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Multiplexing is important for soft-real-time applications like voice/video calls. Even for a one-on-one call, it isn't suited to a single stream. If you don't receive the information in time, it's not relevant anymore, and the most recent information shouldn't be delayed for it.
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Also works well for handling multiple audio / video streams well at the same time without needing a connection for each. TCP is not very good even if you really do only need a single blocking stream and it's horrible for soft real time use cases like this. Result is not pretty.
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Probably to get it shipped in a reasonable time. I don't think SCTP is great but it would probably take a long of time to design something without awful mistakes. QUIC got split from HTTP/3 as part of standardization so that it will be usable for use cases like this. I find it...
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HTTP/2 isn't strictly better than HTTP/1.1 because multiplexing over a single TCP connection has serious drawbacks. It's an overall improvement for browsers but it ends up making the problems with TCP much more apparent. So they made a problem that they had to solve with HTTP/3.
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