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

Efficient network transmission, especially in IPv6 since on-path fragmentation is not possible, depends on selecting the right packet size. The Maximum Transmission Unit (MTU) determines the largest packet size that can traverse a link and the Path MTU (PMTU) determines the size for an end-to-end path without fragmentation. Due to frequent failures in Path MTU Discovery (PMTUD), protocols must fall back to conservative packet sizes to ensure reachability, sacrificing potential throughput. This creates a need for inference based approaches that estimate a suitable path size. Existing PMTU studies are outdated, limited in scope, and rely on indirect MSS inference or PMTUD-dependent signals, offering little insight into where MTU bottlenecks occur along the path. This paper presents a measurement study of PMTUs using RIPE Atlas to characterize their behavior in general, across protocol layers (IPv4/IPv6) and network locations thus covering 76,700 paths across access networks, the Internet core, and web-serving infrastructure. We observe 97 distinct PMTUs, though a small set:[1500 B (80.5%), 1492 B (11.7%), and 1480 B (1.5%)], dominates. PMTU distributions differ across IP versions: while 99% of IPv4 paths support at least 1420 B, IPv6 paths commonly operate at 1280 B. We further show that sub-1500 B PMTUs are primarily located near network edges, while core paths rarely exhibit such constraints. Finally, we measure PMTU vs. Maximum Segment Size (MSS)-inferred PMTU mismatches in 18.5% of paths.