Congestion Control

Described in detail in RFC2581

The "congestion window" fluctuates in response to congestion signals (typically packet losses)

Ideally the low point is just enough data to fill the network, such that there is always a queue somewhere

Requires "active queue management" [RFC2309]
explicit signals (drops or ECN) to regulate queue length
otherwise the control point is queue full
not well deployed (except ATM)

TCP can get it right across many orders of magnitude