HTTP/3 и QUIC: ментальная модель

Question

Accepted Answer

HTTP/3 = HTTP over QUIC (UDP): per-stream ordering kills TCP HOL; TLS folded in; Connection ID survives IP change.

## Разбор

HTTP/2 multiplexing, one flaw: streams share **one TCP**, TCP delivers strictly in order. One lost packet freezes *all* streams — **TCP head-of-line blocking**. Can't patch from inside TCP — no stream concept.

QUIC: **stop TCP.** UDP datagrams + rebuild reliability **per stream**. Lost packet stalls only owning stream; others flow.

```
HTTP/2 over TCP                 HTTP/3 over QUIC (UDP)
──────────────────              ──────────────────────
[A][B][C][ x ][A][B]            [A][B][C][ x ][A][B]
             ▲                               ▲
   packet loss stalls           only stream that lost a
   ALL streams (TCP HOL)        packet stalls; B, C proceed
```

Plus 1-RTT handshake, 0-RTT resume, **connection migration** Wi-Fi ↔ cellular.

## Итог

HTTP/3 = same HTTP semantics, QUIC transport fixes TCP HOL and connection fragility.