing the mtu problem.