Question about propagation and queuing delays

Mon Aug 22 05:17:43 UTC 2005

David Hagel <david.hagel at gmail.com> writes:

> Would there be any data out there on what fraction from this 60ms to
> 80ms RTT is raw propagation delay and what fraction is typical packet
> queuing delay at intermediate switches? Does queuing delay play much
> of a role at all these days? Or is it all just propagation delay?

With any kind of reasonably fast circuit and modern routers, you may
safely ignore queuing delay.  The two following rules of thumb apply:

Queueing delay (time the packet sits in memory waiting to get clocked
out the port) is insignificant when the circuit is not more than "kind
of full".  The cutoff point for "kind of full" ranges from 60% to >97%
full as circuit speed increases from DS1 to OC48.

Clocking delay (time it takes to put the packet on the wire) takes 7
milliseconds for a 1500 byte packet on a DS1, 266 microseconds on a
DS3, and correspondingly less on faster circuits.  Again,
insignificant in the context of a transcontinental link.

You may find Peter Lothberg's presentation at NANOG 22 enlightening.
Check out the slides at http://www.nanog.org/mtg-0105/lothberg.html
especially the queueing delay graphs starting at 9:30 into the
presentation.

                                        ---Rob