HTTP/3 Test

HTTP/3 is the newest version of the protocol that carries the web. It replaces TCP with QUIC, a transport built on UDP, and it is already used by a large share of big sites. Whether your own browser uses it depends on the browser, the server, the network between them and, sometimes, a firewall that blocks UDP. The page reports what happened for this site.

What HTTP/3 changes

HTTP/2 multiplexes many requests over one TCP connection, so a lost packet delays all of them until it is retransmitted, a problem called head-of-line blocking. HTTP/3 runs over QUIC, where each stream is independent, so a lost packet delays only the stream it belongs to. QUIC also folds the handshake and encryption setup together, so connections open in fewer round trips, and it survives a change of network, for example from Wi-Fi to mobile, without breaking the connection. The gains are largest on lossy, high-latency or mobile connections.

How a browser gets to HTTP/3

A browser does not try HTTP/3 blindly. It first connects with HTTP/2 or HTTP/1.1 over TCP. The server’s reply can include an Alt-Svc header advertising HTTP/3 on a UDP port, or the site can publish the same hint in a DNS record. The browser remembers it and attempts QUIC on later connections, falling back to TCP if UDP is blocked or fails. That is why a protocol test can show h2 on the first visit and h3 after a reload.

What the page can read

The test uses this page and a few requests to this site only, and nothing is sent to any other server. It reads nextHopProtocol, the value the browser records in its performance data for each request: h3 for HTTP/3, h2 for HTTP/2, http/1.1 for the old protocol. It also shows whether the page is a secure context (HTTPS, which HTTP/3 needs) and what the Network Information API reports about the connection, such as a type, rough round-trip time and downlink speed, where the browser provides it. The TLS version is not available to web pages in any browser.

Reading a result

h3 means your browser, network and our server agreed on HTTP/3. h2 is not a problem: it may mean the connection was new, the browser has HTTP/3 turned off, the network blocks UDP port 443, or the browser has not yet learned of the HTTP/3 service. Reload the page once and test again. http/1.1 suggests an old browser, a proxy or a security product that terminates and re-creates the connection. Corporate firewalls, and some VPNs, commonly force fall-back to TCP.

A worked example

Open the page in a current browser on an ordinary home network and press the button. The first test may report h2. Reload and run it again: if the connection upgrades, the reported protocol becomes h3, which confirms that UDP on port 443 is reaching the site and QUIC works. If it stays at h2 after several reloads, check the browser’s settings for QUIC and whether a firewall or VPN blocks UDP.

Limits

The check covers this site only, because browsers hide nextHopProtocol for other origins. For other sites use the Protocol column in your developer tools. The page cannot measure the speed gain, report the TLS cipher or diagnose why UDP is blocked. For delay measurements use the Network Jitter Test.

Related tools

• Network Jitter Test measures the delay and its variation to this site.
• Port Number Lookup shows port 443 is used by HTTPS over TCP and QUIC over UDP.
• TCP Window Calculator explains limits of TCP that QUIC avoids.
• HTTP Status Reference lists the status codes carried over any HTTP version.