I understand that http2 uses one tcp connection to serve multiple requests, for example, if I request index.html which contains a.css and a.js, these three requests will be done in one tcp connection.
What happens if user clicks index2.html? does this request still use the same previous tcp connection? If so, will the browser keep the connection open until user closes the browser? And on the server side, does the server keep many connections open all the time?
When using HTTP/2, browsers typically open only one connection per domain.
In your example, index2.html will be sent on the same TCP connection that was used for index.html, a.css and a.js.
In HTTP/2 requests are multiplexed on the same TCP connection, so that the browser can send them concurrently, without waiting for a previous request to be responded to.
Both browsers and servers have an idle timeout for TCP connections. If the connection is idle for long enough, it will be closed by either party - the one that has the shorter idle timeout, to save resources. For example, you may open a connection to a wikipedia.org, perform a few requests, and then leave that tab and work on something else. After a while (typically 30 seconds) the browser will close the TCP connection to wikipedia.org.
On the server side, the server will keep the connections from various clients open, until they are either closed by the client or until the server-side idle timeout fires, at which point it's the server that initiated the close of the TCP connection.
With HTTP/2, the number of connections that a server has to maintain is vastly less than it was with HTTP/1.1. With HTTP/2, a server has to maintain just 1 TCP connection per client; with HTTP/1.1, the server had to maintain typically 2-8 TCP connections per client.
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