Path MTU Discovery
Path MTU discovery finds the smallest link capacity along a route so endpoints send maximally sized packets that avoid fragmentation, now via packetization-layer probes instead of ICMP dependence.
Definition
Path MTU discovery determines the largest packet a path delivers without fragmentation. Classic PMTUD relies on ICMP Destination Unreachable messages from routers that drop oversized packets; packetization-layer PMTUD instead sends probe packets of decreasing size and infers viability from acknowledgment or loss, robust against ICMP blackholes.
Context and scope
Critical for VPN and overlay tunnels that shrink effective MTU, for QUIC which mandates DPLPMTUD-style probing, and for diagnosing the classic large-packet hang where small packets pass but bulk transfers stall.
Common misconceptions
- Fragmentation solves MTU mismatches — modern stacks avoid in-transit fragmentation; endpoints size packets correctly instead
- ICMP unreachable messages always work — firewalls commonly block them, creating PMTUD blackholes
- The NIC MTU is the real limit — tunnels and overlays shrink the effective path MTU below interface values
Anti-patterns
- Ignoring tunnel encapsulation overhead and black-holing large packets over VPN overlays
- Firewalling all ICMP and wondering why large transfers hang while pings of small packets succeed
- Setting a global 1500 assumption and breaking jumbo-frame or encapsulated environments
Related concepts
Related examples
Authoritative source
What it is
Every hop has a maximum transmission unit; a packet larger than some hop cannot pass intact. Endpoints want the largest packet that fits, because bigger packets mean fewer per byte of data. Discovery finds that ceiling: try, observe, and adapt.
How it works
Classic PMTUD sets the do-not-fragment bit and trusts routers to report drops via ICMP. When those messages are filtered, senders never learn to shrink and connections hang. Datagram packetization-layer PMTUD sends application-level probes and infers success from delivery, requiring no ICMP at all.
When to care
Tunnel and overlay operators must account for encapsulation headers on top of the outer path MTU. QUIC implementations use DPLPMTUD by design. The symptom profile — small packets fine, large transfers stalling — should trigger MTU suspicion immediately.
Pitfalls
MTU mismatches masquerade as application hangs, timeouts, or partial file transfer. Allowing the right ICMP types, verifying with probe tools, and clamping MSS on tunnels are standard mitigations. Asymmetric paths can have different MTUs per direction, so test both.