The MPLS WG Archive

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

Aimin Sang wrote:
> 
> Could you please give us a more detailed example (when "this is OK")
> to set overbooking factor? Say: The overbooked reservation is
> temporary because of the re-routing, but who set the overbooking
> factor and how? It is set by the network operator from time to time,
> or the CAC algorithm automatically?

I would hope that every implementation would require the operator to set
this factor.

> Further, I guess you mean that the CAC's decision of overbooking can
> be balanced by traffic engineering. If that is right, are these two
> control at the same time-scale?

The idea here is not that you want the switch to remain overbooked, but
that temporary overbooking is better than lost connections.

If a link fails, a fast-reroute algorithm may shunt the LSP onto another
router.  Later on, when the routing tables stabilize, the ingress router
(perhaps in response to an operator command) may choose to reroute that
LSP again, onto a less-heavily trafficked switch.

If a lot of LSPs get shunted onto a single fallback router, this router
may not have enough resources to maintain all of their original QoS
levels.  At this point, the failover switch has one of two choices.  It
can refuse to accept those LSPs that it can't satisfy, or it can
overbook them.

If it refuses to accept the LSPs, then those connections will go down
until the ingress router can re-signal with a new path.  The LSPs that
did get rerouted will continue to operate at their former QoS levels.

If it overbooks, then none of the connections will go down, but the QoS
on all of them will suffer until the ingress node can reroute them
elsewhere.

There are advantages to either scenario.  For some networks, it is
better that all connections remain up, at reduced quality.  For some
networks, it is better that some connections go down, so that the rest
remain at their normal quality levels.

As for how to actually implement overbooking, I'll leave that part of
the answer to someone with more experience in this area.

-- David