On 5/12/2017 7:55 PM, Les Ginsberg (ginsberg) wrote:
We are assuming a default behavior of PHP for SR-MPLS. If folks
believe this requires explicit specification I would propose language
similar to the following:
/"When Segment Routing is instantiated over the MPLS data plane the
penultimate node performs the NEXT operation (i.e., MPLS pop) unless a
specific indication to do otherwise is advertised by the ultimate node."/
I think this certainly needs to be stated explicitly, perhaps in the
document that specfies the use of the MPLS dataplane for SR.
It should also be noted that there are circumstances under which the
default will not produce the required behavior, so it may not always be
safe to use the default behavior. (For example, if a SID represents a
multicast tree, and the payload is an MPLS packet that begins with an
upstream-assigned label, PHP just won't produce the proper result
because the label that was popped provides the context for interpreting
the upstream-assigned label that has now risen to the top of the stack.)
As to why we are choosing PHP rather than UHP as the default, it is
our belief that most of the platforms which support MPLS today have
been implemented to take advantage of the optimizations that PHP
affords. In some cases it may even be true that UHP is not supported
at all.
I believe you are correct about this.
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