Reliable streams above UDP
UDP supplies datagrams; QUIC adds a stateful connection and ordered unidirectional or bidirectional byte streams. Stream offsets organize data and identify duplicates. UDP packet boundaries do not become application message boundaries.
Streams can be interleaved without one total delivery order across them. Missing data on one stream need not prevent delivery of available data on another. Any application dependency between streams still has to be expressed separately.
References: [1]
A shared connection still has resource limits
QUIC limits individual stream data, aggregate connection data and the number of streams a peer may open. These protect receive buffers and concurrency; spare capacity elsewhere does not authorize exceeding a particular limit.
Independent delivery does not create independent unlimited network capacity. Congestion control still governs the connection, and loss, bandwidth and application processing matter. HTTP/3 uses QUIC streams, but HTTP methods, statuses and caching are not QUIC’s own semantics.
Connection IDs, migration and security
Connection IDs decouple connection identification from one current IP address and port, supporting client migration or NAT rebinding. Handshake state, path validation and peer parameters still constrain migration, and a new path affects congestion and RTT estimates.
QUIC integrates TLS for key establishment and protection without making application effects exactly once. Early 0-RTT data carries cross-connection replay risk; servers can reject it and applications must define replay-safe behavior. Faster resumption is not a duplicate-execution guarantee.