The MPLS WG Archive

["Steven Wright" <steven.wright@snt.BellSouth.com>]

When you talk of overbooking & CAC in an MPLS network are you talking about
the admission of flows to an LSP ( e.g. the packet classification) or the
admission/establishment of LSPs to an interface/link. These seem to be two
separate CAC decisions ...
regards
Steven Wright

> -----Original Message-----
> From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of David
> Charlap
> Sent: Tuesday, August 29, 2000 12:55 PM
> To: mpls@UU.NET
> Subject: Re: Fw: MPLS and CAC
>
>
> 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
>
>