It doesn't need to be as complicated as pathchar, because of a lucky fact: if you measure the end to end bandwidth, you are always measuring the bottleneck band

It doesn't need to be as complicated as pathchar, because of a lucky fact: if you measure the end to end bandwidth, you are always measuring the bottleneck bandwidth :) So you can just count how many packets arrive at the other end between two time points and call that the bottleneck bandwidth.