What does a TCP Window Size of 0 mean?

Window size 0 means "stop sending" - the receiver's buffer is full; the sender pauses until the receiver advertises a non-zero window (a window update).

Window Size = 0 means "Stop sending!"

When the receiver advertises a window size of 0, it's telling the sender to stop transmitting because its buffer is full.

What happens:

1. Receiver buffer fills up → sends Window = 0
2. Sender stops transmitting
3. Receiver processes data → buffer space frees
4. Receiver sends "Window Update" with new size
5. Sender resumes transmission

This is flow control in action!

Tip: Window = 0 is like saying "My inbox is full, wait until I clear some emails!"

Go deeper:

• RFC 9293 §3.8.6 — Managing the Window — the authoritative rules for the zero window, window updates, and the persist timer.
• TCP — Flow control — Wikipedia's explanation of the zero-window stall and the persist timer that breaks the deadlock.