After thinking more about the draft, I think it is purposelessly hostile against innocent operators and end users who are suffering between people filtering ICMP and people insisting on PMTUD. Today, innocent operators often clear DF bit and end users are happy with it, because, today, probability of accidental ID match is small enough. However, as the ID specifies: >> Originating sources MAY set the IPv4 ID field of atomic datagrams to any value. >> IPv4 datagram transit devices MUST NOT clear the DF bit. people insisting on PMTUD are now authorized to set ID always zero, trying to discourage ICMP filtering and DF bit clearing. But, as people filtering ICMP won't stop doing so and if operators can do nothing other than clearing DF, it is end users who suffers. Then, end users may actively act against PMTUD and/or IETF. Masataka Ohta