Subject: terminology

From: "Gross, Gerhard" <[email-masked]>
Date: Thu, 3 May 2001 10:36:09 -0700

Sorry, sent previous message prematurely.
Once again...

The definitions "LSP tunnel" in the docs
seem to be somewhat misleading. For example,
section 3.27.3 of RFC 3031 states:

"It is possible to implement a tunnel as a
LSP, and use label switching rather than
network layer encapsulation to cause the
packet to travel through the tunnel. ...
This is called a "LSP Tunnel."

This is standard MPLS as far as I tell.
Why rename it?

Also, section 1 of draft-ietf-mpls-rsvp-lsp-tunnel-08.txt
states:

"Since the traffic that flows along a label-switched path is defined
by the label applied at the ingress node of the LSP, these paths can
be treated as tunnels, tunneling below normal IP routing and
filtering mechanisms.  When an LSP is used in this way we refer to it
as an LSP tunnel."

Again this seems to be just normal use of MPLS.
The text implies that an LSP tunnel is a special
case of MPLS. Is there any other use of MPLS?

Thanks!
Gery

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---

From: David Charlap <[email-masked]>
Date: Thu, 03 May 2001 15:52:12 -0400

"Gross, Gerhard" wrote:
> 
> The definitions "LSP tunnel" in the docs seem to be somewhat
> misleading. For example, section 3.27.3 of RFC 3031 states:
> 
>     "It is possible to implement a tunnel as a LSP, and use label
>     switching rather than network layer encapsulation to cause the

The definition of "tunnel" seems to be different in different documents.

In RSVP-TE, a tunnel is a collection of one or more LSPs that are all
signaled with the same SESSION object.  (Session being defined as the
triplet egress-address, tunnel-ID, and extended-tunnel-ID.)  If extended
tunnel ID is zero, these LSPs may originate from different ingress
routers.  The distinction is important because all LSPs in a single
tunnel will share resources where possible, when an SE-style reservation
is used.

Colloquially, this is the definition I usually use.  I usually say
"tunnel" when I mean a collection of LSPs that share a common
destination.  If I mean a single LSP, I just say "LSP".

But this is not RFC 3031's definition.  RFC 3031 defines a "tunneled
packet" as any packet that is forwarded to a not-directly-attached
router.  A tunnel is defined as any means for making this work.

It then goes on to define three kinds of tunnels, the first two don't
necessarily involve MPLS.

The first is a hop-by-hop routed tunnel.  The data is encapsulated in a
network-layer header that gets it to the next-hop router.  One possible
non-MPLS example of this might be an IP-in-IP tunnel.

The second is an explicitly-routed tunnel.  The data is encapsulated in
a network-layer header that is explicitly routed to the next-hop
router.  One possible non-MPLS example of this might be an ATM PVC.

The third is the LSP tunnel.  The data is labeled and forwarded using
MPLS.

If my understanding is incorrect, hopefully one of the RFC authors will
be able to correct me.

-- David