What is Path MTU (PMTU)?
The path MTU is the largest packet size that can travel the entire route between two hosts without being fragmented — in other words, the smallest MTU found along the whole path.
The short version
Every link on a network has a maximum packet size, its MTU. The path MTU is the smallest of those values along the route your packets actually take. Send anything bigger and it has to be fragmented — or, if it can't be, it gets dropped.
When the ICMP messages that keep this in sync are blocked, you get a PMTU black hole: small packets sail through, large ones vanish, and connections stall. NetGlobe's Path MTU probe measures the usable path MTU to any endpoint so you can spot exactly that.
What Path MTU actually means.
MTU stands for Maximum Transmission Unit — the largest packet a given link can carry in a single frame. On standard Ethernet that's 1500 bytes, and most home and office networks inherit that number. Different link types use different values.
The path MTU is a narrower idea. A packet from your machine to a server rarely crosses a single hop; it passes through routers, tunnels, and provider links, each with its own MTU. The path MTU is the smallest MTU anywhere along that route — the biggest packet that can cross the entire path in one piece, without being fragmented into smaller ones.
How Path MTU Discovery (PMTUD) works.
Hosts don't guess the path MTU; they discover it. In IPv4, the sender sets the
Don't Fragment (DF) bit in the IP header and starts by sending full-size
packets. If a router down the line has a smaller MTU, it can't split the packet — the DF bit
forbids it — so it drops the packet and sends back an
ICMP "Fragmentation Needed" message (Type 3, Code 4) that
reports the smaller MTU it can handle.
The sender reads that value, lowers its packet size to fit, and retransmits. Repeat as needed, and both ends converge on the largest size the whole path can carry. (IPv6 works the same way in spirit, except routers never fragment at all and signal the limit with an ICMPv6 "Packet Too Big" message instead.)
Why it matters: PMTU black holes.
PMTUD only works if those ICMP messages get through. Plenty of firewalls and routers are configured to block ICMP wholesale — and that quietly breaks things. The sender never receives the "Fragmentation Needed" notice, so it keeps sending packets that are too big. Those packets are silently discarded down the path while small packets pass fine. That failure mode is called a PMTU black hole.
The symptom is distinctive: a connection establishes and then hangs. The initial handshake and other small packets succeed, so the session looks alive — but the moment real data flows, it stalls. Web pages half-load, an SSH session connects and then freezes when you run a command with lots of output, and VPNs behave erratically. Tunnels, VPNs, and PPPoE are common culprits because they add headers that lower the effective MTU — PPPoE typically drops it to 1492 bytes, and VPNs often lower it further.
Classic black-hole symptoms
- A connection opens instantly, then hangs the second a large transfer begins.
- Pages half-load; SSH connects but freezes on verbose output; VPNs act flaky.
- Small pings succeed, but a large-payload ping with DF set fails.
Path MTU in NetGlobe.
NetGlobe includes a Path MTU probe in its Endpoint Focus panel. Click any connection on the live map and NetGlobe measures the usable path MTU to that endpoint — the real largest packet size that survives the trip, not just your local interface's MTU. When the probe reports a value well below 1500, that's a strong hint you're dealing with a tunnel, a PPPoE link, or an outright black hole caused by blocked ICMP.
NetGlobe is network intelligence and diagnostics, not a firewall — it doesn't change your MTU or block traffic. It surfaces the number alongside a live MTR trace, WHOIS/RDAP ownership, and TLS details, so you can pinpoint where large packets are dying and fix it at the right hop.
Keep exploring the glossary
Path MTU — FAQ
What is the difference between MTU and Path MTU?
MTU is the largest packet a single link can carry — 1500 bytes on standard Ethernet. Path MTU is the smallest MTU along the entire route between two hosts, so it's the largest packet that can cross the whole path without fragmentation. One link's MTU can be 1500 while the path MTU is lower because of a tunnel or a slower link somewhere in the middle.
What is a PMTU black hole?
A PMTU black hole happens when the ICMP "Fragmentation Needed" messages that Path MTU Discovery relies on are blocked by a firewall or router. The sender never learns to shrink its packets, so oversized packets are silently dropped while small ones get through. Connections establish but then hang on large transfers.
Why do large downloads stall but pings work?
Pings and the initial handshake use small packets that fit within any link's MTU, so they succeed. Large downloads use full-size packets, and if the path MTU is smaller than the sender assumes — and the ICMP feedback is blocked — those big packets are dropped. The result is a connection that opens fine but stalls the moment bulk data flows. That's a classic Path MTU black hole.
What's a typical MTU?
On standard Ethernet the MTU is 1500 bytes, and most local networks use that value. It drops over other link types: PPPoE connections are typically 1492 bytes, and VPN or other tunnels are often lower still because their encapsulation headers eat into the payload. That's why the path MTU to a remote host is frequently below 1500.
Does NetGlobe check Path MTU?
Yes. NetGlobe includes a Path MTU probe in its Endpoint Focus panel that measures the usable path MTU to an endpoint, which helps diagnose fragmentation issues and black holes. NetGlobe is network intelligence and diagnostics, not a firewall, so it reports the path MTU rather than changing it — a one-time $18.99 on both Windows 10/11 and macOS 11+.
Path MTU, MTU, PMTUD, and ICMP are standard networking concepts described in RFC 1191 and related specifications. This page is an educational glossary entry; exact MTU values and behavior depend on your network hardware, link types, and configuration.
Measure path MTU — and every connection your machine makes.
A one-time $18.99 — on the Microsoft Store for Windows, or a direct download for Mac.
No account. Runs entirely on your device. See the full feature list or the FAQ.