Re: [spring] Updating the SPRING WG Charter

[Eric C Rosen <[e-mail]>]

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.