HTTP/3 compared to HTTP/2

HTTP/3 is designed for QUIC, which is a transport protocol that handles streams by itself.

HTTP/2 is designed for TCP, and therefore handles streams in the HTTP layer.

Similarities

The two protocols offer clients virtually identical feature sets.

  • Both protocols offer streams

  • Both protocols offer server push support

  • Both protocols have header compression, and QPACK and HPACK are similar in design.

  • Both protocols offer multiplexing over a single connection using streams

  • Both protocols do prioritization on streams

Differences

The differences are in the details and primarily there thanks to HTTP/3's use of QUIC:

  • HTTP/3 has better and more likely to work early data support thanks to QUIC's 0-RTT handshakes, while TCP Fast Open and TLS usually sends less data and often faces problems.

  • HTTP/3 has much faster handshakes thanks to QUIC vs TCP + TLS.

  • HTTP/3 does not exist in an insecure or unencrypted version. HTTP/2 can be implemented and used without HTTPS - even if this is rare on the Internet.

  • HTTP/2 can be negotiated directly in a TLS handshake with the ALPN extension, while HTTP/3 is over QUIC so it needs an Alt-Svc: header response first to inform the client about this fact.

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