What is Path MTU Discovery and why must it be dynamic?
Path MTU Discovery finds the smallest MTU along the route so the source can size packets to avoid drops — and it must adapt because the path (and thus the MTU) can change mid-conversation.
* PMTUD probing loop driven by ICMPv6 Packet Too Big. *
Since routers won't fragment, the source must learn the smallest MTU between itself and the destination (the "path MTU"):
- The source sends packets and, if one is too big, a router replies ICMPv6 "Packet Too Big" with the link's MTU
- The source lowers its packet size accordingly and retries
- It keeps probing until packets reach the destination
It has to be dynamic because routing can change during a session — a new path may have a smaller MTU, so the source must re-discover and adjust on the fly.
Tip: This is exactly why blocking ICMPv6 "Packet Too Big" causes the infamous "PMTU black hole" — connections that hang because the source never learns to send smaller packets.
Go deeper:
Path MTU Discovery (Wikipedia) — ICMPv6 "Packet Too Big", dynamic re-discovery, and the PMTU black-hole failure mode.- RFC 4443 §3.2 — ICMPv6 Packet Too Big — the exact message that drives PMTUD.