A fast sender can easily overwhelm a slow receiver. If a server is downloading a massive file over a gigabit connection, but its hard drive can only write at 10 MB/s, the OS kernel will quickly run out of memory buffering the incoming packets.
TCP solves this with Flow Control. Every single ACK packet sent by the receiver includes a 16-bit field called the Receive Window (rwnd). This field tells the sender: "Here is exactly how many more bytes of buffer space I have available right now."
The Zero-Window
The sender is strictly forbidden from sending more unacknowledged data than the receive window allows. As the receiver's buffer fills up, its advertised rwnd shrinks. If the buffer gets completely full, the receiver advertises a Zero-Window (rwnd=0), which acts as a hard stop.
Receiver Overwhelm Simulator
Click "Send 200 Bytes" rapidly. Watch the receiver's buffer fill up and its advertised window shrink to 0. The sender will become blocked until you click "App Reads Data" to free up receiver buffer space.
Ready.
When the window hits zero, the sender stops transmitting data. However, how does the sender know when the window opens back up? If the receiver sends an ACK saying "rwnd is now 400", but that ACK is lost in the network, the sender would wait forever, and the receiver would wait forever for data.
To prevent this deadlock, when a sender is blocked by a zero-window, it periodically sends a Zero-Window Probe: a packet containing just 1 byte of data. The receiver is forced to ACK the probe and advertise its current window size. If it's still zero, the sender backs off and probes again later.
Window Scaling: The original TCP specification allocated 16 bits for the window size, meaning a maximum buffer of 65,535 bytes. On modern high-speed networks, a 64KB buffer is dreadfully small and heavily throttles throughput. To fix this, RFC 1323 introduced Window Scaling: a multiplier negotiated during the 3-way handshake that allows the window to scale up to 1 Gigabyte.
Check yourself
1. What happens when the receiver advertises a Receive Window (rwnd) of 0?
A zero-window simply means the receiver's OS buffer is full. The sender politely waits for the application on the receiver side to process some data and free up memory.
2. How does the sender discover that a zero-window has opened back up if the receiver's "Window Update" ACK gets lost in the network?
Because TCP does not retransmit ACKs (only data), a lost Window Update would cause a deadlock. Zero-Window Probes solve this by forcing the receiver to respond.