Expose the QUIC connection stats as an Extension

[passcod]

With HTTP/1 and HTTP/2, it's possible to get information about the TCP connection by obtaining the (local_socket, remote_socket) tuple. However, with HTTP/3 it's not possible, as the connection exists entirely in userspace.

Would it be possible to add an extension retrievable through the Response that exposes https://docs.rs/quinn/0.11.7/quinn/struct.ConnectionStats.html? I'm happy to do the work, just wanted to ask first.

[seanmonstar]

Oh yea, interesting idea! So, while I like the thought, here's a constraint we'd need to keep: not exposing (most) internal dependencies as public types. Basically, we want to be able to upgrade TLS, quinn, h3, etc without that being a 'breaking change' to our public API.

[passcod]

Oh, that's true. We could "simply" copy the entire struct in a different type. Looking closer at that struct, there's a lot of information, but the stuff that maps most closely to TCP connection info is just the PathStats structure.

That leads to a different thought: perhaps instead of having it be H3-only, have reqwest extract a small subset of connection information for both TCP and QUIC (not available for unix sockets or windows named pipes) to start with e.g.

#[non_exhaustive]
pub struct ConnectionKind {
	Tcp,
	Quic,
}

#[non_exhaustive]
pub struct ConnectionInfo {
	kind: ConnectionKind,
	local_addr: SocketAddr,
	remote_addr: SocketAddr,
	created: Instant,

	rtt: Duration, // maps to quinn's rtt and tcpinfo's rtt_us
	cwnd: u64, // maps to quinn's cwdn and tcpinfo's cwdn
	lost_packets: Option<u64>, // maps to quinn's lost_packets and tcpinfo's lost (Linux-only)
}

and as it's non-exhaustive, later be able to add more fields mapped from whatever transport.