"One thing that appears to be missing from the IP protocol is a concrete way to indicate network congestion to endpoints by signals originating at the affected point in the intervening infrastructure."
There's always ECN, although there's the problem that some NAT routers discard every TCP packet that has that "unknown option" of "I support ECN". Apparently apple are going ECN enabled by default which might be interesting.
"It is achieved by ramping up the packet rate until the round trip time starts to increase, as that is when the packets start to fill the queues in the routers along the route."
Detecting congestion has nothing to do with monitoring latency. Increased latency will not cause TCP to slow down - it will assume there is still more bandwidth and keep increasing the data rate until the buffers completely fill and start dropping packets. Oversized buffers in networking equipment is a serious problem.