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

Alexandru Petrescu <alexandru.petrescu@gmail.com> Thu, 23 July 2015 12:49 UTC

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From: Alexandru Petrescu <alexandru.petrescu@gmail.com>
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Date: Thu, 23 Jul 2015 14:49:09 +0200
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Subject: Re: [v6ops] ref Hosts dont MLD to join LL groups
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Le 23/07/2015 14:27, Enno Rey a écrit :
> Hi,
>
> On Thu, Jul 23, 2015 at 02:16:43PM +0200, Alexandru Petrescu wrote:
>> If one disallows all MLD joins for all link-scoped IP multicast
>> addresses - why does one need IP multicast addresses with link
>> scope?
>>
>> Why does one need the lino-layer  33::1 address?
>>
>> If one doesnt need 33::1 then why Ethernet provides it?
>>
>> I guess what I am trying to say is that it's useless to have
>> multicast groups if one can't join them.
>
> define "join a multicast group"...

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.


> 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.

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.

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
>>>
>>
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>