nanog mailing list archives
Re: Is latency equivalent to RTT?
From: Michael.Dillon () radianz com
Date: Wed, 14 May 2003 15:26:27 +0100
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
Current thread:
- Is latency equivalent to RTT? Michael . Dillon (May 14)
- Re: Is latency equivalent to RTT? Stephen J. Wilcox (May 14)
- Re: Is latency equivalent to RTT? Andrew Bangs (May 14)
- Re: Is latency equivalent to RTT? John Kristoff (May 14)
- Re: Is latency equivalent to RTT? Henk Uijterwaal (RIPE-NCC) (May 14)
- Re: Is latency equivalent to RTT? Barney Wolff (May 14)
- <Possible follow-ups>
- Re: Is latency equivalent to RTT? Michael . Dillon (May 14)
- NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) Steve Francis (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) Henk Uijterwaal (RIPE-NCC) (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) Joel Jaeggli (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) Sean Donelan (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) just me (May 14)
- NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) Steve Francis (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent bmanning (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) David G. Andersen (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) David G. Andersen (May 14)
- Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?) Rafi Sadowsky (May 14)