NAT type, path MTU, HTTP/3 and the route back to you
NAT, path MTU, HTTP/3 and the route back
A NAT router rewrites the addresses and ports of your traffic, and how it maps them decides whether peer-to-peer connections, calls and games work behind it. STUN binding requests to two ports of our own responder reveal the mapping and filtering behaviour, the public address seen over UDP and the source port on the wire.
The path itself is probed too: the largest packet that reaches you (path MTU), whether IPv6 extension headers survive the trip, whether HTTP/3 over QUIC gets through, and a reverse traceroute from this server back to your address, hop by hop, with the operators along the way.
What NAT type do I have?
Your browser sends STUN requests to two ports of our responder, and the reply tells you which public address and port each one was seen from. If both use the same mapping, the NAT is endpoint-independent, the friendly kind for peer-to-peer connections, calls and games. If the port changes with the destination, the NAT is address- or port-dependent, and connections have to be relayed more often.
What is path MTU?
The largest packet that reaches you without being fragmented. Plain Ethernet allows 1500 bytes, PPPoE lines 1492, and VPNs and tunnels take more off. The probe sends packets of decreasing size and reports the largest that arrived, for the TCP connection and separately for UDP, where QUIC has to live within it.
Does HTTP/3 work on my connection?
HTTP/3 runs on QUIC over UDP port 443, and some networks block or throttle UDP entirely. The page tries an HTTP/3 request to our server; if it succeeds, the QUIC transport parameters your browser sent, its HTTP/3 settings and the round-trip time QUIC measured appear next to the TCP figures.
What is a reverse traceroute?
A traceroute run from this server back to your address, hop by hop, with the operator of each router. An ordinary traceroute stops at your router because it drops unsolicited probes; here the probes travel inside the TCP connection your browser already holds open, shaped like the keepalives every stack sends, so the router forwards them to your device. It runs only when you start it and is rate-limited.
Why do IPv6 extension headers matter?
IPv6 puts options such as fragmentation and routing information into extension headers between the IP header and the payload. A share of networks drop any packet that carries one, which silently breaks fragmentation and some protocols. The test sends packets with extension headers and reports which of them got through.
Does ECN survive my path?
Explicit Congestion Notification lets routers mark packets instead of dropping them when a queue fills. Some middleboxes clear the bits or drop marked packets, which forces endpoints to fall back. The view shows whether ECN was negotiated on the TCP connection and whether the marks survive the UDP path that QUIC uses.
Related terms: NAT, NAT mapping behaviour, STUN, Path MTU, IPv6 extension headers, HTTP/3, Traceroute