MPLS VPNs or not?

* Per hop policy decisions can be made more effectively in MPLS than
  in IP. Not true in theory unless you want to look very deep in
  the packet to identify the policy association, though it may be
  true in practice on certain current systems.

MPLS doesn't require per-hop policy decisions. Policy decisions only
need to be made at the edge, re FEC inclusion. Intelligence at the edge
etc. Parallels with the diffserv model of classifying & marking packets
at the edge so you only need to look at PHBs in the middle.

Hi Scott:

Sorry I was too cryptic here -- sure MPLS makes a policy decision -- it
decides how to forwarding based on the tag (e.g. the policy is embedded in
the tag). My point is that you could just as easily associate the forwarding
rule with a key, made up, say from source and destination address (which in
some route lookup schemes requires only one more memory access than looking
up purely on destination).

* Instantiation of per-hop policy information via MPLS is more scalable
  than it would be in IP (not quite said above but an implied issue).
  Almost certainly not true (see above about general policy being hard
  being why IP doesn't do it).

Instantiation of per-hop policy in MPLS consists of forwarding by LSP,
except at the edge router.

Except that something has to decide where the the path goes (and thus,
has to execute the policy at something close to a network wide level in
terms of analyzing the network and instantiating the path). If you're
suggesting we can do policy purely at the edges, then presumably a
routing protocol could equally well force its policy information to only
be computed at the edges. Yes? Or am I missing something?