On Dec 7, 2007, at 2:17 AM, Pascal Thubert (pthubert) wrote:
Hi
I tend to agree with Tim here; multicast is a complex issue in the low
power / sleepy space. It's even unclear which WG should be responsible
to define it.
Back to basics, there are basically 2 extreme models to locate
somebody;
cry out loud or white board. ND on Ethernet uses the first model based
on multicast. Mobile IP uses the second. When you look up a mobile
node
on the Home Link, the Home Agent is the reference that responds the ND
requests on behalf of the mobile nodes.
So my question is: Should we take as granted that ND on the LoWPAN
requires multicast? Maybe the white board model based on unicast is
enough, in which case we get rid of a very difficult dependency.
Considering that there is a need for a router that understands 6LowPAN
to connect the LoWPAN to the Internet, that router is the natural
location for a white board.
The issue here is whether sleep schedules are visible to the network
layer, or whether it observes longer latency. MACs which do not
support a "cry out" approach, e.g., TDMA with no broadcast slot,
typically emulate broadcast through unicast to each neighbor for
which a slot exists. The network layer is not made aware of this, as
the TDMA schedule will by necessity constrain the set of neighbors
that you have a "link" to. Or are you assuming mesh under?
So the concept of backbone router is this: cry out loud on the high
speed backbone that federates LoWPANs, and white board on the LoWPAN,
and ND proxying to federate the whole thing. The backbone router
implements ND proxying in a fashion that is compatible with mobile
IP so
one day, sensors with a global address can move away and stay
virtually
there.
I guess I don't quite understand why you think the whiteboard
approach is necessary. Can you describe a use case where "cry out"
doesn't work? The "nodes are off" case seems very suspect to me: if
nodes are *really* off, e.g., for long, application-level periods,
you would not expect multicast to reach them. Instead, you would
depend on higher-layer protocols (e.g., SRM) for improving
reliability. If nodes are off for very short, link-layer periods, the
MAC should take care of this. Networks in deployment today tend to
solely use the latter approach, as it's quite sufficient.
It might be the best approach for performance reasons, but it may
not be. The fact that such a tradeoff might depend on the actual
media access approach used suggests to me that this is something e
In the meantime, a link local address is enough to connect to any node
in the network federated by backbone routers,
So this assumes mesh under?
and a mote can move from a
backbone router to another within the federated network without
renumbering.
This is definitely a desirable property.
Phil
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