round_trip测量方法是将两条相差很大的路径的性能一起测量，这样我们只知道总体性能，却不知道每条路径各自的性能，单独测量每条路径可以看出两条路路径的性能差异。即便这两条路径是对称的，由于排队时间的不同，他们的性能差别可能会很大。-round-trip measurements are two big difference to the performance of the path with the measurement, so we only know the overall performance, but do not know their performance each path, a single measurement of each path can be seen the performance of the two way path difference. Even if these two paths are symmetrical, due to the different queuing time, their performance difference may be significant.