PMTUD

Path MTU Discovery (PMTUD) is a networking technique used to determine the maximum transmission unit (MTU) size that can be used on the path between a source and destination without requiring fragmentation. By learning the smallest MTU along the route, hosts can send packets large enough to maximize efficiency while avoiding fragmentation at routers.

In IPv4 PMTUD, the sender marks packets with the Don't Fragment (DF) bit set. If a router cannot forward a packet without fragmenting, it drops the packet and returns an ICMP "Fragmentation Needed" message that includes the MTU of the next hop. The sender uses this information to reduce its perceived path MTU and retransmit. The process typically starts with a common default MTU (such as 1500 bytes on Ethernet) and iteratively adjusts downward as needed. PMTUD is defined in RFC 1191 for IPv4.

In IPv6, routers do not fragment packets. PMTUD relies on ICMPv6 "Packet Too Big" messages to inform the sender of the path MTU. The minimum MTU for IPv6 links is 1280 bytes, and hosts must discover path MTU down to that size. As with IPv4, PMTUD in IPv6 is vulnerable if ICMPv6 messages are blocked or filtered.

Limitations and risks include dependence on unblocked ICMP traffic, potential black holes when ICMP is filtered, and complications from tunneling, VPNs, or encapsulation that conceal the true path MTU. In such cases PMTUD may fail to converge or report an inaccurate MTU. Standards and related concepts include RFC 1191 (IPv4) and RFC 1981 (IPv6).