I'm trying to debug a pesky PMTUD issue with IPv6 on Mac OS-X 10.6. It happens only from home, on wireless, when connected to a mac aiport that does an automatic tunnel (teredo) to IPv6 backbone. There are IPv6 web site that I cannot browse until I lower the MTU to 1400. My Linux desktop in similar config works fine... With tracepath6, I see one end of the tunnel tells me MTU should be 1280, that's using linux tools. On mac, tracepath6 does not seem to exist, and I don't know what tool to use... I have seen scamper, but no binary... I have used wireshark (on mac) to detect the PTB packet, but don't see it either... I have not compared with my linux box... I'm doing this testing on the side, slowly because it is fun to try to understand what's wrong here... and I wonder if others have had similar issues... May be not fully on topic, but to bring it back to topic see this presentation: http://www.ripe.net/ripe/meetings/ripe-60/presentations/Stasiewicz-Measureme... Which states the PMTUD failure rate on IPv6 is about 1.9%