The L2TPv3 protocol specification RFC 3931refers to the necessity of considering IP fragmentation in the protocol implementation
but fails to give a specific solution.On the basis of the existing solutions to Path Maximum Transmission Unit Discovery(PMTUD)and the advantages of several solutions
it proposes a specific solution suitable for L2TPv3
which takes into consideration the existence of both old and new formats of Internet Control Message Protocol(ICMP)messages in the existing networks