Every link limits how large one transmitted frame or packet can be. At the IP layer this practical limit is expressed as a maximum transmission unit (MTU).

If an IP packet is too large for a link, the network must either fragment it where the protocol permits or report that it cannot be forwarded at that size. IPv6 routers do not fragment transit packets; endpoints are expected to adapt.

Path MTU Discovery tries to learn the smallest usable MTU along an end-to-end path. A sender transmits appropriately constrained packets and reacts to feedback indicating that a packet is too large.

For example, if an endpoint assumes 1500-byte IP packets but a tunnel along the path only carries 1400 bytes, it must reduce packet size or segment transport data differently.

MTU problems illustrate why end-to-end behavior depends on constraints of every link along the route, not only the sender's local network.