HTTP/2 & HTTP/3 Test
Check whether a website supports modern HTTP/2 and HTTP/3 protocols for faster, more efficient connections.
What This Tool Checks
A comprehensive HTTP protocol compatibility test for any website.
HTTP/2 Detection
Establishes a real HTTP/2 connection to the server to verify support, not just header sniffing. Tests actual protocol negotiation.
HTTP/3 Discovery
Checks the Alt-Svc response header for h3 advertisements, which indicate the server supports the latest QUIC-based HTTP/3 protocol.
ALPN Negotiation
Inspects the TLS ALPN (Application-Layer Protocol Negotiation) extension to see which protocol was negotiated during the handshake.
Performance Timing
Measures the total time to connect, negotiate protocols, and receive the first response from the server.
How It Works
Three simple steps to test HTTP protocol support.
Enter URL
Type a domain or URL. The tool automatically adds HTTPS if needed and extracts the hostname for testing.
Test Protocols
An HTTP/2 connection is established and a TLS ALPN check runs in parallel to verify protocol support.
View Results
See clear pass/fail indicators for HTTP/2 and HTTP/3, plus ALPN, Alt-Svc, server header, and timing details.
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Frequently Asked Questions
Common questions about HTTP/2 and HTTP/3 protocols.
What is the difference between HTTP/2 and HTTP/3?
HTTP/2 runs over TCP and uses multiplexing to send multiple requests over a single connection, eliminating head-of-line blocking at the HTTP level. HTTP/3 replaces TCP with QUIC (based on UDP), which eliminates head-of-line blocking at the transport level too, provides faster connection setup with 0-RTT, and handles packet loss more gracefully. HTTP/3 is especially beneficial on mobile and unreliable networks.
What are the performance benefits of HTTP/2?
HTTP/2 offers multiplexed streams (multiple requests/responses on one connection), header compression (HPACK), server push, and stream prioritization. These features reduce latency, eliminate the need for domain sharding and sprite sheets, and typically improve page load times by 10-50% compared to HTTP/1.1, especially for resource-heavy pages.
How is HTTP/3 support advertised?
Servers advertise HTTP/3 support through the Alt-Svc (Alternative Services) response header. When a browser sees an Alt-Svc header containing 'h3' (e.g., 'h3=":443"'), it knows it can upgrade future connections to HTTP/3 over QUIC. The initial connection still happens over HTTP/2 or HTTP/1.1, with the upgrade happening transparently on subsequent requests.
What is ALPN and why does it matter?
ALPN (Application-Layer Protocol Negotiation) is a TLS extension that allows the client and server to agree on which application protocol to use during the TLS handshake. Without ALPN, an extra round trip would be needed to negotiate HTTP/2. ALPN returns 'h2' for HTTP/2 or 'http/1.1' for HTTP/1.1, and is required for HTTP/2 support over TLS.