MPLS VPNs or not?

Wed Aug 8 12:34:35 UTC 2001

In message <20010808103821.H1592 at SBRIM-W2K>, Scott Brim writes:

>> * 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?

Craig