The Largest Packet a Path Allows
The MTU (maximum transmission unit) is the biggest packet a link can carry. Different links along a route have different MTUs, so the safe size end to end is the smallest one, called the path MTU.
Discovering the Path MTU
Path MTU discovery finds this value without trial and error fragmentation.
- The sender marks packets do not fragment.
- If a router needs to forward a packet larger than its link MTU, it drops it and returns an ICMP fragmentation needed message stating the smaller size.
- The sender shrinks its packets to that size and continues.
When It Breaks
Some firewalls block the ICMP messages, so the sender never learns it must shrink. Packets vanish silently, causing connections that hang on large transfers, a black hole. Mitigations include packetization layer probing that infers the limit from lost data, and clamping the TCP maximum segment size at the edge.
Key idea
Path MTU discovery uses do not fragment packets and ICMP feedback to find the smallest MTU on a route, but blocked ICMP can create black holes that need clamping or probing to fix.