The Internet's Path MTUs: How Big Can You Go?#

Tobias Fiebig, TU Wien, and Vaishnavi Raghavajosyula, Max Planck Institute for Informatics

• Why: PMTUD is unreliable, so protocols fall back to conservative sizes; knowing where the real MTU bottlenecks are — and that they're at the edges, not the core — is directly useful for interconnect and MSS-clamp decisions.
• What: A measurement of 76,700 paths (access, core, and web-serving) using a PMTUD-independent traceroute-based method: 97 distinct PMTUs, but three dominate — 1500 B (80.5%), 1492 B (11.7%), 1480 B (1.5%). 99% of IPv4 paths support at least 1420 B; IPv6 paths commonly operate at 1280 B. Sub-1500 breaks concentrate near network edges (1492 B maps cleanly to PPPoE overhead in fiber/DSL ASes) while core paths are rarely constrained. PMTU is asymmetric: reverse-direction PMTUs are larger in >92% of sub-1500 pairs (~64% of those are 1500 B). 18.5% of paths show PMTU-vs-MSS-inferred mismatches.
• How: RIPE Atlas anchors and probes, traceroute-based PMTU discovery independent of PMTUD (iterative search over a coarse size list), cross-checked against ICMP PTB hints and TCP-MSS, with heuristic attribution of break locations using AS, TTL and external network documentation.
• Where applicable: WAN/DC-interconnect MTU planning and any jumbo-vs-1500 end-to-end decision; the edge-vs-core finding means the core can safely run large MTUs while edge paths need clamping.