The MPLS WG Archive

[Eric Gray <EGray@zaffire.com>]

Ling-chih,

	See below.

You wrote:
> 
> hi,
> 
> Can somebody tell me the differences between LSP and LSP tunnel?
> 
> I see the definition in "draft-ietf-mpls-arch-0.6txt".
> 
> 1.
> LSP tunnel example:
> 
> 3.27.4. Hierarchy: LSP Tunnels within LSPs
> 
>    Consider a LSP <R1, R2, R3, R4>. Let us suppose that R1 receives
>    unlabeled packet P, and pushes on its label stack the label to cause
>    it to follow this path, and that this is in fact the Hop-by-hop path.
> 
>    However, let us further suppose that R2 and R3 are not directly
>    connected, but are "neighbors" by virtue of being the endpoints of an
> 
>    LSP tunnel. So the actual sequence of LSRs traversed by P is <R1, R2,
> 
>    R21, R22, R23, R3, R4>.
> 
>    When P travels from R1 to R2, it will have a label stack of depth 1.
>    R2, switching on the label, determines that P must enter the tunnel.
>    R2 first replaces the Incoming label with a label that is meaningful
>    to R3.  Then it pushes on a new label. This level 2 label has a value
> 
>    which is meaningful to R21. Switching is done on the level 2 label by
> 
>    R21, R22, R23. R23, which is the penultimate hop in the R2-R3 tunnel,
> 
>    pops the label stack before forwarding the packet to R3. When R3 sees
> 
>    packet P, P has only a level 1 label, having now exited the tunnel.
>    Since R3 is the penultimate hop in P's level 1 LSP, it pops the label
> 
>    stack, and R4 receives P unlabeled.
> 
> but from the LSP definition
> 
> In other words, we can speak of the level m LSP for Packet P as the
>    sequence of routers:
> 
>       1. which begins with an LSR (an "LSP Ingress") that pushes on a
>          level m label,
> 
>       2. all of whose intermediate LSRs make their forwarding decision
>          by label Switching on a level m label,
> 
>       3. which ends (at an "LSP Egress") when a forwarding decision is
>          made by label Switching on a level m-k label, where k>0, or
>          when a forwarding decision is made by "ordinary", non-MPLS
>          forwarding procedures.
> 
> So my questions are
> 
> 1. Is  <R2, R21, R22, R23, R3, R4>  a  LSP?

There is an LSP (let's call it LSP-A) consisting of R1, 
R2, R3 and R4 that includes an LSP (we'll call it LSP-B) 
consisting of R21, R22 and R23 as the logical link 
between R2 and R3.

In the example you cite, LSP-B is used to tunnel labeled
packets from R2 to R3, so it is referred to as an LSP
tunnel.  In fact, LSP-B could easily be an IGP-based LSP
setup using hop-by-hop LDP which would mean it could move
around a lot and would not be an LSP tunnel in the sense
that the term is used in both RSVP-TE and CR-LDP.  For
this reason, I would tend to refer to LSP-B as an LSP
that is being used to tunnel labeled packets rather than
the more easily said "LSP tunnel".

> 2. Is  <R1,R2, R21, R22, R23, R3, R4> a LSP tunnel?

LSP-A (R1, R2, R3 and R4) is an LSP and LSP-B (R21, R22 and
R23) is an LSP. Any LSP may be used as a tunnel - to tunnel
either labeled or unlabeled packets.  In the sense that TE
uses the term tunnel, however, it is not certain that either
of these LSPs is a traffic engineered tunnel.

> 3. Trsffic engineering works on LSP or LSP tunnel?

Yes.

> 
> Sincerely yours
> 
> Ling-chih Kao

--
Eric Gray