The problem is physics can't even distinguish whether light travels instantaneously in direction x, but at c/2 going in the opposite direction. There is no know

The problem is physics can't even distinguish whether light travels instantaneously in direction x, but at c/2 going in the opposite direction. There is no known way to synchronize clocks or come up with a clever apparatus to prove that light moves at the same speed in every direction. You need to make some strong assumptions in order to try and measure asymmetrical latency on a network. I believe the method you are proposing is to measure RTT when RTT differs depending on the starting node. The difficult problem is determining how much of your RTT time comes from node A to node B as opposed to node B to node A. I believe you need to at least have some sort of oracle on both sides which can synchronize time without using the network path you are trying to measure (I think you can do this if you have node C which both A and B have a fast connection to relative to each other, this implies that the delay between A and B is significantly longer than the speed of light delay).