MTU Discovery
Path MTU Discovery (PMTUD, RFC 1191) sends probe packets with the DF flag and steps down to the PMTU based on ICMP "too big" feedback.
Purpose
It dynamically probes the path MTU so TCP sends packets no larger than the PMTU, avoiding fragmentation.
Mechanism
Packets are sent with DF=1; if one exceeds the PMTU, the router returns ICMP "fragmentation needed" carrying the next-hop MTU, and the source lowers the packet size and retries, eventually converging on the PMTU.
Use cases
- Automatic TCP MSS
- Avoiding fragmentation
- Tunnels and VPNs
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