$ path mtu discovery
Find the largest packet that reaches a host without fragmentation, using ICMP echo with the Don't-Fragment bit set and a binary search (about 11 probes). It measures the path from this server to the target, not from your own connection. Below it: an MSS calculator for common link types. The target must answer ping.
About the mtu tool
Path MTU discovery finds the largest packet that travels from this server to a target without being fragmented. The tool sends ICMP echo requests with the Don't-Fragment bit set and binary-searches the size between 576 and 1500 bytes, then reports the MTU and the matching TCP MSS. A calculator below lists MTU and MSS values for common link types.
How to read the results
A path MTU of 1500 is normal Ethernet. Lower values mean a link on the path, often a VPN, tunnel or PPPoE line, shrinks packets. If large transfers stall while small requests work, the MSS or MTU on that tunnel is usually wrong and ICMP fragmentation-needed messages are being blocked. MSS is MTU minus 40 for IPv4 (60 for IPv6).
FAQ
- What is MTU?
- The maximum transmission unit: the largest packet, in bytes, a link carries without fragmenting. Ethernet is 1500.
- What is MSS and how is it related?
- The TCP maximum segment size is the largest payload per TCP segment: MTU minus the IP and TCP headers (40 bytes on IPv4).
- Why does my VPN need a lower MTU?
- Tunnels add headers, so the inner packet must be smaller. WireGuard commonly uses 1420 and PPPoE 1492.
- Why can this tool not measure some hosts?
- It needs the target to answer ping. Hosts that drop ICMP, or paths that filter fragmentation-needed errors, cannot be measured this way.