Constant HEADER_READ_TIMEOUT 

const HEADER_READ_TIMEOUT: Duration;

How long a client may take to send the head of its request, which only applies with a timer. A connection which never sends one would stay open otherwise. Clients send it right after they connected, and every connection serves one request, see MAX_CONNECTIONS. Far below the 30 seconds of hyper, so that a connection without a request holds its permit for a short time only. That does not bound how long a local process can hold the server off: one which keeps opening connections, holds all permits and keeps the backlog of the listener full makes the connection of another client wait behind its own ones, a few times UPGRADE_TIMEOUT, or get its handshake dropped and retried while the backlog is full, for as long as it keeps that up. It cannot use up the file descriptors of the server.