Subject: cr-ldp doubts

From: Mohit Misra <[email-masked]>
Date: Thu, 17 May 2001 13:57:58 +0530

I have some basic doubts about CR-LDP

draft-ietf-mpls-cr-ldp-05.txt

Sec. 2 (Constraint based routing overview)
... The constraint based route is calculated at one point at the edge of the
network based on criteria including but not limited to routing information.
           Does that mean that the ingress node has to always calculate the
path (loose or strict) which could be based on policy or traffic parameters
& routing information. If this is true then CR-LDP should always use ER-Hop
TLV. Then why it is mentioned as optional in the draft ?.

Suppose I want to setup a CR-LSP based totally on traffic parameters and and
I don't care which path it follows. In that case can't intermediate nodes
choose the path that fulfils the constraints. (like LDP). In this case
ER-TLV may not be required.

2.4 Route Pinning
... A CR-LSP may be setup using route pinning if it is desirable to change
the path used by an LSP even when a better next hop becomes available at
some LSR alsong the loosly routed portion of the lsp.

In case route pinning is not used then who decides that a better next hop
has become available . From the lines above looks like it is the
intermediate node. Is it something like 'Detect change in FEC next hop'
described in RFC 3036 (LDP). That means whenever there is a routing table
change happens , somebody needs to find out the better next hop(s) for all
the LSPs for which route pinning is not required. That looks like a lot of
effort for me if it is done by each intermediate node of the LSP. 
If the above mentioned scenario is true then from the node where a better
next hop is available a second LSP needs to be setup 
before it can start sending the packets on the new path . The draft doesn't
say anything about that.
                      Suppose I set route pinning for an LSP. What happens
if a new LSP is setup with higher setup priority than the holding priority
of the LSP which has been pinned. In case resources are not available ,
should new LSP be setup and the old one is teared down as you can not
re-reoute it.
					thanx in advance

Mohit Misra
Netbrahma Technologies
www.netbrahma.com

---

From: [email-masked]
Date: Thu, 17 May 2001 17:15:51 +0530

Hi,

Please see the comments inline.

>>some LSR alsong the loosly routed portion of the lsp.

A small correction here route pinning is used when it "undesirable" to change
the path used by an LSP even when a better next hop becomes available at some
LSR along the loosely routed portion of the LSP.

>>effort for me if it is done by each intermediate node of the LSP.

MPLS is mainly used in the backbone networks(like core ISP networks) where
topological changes are very less.So on an average there is not very high
overhead involved.

>>before it can start sending the packets on the new path . The draft doesn't

This draft is basically an extension of LDP draft, so whenever a better next hop
is found, the same algorithm of section A.1.7 (Detect change in FEC next hop)
should be executed with all the messages for CR-LSP setup rather than plain LSP
setup.

>>should new LSP be setup and the old one is teared down as you can not

In my opinion the old LSP should be teared down in that case.

Pls correct me if I am wrong.

Regards
Mohit Goel