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Flow Control

Overview

HTTP/2 uses two levels of flow control (RFC 7540 Section 5.2):

  1. Connection-level: shared across all streams
  2. Per-stream: independent for each stream

Both start at 65535 bytes (default initial window size).

Receive Side (client → server)

When the server receives DATA frames:

  1. Check payload size against connection receive window
  2. If exceeded → GOAWAY with FLOW_CONTROL_ERROR
  3. Deduct from connection window
  4. Send WINDOW_UPDATE on stream 0 (connection) to replenish
  5. Send WINDOW_UPDATE on stream ID to replenish
  6. Accumulate data in request body

The server immediately replenishes windows after consuming data, preventing backpressure from blocking clients.

Send Side (server → client)

When sending response DATA frames:

  1. Calculate sendable = min(remaining, maxFrameSize, connWindow, streamWindow)
  2. If sendable = 0 → stop; caller must retry after WINDOW_UPDATE
  3. Consume from stream send window via ConsumeSendWindow
  4. Deduct from connection send window
  5. Encode DATA frame header + payload
  6. Advance body offset

If the send window is exhausted mid-response, SendResponse returns early. The caller must call FlushData after receiving WINDOW_UPDATE frames from the client.

WINDOW_UPDATE Handling

Incoming WINDOW_UPDATE frames:

  • Stream 0: updates connRecvWindow (connection level)
  • Stream N: calls UpdateSendWindow on the stream's H2Stream

Zero-increment WINDOW_UPDATE is a protocol error.

Settings

The SETTINGS_INITIAL_WINDOW_SIZE parameter affects new streams. When the remote peer changes this setting, existing streams are NOT retroactively adjusted (simplified implementation).