TCP Congestion Control

The receiver may be perfectly capable of handling 1 Gbit/s. But the network between you might only have 20 Mbit/s available. Sending faster creates queues and packet loss. How does TCP avoid overwhelming the network itself?

The sender maintains a congestion window (cwnd), limiting how much data can be sent but still unacknowledged. In loss-based algorithms, acknowledgments allow this window to grow; signs of packet loss cause it to shrink. The sender obeys the smaller of the receive window and congestion window: if rwnd is 32 KB and cwnd is 10 KB, at most 10 KB can remain unacknowledged.