Colin O'Flynn a écrit :
Another way of saying this is that mesh under is more often than
not akin to NBMA. Fully mesh might exist and multicast emulation
might exist though I've seen neither so far.
FYI 802.15.5 does have multicast support (15.5 being the mesh
standard). If you have IEEE Xplore you can get 15.5 from there,
otherwise it should be out around Dec 8th at
http://standards.ieee.org/getieee802/802.15.html by my count if they
keep up with the '6 months after publication in PDF it's free'.
I also have access to it.
Multicast is done with a combination of broadcast & unicasting
messages. It has systems to avoid a forwarding loop and some
considerations to low-power.
Despite the addition of the multicasting support, it does remain
wasteful of network air time & node power. So even when we do have
support for Multicasting at the mesh layer, there are significant
advantages to avoiding it.
And what makes one think that the implementation of multicast at IP
layer (instead of reusing the 802.15.5 link-layer multicast support) is
less wasteful?
For example, one version of the 6lowpan-ND spec (the current?) said at
one point that the Edge Router performs the link-layer multicasted ND
messages on the backbone (supposedly link-layer multicast capable), on
behalf of the 6lowpan nodes, which don't speak link-layer multicast - is
it efficient to have these numerous MLD messages on the backbone for
nodes which are not on the backbone?
I mean this paragraph:
In the Extended LoWPAN case, the Edge Router also performs proxy ND
operations over the Backbone Link on behalf of the LoWPAN Nodes that
are registered to it.
Is it efficient for the Edge Router to send a myriad of Neighbor
Advertisement messages (which are addressed to a link-layer multicast
address, and a L3 multicast address) on the backbone?
Why cluttering the backbone?
Alex
Regards,
-Colin O'Flynn
-----Original Message----- From: [email protected]
[mailto:[email protected]] On Behalf Of Pascal Thubert
(pthubert) Sent: November 16, 2009 7:08 AM To: Carsten Bormann; Ralph
Droms (rdroms) Cc: 6lowpan Subject: Re: [6lowpan] Thoughts on
draft-ietf-6lowpan-nd-07
Hi Ralph
1. In a mesh-under network, the L2 characteristics of the lowpan
are close to those usually assumed for implementation of IPv6; in
particular, there are no "lowpan routers" at L3 and L3 messages
*can* be delivered (perhaps with lower probability of success)
directly between lowpan nodes. Why is ND not sufficient in this
model?
RFC 4861 ND works in a pure RFC 4944 mesh-under network if: 1) the
mesh-under (L2) routing protocol provides subnet-wide multicast, 2)
that is efficient enough to be used for routine ND messages, 3)
nodes are awake often enough to detect and reply to NS messages.
Such networks do exist, but these assumptions are not necessarily
compatible with many application
scenarios we have in mind; this is the reason we started with ND
optimizations.
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