Moving to HTTP/3: Alt-Svc

HTTP/3 runs on QUIC, and QUIC is not something a TCP connection can turn into: there is no Connection: Upgrade on the way to HTTP/3, the way there is one from HTTP/1.1 to HTTP/2. What there is instead is the server saying where else it can be reached (RFC 7838), and the client making its next connection there.

The server does that for its own HTTP/3 endpoint, which shares the address and port the TCP acceptor is listening on, so the advertised alt-authority is a port and nothing else. An empty host in one means “the host of the origin”:

Alt-Svc: h3=":8080"; ma=86400

It goes into every response sent over HTTP/1.1 and HTTP/2, but never over HTTP/3, which is already there. server::Config::alt_svc_max_age is how long a client may remember it, and 0s advertises nothing at all.

A client only acts on it with client::Config::follow_alt_svc set, and then it takes precedence over client::Config::protocol:

client::Client client(executor, {.url = url, .protocol = Protocol::h2, .follow_alt_svc = true});

auto [ec1, first] = co_await client.connect();  // HTTP/2, and learns about the alternative
auto [ec2, second] = co_await client.connect(); // HTTP/3

The session that learns about the alternative keeps speaking what it speaks, as a connection in the middle of a request can not be moved, and the alternative is remembered for as long as ma says, but only for the lifetime of the client::Client: there is no cache on disk. The origin does not change with any of this, only where it is reached: requests still go out with the authority of client::Config::url.

Over HTTP/2, an alternative may also arrive in an ALTSVC frame instead of a header field, which lets a server advertise before the first request has even been sent. anyhttp’s client reads both, and its server sends the header field only. parse_alt_svc() is the parser of either.

curl does the same thing, which is the easy way to watch it happen. It honours Alt-Svc for https:// origins only, so this needs TLS, and a cache file to remember the alternative between invocations:

build/src/server -p 8080

curl --alt-svc altsvc.txt --cacert pki/out/root.pem https://localhost:8080/echo -d hello -so/dev/null -w '%{http_version}\n'

The first run answers 2, and every one after it 3.