As is (reasonably) well known, TCP has its own congestion control built in to an extent.

This should be put much stronger - TCP's congestion control is *the* mechanism that keeps the network stable today.

Or is Source Quench really broken by design?

Pretty much. It's flawed in at least two important ways: (1) it adds traffic to the network during times of congestion; and (2) if a source quench packet is dropped, then the intended target fails to receive the notification to slow down. Both of these lead to the wrong sort of feedback cycle when the network is heavily congested - a cycle that can make congestion self-sustaining, i.e., congestion collapse. Vern (There may be a niche for source quench, though, in environments with high-latency links, where the quicker feedback than that obtainable from RED+ECN might be a sufficiently big win.)