The MPLS WG Archive

["Don Fedyk" <dwfedyk@nortelnetworks.com>]

Title: RE: MPLSOAM BOF revised minutes

Yakov

I think MPLS is a layer in its own right.

You replied to Shahram that forwarding information is not
in the shim header. My experience with many routers that
use ATM is in practice that they ONLY put the forwarding
information in the SHIM header and use the ATM VC/VP as a
pipe. Things would be better if they would follow the drafts
you quote from below. I don't know yet how they will interpret
the GMPLS specs but if they follow the ATM example there will
be a extra shim header there as well.

Don

> -----Original Message-----
> From: Yakov Rekhter [mailto:yakov@juniper.net]
> Shahram,
>
> > > While we are busy arguing about whether MPLS is a separate
> > > layer or not,
> > > please bear in mind that the whole concept of a layer assumes
> > > (among other
> > > things) a single format for transport of the data from the
> > > layer above.
> > > Since MPLS doesn't make such an assumption (as in some
> cases it uses
> > > the "shim" header, in other cases it uses the VCI/VPI,
> > > etc...) one could
> > > conclude that perhaps MPLS doesn't fit into the
> traditional layering
> > > model at all.
> >
> > It seems that you have confused the "layer above" with the
> "layer below".
> > X over MPLS always requires a Shim header.
>
> First of all, in the case of ATM-LSRs the presence of the shim header
> strictly speaking is not mandatory. Second, even when the shim header
> is present with ATM-LSRs, the forwarding information used by
> ATM-LSRs is
> *not* in the shim header, but in VCI/VPI fields of the ATM
> cell header.
> Finally, in GMPLS there is no shim header at all.
>
> Yakov.
>