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.

What is the HTTP Alt-Svc & Upgrade Reference?

Reference for HTTP protocol-upgrade headers including Upgrade, Alt-Svc, ALPN and 101 Switching Protocols, plus a live Alt-Svc parser that splits entries into protocol, authority and max-age. It runs free in your browser on Gera Tools, with nothing uploaded.

HTTP Alt-Svc & Upgrade Reference

Name: HTTP Alt-Svc & Upgrade Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

When each mechanism is used

Mechanism	When it applies	Protocol examples
TLS ALPN	During TLS handshake — negotiates protocol before first byte	HTTP/2, HTTP/3
`Upgrade` header	On an existing clear-text connection; switches in place	WebSocket, h2c (cleartext HTTP/2)
`Alt-Svc` header	Advertises a better endpoint for future connections	HTTP/3 via QUIC
`101 Switching Protocols`	Server response confirming an `Upgrade`	WebSocket

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.

How browsers adopt HTTP/3 via Alt-Svc

A typical HTTP/3 advertisement looks like this in the response from a server that supports both HTTP/2 (current connection) and HTTP/3 (preferred future):

HTTP/2 200 OK
Alt-Svc: h3=":443"; ma=86400

The browser receives this on an HTTP/2 connection, notes the advertisement, and on its next request to this origin tries QUIC/HTTP/3 on port 443. If the QUIC connection succeeds, it switches and caches the result for the ma duration (86400 seconds = one day). If QUIC fails (firewall, middlebox), the browser falls back to HTTP/2 and suppresses the alternative for a while.

The persist=1 flag asks the browser to keep the alternative across network changes (for example, Wi-Fi to cellular handover). Without it, the cache is tied to the current network interface.

WebSocket upgrade flow

The WebSocket protocol repurposes HTTP/1.1’s Upgrade mechanism. The client signals intent by sending two required headers:

GET /socket HTTP/1.1
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

The server confirms by echoing Upgrade: websocket and returning 101 Switching Protocols. After that, the connection is no longer HTTP — it is a bidirectional WebSocket stream. The Connection: Upgrade header is hop-by-hop and must not be forwarded by proxies, which is why WebSocket connections require Connection: Upgrade rather than using a different mechanism.

Tips

Connection: Upgrade must accompany Upgrade, or proxies will strip it and the switch will fail.
An empty host in Alt-Svc (e.g. h3=":443") means the same host on that port — the colon is required.
ma defaults to 24 hours; persist=1 keeps the alternative cached across network changes.
Cleartext h2c upgrades are essentially unused on the public web — browsers negotiate HTTP/2 and HTTP/3 via TLS ALPN instead.
Use Alt-Svc: clear to tell clients to forget any advertised alternatives (useful during a migration rollback).