The TCP Bottleneck
In traditional HTTP/2, multiplexed streams travel over a single TCP connection. If one packet is dropped, TCP halts all streams until that packet is retransmitted. This issue, called Head-of-Line (HoL) blocking, degrades latency on unstable networks.
QUIC: Rebuilding Transport over UDP
QUIC shifts transport to UDP. It implements packet recovery and congestion management inside the application layer. Each HTTP stream inside a QUIC packet is handled independently; if one packet fails, only that stream halts, while others continue transferring.
Integrated TLS Handshake
QUIC combines the transport handshake and the TLS cryptographic handshake into a single exchange. This reduces connection startup from 2 or 3 round-trips to just 1. On reconnecting networks, QUIC supports 0-RTT, sending encrypted HTTP payload data in the first message.