Subject: Re: Some Queries

From: Lyndon Ong <[email-masked]>
Date: Mon, 8 Jan 2001 10:19:00 -0800

----- Original Message -----
From "Vijay Gill" <[email-masked]>
> This approach is perfectly analogous to just buying point to point
> circuits and gives up a very important feature that some ISPs currently
> use ATM for - the ability to get constrained based routing, so as to do
> effective automatic traffic engineering.
>
> To allow for automatic traffic engineering, some organizations terminate
> their SONET handoffs into ATM switches and terminate routers into the ATM
> switches as well. The ATM switches then talk amongst each other (various
> levels of hierarchy go in here), and export a virtualized transport fabric
> to the routers. The routers are then fully meshed with subinterfaces and
> sPVCs to all the other routers in that particular area, such that no
> router _transits_ another router. All routers appear to be directly
> connected to each other, so that the IP traffic always takes what appears
> to be the shortest path to any other router (a direct connection).
>

MPLS will also allow you to do this, essentially make a physical topology of
multiple links look like a logical topology of a single link between router
pairs
so that IP always picks this as the shortest path.

The IP part is "automatic", but how is the logical topology (ATM or MPLS) 
created?  Is this through some offline tools and configuration, or done
directly 
by switches?

Cheers,

L. Ong

---

From: David Charlap <[email-masked]>
Date: Mon, 08 Jan 2001 15:07:00 -0500

Lyndon Ong wrote:
> 
> ... how is the logical topology (ATM or MPLS) created?  Is this
> through some offline tools and configuration, or done directly
> by switches?

There are two common ways:

1: Each switch can have its forwarding tables hand-configured (or via
   an external network management package).  In the ATM world, this is
   how PVC's (permenant virtual circuits) are created.

2: The network can be given a list of endpoints for the virtual circuits
   and let the switches set up the actual circuits via a routing and
   signalling protocol.  In the ATM world, this is the SPVC (signalled
   PVC), using PNNI for a routing protocol and UNI/NNI as a signalling
   protocol.  In the MPLS world, it would use an IGP (like OSPF or ISIS)
   as the routing protocol and an IP signalling protocol (like RSVP or
   LDP.)

There are other ways as well.  For instance, ATM supports SVCs
(signalled VC) which are created on-demand by edge hosts/routers and are
torn down by those hosts/routers when no longer needed.  This would not
be a good idea in an ISP's core, but is often used in enterprise
networks.

-- David

---

From: Vijay Gill <[email-masked]>
Date: Mon, 8 Jan 2001 16:43:02 -0500

On Mon, 8 Jan 2001, Lyndon Ong wrote:

> MPLS will also allow you to do this, essentially make a physical
> topology of multiple links look like a logical topology of a single
> link between router pairs so that IP always picks this as the shortest
> path.

Whilst I would be the first to say that MPLS has its place, it is far from
being a protocol that can solve all problems for all people, unlike the
perception that is being foisted upon the world by the marketing
departments (hardware vendors AND service providers are both guilty of
this). Here is a small illustration of why MPLS will also NOT allow us to
do this with the same cleanliness (in the ip portion of things) as Frame
or ATM.

One of the problems with MPLS is the fact that most documents and "high
level" overviews show something like an edge-to-edge mpls LSP setup.

Like so:
       ---[rtr1]---      -------[rtr4]
     /              \  /         |
[rtr2]-----lsp------[rtr3]---lsp-+

Which makes for all sorts of easy to design networks. Reality
unfortunately, tends not to care too much about fitting into neat
marketing diagrams and looks something much like this:

                           /---[rtr]--[edge] * 200-300 more of these.
  {huge glob of networking}---[rtr]---/
     |        |     |    |
   [rtr]----[rtr]  [rtr] [rtr]
     \       /      \      /
       \   /          \  /
       [edge]         [edge]

Now, how do you run an LSP from the edge to the upper level clouds? Where
do you terminate them? Do you go edge-to-edge (causing a full mesh
explosion of state in your network?) 700 edge device networks are common,
and they all have multiple uplinks. Do we go with two LSPs from every edge
device to every OTHER edge device so we can use both uplinks effectively?

Then we start to add hierarchy here. One design being the lets mesh the
aggregation routers instead of the edge routers. Problem with that design
is that now the MPLS domain does not go all the way to the edge, causing
the edge to slosh ip traffic flows based on topology changes inside your
network, causing your bandwidth settings on LSPS to fail miserably, as
traffic may now start traversing LSPs which were previously (thanks to the
wonders of SPF) never loaded, thus the automatic TE scripts set their
reserved bandwidth to 0. The same issue arises when you have regionalized
meshing. As soon as you break away from the edge-edge full mesh model, you
introduce a discontinuity into the mpls/igp domain, causing traffic flows
to slosh around unexpectedly in responses to topology changes,
invalidating your schemas, and your neat bandwidth reservations.

Not all is lost here, given sufficient planning and small enough flows,
most of these can be worked around. The key thing to take away is that
automatically saying technology XXX will do what technology YYY did is not
necessarily a valid statement.

> The IP part is "automatic", but how is the logical topology (ATM or
> MPLS)  created?  Is this through some offline tools and configuration,
> or done directly by switches?

There are multiple ways to do this. 

/vijay

---

From: Kireeti Kompella <[email-masked]>
Date: Mon, 8 Jan 2001 14:33:27 -0800 (PST)

Hi Vijay,

> reserved bandwidth to 0.

So you advertise the LSPs into your SPF, and give them a fixed metric.
Now your SPFs behave: end of slosh.  (Confess!)

> The key thing to take away is that

Excellent point.  Explicitly: ATM is not MPLS; MPLS is not ATM.
They are close in many respects, but not identical.

Kireeti.

---

From: Vijay Gill <[email-masked]>
Date: Mon, 8 Jan 2001 18:00:06 -0500

> Now your SPFs behave: end of slosh.  (Confess!)

I do not have any idea what you are talking about it. No one saw me do it,
I deny _all_ knowledge of such heinous hacks.

> They are close in many respects, but not identical.

vijay "plausible deniability" gill

---

From: "Cheng-Yin Lee" <[email-masked]>
Date: Tue, 09 Jan 2001 14:04:52 -0500

Kireeti,

> Now your SPFs behave: end of slosh.  (Confess!)

Could you elaborate how this scheme works, how this solves the problem
described by Vijay above?

thanks,
cyl