Streams in QUIC provide a lightweight, ordered byte-stream abstraction.
There are two basic types of stream in QUIC:
Unidirectional streams carry data in one direction: from the initiator of the stream to its peer.
Bidirectional streams allow for data to be sent in both directions.
Either type of stream can be created by either endpoint, can concurrently send data interleaved with other streams, and can be canceled.
To send data over a QUIC connection, one or more streams are used.
Streams are individually flow controlled, allowing an endpoint to limit memory commitment and to apply back pressure. The creation of streams is also flow controlled, with each peer declaring the maximum stream ID it is willing to accept at a given time.
Streams are identified by an unsigned 62-bit integer, referred to as the Stream ID. The least significant two bits of the Stream ID are used to identify the type of stream (unidirectional or bidirectional) and the initiator of the stream.
The least significant bit (0x1) of the Stream ID identifies the initiator of the stream. Clients initiate even-numbered streams (those with the least significant bit set to 0); servers initiate odd-numbered streams (with the bit set to 1).
The second least significant bit (0x2) of the Stream ID differentiates between unidirectional streams and bidirectional streams. Unidirectional streams always have this bit set to 1 and bidirectional streams have this bit set to 0.
QUIC allows for an arbitrary number of streams to operate concurrently. An endpoint limits the number of concurrently active incoming streams by limiting the maximum stream ID.
The maximum stream ID is specific to each endpoint and applies only to the peer that receives the setting.
Endpoints use streams to send and receive data. That is after all their ultimate purpose. Streams are an ordered byte-stream abstraction. Separate streams are however not necessarily delivered in the original order.
Stream multiplexing has a significant effect on application performance if resources allocated to streams are correctly prioritized. Experience with other multiplexed protocols, such as HTTP/2, shows that effective prioritization strategies have a significant positive impact on performance.
QUIC itself does not provide frames for exchanging prioritization information. Instead it relies on receiving priority information from the application that uses QUIC. Protocols that use QUIC are able to define any prioritization scheme that suits their application semantics.
There have been criticisms of the HTTP/2 prioritisation model, and concerns it is overly complex and not used and implemented by many HTTP/2 servers. For now prioritisation in HTTP/3 has been removed from the main HTTP/3 specification and is being worked on as a separate specification.