Subject: Hierarchical Routing

From: "Christopher Poh" <[email-masked]>
Date: Tue, 30 Apr 2002 02:31:32 +0000

Hi experts,

I am currently pursuing my research study on traffic engineering with MPLS. 
I am interested in exploring into hierarchical routing that hides certain 
level of details about network topology when establishing a path. MPLS 
offers 2 solutions for this, namely, through explicit peering and implicit 
peering. My preference is towards implicit approach to avoid explosive 
number of remote peering. I consulted RFC 3031, but section 4.3 is quite 
brief about implicit peering and I am doubtful about its correct usage. I 
hope some experts out there can shed some light to my doubts.

1. Implicit peering is not possible for non-merging capable LSR domain. Is 
this true?

2. Given a border router, which is an egress LSR, originates a stack 
attribute for a label and distributes the information through implicit 
peering technique.

i) How does the LSR manage the stack attribute? (my definition of 'manage' 
in this context is distributing the stack attribute when required and 
withdrawing it when not inuse). The RFC does not state this clearly. I find 
out that the originator of the stack attribute cannot easily determine 
whether the stack attribute is still being used by any LSR in the domain or 
not such that it may safely withdraw the attribute to conserve memory and/or 
label usage. Furthermore the originator cannot easily ensure that no LSR in 
the domain will ever continue to use the stack attribute after it has issued 
the withdrawal of the stack attribute unless there is a mechanism to 
guarantee this. In my opinion when the originator broadcast or multicast the 
withdrawal message, there may be some LSRs that may not receive it due to 
link failure or packet lost or etc. So this will create problems.

ii) Section 3.27.5 of RFC3031 states that the intermediate LSRs need to 
store information about the label and its attribute even they do not require 
it. Is this a MUST? From my point of view an intermediate LSR may choose not 
to store the information provided that it does not use it because the stack 
attribute will never be visible when it relays any related tagged packets 
across. Am I missing important points here?

3. From label distibution protocol standpoint, how does a label be 
distributed with its stack attribute? I can't remember seeing this in LDP 
Specification.

I would also appreciate if anyone could provide me some links to informative 
guidelines on implicit peering and its usage.

Thank you in advance.

-Chris

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