Subject: a simple question!

From: [email-masked]
Date: Mon, 7 May 2001 09:26:50 +0530

hi!
     why was downstream label assignment chosen in MPLS over upstream label
assignment as is the case with generalised MPLS? Is there any specific
advantage/constraint posed by the downstream label assignment method?

regards
sumit

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From: Guangzhi Li <[email-masked]>
Date: Mon, 07 May 2001 09:16:27 -0400

Hi,

I can figure out one reason:

(1) It is convenient to achieve LSP merging, i.e., to generate a destination
rooted tree.

--- guangzhi

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From: Eric Gray <[email-masked]>
Date: Mon, 07 May 2001 10:11:17 -0400

Sumit,

    First of all, GMPLS defines extensions to MPLS signaling to allow
upstream label 'assignment' but the default mode is still downstream
allocation.  The extensions are provided to cover specific instances in
the general case where it is possible to gain significant optimization of
system behavior at little cost.  In these instances, use of extensions
will likely be the normal case - however, that is not the same as saying
that GMPLS somehow turns signaling around in general.

    The key reason for downstream label allocation is that - in choosing
between a downstream (which needs to interpret the labels it receives)
and an upstream (which only needs to write the label into the packet), we
(the MPLS working group) felt that it makes the most sense to allow the
device that has to interpret labels be the one that allocates them.  This
paradigm extends generally - in most cases even extending to behavior
'inside the box'.

    GMPLS, on the other hand, deals with a number of instances in which
there is no 'label' per-se.  For example, a TDM time slot, a lambda or a
physical fiber interface may stand in for a 'label'.  In these cases, the
usage of a 'label' are fairly well understood and the need to allow the
receiver to 'control' allocation of 'labels' does not exist.  In addition, it
is even possible that allowing the upstream device to assign the 'label'
lends a significant advantage.

    For example - if the upstream device needs to undergo a physical
re-alignment in order to use the 'assigned' label, and this alignment
process consumes some non-zero time, then there is a clear benefit
to be realized from allocating the corresponding 'label' at the upstream
device.  This would be the case, for instance, if a device uses some
technology (e.g. - perhaps twisted multi-laminate nano-technology)
to physically position a mirror or prism in the optical transmission path.

--
Eric Gray