Subject: TTL draft: draft-ietf-mpls-ttl-00.txt

From: Puneet Agarwal <[email-masked]>
Date: Fri, 8 Feb 2002 11:37:10 -0800

draft-ietf-mpls-ttl-00.txt has been submitted. This is the reissue (with
minor updates) of draft-agarwal-mpls-ttl-01.txt as a WG document.

Looking forward to comments/suggestions.

-Puneet

 <<draft-ietf-mpls-ttl-00.txt>>

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From: [email-masked]
Date: Fri, 8 Feb 2002 13:04:45 -0800

Hi, 

OSPF-TE (draft-katz-yeung-ospf-traffic-06.txt) uses type 10 LSAs, which have
intra-area flooding scope. Does it mean that constraint-based routing only
works for single area? How to make it work for multiple areas?

Thanks!

Cheng-hong

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From: Puneet Agarwal <[email-masked]>
Date: Mon, 11 Feb 2002 10:59:26 -0800

The general internet philosophy is 
"be conservative in what you do, be liberal in what you accept from others".

In the specific case you pointed out for pipe/short-Pipe LSPs, the LSP
terminates at the egress LSR (where you are performing the pop). Hence the
incoming TTL can be 0 (liberal principle) as the packet is meant for that
egress LSR (as the tunnel conceptually terminates there). The egress LSR may
forward based on the exposed label but the exposed TTL is completely
orthogonal to the TTL of the Pipe/short-Pipe LSP label(s) that you popped.
Hence even though the incoming TTL may be 0, it is OK to accept the packet
as the forwarding is not based on the terminated tunnel. Hence the short
answer to your question is yes.

However contrast this case with the PHP at the Penultimate LSR for a short
Pipe Model LSP. The forwarding label is popped and we do recommend that you
perform the "oTTL check" even though the oTTL is not set in the outgoing
packet (conservative principle). This is because the tunnel conceptually
ends at the egress LSR and not at the penultimate LSR.

If one looks at IP TTL handling, RFC 1812 allows the following: 

If you receive a packet with iTTL of 0 and you are not forwarding (ie the
packet is meant for you), then still accept the packet (the liberal
principle). However do not send a packet out with TTL of 0 (the conservative
principle). 

Hope this helps.

-Puneet  

-----Original Message-----
From: Per Flemming Hansen [mailto:[email-masked]]
Sent: Friday, February 08, 2002 11:58 PM
To: Puneet Agarwal; [email-masked]
Subject: Re: TTL draft: draft-ietf-mpls-ttl-00.txt

POP Question/Clarification decrement of  iTTL and check, -  for pipe and
short pipe model:

When a packet arrives where the top label i popped (pipe and short pipe
model). Is the TTL in 
that label not checked at all, such that a TTL=0 could arrive and pass. 

The following relevant text is directly from the draft document, and seems
to answer NO to my question:

 2.3. New Terminology 
    
   iTTL: ............................................................ No
checks are 
   performed on the iTTL.
    
   oTTL Check: Check if oTTL is greater than 0.........................
  
3.7. Implementation Remarks 

           2) Whenever iTTL is decremented, the implementor must 
                 make sure that the value does not go negative.