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QUIC Folds Transport and Crypto Into One Handshake
TLS 1.3 runs inside QUIC, so the first flight already carries application data — and loses TCP head-of-line blocking.
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QUIC integrates TLS 1.3 as its handshake protocol instead of layering it on top of TCP. The result is 1-RTT connection setup (0-RTT on resumption), per-stream delivery, and connection IDs that survive IP changes.
The structure
- Initial packets carry the ClientHello; the server responds with its handshake and can send application data immediately.
- Streams are independent ordered byte streams; a lost packet blocks only its stream, not the connection.
- Packets are encrypted; header protection hides most of the header fields.
Costs
- Userspace implementation means CPU cost per packet and a large codebase.
- UDP is blocked or throttled on some networks; fallback to TCP/TLS remains necessary.
- 0-RTT data is replayable, same as TLS 1.3 early data.
Takeaway: QUIC's win is latency and multiplexing without TCP head-of-line blocking. It is not automatically faster on a clean, high-bandwidth link — measure.