The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Erickson Trejo-Reyes wrote:
> 
> By overbooking you mean that, for example, if a voice source can be
> treated as a VBR source with 64 KB peak bit rate and activity factor
> of 0.4, then the reservation should be made to a higher value than the
> computed effective bandwidth (and obviously higher than the average
> bit rate of 0.4 times 64KB)?

The idea behind overbooking is simple.  You accept requests for
resources that you don't have.

Under certain circumstances, this is OK.  For instance:

- You know that the overbooked reservations are temporary and will go
  away (eg. the overload is due to a failure somewhre and will be moved
  to another switch when the rest of the network realizes this and
  reroutes the tunnels.)

- You know that most of these tunnels aren't going to be running at peak
  capacity.  This is dangerous for an ISP to assume - especially when
  legal contracts demand a certain service level and you don't know the
  nature of the data, but it can be acceptible in a situation where you
  do know what kind of data will be flowing through the tunnels.

- You know that the tunnesl aren't used much during certain times of the
  day.  For instance, many corporate links (especially those that are
  not behind a publicly-accessible server) almost go idle at night, when
  most employees have gone home.  Again, you have to know your customers
  here.

> I had thought that, if an MPLS node is due to use only certain
> percentage of its total capacity to serve, for example, guaranteed
> services, it would not be too harmful to make reservations (up to the
> available percentage) only based on the sustained rate, with the only
> possible consequence of temporary squeezing the throughput of
> best-effort services. Comments?

Squeezing best-effor services due to a lot of reservations is not
overbooking.  That's just the nature of best-effort traffic.

Overbooking is where (for instance) a 10G switch grants an aggregate of
20G among all of its tunnels.  If the tunnels don't actually use more
than 10G total, you're OK - if not, then you cause their guarantees to
be violated.

Whether or not this is acceptible will depend on why you're overbooking
(is it something transitory that will quickly go away, or is it a
deliberate management decision), and what kind of service your customers
are actually paying for.

-- David