Assuming that one has measuring devices in every PoP, do you think it is harder to measure a full matrix of one way latency compared to measuring a full matrix of RTT?
Your problem with 1-way is finding a way to do it thats reliable, for rtt ping is about as simple as you can get and works a treat but for 1-way you have to be a bit more creative....
I assume that it's fairly common for people to have Solaris or Linux boxes in every PoP to do measurements. In that case, the difficulty isn't in measuring one-way latency, it's in synchronizing the time on all the servers. And with fairly cheap GPS and CDMA clocks that is a lot easier/cheaper than it once was. I really think that there are a lot of people out there who do not understand that RTT is not the same thing as two times the one-way latency. In other words if you measure (A-to-B + B-to-A) / 2 then you have failed to learn anything about one-way latency on either path A-to-B or B-to-A. Yet that's precisely what people are doing when they measure RTT and then assume that RTT/2 is equal to the one-way latency A-to-B. --Michael Dillon