HTTP 2.0 - one TCP/IP connections vs 6 parallel

Question

Its said that one of the advantages of HTTP 2 over HTTP 1 is that HTTP2 has streams of data. It can be up to 256 different streams in ONE TCP/IP connection. However, in HTTP 1 there can be up to 6 parallel connections. It's an improvement that HTTP 2 enables reading data from 256 resources, but still I think that 6 connections (in HTTP 1) have a better throughput that one TCP/IP connection (in HTTP 2). Still, HTTP2 is considered faster than HTTP 1. So... what don't I understand correctly ?

Answer 1

6 physical connections would have more throughput than one physical connection, all else being equal.

However the same doesn't apply to 6 different TCP/IP connections between the same computers, as these are virtual connections (assuming you don't have two network cards). The limiting factor is usually the latency and bandwidth of your internet connection rather than TCP/IP protocol itself.

In fact, due to the way TCP connections are created and handled, its actually much more efficient to have one TCP/IP connection. This is because of the cost of the initial connection (a three way TCP handshake, the HTTPS handshake and the fact that TCP connections use a process called Slow Start to slowly build up its capacity to the maximum speed that the network can handle) but also in the ongoing upkeep of the connection (as the Slow Start process happens again periodically unless the connection is fully utilised all the time - which is much more likely to happen with one connection that is used for everything, than it is to happen when your requests are split across 6 connections).

Also HTTP/1.1 only allows one request in flight at a time, so the connection is not able to be used until the response is returned (ignoring pipelining which is not really supported at all in HTTP/1.1). This not only limits the usefulness of the 6 connections, but also means that it's more likely that the connections will be underused which, given the issues with underused connections in TCP mentioned above, means they are likely to be slower as they throttle back and have to go through the Slow Start process again to build up to maximum capacity. HTTP/2 however allows those 256 streams to allow requests to be in flight at the same time. Which is both better than just 6 connections, and allows true multiplexing.

If you want to know more, then Ilya Grigorik had written an excellent book on the subject called High Performance Browser Networking which is even available online for free.