Path MTU
The path MTU is the smallest link MTU along the path from source to destination; it determines the largest packet that can be sent without fragmentation and requires a discovery mechanism to detect.
Purpose
It identifies the largest sendable packet size to avoid IP fragmentation, improving efficiency (fragmentation adds reassembly overhead and amplifies loss).
Mechanism
Path MTU = min(link MTUs) along the path. A packet larger than the PMTU is dropped by some router with an ICMP "packet too big" (IPv6), or, in IPv4, fragmented if the DF flag is not set.
Use cases
- TCP MSS negotiation
- Avoiding IP fragmentation
- Tunnel MTU
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