Is latency equivalent to RTT?

Wed May 14 14:35:16 UTC 2003

On Wed, 14 May 2003 15:01:06 +0100
Michael.Dillon at radianz.com wrote:

> Has it become common usage to define latency in an IP network as the
> round trip time in that network?

It probably depends on the context.

> 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?

Yes probably, because of the potential assymetric paths, changing paths
and synchronization issues.  See below.

> Does it even make sense to measure a full matrix of RTT when the 
> measurement of A to B to A should be equivalent to the measurement of
> B to A to B?

I think so.  It would probably be nice to do both if you could though. 
RTT may be particularly nice to measure, because the path between A to B
or between B to A may not be stable.  In addition, the endpoints may not
be stable either (e.g. if some type of load balancing is happening).

The problem with measuring one way latency is that the receiver has to
be synchronized with the sender.  The receiver also has to either
perform the collection of the data or return at minimum the received
timestamp.  This is all a little more complicated than a simple 'echo'
or 'ack' service used for RTT measurements.

John