nghttp (HTTP/2 probe)
Negotiate HTTP/2 via ALPN and decode the server's SETTINGS frame (window size, max streams, etc.).
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What does the HTTP/2 Probe check?
It performs a TLS handshake with ALPN negotiation to confirm the server supports HTTP/2 (h2), then sends a client preface and decodes the server's SETTINGS frame. The result shows negotiated settings: initial window size, maximum concurrent streams, header table size, and maximum frame size — the parameters that govern HTTP/2 performance.
What is ALPN and why does it matter for HTTP/2?
ALPN (Application-Layer Protocol Negotiation) is a TLS extension that lets the client advertise supported protocols during the TLS handshake. For HTTP/2 over TLS (h2), the client includes h2 in its ALPN list; if the server supports it, the connection upgrades to HTTP/2 without an additional round trip. Without ALPN, HTTP/2 would require an extra HTTP Upgrade round trip on plaintext connections.
What does 'max concurrent streams' mean?
HTTP/2 multiplexes multiple requests over a single TCP connection using streams. The server's SETTINGS_MAX_CONCURRENT_STREAMS caps how many concurrent streams the client may open. A common default is 100–250. When a browser has more requests than the limit, it queues the extras — a bottleneck for very asset-heavy pages.
Does my site support HTTP/2?
Enter your hostname above. If ALPN negotiation succeeds with h2, your server supports HTTP/2. Most modern web servers (nginx ≥1.9.5, Apache ≥2.4.17, Caddy, LiteSpeed) support HTTP/2 when configured with a TLS certificate. HTTP/2 is not supported over plaintext (h2c) by browsers.