Subject: Re: several questions

From: "Navid Yazdani" <[email-masked]>
Date: Fri, 7 Sep 2001 15:53:16 -0400

Thank you all for your responses.  Here is what I understood.  PLEASE
correct me if I am wrong.

1) Do I have to use CSPF (a constraint-based shortest path first algorithm)
rather than just normal OSPF?

**Yes

2) Does that mean that I can't use the concept of areas in OSPF to
summarize routes?  How would a summary advertise the CR-LDP traffic
constraints?

**Currently there is way to send summary LSPs out of an area.
draft-venkatachalam-ospf-traffic-01.txt somehow tries to send CSPF summary
LSPs.

3) If I can't use route summaries, is MPLS with dynamically setting up LSP
not scalable?

**Currently CSPF isn't scalable with respect to areas and summary routes

4) When an Edge LSR wants to setup an LSP using CR-LDP, does it look at the
traffic requirements, choose a path, and then mark that path as an explicit
route?  I want to know if the entire path is choosen by the Edge LSR and
the other LSRs just reserve the requested resources and forward the label
request message along the explicit route.

**One approach is for the edge LSR to decide the entire path and set the
explicit route.  Juniper does this with CSPF.  You could also not use ERO
and have each LSR decide the next hop.
--my comments:  if you don't use ERO then there may be trouble if the route
tables are not in sync.  if you don't use ERO you may have trouble if a LSP
setup fails - there is nothing stopping the reattempt to setup from going
through the same path.

5) If the resources have changed for whatever reason at an intermediate LSR
and there are not enough resources to setup the LSP, is there any mechanism
for the intermediate LSR to find a new path from that point on that meets
the constraints and forward the label request message along that path?  Or
is that LSP setup completely abandoned and the edge LSR must try to setup
another LSP?

** If it can't be setup the whole thing fails and the head can retry.
There is no concept similar to crankback in PNNI.

++ more questions added below

-- important comment that was made
6) CR-LDP is dead

Not the kind of comment that will easily see closure but I wonder what
people are actually supporting.
vendors support LDP.  Who supports RSVP-TE? Who supports CR-LDP? While
mundane these are practical issues to consider.

-- another question regarding relevance
7) Is setting up constrained LSPs dynamically due to application/user needs
a pipe dream?  Do most people use MPLS with ERO to do some traffic
engineering but nothing on demand?

-Navid

---

From: Eric Gray <[email-masked]>
Date: Fri, 07 Sep 2001 16:37:12 -0400

Navid,

    See below...

> rather than just normal OSPF?

This is an observation based on circular logic.  Yes, you need to use some
routing protocol that supports the notion of finding a reasonable path given
a set of constraints for a set of packets to which these constraints apply,
IF you want to constrain the way that this set of packets is forwarded.

Question: I want to forward a certain set of packets along a path which
meets certain constraints.  Do I have to use a routing protocol that selects
a path based on these constraints?

Answer: left as an exercise to the initiate.

> **Currently CSPF isn't scalable with respect to areas and summary routes

Is there a formal definition of, and specification for, CSPF on which we can
base this assertion?

> setup fails - there is nothing stopping the reattempt to setup from going

Here you have skipped over the notion of abstract nodes.  An AS can be an
abstract node and a path consisting of such nodes can be entirely chosen by
an ingress LSR based on BGP routing.  At each AS, however, an ASBR can
select a constrained path that meets the resource requirements of the LSP
being requested and expand the ERO to include the nodes within that AS to
be traversed by the LSP.

> There is no concept similar to crankback in PNNI.

This is not necessarily true.

> ...

--
Eric Gray

---

From: Giles Heron <[email-masked]>
Date: Sat, 08 Sep 2001 00:05:00 +0100

> Is there a formal definition of, and specification for, CSPF on which we can

Do we need a formal definition?  Let's just look at current
implementations.  Do these allow you to scale wrt area boundaries?  I
think not...

> being requested and expand the ERO to include the nodes within that AS to

Does anyone implement this yet?

> This is not necessarily true.

Again there's an issue here of tense.  Is Navid asking the questions in
the present or the future tense?  I'm not sure of the answer to that,
but I do get the feeling that I am answering them in the present, whilst
you are answering them in the future, and Marelene in the theoretical
(and hence the differences in our answers).

Giles

-- 
=================================================================
Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
=================================================================

---

From: Eric Gray <[email-masked]>
Date: Fri, 07 Sep 2001 20:57:02 -0400

Giles,

    We're both picking apart the language, I suspect.  We DO need a formal
definition of X to say that X isn't 'scalable'.  The term 'currently' is not
especially meaningful when using this term.  If it will ever be made to scale,
then it is 'currently scalable'.  'Currently doesn't scale' would - of course -
be a horse of a different color.  But - in the absence of at least a better
definiton for the term CSPF - what exactly does 'currently' mean?   We
could as well say that Momroths currently are more Outgrabing then
previously was the case.

--
Eric Gray

---

From: "Mareline Sheldon" <[email-masked]>
Date: Sat, 8 Sep 2001 10:27:58 +0530

> draft-venkatachalam-ospf-traffic-01.txt somehow tries to send CSPF summary

You might also want to take a look at the following draft, which uses new
TE-LSAs for inter-area communication. The draft does not use Opaque LSAs and
does not assume that TE properties across area boundaries to be linearly
summarizable. <draft-srisuresh-ospf-te-01.txt>

~Mary

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