Re: [Roll] multicast & MLD on LLN

Don Sturek <> Thu, 16 October 2014 13:09 UTC

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Date: Thu, 16 Oct 2014 06:09:06 -0700
From: Don Sturek <>
To: "Pascal Thubert (pthubert)" <>, "Turner, Randy" <>, Kerry Lynn <>
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Thread-Topic: [Roll] multicast & MLD on LLN
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Cc: "Greg Shepherd (shep)" <>, Routing Over Low power and Lossy networks <>, "IJsbrand Wijnands (iwijnand)" <>
Subject: Re: [Roll] multicast & MLD on LLN
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Hi Pascal,

Currently, all 5000 devices are members of the same DODAG.


On 10/16/14 5:54 AM, "Pascal Thubert (pthubert)" <>

>Hello Michael
>I think that saying that MPL is the default is quite exaggerated.
>RPL has a classical multicast built in, which is a simple extension to
>storing mode (
>The trouble is that this hardly fits with non-storing.
>BIER (draft-wijnands-bier-architecture-00.txt) seems an appropriate
>approach for non-storing devices, considering that it makes the same
>tradeoff of bytes in packets vs. state in the nodes. BIER could be used
>for both unicast and multicast, unicast being seen as an extreme form of
>multicast. Each address, or group, or (source, group),  the node
>registers unicast to the root in pretty much the same way as we do today
>in non-storing. There is a bit offset associated to each node. When a
>packet comes in, the root looks up the list of interested nodes, sets a
>bit map at all the relevant bit offsets, and encapsulates the packet
>adding that bitmap, e.g. in an address or an option header. Each node
>maintains a bitmap per child down the RPL preferred tree, which has the
>bits set for the end nodes that are reachable via this child. These
>bitmaps could be built and re-advertised over classical storing-mode DAO
>messages whereby a node just advertises the AND of all its bitmaps to its
>preferred parent.
>It would seem to be the right approach for Don's problem if it were not
>for the sheer size of " Around 5000 devices in the network total ".
>BIER needs one bit per device in the routing bitmap. We cannot have a 5K
>bitmap in every packet. If we steal one bit in the ff1 (RFC 7371) to
>indicate a BIER routing, we could fit up to 112 hosts in a multicast IPv6
>address.  To reach 5K, we'll need to subnet. For instance, by using 8
>bits as a prefix, we could carve out 256 sets of 104 devices. For 5000
>nodes, this means roughly 50 groups, and could multiply the state in
>every node by 50, though the numbers are probably a lot less if the sets
>are formed based on geography. But then, the root may have to send
>multiple copies of a packet, as many as there are groups with at least
>one listener for that packet.
>Don: are these nodes all in a same DODAG? Or are they partitioned in
>smaller groups through multiple roots, or multiple instances?
>> -----Original Message-----
>> From: Roll [] On Behalf Of Michael
>> Sent: mercredi 15 octobre 2014 23:57
>> To: Routing Over Low power and Lossy networks
>> Cc: IJsbrand Wijnands (iwijnand)
>> Subject: Re: [Roll] multicast & MLD on LLN
>> Kerry Lynn <> wrote:
>>     > On Sat, Oct 11, 2014 at 2:39 AM, Carsten Bormann <>
>> wrote:
>>     >> Right, MLD is a host-router protocol.  Routers among them speak a
>>     >> routing protocol, so they wouldn¹t exchange MLD.  (A RPL ³leaf²
>>is a
>>     >> router.)
>>     >>
>>     >> > Finally I'm looking at BIER see how their ideas could apply to
>>     >>
>>     >> Yep.  We (TZI) have done (specified, implemented, analyzed) an
>>     >> efficient BIER-like multicast forwarding protocol for
>>non-storing mode
>>     >> a while ago.
>>     >>
>>     > I admit to some confusion here.  Is (topology-free) MPL no longer
>>     > default multicast forwarding protocol for LLN?
>> I don't know anything about BIER really... MPL is still the default at
>> point.
>> (proceedural delays getting trickle-mcast out, not-with-standing) My
>>take on
>> the conversation is that BIER has some kind of helper node that unicasts
>> everything out as appropriate.
>> --
>> Michael Richardson <>, Sandelman Software Works
>> IETF ROLL WG co-chair.