Subject: Re: Hierarchical Tunnel Establishment in RSVP-TE

From: Kireeti Kompella <[email-masked]>
Date: Mon, 12 Jun 2000 23:58:39 -0700 (PDT)

> Two POPs are needed to terminate a VPN tunnel.

Not if you use PHP.  You start popping one hop earlier, e.g., if
X is the PE attached to the customer, and Y the penultimate hop,
Y pops the "outer label" and forwards the packet with the "inner
label" identifying the VPN to X.  X looks at the inner (only!)
label, pops it and knows which customer to forward the packet to.

Corner case: a one-hop LSP carrying VPN info: don't put any "outer
label" in this case.

> Three might be needed
> if the outer tunnel on the VPN took its last hop through a FA-PSC.

Picture time:

    A ---- B ==== C ==== D ==== E ~~~~ customer
            <-43-  <-28-  <--3-
    <--71-- <-------- 3 -------

    <-- 32 for VPN to customer --

There's an FA-LSP from B to E.  E sends D an implicit NULL label,
D sends C label 28, C sends B 43.

Now, there's another LSP that wants to tunnel through the above
FA-LSP.  This LSP has 2 hops, A->B and B->E.  E sends B label 3,
B sends A label 71.

A and E agree on label 32 for the VPN.

The label stack for a packet to the customer is as follows:

    A ---- B ==== C ==== D ==== E ~~~~ customer
              43     28
       71
       32     32     32     32

Note that 71 -> 43 is not really a swap as much as a pop (PHP),
followed by a push (equivalent, but you get there differently).

It is interesting to first work out the case where the tunneled
LSP terminates beyond the tail of the FA-LSP, as the above example
is another 'corner case'.

Kireeti.

---

From: Curtis Villamizar <[email-masked]>
Date: Tue, 13 Jun 2000 10:15:49 -0400

>     A ---- B ==== C ==== D ==== E ~~~~ customer

Kireeti,

PHP does remove POPs and make it easier to implement an egress LSR but
it makes it very hard to collect statistics of packets and bytes
received on an LSP at the egress.  Recall the discussion of OAM and
loss measurements.  There is no way to count at the egress if the
labels are lost.  Therefore in your example there is no way to
determine loss over the FA-PSC tunnel or over the tunnel from A-E.

Curtis