The MPLS WG Archive

[Curtis Villamizar <curtis@avici.com>]

In message <14569.26997.850641.44794@lohi.eng.telia.fi>, Juha Heinanen writes:
> Lee Thomas writes:
> 
>  > I like having the static overlay model identified as this
>  > reflects many current network implementations and I like the Signaled
>  > Overlay Model as it describes a model that may be implemented as a new
>  > network, or a Static Overlay network could be transitioned to a SOM.
> 
> opposite of static is not signaled but dynamic.  we do all of our ip/atm
> overlays using signaled svcs between the routers, but the topology is
> still static.  so better names would be static and dynamic overlays.
> 
> an example of dynamic overlays was documented as
> 
> ftp://lohi.eng.telia.fi/tmp/draft-ietf-ion-intra-area-unicast-01.txt
> 
> which unfortunately never got progressed because in the atm world it had
> a dependency with the ospf ara option.  since signaling in mpls is based
> on ip addresses, the ara thing is not anymore needed and i may respin
> the i-d for mpls.
> 
> -- juha


We are splitting hairs but please recall that there were three cases:

  static
  signaled
  peer

One could argue that the latter two are dynamic and that the peer
model is just a bit more dynamic that the signaled model.  The PVC
style is certainly the most static.

Since the first two have limited or no visibility into the underlying
topology they are overlay.

Juha may have added a fourth case.  The following table might help.

   router's topology  type of
   visibility         path setup      model name           example

    configured*        none            static overlay       ATM-PVC
    configured*        endpoint        signaled overlay     ATM-SVC
    endpoints          endpoint        dynamic overlay      MPLS-loose
    full               full path       peer                 MPLS-strict

   *configured here means endpoints only are configured in the router.

The model Juha proposed is a slight improvement over the pure ATM SVC
model in that the endpoints to the ATM are advertised for the purpose
of making ATM call setups.  This model seems similar to the use of
MPLS with a single loose hop in the explicit route.  It is similar in
that the ingress knows about the endpoint through signaling rather
than configuration but leaves it to the next (signaling) hop (which
would be an optical device) to figure out how to get there.  If the
ingress had enough information to compute the path, we'd have the peer
model and the ingress would use strict hops in the explicit route.

It wouldn't hurt to add the distinction that Juha has pointed out and
adapt the term "dynamic overlay" that he suggested.  I hope this
terminology is clear.  We seem to have some growing consensus on these
model names.

Curtis