The MPLS WG Archive

[Ramesh Bhandari <bhandari1@lucent.com>]

Hi Kireeti,

I think the problem you are trying to pose pertains to the calculation of the
pair of disjoint paths from A to B through an intemediate OTN network. The
solution consists of three parts:

1) Calculate diverse paths from A to ingress points X and W, i.e., path A to X
and path A to W, which are physically-disjoint from each other, except at the
common source point A
2) Calculate two disjoint paths paths through the OTN network connecting the
ingress points X and W to the egress points Y and Z (two possibilities arise in
general: the diverse paths within the OTN are i) X to Y and W to Z ii)  X to Z
and W to Y)
3) Calculate paths Y to B and Z to B, which are diverse from each other, except
at the end point B.

For A to do all the calculation would require a massive amount of data at B,
including all the necessary details of the (physical) OTN ( not to mention the
fact that in a catastrophic disaster like a fiber cable cut, thousands of LSP's
with different pairs of endpoints would be affected simultaneoulsy, and need to
be also restored simultaneously by the routers for fast restoration).

In the Overlay/Client model, the requests are received at the ingress points X
and W from the router A, and diverse paths are computed within the OTN, using
network information at each of the OXC controllers (basically part 2 above of
the solution for the peer model).

Regards,

Ramesh

PS Algorithms for finding disjoint paths of the type - parts 1), 2) and 3) of
thesolution above- exist, and can be found in my book "Survivable Networks -
Algorithms for Diverse Routing", Kluwer Academic Publishers (1999).

Kireeti Kompella wrote:

> > > Suppose router A wants to get to router B, and wants to take two
> > > different ingress and egress points in the optical domain, X->Y
> > > for the primary LSP, and W->Z for the backup.  A does not require
> > > optical protection for the X->Y path, nor for the W->Z path.  A
> > > *does* require that the X->Y path and the W->Z path do not share
> > > common links.  How is this to be done?
> > >
> > > If A did the full path computation, this is simplicity itself.
> > >
> >       NH=> Kireeti, I don't this can be a general answer.  The request
> > (from whatever the client) is for two phyically disjoint paths between two
> > points A and B.
>
> Neil, A *is* the client.  And A wants two disjoint paths to B via
> *different ingress points in the OTN*.  This is a very reasonable
> request.
>
> Also, A doing a full path computation is *not* a general answer.
> It is an illustration of an advantage of the peer model, and of
> routers doing a full path computation across the OTN.  To achieve
> what A wants in an overlay/client/UNI model would require a fair
> amount of work in extending routing/signalling/UNI protocols.
>
> Kireeti.