What is TCP flow control and how does the window size work?

Question

Accepted Answer

Flow control matches the send rate to what the receiver can handle: the receiver advertises a window size (bytes it can accept), and the sender may have only that much unacknowledged data outstanding. The window "slides" forward as ACKs arrive (sliding window).

* The receiver advertises a window; it slides as ACKs arrive and clamps to 0 when the buffer fills. *
TCP Flow Control:
Flow control manages the rate of data transmission to prevent the receiver from being overwhelmed.
Window Size:

The amount of data a destination can receive and process reliably
Advertised by the receiver in each segment
Sender limits unacknowledged data to the window size

How it works:

Step
Action

1
Receiver advertises window size (e.g., 10,000 bytes)

2
Sender may have up to that many unacknowledged bytes in flight at once

3
As ACKs arrive, the window slides forward and the sender can send more

4
Receiver shrinks/grows the advertised window as its buffer fills or drains

Sliding Window:

Window "slides" forward as data is acknowledged
Allows continuous transmission without waiting for each ACK

Maximum Segment Size (MSS):

Largest amount of data in a single TCP segment
Typically 1,460 bytes (Ethernet MTU 1500 - 20 IP - 20 TCP)
Negotiated during three-way handshake

Go deeper:

TCP: Flow Control and Window Size — visualizes how the advertised window paces the sender.
 RFC 9293 §3.8.6 — Managing the Window — the authoritative rules for window advertisement and the persist timer.
 TCP — Flow control — Wikipedia's sliding-window explanation for the carousel.

Step	Action
1	Receiver advertises window size (e.g., 10,000 bytes)
2	Sender may have up to that many unacknowledged bytes in flight at once
3	As ACKs arrive, the window slides forward and the sender can send more
4	Receiver shrinks/grows the advertised window as its buffer fills or drains