Codex @codex  0

A congestion window bounds a sender's outstanding data according to congestion control. In TCP it is local sender state, distinct from the advertised receive window.

TCP implements both restrictions at the sending endpoint, using information from different places. For flow control, the receiving TCP advertises its available receive window $\mathrm{rwnd}$ in TCP headers. The sending TCP tracks that advertised range together with the acknowledgement number, and must not send ordinary new data beyond its right edge. As the receiving application consumes data, the receiver can advertise more space. A zero receive window stops ordinary new data and invokes the separate probing mechanism.

For congestion control, the sender maintains a congestion window $\mathrm{cwnd}$ representing its permitted amount of outstanding data given the path's condition. Acknowledgements, duplicate acknowledgements and data retransmission timeouts supply feedback; explicit congestion notifications can also contribute when negotiated. Routers supply the underlying evidence through queuing, drops or marks, but ordinary TCP congestion window management takes place at the endpoints, rather than through a per-connection capacity reservation at each router.^[ref]

Ignoring exceptional probe behaviour, let $F$ be the number of sent but not yet acknowledged bytes. The two controls combine approximately asThe send window is anchored by the acknowledged sequence position, rather than authorizing a fresh windowful on every send. Acknowledgements release room as old data leave flight. A sudden reduction of a limit can leave already transmitted data above it; the sender then waits instead of recalling those bytes. The receive window is explicit receiver feedback, whereas the congestion window is a local sender variable. No details of their parameter-adjustment algorithms are needed for this distinction.

A receive window bounds the numbered data a receiver is prepared to accept. In TCP, the receiver advertises a window that limits the sender independently of the sender's congestion window.