Subject: What to do with an illegal label stack?

From: NoZizzing OrDripping <[email-masked]>
Date: Fri, 22 Jun 2001 11:56:08 -0700 (PDT)

An MPLS packet arrives with a label stack with no
bottom of stack bit set (e.g. 88 47 22 22 20 60 00 00
00 00 00 00...).

Is any behavior mandated here?  Is it OK to forward
this packet, or must it be discarded?

-Zizz

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From: "Hemant P. Kelkar" <[email-masked]>
Date: Mon, 25 Jun 2001 07:47:28 -0400 (EDT)

Hi Zizz,
If the MPLS forwarding plane receives a label stack which is illegal, it
will silently (ideally it should) process the upper layer of the label
stack, WITHOUT checking for the validity/presence of the "S" bit. The
reason for this behaviour is that if the forwarding plane checks everytime
about the sanity of the label stack, the desired feature of MPLS (i.e.
fast switching using labels) will not be achieved in practice.
Therefore, the better idea is to keep the responsibility of
assigning/determining the correct label stack, with the LDP (or any other
label distribution protocol). The control plane of every LSR, 
participating
in the label distribution process should keep this check.
Further, avoiding such errors is a must, since a case may arise, when the
MPLS forwarding plane will process the "header of IP packet" assuming it
to be the next layer of the label stack(when the S bit is not set in the
bottom of the stack.)
Hope that was useful to you,
regards,
--
Hemant

------------------------------------------------------
An MPLS packet arrives with a label stack with no
bottom of stack bit set (e.g. 88 47 22 22 20 60 00 00
00 00 00 00...).

-Zizz