The MPLS WG Archive

["Chiu, Angela L, ALSVC" <alchiu@att.com>]

Hansen,

See comments in line.

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:	HANSEN CHAN [SMTP:hchan@newbridge.com]
> Sent:	Monday, April 24, 2000 6:17 PM
> To:	Chiu, Angela L, ALSVC
> Cc:	Daniel Awduche; mpls@UU.NET
> Subject:	Re: TE Extension of IGP
> 
> Angela,
> 
> > 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.
> 
> If I understand the picture correctly, that offline tool should do the
> following:
> 
> 1. collect statistics from the network and analyze network trunk bandwidth
> utilization to understand where the hot spot is
> 2. compute the LSP paths (offline) with certain algorithm for the optimal
> use of
> trunk bandwidth in the network
	[Chiu, Angela L, NNAD]  Yes. As I pointed out before, between your
steps 1 and 2, the offline tool needs to identify, for each hot spot, a set
of traffic trunks that contribute to the congestion and need to be rerouted.


> But as Dan said in his message, LSPs are more likely to be computed
> online. I
> cannot see how a node can compute all LSPs for the whole network. It might
> be
> able to do a good job for LSPs originating from itself. But for LSPs
> originating
> from other edge LSR? I would think a offline tool is a better place to
> plan for
> all LSPs.
	[Chiu, Angela L, NNAD]  Actually, it is the ingress node (the node
where an LSP originates) that is responsible for the path computation based
on the link state information propagated via IGP extension.

	Angela

> Cheers,
> Hansen
> 
> >
> >
> > 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.
> >
> > 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.
> > > > > > > >