Subject: RE: Label Stack query

From: Rajesh Kumar <[email-masked]>
Date: Fri, 3 Dec 1999 11:16:36 -0800

Hello Eric,
Please see my answers marked by [rajesh]
Hope this helps some..
Rajesh

-----Original Message-----
From:	Eric Ng [SMTP:[email-masked]]
Sent:	Friday, December 03, 1999 12:55 AM
To:	[email-masked]'
Subject:	Label Stack query

Hi all,

I have a few fundamental questions on MPLS label stack.

From draft-ietf-mpls-arch-06.txt section 3.15,
# A "Label Switched Path (LSP) of level m" for a 
# particular packet P is a sequence of routers,
#                 <R1, ..., Rn>

 Q1 Does this "level m" mean "m Autonomous Systems" that 
    do not share each other internal routing information?

[rajesh] This is one of the cases where a label stack is useful
although not the only one. Another very good example is the
VPN application. Consider a 2-level label stack. The top level
is used to carry packets across the internet to the correct gateway.
The 2nd level could be used to switch within the enterprise network
itself.

 Q2 The LSR uses the top label(i.e. level m) to make 
    forwarding decision.  What is the purpose of the 
    "level m-1, ... ,2,1" labels?  Is there any situation 
    where an LSR would look beyond the "level m" label?  

[rajesh] I think the answer to the previous question answers
this too. In the same example (the VPN), the routers
in the internet use the top-level (level-2) label; the routers
within the enterprise use the level-1 label.
 
# At no time during P's transit from R1 to R[n-1] does 
# its label stack ever have a depth of less than m;
also,
# which ends (at an "LSP Egress") when a forwarding 
# decision is made by label Switching on a level m-k label, 
# where k>0, or when a forwarding decision is made by 
# "ordinary", non-MPLS forwarding procedures.
    
 Q3 Why does MPLS not allow a label stack depth < "m" as the
    labeled packet traverse along the LSP?

[rajesh]The LSP will be setup based on the m-level label. That's
why all the switches/routers along the LSP must use the
m-level label to switch a packet. So, the packet must always
have a m-level label in the LSP.
  
 Q4 Why the intermediate LSRs cannot pop labels that are
    meaningless to the subsequent downstream LSRs?  
    Consequently, making the label stack depth less than "m".  

[rajesh] This is in fact allowed for the penultimate LSR (see
Penultimate Hop Popping). All the LSRs on the m-level LSP
have label bindings based on the m-level label. If an intermediate
LSR popped the m-level label, the downstream LSR would not
be correctly forwarding the packet.
     
 Q5 Does this mean only the (n-1)th LSR can pop labels
    without pushing the same number labels or no label 
    at all onto the stack? 

[rajesh] Yes.

thanks in advance,
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