Subject: which layer does the label encapsulation...

From: Brahma Naidu Golla <[email-masked]>
Date: Thu, 25 Nov 1999 16:58:27 +0530 (IST)

Hi

I have a doubt regarding the layer changes when we are implementing the
MPLS.

Which layer does the label encapsulation either Network or Data link layer
or is there any separate layer like MPLS layer? While implementing MPLS,
what could be the changes that have to be done to IP?

quick responses are appreciated.

thanks
-- gbnaidu.

---

From: Shahram Davari <[email-masked]>
Date: Thu, 25 Nov 1999 10:22:03 -0800

Hi,

I think since MPLS uses L3 routing information, it should reside at or below
L3. But MPLS can use the services (label encapsulation)of either L3 or L2 or
none based on the media. For example in IPV6 you may have the label inside
the IPV6 header (L-3), while in ATM it is part of ATM header (L-2), and in
PPP case it is neither in L2, or L-3. Therefore I think theoretically MPLS
can be part of L3 or L2 or a separate layer in between. But most commonly it
is referred to as layer 2.5.

I hope that helps.

Shahram

> -----Original Message-----
> From: Brahma Naidu Golla [mailto:[email-masked]]
> Sent: Thursday, November 25, 1999 3:28 AM
> To: [email-masked]
> Subject: which layer does the label encapsulation...
> 
> 
> Hi
>

---

From: "J. Noel Chiappa" <[email-masked]>
Date: Sat, 27 Nov 1999 10:53:13 -0500

> From: <[email-masked]>

    > this layer business has me constantly confused. What layer model are we
    > talking about? ... None of them fit neatly in the OSI stack but that
    > what we usually use, right? ...  we should probably look at what
    > function it performs rather than what format it has. ... This is a
    > network function and thus MPLS belongs in layer 3.

Err, don't get too hung up on layer models - particularly the OSI reference
model. That model was defined over 20 years ago, and we now understand a lot
in networking that was not understood back then. So, whether the OSI model
assigns a particular function to layer N or layer M is not really that
definitive - we may have since decided that it's better done at layer
Q.

In fact, a lot of people find the OSI model problematic because it doesn't
have an "internetwork" layer - i.e. a layer which provides end to end
unreliable service, separate from a particular network technology's (e.g.
ATM) service layer. That is why you see people talking about "layer 2.5" -
they need another layer in the model, but there isn't one, so they've taken
over the old layer 3 and redefined it (effectively) to be an "internetwork
layer (although, confusingly, they retain the name "network"), and then
this new '2.5' layer takes over the function that used to be at layer 3.

Other people find the whole layer model fundamentally broken, because in truth
the functional relationship of various protocol subsystems is more complex
than a simple stack - rather, it's at least a tree, and in reality more of a
graph. E.g. consider MPLS - some people want to use it to join together
various non-contiguous networks by bridging (e.g. at the Ethernet layer),
while others want to carry IP directly on MPLS. IP can also run directly on
Ethernet, or on Ethernet bridged over MPLS!

So stop trying to jam round pegs into square holes, and throw out the OSI
model, and just draw yourself a whole new diagram, one which accurately shows
the functional relationships between all these protocols. It will be rather
complex, unfortunately - but that's the reality of networking today.

	Noel