Subject: A philosophical question about FA and FA-LSP

From: "Igor Bryskin" <[email-masked]>
Date: Fri, 4 Mar 2005 11:42:31 -0500

Hi,

I have a question to Kireti and Yakov.

This is from draft-ietf-ccamp-gmpls-routing-09.txt: 

Traditionally, a TE link is advertised as an adjunct to a "regular"   link, i.e., a routing adjacency is brought up on the link, and when   the link is up, both the properties of the link used for Shortest   Path First (SPF) computations (basically, the SPF metric) and the TE   properties of the link are then advertised.    GMPLS challenges this notion in three ways.  First, links that are   not capable of sending and receiving on a packet-by-packet basis may   yet have TE properties; however, a routing adjacency cannot be   brought up on such links.  Second, a Label Switched Path can be   advertised as a point-to-point TE link (see [LSP-HIER]); thus, an   advertised TE link may be between a pair of nodes that don't have a   routing adjacency with each other.  Finally, a number of links may be   advertised as a single TE link (perhaps for improved scalability), so   again, there is no longer a one-to-one association of a regular   routing adjacency and a TE link.    Thus we have a more general notion of a TE link.  A TE link is a   "logical" link that has TE properties. . My question is:  can I draw a conclusion from the above that it is not required anymore for an FA to be advertised into the same instance of IGP-TE as was used for the advertising of the FA-LSP constituent TE links. In other words, if an LSP created by one instance of the control plane (say, L1VPN Provider) and advertised as a TE link into another instance of the control plane (say, VPN), would/should such TE link be treated anyhow different from the LSP advertised into the same instance of control plane. Thanks,Igor

---

From: Kireeti Kompella <[email-masked]>
Date: Fri, 4 Mar 2005 09:53:29 -0800 (PST)

> I have a question to Kireti and Yakov.
...
> My question is:  can I draw a conclusion from the above that it is
> not required anymore for an FA to be advertised into the same
> instance of IGP-TE as was used for the advertising of the FA-LSP
> constituent TE links.

Correct -- LSPs created in IGP A can be advertised as TE links in IGP B.
This can be used (say) for inter-area signaling.

> In other words, if an LSP created by one instance of the control
> plane (say, L1VPN Provider) and advertised as a TE link into another
> instance of the control plane (say, VPN), would/should such TE link
> be treated anyhow different from the LSP advertised into the same
> instance of control plane.

The advertisement into the VPN control plane is just a TE link for the
VPN IGP-TE.

Kireeti.
-------

---

From: "Igor Bryskin" <[email-masked]>
Date: Fri, 4 Mar 2005 13:07:18 -0500

Hi,

My question was what is (if any) special about FA TE link apart from the
fact that it is dynamically created/destroyed?

Igor

---

From: Kireeti Kompella <[email-masked]>
Date: Fri, 4 Mar 2005 10:40:45 -0800 (PST)

On Fri, 4 Mar 2005, Igor Bryskin wrote:

> My question was what is (if any) special about FA TE link apart from
> the fact that it is dynamically created/destroyed?

Actually, an FA TE link doesn't have to be dynamically
created/destroyed.

What's special?  Nothing really.  A "normal" TE link usually has a
(single) physical interface associated with it; an FA TE link is
usually multi-hop.  A "normal" TE link usually is accompanied by an
IGP adjacency (at least, in the packet domain); an FA TE link usually
doesn't.

But from the point of view of the IGP, or an LSR in that IGP, they
can't tell an FA TE link from any other TE link.

Kireeti.
-------

---

From: Kireeti Kompella <[email-masked]>
Date: Fri, 4 Mar 2005 11:22:43 -0800 (PST)

> can't tell an FA TE link from any other TE link.

Correction: it has been pointed out that the above statement about
LSRs applies to all LSRs except those at the two ends of the FA LSP.

Kireeti.
-------

---

From: dimitri papadimitriou <[email-masked]>
Date: Sat, 05 Mar 2005 15:58:29 +0100

hi kireeti,

> IGP adjacency (at least, in the packet domain); an FA TE link usually

but if a couple this with your answer "Correct -- LSPs created in IGP A 
can be advertised as TE links in IGP B. This can be used (say) for 
inter-area signaling." don't you have now an adjacency accompanying the 
TE link advertisement in IGP B - therefore why is there a need to rename 
the canonical inheritance i.e. why not just speak in that case about a 
TE link or a "hierarchical TE link" as it was the case in one of the 
earlier versions of the LSP-HIER document and keep the FA concept when 
dealing with a unique instance ?

thanks,
- dimitri

> .

---

From: Kireeti Kompella <[email-masked]>
Date: Sat, 5 Mar 2005 09:45:55 -0800 (PST)

On Sat, 5 Mar 2005, dimitri papadimitriou wrote:

> TE link advertisement in IGP B

Not necessarily.  Say you have:

       IGP B - X == IGP A == Y - IGP B

X and Y participate in both IGPs; X has an LSP to Y, and advertises it
into IGP B.  All's fine.

I'm guessing you are referring to the fact that IGP B appears
partitioned?  Well, to connect up the piece on the right with the
piece on the left, one can:
 a) create an adjacency over the FA LSP (only possible if LSP is PSC)
 b) have a virtual link (OSPF) from X to Y
 c) have some other "backdoor" connection between the two parts.

> - therefore why is there a need to rename

You can call it what you like :-)  "FA" in particular refers to the
idea that the "adjacency" is primarily for forwarding, and not
necessarily for the control plane.  However, the latter is not ruled
out (as in case (a) above.)

Kireeti.
-------

---

From: dimitri papadimitriou <[email-masked]>
Date: Sun, 06 Mar 2005 19:27:51 +0100

hi - see in-line:

>  a) create an adjacency over the FA LSP (only possible if LSP is PSC)

ok but is that not another adjacency accompanying the TE link 
advertisement for (or in) IGP B as stated in my previous reply ?

>  c) have some other "backdoor" connection between the two parts.

the same applying for the other cases

> out (as in case (a) above.)

initially the term has been introduced to implicitly allow for the 
creation of a forwarding adjacency *without introducing a control plane 
adjacency* solving the facto scaling of the control plane both in terms 
of routing (creation of a data plane adjacency does not trigger creation 
of a routing adjacency therefore the number of routing adjacencies 
remains constant independently of the number of data plane adjacencies) 
and signaling (resulting from the LSP nesting operations), and the 
inherent problem associated to the so-called "unknown adjacency" problem 
that was also the achille heel of the IPoverATM approach where one had 
to create a full-mesh of VCs to determine the reachable end-points from 
the IP level

i do believe we are in agreement on this point - if not please let me 
know - now on the issue concerning not ruling out case a) as described 
here above how do you position this with the following paragraph in the 
LSP-Hierarchy document

"   It is expected that FAs will not be used for establishing ISIS/OSPF
    peering relation between the routers at the ends of the adjacency."

probably we do not have to preclude this but it would be advisable to 
consider a self-consistent terminology to describe the sub-cases we are 
dealing with

thanks,
- dimitri.