PMTUD

RU: PMTUD

Path MTU Discovery (RFC 1191 for IPv4, RFC 1981 for IPv6): a mechanism that finds the smallest MTU on a path by sending packets with the Don't Fragment bit set. Any router that cannot forward the packet without fragmentation drops it and returns an ICMP Fragmentation Needed message carrying its local MTU; the sender then reduces its packet size accordingly. Middleboxes that silently drop ICMP cause PMTUD blackholes.