The MPLS WG Archive

[Daniel Awduche <awduche@UU.NET>]

Angela,

See comments inline...

On Mon, Apr 24, 2000 at 04:42:21PM -0400, Chiu, Angela L, ALSVC wrote:
> Hansen and Dan,
> 
> I just want to add one other point. When a network operator wants to use a
> small set of LSPs to remove a few hot spots in the network assuming all
> other links having satisfactory performance, an offline tool may be the
> preferred way to identify which traffic trunks that contribute to those
> congestion points should be re-routed away from those hot spots, and then
> figure out what the new non-shortest path should be, and leave all the rest
> of the traffic on its normal shortest path. 

Yes, if specific pathologies exist (e.g. hot-spots) within 
the network, then intervention is usually necessary to 
address the situation. In such cases, analysis can be
conducted offline to diagnose the problem and prescribe 
an appropriate course of action, which may involve defining 
or identifying LSPs to route/re-route through alternative 
paths with adequate capacity. If frequent intervention 
is warranted, then perhaps there are more fundamental 
issues at play which demand a stronger solution, e.g., 
augment capacity, review and amend the operational 
process models, etc.

> 
> As far as I know, all the online LSP path computation algorithms provided by
> the vendors are not capable of taking the normal IP traffic into account
> since the algorithms only see the bandwidth requirements from traffic on
> LSPs. If one has to use the online LSP path computation algorithm, he/she
> needs to use some offline tool to figure out the aggregate bandwidth
> requirement by the normal IP traffic on each link (excluding the traffic
> trunks that will be routed via LSPs), substract that from the total
> available bandwidth for that link, and make that the new available bandwidth
> for the online LSP path computation algorithm to use.

Yes, if "normal IP traffic" (traffic that do not traverse through 
LSPs) constitute a reasonable proportion of the workload through 
an MPLS domain, then online constraint-based routing becomes
less effective and additional offline effort may be required. 
The operational model can also be amended to reduce the amount
of effort required...

Cheers,
/Dan

> 
> Regards,
> Angela
> 
> AT&T Labs
> Room C4-3A22
> 200 Laurel Ave.
> Middletown, NJ 07748
> Tel. (732) 420-2290
> Fax (732) 368-1746
> Email: alchiu@att.com
> 
> > -----Original Message-----
> > From:	Daniel Awduche [SMTP:awduche@UU.NET]
> > Sent:	Monday, April 24, 2000 1:59 PM
> > To:	HANSEN CHAN
> > Cc:	Anoop Ghanwani; mpls@UU.NET; Daniel Awduche
> > Subject:	Re: TE Extension of IGP
> > 
> > Hansen,
> > 
> > Online LSP path computation is the preferred operational model in many
> > contexts -- for many good reasons.  Obviously, service providers
> > have the option to activate these aspects according to their
> > circumstance and needs...
> > 
> > As a simple rule of thumb, in networks with adequate capacity, online
> > constraint-based routing should suffice for LSP path placement. In
> > relatively under-capacitated networks, however, significant 
> > offline effort may be required to squeeze additional utility from the
> > infrastructure.
> > 
> > Cheers,
> > /Dan
> > 
> > On Fri, Apr 21, 2000 at 11:06:08PM -0400, HANSEN CHAN wrote:
> > > Dan,
> > > 
> > > I agree that most MPLS implementations perform LSP path computations
> > online. But I
> > > always thought the working LSPs deployed in the networks are still
> > computed
> > > offline. You only use online computations when you're
> > re-routing/repairing the LSPs
> > > around some failure. Is my understanding correct?
> > > 
> > > Cheers,
> > > Hansen
> > > 
> > > Daniel Awduche wrote:
> > > 
> > > > Hansen,
> > > >
> > > > Yes, many (perhaps most) contemporary implementations perform
> > > > LSP path computations online. This is a mandatory requirement
> > > > in some operational contexts. It's also possible to augment
> > > > the online system with offline software tools.
> > > >
> > > > Cheers,
> > > > /Dan
> > > >
> > > > On Thu, Apr 20, 2000 at 06:04:13PM -0400, HANSEN CHAN wrote:
> > > > > Dan,
> > > > >
> > > > > To make sure I understand. Do you mean the path of LSPs is computed
> > on the
> > > > > node, not by software tools?
> > > > >
> > > > > Thanks,
> > > > > Hansen
> > > > >
> > > > > Daniel Awduche wrote:
> > > > >
> > > > > > Actually, the original assertion/generalization is false
> > > > > > (i.e. that "LSPs in today's MPLS network are usually computed
> > > > > > off-node").
> > > > > >
> > > > > > Cheers,
> > > > > > /Dan
> > > > > >
> > > > > > On Thu, Apr 20, 2000 at 02:08:22PM -0400, Anoop Ghanwani wrote:
> > > > > > >
> > > > > > >
> > > > > > > > I am trying to understanding the use of TE extension of IGP in
> > a MPLS
> > > > > > > > network. From my understanding, you need TE extension when
> > you're doing
> > > > > > > > on-node path computation. However, since LSPs in today's MPLS
> > network
> > > > > > >
> > ^^^^^^^^^^^^^^^^^^^^
> > > > > > >
> > > > > > > We're hoping it won't stay that way forever because it's
> > limiting
> > > > > > > to have to rely on offline tools for all traffic engineering :-)
> > > > > > >
> > > > > > > That means that traffic engineering would need to be more
> > dynamic,
> > > > > > > and the routers would play a more active role in determining
> > paths
> > > > > > > and possibly doing network optimization.  Hence the IGP
> > extensions.
> > > > > > >
> > > > > > > > are usually computed off-node (in software tool), why would
> > the use of
> > > > > > > > TE extension be critical?
> > > > > > > >
> > > > > > > > Appreciate if someone can shed some light on this question.
> > > > > > >