Date: Wed, 10 Jun 2009 18:53:43 +0300
From: Zach Shelby <[email protected]>
Yep, DAD is the core feature.
So apart from a generic discussion about whiteboards - how about we
concentrate on technical details. As a WG we really need to check that
all the fine details here work - and if not suggest a way to fix/improve
them. We would like to submit -04 before the Stockholm cutoff.
Zach,
Requiring that Edge Routers have a whiteboard was a
significant change from -02 to -03. I am concerned because
the whiteboard is a single point of failure and because of
the memory required to store it.
Yes, if an Edge Router has a full IPv6 stack, or generally
has plenty of memory, it should certainly have a whiteboard.
In an extended LoWPAN, the Edge Routers clearly have to
have whiteboards. My concern is with ad-hoc networks of
small devices.
It looks to me as if a whiteboard entry requires at least 20
bytes or so, or 2k bytes for a 100-node network. Fitting
that into an embedded device with, say, 6k of RAM is going
to be difficult. If the core benefit is detecting duplicate
EUI64s, it really doesn't seem worth the trouble. As I
mentioned before, having the Edge Router assign 16-bit IDs
helps with header compression, but is not required for
routing.
As to the technical details, if DAD is the main purpose of
the whiteboard, it seems to me that it might be able to do
so in a more timely fashion. As I understand the protocol,
if a second node appears with the same EUI64 as a device
already in the network, the collision will not be detected
until the existing device renews its registration. The
first appearance of the new device will be treated as a
reboot by the original one. Would it be OK to require that
routers keep address data in non-volatile memory? If not,
is the collision going to have to be re-detected every time
the colliding device reboots?
By the way, if two devices with the same EUI64 are powered
up or reboot at the same time, it looks to me as if the
conflict might never be detected. Assuming that the Edge
Router gives them the same registration lifetimes, the two
TIDs will move forward in leap frog fashion, never getting
more than one removed from each other.
-Richard Kelsey
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