Is latency equivalent to RTT?

Wed May 14 14:26:27 UTC 2003

>> 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