tags: web, http, quic, udp
QUIC is initially designed at google and aims to be successor of HTTP/2. It provides
reliable, ordered streams on top of unreliable, unordered UDP protocol.
UDP is connectionless so that the three-way handshake is avoided. And QUIC uses TLS 1.3 so that only 1-RTT first page latency. This improves loading time of new site dramatically.
UDP has small header and it gives chance to improve throughput a little.
UDP has no congestion control and flow control. I don't mean that this is advantage. But it do give opportunity to high level protocols like QUIC to implement their own congestion control and flow control.
Another important reason is the head byte block of TCP. We've heard of head byte block
of HTTP 1.1. However, this also affects HTTP/2. If one packet is lost or delayed in
TCP, all later packets are blocked since TCP gurantees reliability and ordering. Then
even though HTTP/2 supports multiplex, all streams may be blocked if one packet got lost.
With UDP, one packet losing or delaying only affects streams packed in this packet. It doesn't block all streams.
QUIC learns the multiplex feature of HTTP/2. A QUIC connection can carry many logical ordered streams, each stream is like an ordinary TCP connection. Both client and server can open a new stream. To avoid collision, server side uses even number from 2 as stream id while client side uses odd number. 0 is invalid and 1 is used for crypto handshake.
We know that a TCP connection is identified by 4 tuples of source ip, source port, destination ip and destination port. It will fail if any one of the 4 changed and then you need to reconect.
QUIC doesn't depend on any of them. It uses connectionId as identifier so that it will continue smothly while network migration.
We often see QUIC together with 0-RTT (round trip time). Is QUIC really 0-RTT all the time?
Actually, QUIC 0-RTT only works for resumption while 0-RTT was enabled. There is no connection handshake overhead since UDP but not TCP is used. And due to TLS 1.3, TLS handshake RTT is reduced to 1-RTT for new connection. So 1-RTT for new connection and 0-RTT for resumption if enabled (for security reason, 0-RTT may be disabled, see [here](./../../algorithm/crypto: TLS 1.3 and 0-RTT.md)).