Question about propagation and queuing delays

Mon Aug 22 15:14:04 UTC 2005

This is interesting. This may sound like a naive question. But if
queuing delays are so insignificant in comparison to other fixed delay
components then what does it say about the usefulness of all the
extensive techniques for queue management and congestion control
(including TCP congestion control, RED and so forth) in the context of
today's backbone networks? Any thoughts? What do the people out there
in the field observe? Are all the congestion control researchers out
of touch with reality?

- Dave

On 8/22/05, Robert E. Seastrom <rs at seastrom.com> wrote:
> 
> 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
> 
> 
>