Re: [v6ops] ref Hosts dont MLD to join LL groups

Enno Rey <erey@ernw.de> Thu, 23 July 2015 13:01 UTC

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Date: Thu, 23 Jul 2015 15:01:52 +0200
From: Enno Rey <erey@ernw.de>
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Subject: Re: [v6ops] ref Hosts dont MLD to join LL groups
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Hi,

> 
> Inform the sender willingnes to be part of the group.  If not part of
> group, then sender will not send it.  Same concept at L2 and at L3.

wrong.
the sender of a MC message just sends it out, regardless if somebody listens or not.
the router(s) helping to distribute this message (in case of interdomain MC) only have to know (= are interested) if there's at least one interested listener on an adjacent link.
neither the sender nor the router(s) have any interest (or need to have that) in individual listeners.


>
> 
> > strictly speaking MLD as a whole is only needed for interdomain
> > multicast anyway.
> 
> Disagree - beyond just interdomain multicast, Ethernet has link-layer
> multicast builtin.  There is no 'broadcast' address anymore in Ethernet
> since some time, replaced by 'multicast': supposedly better at saving
> battery and other advantages.
> 
> > on the local link you join a MC group by kind-of self declaration
> > ("hey I consider myself part of that group
> 
> That is an Ethernet message.

no. that's a decision taken somewhere in the software/stack. no need for any packet here.

> 
> Some cards send that message, others don't.  All should.
> 
> > so I'm interested in
> > certain traffic and hence instruct my stack to listen to/process
> > packets with certain addresses"). no need of MLD for that action.
> 
> How does the Ethernet layer know this is a Router, and not a Host?  Only 
> by knowing that it can join the all-hosts or all-routers link-layer 
> address.  And only the IP stack knows whether it's a Host or a Router. 
> So the IP stack should tell the Ethernet layer to make that link-layer join.

which it does, as a stack-internal decision.
"once I'm a router get me everything sent to ff02::1/33:33:00:00:00:01. in addition - given I'm a node anyway - get me everything for ff02::2". no need to send any packet for all this.

best

Enno





> 
> Yours,
> 
> Alex
> 
> > but
> > "joining" on the local-link doesn't need MLD.
> >
> > best
> >
> > Enno
> >
> >
> >
> >
> >
> >>
> >> Alex
> >>
> >> Le 23/07/2015 13:51, Ole Troan a ??crit :
> >>> Alexandru,
> >>>
> >>> I???m afraid I couldn???t interpret your message. would someone
> >>> else be able to translate?
> >>>
> >>> cheers, Ole
> >>>
> >>>>>
> >>>>> I do wonder if we should expand that exception to all
> >>>>> link-scope multicast addresses.
> >>>>>
> >>>>
> >>>> I would beg to disagree.
> >>>>
> >>>> If we expand that to all link-scoped groups, may lead to
> >>>> dismantling IPv6 dependence on 33::1 - ff:ff:ff:ff:ff would be
> >>>> sufficient.
> >>>>
> >>>> My oppinion would rather be to modify the MLD RFC to mandate
> >>>> MLD joins for all scopes.
> >>>>
> >>>> Ethernet has primitives for joining the corresponding
> >>>> link-layer groups, and in some cases they are used. Maybe all
> >>>> should use them.
> >>>>
> >>>>> the bridge implementors I speak to tell me that they don???t
> >>>>> have enough state to do MLD snooping for link-local scoped
> >>>>> multicast addresses anyway...
> >>>>
> >>>> This may be dumb from my side, but why dont bridge
> >>>> implementers use link-layer multicast?  They shouldnt implement
> >>>> MLD, and not snoop it. The Hosts should send the necessary
> >>>> link-layer multicast joins (triggered by themselves sending MLD
> >>>> REPORT for these groups) to the bridge addresses.
> >>>>
> >>>> Alex
> >>>>
> >>>>> cheers, Ole
> >>>
> >>
> >> _______________________________________________ v6ops mailing list
> >>  v6ops@ietf.org https://www.ietf.org/mailman/listinfo/v6ops
> >
> 
> _______________________________________________
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> v6ops@ietf.org
> https://www.ietf.org/mailman/listinfo/v6ops

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Enno Rey

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