Path MTU Discovery (PMTUD)
Path MTU Discovery (PMTUD), defined in RFC 1191 for IPv4 and RFC 8201 for IPv6, is how a sender finds the largest packet size -- the maximum transmission unit, or MTU -- that every link along a route to a destination can carry without fragmenting it. A sender transmits packets with the "don't fragment" flag set; if a router along the path has a smaller MTU than the packet, it drops the packet and sends back an ICMP "fragmentation needed" (IPv4) or "packet too big" (IPv6) message naming the smaller size, and the sender retries at that size. The common failure mode is a firewall or middlebox blocking that ICMP message outright, intending to block unwanted ICMP traffic generally: the oversized packet is still silently dropped, but the sender never learns why, so a connection can complete its initial handshake with small packets and then stall indefinitely once it tries to send something larger -- a large TLS certificate chain or a file transfer, for example -- a symptom often mistaken for an unrelated problem. QUIC avoids relying on ICMP for this and instead probes path MTU directly at the transport layer, a technique called DPLPMTUD, defined in RFC 8899.
By AdminKit team, Netbase JSC · Updated