From: Carsten Bormann <[email protected]>
Date: Wed, 14 Oct 2009 21:30:26 +0200
On Oct 14, 2009, at 20:45, Zach Shelby wrote:
> I agree that within the LoWPAN some kind of 1-hop NUD may be useful,
> and that should be discussed.
A couple of thoughts about that:
For route-over (where IP hop = MAC hop), much of that need is filled
by 802.15.4 ACKs.
(If there are no actual data, just a probe, it might be beneficial to
add a NOP dispatch type to 4944 at some point...)
One disadvantage of 802.15.4-2006 ACKs is that they are not secure.
15.4e is scheduled to fix this, but we may not want to wait for that.
I have generally found 802.15.4 ACKs unhelpful for NUD.
There a lots of ways in which an ACKed packet can fail to
reach the higher layers. For example, any kind of MAC
security confusion can cause packets to be ACKed but
otherwise ignored.
What has worked for NUD, between routers at least, is
relying on link quality measurements. In the systems that I
have used each router periodically informs its neighbors of
its own link quality estimates in order to detect asymmetric
links. This has the side effect of removing any need for
NUD above the link layer.
In summary, I think that 1-hop NUD is important, but that
it doesn't necessarily have to be done at layer 3.
-Richard Kelsey
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