Contention/Oversubscription maths

Jay Ashworth jra at
Fri May 27 14:23:04 UTC 2011

----- Original Message -----
> From: "Adam Armstrong" <lists at>

> I'm more interested in the levels of traffic that we will see
> consistently.

You're planning to engage in Statistical Multiplexing, or what I've always
termed "bandwidth surfing": how hard can I oversubscribe my uplink without
pissing off the paying customers?

As others have suggested, it depends on quite a number of things, primarily:
whether you're offering an SLA to the customers or not.  Whether you have
a caching proxy or not and which CDNs you solicit to provision your node
are also big factors, of course.

In the final analysis, though, it depends on the customer class.

Residence customers will tolerate a lot more oversubscription than business,
enterprise, and server going on down the list of oversubscription, but
happily *up* the list of "how much can I charge".  Remember that QoS and 
load shaping don't work all that will across the internet at large,
but they work pretty decently inside a single switch; you can prioritize
customers who are willing to pay extra for it.  The problem is similar
to airline bookings; it is possible, in the immortal words of Dave Barry,
to envision a situation -- this will happen in your lifetime -- where
no two customers pay exactly the same price.  :-)

-- jra
Jay R. Ashworth                  Baylink                       jra at
Designer                     The Things I Think                       RFC 2100
Ashworth & Associates         2000 Land Rover DII
St Petersburg FL USA             +1 727 647 1274

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