HTTP/3 Reachability
Probes six well-known endpoints and reports which protocol your browser actually negotiated. HTTP/3 (QUIC) runs over UDP and is often filtered or simply unreachable from networks that aggressively manage UDP traffic.
| Target | Protocol | Latency |
|---|---|---|
| Cloudflare anycast | … | — |
| Logrus | … | — |
How to read the table — h3 / h3-29 — HTTP/3 over QUIC, the path supports UDP-443. h2 — HTTP/2 over TCP, the fallback when QUIC isn't reachable or the server doesn't advertise Alt-Svc. http/1.1 — ancient, suggests a proxy stripping protocol upgrades. unknown — cross-origin response without Timing-Allow-Origin, so the browser hides the field.
Why HTTP/3 reachability matters
HTTP/3 carries traffic over QUIC, a UDP-based transport with 0-RTT handshake and built-in multiplexing. Most major CDNs advertise it via Alt-Svc, and modern browsers negotiate it automatically on the second visit to a domain. The catch: QUIC needs UDP-443 to pass cleanly. Many corporate firewalls, some ISPs, and most DPI systems either block UDP-443 outright or rate-limit it severely. In those environments the browser detects the failure and falls back to HTTP/2 over TCP-443 silently — the user only notices through slower connection setup. This tool fetches a tiny asset from each target and reads the negotiated protocol via the Performance API's `nextHopProtocol` field. h3 means QUIC won; h2 means the fallback fired. If a row shows 'unknown' it's not a failure — it's just that the cross-origin response didn't include Timing-Allow-Origin, so the browser hides the field per the spec.
Technical tool for diagnosing the quality of a network connection. Does not provide means of access to information resources whose access is restricted under applicable law.