Subject: Re: What to do when label has error?

From: "Sherry Wei" <[email-masked]>
Date: Wed, 23 May 2001 14:48:21

Hi,

ILM is incoming label mapping. I guess if the route can't find ILM entrance 
according to the incoming label. It still can strip off the label stack and 
send to IP forwarding processing. Only in the case of FEC can't find, the 
packet has to be dropped definitely.

sherry

>From: David Charlap <[email-masked]>
>To: [email-masked]
>Subject: Re: What to do when label has error?
>Date: Wed, 23 May 2001 10:37:05 -0400
>
>Sherry Wei wrote:
> >
> > I have a doubt about the real operation in the case that the LSR
> > can't find the according ILM or NHLFE.
>
>If the ILM is not found, then the packet is dropped.  There is no
>question about this.
>
>If there is an ILM but no NHLFE, then the router will probably drop the
>packet.  If the router is able to forward the payload without looping
>(which implies that the router knows what the L3 protocol is), then it
>may pop the label and forward the payload as an unlabeled packet.  If it
>can not be certain what the payload is, or can not guarantee that it
>won't loop, then it must drop the packet.
>
>-- David

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---

From: David Charlap <[email-masked]>
Date: Wed, 23 May 2001 12:23:57 -0400

Sherry Wei wrote:
> 
> ILM is incoming label mapping. I guess if the route can't find ILM
> entrance according to the incoming label. It still can strip off the
> label stack and send to IP forwarding processing. Only in the case
> of FEC can't find, the packet has to be dropped definitely.

If the ILM is missing, then you have absolutely no idea what kind of
data may be in that packet.  This is by definition.  If you knew, then
you would have an ILM entry.

Since you don't know what the packet contains, you MUST NOT assume that
it contains IP data.

The best you can hope for in this case is to pop the label stack, and
see if there's another label underneath that you do have an ILM for.  If
not, then you have no choice but to drop the content.

-- David

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From: Eric Gray <[email-masked]>
Date: Wed, 23 May 2001 13:41:18 -0400

David,

    If you did not give out the top label, it could be a very
bad thing to use the second label - just because it is one
you recognize.  Labels are not globally distinguishable -
and you are deciding to trust the use of labels by a sender
who is already demonstrating untrustworthiness.  Could
be very bad...

--
Eric Gray

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From: Nagendra Singh Tomar <[email-masked]>
Date: Thu, 24 May 2001 07:57:12 +0530 (IST)

David,
      What do u mean by "popping the label and checking whether we have an
ILM for the next label on the stack if any".
Even if we have an ILM entry for that label that might not be meaningful
to us at all as it would not have been advertised by us (the fact that we
have an ILM entry for that label is purely coincidental and we might
have advertised that label for an altogether different FEC to an
altogether different peer), this might be
the case when we are an intermediate router in an LSP tunnel. 
In that case "checking the nextlabel" will do more harm than good !!!

cheers
tomar