North American Network Operators Group
Date Prev | Date Next |
Date Index |
Thread Index |
Author Index |
Historical
Re: Is latency equivalent to RTT?
- From: Michael.Dillon
- Date: Wed May 14 10:29:05 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
|
