[58446] in North American Network Operators' Group
Re: Is latency equivalent to RTT?
daemon@ATHENA.MIT.EDU (Eric Gauthier)
Wed May 14 11:04:40 2003
Date: Wed, 14 May 2003 11:02:30 -0400
From: Eric Gauthier <eric@roxanne.org>
To: nanog@merit.edu
In-Reply-To: <OF95FB83A0.8B2C8F7E-ON80256D26.004E9CE2-80256D26.004F537B@radianz.com>; from Michael.Dillon@radianz.com on Wed, May 14, 2003 at 03:26:27PM +0100
Errors-To: owner-nanog-outgoing@merit.edu
> 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.
What you've said is true, but there are times when RTT measurements can be
useful. If you are making measurements on your own backbone and know that
paths are either symmetric or have a well-know latency ratio, then RTT
might not be a bad measurement. Moreover, though this isn't really what
the original question regarding "latency" measures was about, there are
times when you don't care much about the absolute value of the RTT, but
rather you're looking for latency spikes (i.e. RTT(t) - RTT(t-1) > 20ms )
as a crude warning of either a topology change (i.e. my traffic that normally
goes direct from Boston to NYC is now going via Chicago) or of some sort of
link/router issues. When this happens, it might not tell you which path or
device is having a problem, but it is an easy monitor to set up to tell you
that something has changed...
Eric :)
