Subject: Re: Hierarchical Tunnel Establishment in RSVP-TE

From: Kireeti Kompella <[email-masked]>
Date: Sat, 10 Jun 2000 22:41:48 -0700 (PDT)

Hi Rahul,

> A forwarding adjacency LSP (FA-LSP) is advertised into the
> IGP. However from what I have seen 
> there is no way to include a FA-LSP in an ERO.

When an LSP is advertised into the IGP, it is given IP addresses
for the head and tail ends (this defaults to the router ids).  So,
to include the LSP in an ERO, just use those addresses.

A-->B         E-->F
     \       /
      C---->D

> Thus in the case being discussed if the LSP fron B to E 
> is advertised as FA-LSP1 the ERO for the LSP
> from A to F (LSP2) could include FA-LSP1. So the ERO could look
> like: A->B->FA-LSP1->F. 

The ERO could be (for example) A, B, E (strict!), F.  B being wise
to FA-LSPs realises that E is the end point of an LSP, and send the
Path message to E (either through the LSP, or by unicasting it).

To go back to prior questions:

> 1. Do the RSVP messages for the second (hierarchical) tunnel get
>    forwarded via the previously established LSP or go hop by hop?

Signalling hop-by-hop seems to negate some of the benefits of
tunneling through the LSP.  The alternatives are: signalling through
the LSP, and unicasting (*without router alert*) from B to E.

> 2. What happens to the bandwidth reserved for the original tunnel when a
>    hierarchical tunnel rides over it?

That's a good question.  With FA-LSPs, we thought that the original
LSP should be provisioned for the tunneled LSPs, so its bandwidth
doesn't change.  What changes is the bandwidth announced in the IGP
for the FA-LSP, especially the 'unreserved bandwidth'.

> 3. In the following example:

> When a PATH message goes from A to F, how does B, C, D, E know that
> this is a hierarchical tunnel vs a regular tunnel (related to question
> 1 above).

B and E have to know.  If the signalling is not hop-by-hop, there
is no need for C and D to know.

> What exactly are the steps that are performed?

One answer is laid out in the draft Yakov and I wrote.

Kireeti.

---

From: Robert Raszuk <[email-masked]>
Date: Sat, 10 Jun 2000 22:50:15 -0700

Kireeti,

If you only can specify router_ids from FA-LSP in the ERO from A to F in the
case where there is more then one B-E LSP advertised as a link you don't have
too much of control which one to pick.

Therefor I would be in favour of defining new ERO sub-object to allow addition
of the FA-LSP id for the more granular path selection control from A point of
view.

R.

> Kireeti Kompella wrote:

---

From: Rahul Aggarwal <[email-masked]>
Date: Sat, 10 Jun 2000 23:26:58 -0700

Kireeti,
Thanks for the response. I have a comment inline.

> Path message to E (either through the LSP, or by unicasting it).

There are two cases:
1. B happens to be the edge of a domain (with the domain definition taken
from draft-kompella-mpls-optical-00.txt). In this case B knows that it has
to use a FA-LSP for this LSP request and it can look up the FA-LSP to use
or create one if it doesn't exist. Hence there is no value to explicitly
specifying a FA-LSP in the ERO.

2. All the links are in the same domain. In this case if B receives an 
ERO A, B, E (strict), F do we say that since E is a strict next hop and
is not adjacent to B, a LSP should be used to send it E ? Apparently there
is no other way to interpret this case other than sending an error back!
Furthermore if E is a loose next hop, should B use a LSP to send it to E
or should it just compute a route and add it to the ERO? It might be 
nice to have the flexibility to specify the usage of a FA-LSP explicitly
in such a case. 

Ofcourse as you described the desired behavior can be achieved
by having certain implicit processing rules (at A, the originator and B), 

thanks,
rahul