HTTP/3 and QUIC support

Whether the server offers HTTP/3. It opens connections in fewer round trips, which shows up most on mobile networks. Optional.

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HTTP/3 is the newest version of HTTP. Instead of TCP it runs over QUIC, a UDP-based protocol with TLS 1.3 built in. A new connection needs fewer round trips before data flows, and one lost packet no longer holds up every other file on the same connection. The difference is most noticeable on mobile networks with high latency and packet loss; on a fast wired connection it is small.

How SiteScan checks it

Browsers learn about HTTP/3 from the Alt-Svc header, which advertises it for the next connection. The scanner reads that header on the home page: the HTTP/3 row of the Delivery & reputation group then reads “Offered” with a green tick, and HTTP/3 is also listed in the protocols row of the Technology group. “Not offered” comes with a grey dash, not a warning — HTTP/3 is optional, and a site on HTTP/2 is perfectly healthy.

How to enable it

• CDNs such as Cloudflare, Fastly or CloudFront offer HTTP/3 as a switch in the settings — the simplest route.
• nginx 1.25 and later: listen 443 quic reuseport; alongside the usual listen 443 ssl;, http3 on; and add_header Alt-Svc 'h3=":443"; ma=86400';.
• Open UDP port 443 in the firewall, otherwise browsers silently fall back to HTTP/2.

Browsers always keep HTTP/2 as a fallback, so enabling HTTP/3 does not lock anyone out. Speed itself is measured in Core Web Vitals.