On Fri, 12 Jun 2020 at 18:42, David Sinn <dsinn(a)dsinn.com> wrote:
> But why do you need full table lookup in the middle of the network? Why place that class of gear where it's not needed?
Some people do collapsed core. But this is getting bit theoretical,
because we definitely could do this on IP, we could do some lookups on
on-chip and some off-chip to do both, should market want it.
> The label stack question is about the comparisons between the two extremes of SR that you can be in. You either label your packet just for it's ultimate destination or you apply the stack of the points you want to pass through.
Quite, but transit won't inspect the stack, it doesn't have to care
about it, so it can be very deep.
> In the former case you are, at the forwarding plane, equal to what you see with traditional MPLS today, with every node along the path needing to know how to reach the end-point. Yes, you have lowered label space from traditional MPLS, but that can be done with site-cast labels already. And, while the nodes don't have to actually swap labels, when you look at commodity implementations (across the last three generations since you want to do this with what is deployed, not wholesale replace the network) a null swap still ends up eating a unique egress next-hop entry. So from a hardware perspective, you haven't improved anything. Your ECMP group count is high.
I don't disagree. What I'm trying to say, however you tunnel, you have
the same issues. If you need to tunnel, then MPLS is better tunnel
than IP. Ultimately both can be made LEM on-chip should market want
it, so difference left is what is the overhead of the tunnel, and MPLS
wins here handsdown. This is objectively true, now what practical
market reality is, that may be different, because market doesn't
optimise for best solution.
> So, yes, MPLS works fine if you want to buy big iron boxes. But that come at a cost. So the point about MPLS is always better is not accurate. Engineering is about trade-offs and there are trade-offs to be made when you optimize in a different direction and that points away from MPLS and back to IPIP
Always if you need tunnel. Because the fundamental question is how
much overhead we have, and what is our key width. In both IP tunnel
and MPLS tunnel cases we will assume LEM lookup, to keep the lookup
cheap.
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++ytti
