On 6/7/2018 10:55 AM, Robert Raszuk wrote:
Imagine a segment with embedded meaning that packets received with
such value X should be replicated to interfaces Y & Z. Such decision
can be configured from controller or locally computed.
Nothing there is per-path as well as nothing there is per flow or per
multicast group. It is a local function.
How can you say "nothing there per-path"? The label X represents a
multicast tree, and thus constitutes per-path state. If some packets
need to go to Y and Z, some need to go to Y, Z, and W, some need to go
to Y, U, and V, etc., you obviously need a different label for each
different multicast tree, and appropriate per-tree state.
Using a controller to set up multicast paths may be a good idea in some
scenarios, but let's not pretend it doesn't create per-path state in the
router. Per-path (per-tree) is not the same as per-flow or per-group,
of course, but that's true of any technique that aggregates flows into
multicast tunnels.
Note also that if the label X is domain-wide unique and there is no RPF
check, there is the possibility of nasty multicast loops. These is some
discussion of this in draft-zzhang-bess-bgp-multicast-controller-00.
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