QUIC
/ quick /
Imagine you order several different things from one shop in a single trip. With the old way of doing things (TCP), the delivery van carries everything in one long queue: if the first box gets lost and has to be re-sent, every box behind it must wait too, even the ones that already arrived safely. That stuck-behind-the-front-of-the-line problem is called head-of-line blocking. QUIC is a newer transport designed so that a delay on one item does not freeze all the others — each stream of data moves on its own track.
Concretely, QUIC is a reliable, encrypted, multiplexed transport that runs on top of UDP, but does the clever work in user space (inside the application or its library) rather than in the operating-system kernel. It gives you what TCP gives you — in-order, reliable delivery and congestion control — but it carries many independent streams inside one connection, so a lost packet only stalls the stream it belonged to, not the rest. It also folds the connection setup and the TLS encryption handshake together: where old TCP-plus-TLS needed a round trip to set up the connection and then more round trips for encryption, QUIC sets up a secure connection in as little as one round trip (and zero round trips when reconnecting to a server you have spoken to before). And because every QUIC connection has its own ID, your session can survive your phone switching from Wi-Fi to cellular without breaking.
Why it matters: QUIC is the foundation of HTTP/3, the version of the web's main protocol now used by a large share of traffic to big sites. A common misconception is that QUIC abandons reliability because it sits on UDP — it does not. UDP is just the thin, unencrypted base it builds on; QUIC re-implements reliability, ordering, and congestion control itself, plus mandatory encryption. The real reason it lives on UDP and in user space is that TCP is baked into operating-system kernels and middleboxes worldwide, which makes it almost impossible to change; building on UDP let QUIC's designers evolve a modern transport quickly without waiting for the whole Internet to upgrade its kernels.
A web page loads images, scripts, and stylesheets at once. Over HTTP/2 on TCP, if a single packet carrying one image is lost, the whole connection stalls until it is re-sent — every other resource waits. Over HTTP/3 on QUIC, only that one image's stream pauses; the scripts and stylesheets keep arriving and the page keeps rendering.
Per-stream delivery removes TCP's head-of-line blocking.
QUIC's encryption is not optional the way TLS is bolted onto TCP — it is built in and mandatory, including encrypting most of QUIC's own headers. But QUIC is not magically faster for everything: on a clean, low-loss link a well-tuned TCP+TLS connection performs similarly, and QUIC's user-space processing can cost more CPU per byte.