5. Streams and Multiplexing

A "stream" is an independent, bidirectional sequence of frames exchanged between the client and server within an HTTP/2 connection.

Key Characteristics

• Streams can be established and used unilaterally or shared by either client or server
• Streams can be closed by either endpoint
• Order of frames sent on a stream is significant
• Streams are identified by an integer (Stream Identifier)
• Stream Identifiers initiated by clients are odd; by servers are even
• Stream ID 0 is reserved for connection control messages

5.1 Stream States

Streams transition through various states during their lifetime:

                         +--------+
                 send PP |        | recv PP
                ,--------|  idle  |--------.
               /         |        |         \
              v          +--------+          v
       +----------+          |           +----------+
       |          |          | send H /  |          |
,------| reserved |          | recv H    | reserved |------.
|      | (local)  |          |           | (remote) |      |
|      +----------+          v           +----------+      |
|          |             +--------+             |          |
|          |     recv ES |        | send ES     |          |
|   send H |     ,-------|  open  |-------.     | recv H   |
|          |    /        |        |        \    |          |
|          v   v         +--------+         v   v          |
|      +----------+          |           +----------+      |
|      |   half   |          |           |   half   |      |
|      |  closed  |          | send R /  |  closed  |      |
|      | (remote) |          | recv R    | (local)  |      |
|      +----------+          |           +----------+      |
|           |                |                 |           |
|           | send ES /      |       recv ES / |           |
|           | send R /       v        send R / |           |
|           | recv R     +--------+   recv R   |           |
| send R /  `----------->|        |<-----------'  send R / |
| recv R                 | closed |               recv R   |
`----------------------->|        |<----------------------'
                         +--------+

5.2 Flow Control

HTTP/2 provides flow control using WINDOW_UPDATE frames to prevent sender from overwhelming receiver.

Principles:

• Flow control is specific to a connection
• Flow control is directional
• Flow control is credit-based
• Initial flow-control window size is 65,535 octets
• Flow control applies to DATA frames

5.3 Stream Priority

Clients can assign a priority to streams using PRIORITY frames or HEADERS frames with priority information.

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