What does the Alt-Svc header do?

Alt-Svc advertises alternative endpoints or protocols that serve the same origin — typically HTTP/3 over QUIC. The client may switch to the advertised service on its next request and remember it for the ma (max-age) duration, defaulting to 24 hours.

How does an HTTP Upgrade work?

The client sends Upgrade: together with Connection: Upgrade. If the server agrees it replies 101 Switching Protocols and both sides continue over the new protocol on the same TCP connection. This is how WebSocket connections are established.

What does h3=":443" mean in Alt-Svc?

It advertises HTTP/3 (h3) reachable on port 443 of the same host — the empty host before the colon means reuse the current origin's host. h2="alt.example:443" would point HTTP/2 at a different host and port.

Is HTTP2-Settings still used?

Rarely. HTTP2-Settings carried a base64url SETTINGS frame for cleartext HTTP/2 (h2c) upgrades. Browsers never adopted h2c and negotiate HTTP/2 and HTTP/3 via TLS ALPN instead, so the header is effectively legacy.

Does this tool send my header anywhere?

No. Alt-Svc parsing happens entirely in your browser with JavaScript. Nothing is uploaded, logged or stored.

HTTP Alt-Svc & Upgrade Reference

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Switching protocols mid-connection

Modern HTTP can negotiate a better protocol — HTTP/2, HTTP/3 or WebSocket — after a connection starts. The Upgrade mechanism switches the current connection in place and returns 101 Switching Protocols, while Alt-Svc points the client at an alternative endpoint to use on future requests. This reference covers those headers plus the TLS ALPN extension that actually drives h2/h3 selection in browsers, with a parser for any Alt-Svc value.

How it works

An Alt-Svc value is a comma-separated list of alternative services, each with a protocol id, an authority and optional parameters:

Alt-Svc: h3=":443"; ma=86400; persist=1, h2="alt.example.com:443"; ma=3600

Here h3 (HTTP/3 over QUIC) is offered on port 443 of the same host for one day and persisted across network changes, while h2 points to a different host. The parser splits each entry and reads its ma (max-age) and persist flag. The classic Upgrade flow is different — it is hop-by-hop and switches the live connection:

GET /chat HTTP/1.1
Connection: Upgrade
Upgrade: websocket
→ HTTP/1.1 101 Switching Protocols

In browsers, HTTP/2 and HTTP/3 are chosen during the TLS handshake via ALPN, so Upgrade survives mainly for WebSocket and Alt-Svc for steering to QUIC.

Tips and notes

Connection: Upgrade must accompany Upgrade, or proxies will forward it incorrectly.
An empty host in Alt-Svc (h3=":443") means the same host on that port.
ma defaults to 24 hours; persist=1 keeps the alternative across network changes.
Cleartext h2c upgrades are unused on the web — rely on ALPN and Alt-Svc.