Subject: Re: [manet] DLEP ip multicast destinations

From: Toerless Eckert <[email-masked]>
Date: Sat, 26 Jul 2014 10:06:49 -0700

Some thoughts about IP multicast and DLEP:

a) As mentioned in another thread, there seems to be no indication in
   DLEP whether or not the radio network supports L2 multicast. I
   think it would be great if there was such an indication, so the
   router(s) would know whether they need to send L3 multicast as
   L2 unicast (so called NBMA mode). It would also help to indicate
   that unicast destination entries MUST have IP address entries.

b) The current Multicast dfefinition in DLEP is very lightweight,
   which is nice because modem vendors could implement a lot of different
   things. It is bad though, because to router vendors, the current
   DLEP mechanisms do not expose what the modems do and therefore router
   vendors can not automatically adopt to what modems need.

   A couple of different situations:

   b.1) The modem connected to the router is set up to only forward
   specific multicast traffic to that router, if the router indicates
   interest in the multicast via:
     I) DLEP destination
     II) IGMP
     III) PIM
     IV) GARP/GMRP
   
   b.2) The RF network is set up to indicate multicast destinations
   via DLEP to routers only if the routers indicate interest in the multicast
   via:
     I) DLEP destination
     II) IGMP
     III) PIM
     IV) GARP/GMRP
   
   Of course, b.2) does not imply b.1). The DLEP modems may not
   constrain any IP multicast, but they may indicate the multicast
   path characteristic based on what other modem/router need to receive
   it.

As you can see, without DLEP modems telling routers how they
operate for multicast, the DLEP router setup for multicast is a
mess of nerd knobs and a mess of protocols that likely need to be
supported.

Traditionally, modems have primarily supported IGMP snooping, which
is a hack, but they have done it because "hosts" normally speak IGMP.
But then a lot of these modems (eg; HAIPE) started to get used not
in conjunction with hosts, but with routers, and that made life really
really miserable for those routers: They had to do IGMP where they
really wanted to do PIM. And they could not give up PIM, because the
router on the other side of the modem still needed it. So we ended
up having to do both. 

PIM snooping is not much better. Unlike IGMP snooping in modems/switches,
which at least has an information RFC, PIM snooping (luckily ?) does not
even have one.

Long time ago, we did put in RFC3488 (RGMP) for router<->switch/modem
communications that helps to overcome those snooping issues. Alas,
it was done so early that we did then not get SSM support in.

Long story short: It would be very helpfull to understand better,
what the stakeholder in DLEP really expect to get done with multicast,
and/or what already exists or is planned in modem implementations. That
would help a lot to decide what DLEP should do, and how we can best
document it.

IMHO, Ideally, DLEP should allow to run multicast over a radio network
with the optional ability of modems to to constrain the forwarding
of IP multicast to only those routers that need it. And without forcing
routers to do IGMP. 

Cheers
    Toerless