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HTTP/3 Migration: Connection Establishment & Fallback

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HTTP/3: upgrading transparently from HTTP/2

HTTP/3 isn't the default — browsers start with TCP-based HTTP/2, then upgrade. The upgrade path involves Alt-Svc headers (server advertises HTTP/3 support) or HTTPS DNS records (discover HTTP/3 before the first TCP connection).

Once HTTP/3 is established, QUIC's connection IDs enable features TCP can't: 0-RTT resumption and seamless migration when the client's IP changes.

Packets in motion — click to inspect:

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