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Started by kasperd, October 08, 2012, 07:06:18 AM
Quote from: kasperd on October 08, 2012, 07:06:18 AMI started wondering about how 6to4 deals with certain MTU issues. In particular I am wondering about this scenario:1500 bytes packet is sent from 2001:db8::1 to 2002:c000:22a::1Packet arrives at 6to4 relay, which sends too-big error message back to 2001:db8::12001:db8::1 receives error message and retransmits packet in two fragments of 1480 and 76 bytesBoth packets arrive at some 6to4 relay, likely the same as before.6to4 relay encapsulates both packets in IPv4 packets from 188.8.131.52 to 192.0.2.42 of 1500 and 86 bytes
QuoteBoth IPv4 packets makes it most of the way, but link from 192.0.2.1 to 192.0.2.42 has an MTU of only 1492 bytes.
Quote192.0.2.1 sends fragmentation-needed error to 184.108.40.206 for the first packet.ICMP error arrives at a different 6to4 relay, which never saw the first packet to 192.0.2.42This 6to4 relay remembers that packets to 192.0.2.42 must not be larger than 14922002:c000:22a::1 receives second fragment and discards it after timeout. (No error message is sent because first fragment is missing.)2001:db8::1 retries after a retransmission timeout.Packet is received by 6to4 relay, but this relay did not see the ICMP error and sends a 1500 bytes packet again.
Quote from: plugwash on January 02, 2013, 05:26:30 PM6to4 relays SHOULD NOT set the don't fragment bit in their IPv4 packets.