Routing & Path
Path MTU Discovery: Find Your Largest Unfragmented Packet
The Path MTU Discovery tool finds the largest IP packet size that can travel from your device to a destination without being fragmented. MTU mismatches cause stalled file uploads, broken VPN tunnels, and websites that refuse to load even though ping works fine.
What It Measures
This tool measures Path MTU (PMTU) in bytes by sending probes of decreasing size with the Don't Fragment bit set and observing which sizes succeed. It identifies the smallest MTU on any link in your path between client and server.
How It Works
- Sends probe packets at common MTU sizes (1500, 1492, 1452, 1400, 1280)
- Marks each probe with the Don't Fragment flag to force end to end delivery
- Detects which sizes succeed and which trigger ICMP Fragmentation Needed
- Reports the largest working MTU and identifies if a black hole is present
Why It Matters
PPPoE links, VPN tunnels, and IPv6 transition technologies all reduce MTU below the standard 1500 bytes. When firewalls block the ICMP Needs Fragmentation message, hosts never learn the smaller MTU and large packets silently disappear. Symptoms include hung uploads, broken HTTPS sites, and dead SSH sessions after the first prompt.
Understanding Your Results
1500 bytes is the standard Ethernet MTU and is ideal for most connections. PPPoE links use 1492. IPsec and OpenVPN typically reduce MTU to 1400 or lower. Anything under 1280 violates the IPv6 minimum and indicates a misconfigured tunnel. Consistent results across multiple destinations confirm correct discovery.
Frequently Asked Questions
What is an MTU black hole?
An MTU black hole is when a link in your path drops oversized packets without sending the ICMP Fragmentation Needed message back to the sender. The sender keeps retransmitting at the same size and never learns to shrink. Symptoms include working ping but failing large transfers. The fix is usually to lower MSS via TCP MSS clamping on your router.
Why does my MTU matter for VPNs?
VPNs add encapsulation overhead, reducing the effective payload size. If your VPN client does not negotiate MTU correctly, packets larger than the tunnel's MTU get dropped or fragmented. This is the most common cause of VPN connections that work for ping but fail for web browsing or file transfers.
How do I fix MTU problems?
On home routers, enable MSS clamping (often labeled MTU Fix or Clamp MSS to PMTU). For VPNs, manually set the MTU on the tunnel interface to a known good value like 1400. On Windows or Linux, you can configure interface MTU directly. Always retest after changes to confirm.
What is the difference between MTU and MSS?
MTU is the largest IP packet size for a link, including IP and TCP headers. MSS (Maximum Segment Size) is the largest TCP payload that fits inside an MTU sized packet, which is MTU minus IP and TCP header sizes (typically 40 bytes). MSS clamping rewrites the TCP MSS option in SYN packets to prevent oversized segments.
Related Tools
Browse all tools
Explore all 50+ network tools, from speed and latency to DNS, routing, and CDN diagnostics. Free, instant, no installation required.
See all network tools