The MPLS WG Archive

[Eric Gray <ewgray@GraIyMage.com>]

Eric,

    There remains the question of whether a specific "explicit NULL"
label might occur or in this case or either IPv4 or IPv6 explicit NULL
is acceptable.  Realistically, it is going to be an IPv4 explicit NULL
label now and for much of the forseeable future, but this is a hole we
might also want to address.

    One reason for bringing this up is that it is conceivable that some
implementations might not check the bottom of stack bit when dealing
with either explicit NULL label type while others may have hard-coded
the condition in which an explicit NULL label is not bottom of stack as
an error.  If we are going to potentially require people to "fix" these
implementations, we should make sure we are not setting them up for
more "fixes" down the road.  Or at least as sure as possible...

--
Eric Gray

Eric Rosen wrote:

> Francis> Should the statement  that <the explicit null label  only occurs at
> Francis> the bottom of the stack> be relaxed?
>
> This statement causes a lot of problems, and no one can seem to remember why
> it is there.  Further, there are a number of cases in which it is awkward to
> prevent explicit  null from appearing in  mid-stack. Not only  that, but the
> specs  don't really say  what you  are supposed  to do  if you  do encounter
> explicit null above another label, or if you are instructed (via LDP) to put
> explicit null on a packet which already has a label stack.
>
> So I would favor removing the  requirement that explicit null only appear at
> the bottom of the stack.