Hi Pars,
Pars Mutaf wrote:
I believe understand the motivations behind avoiding broadcast NS messages
upon incoming session. However, what happens if the whiteboard approach fails
i.e. if whiteboard bindings are lost due to edge router crash, reboot
or changing
the edge router for whatever administrative reason?
This is a good question. As whiteboards have soft state, and nodes
refresh the addresses they are using periodically, a whiteboard failure
is not a disaster to the network. If an Edge Router crashes:
1. If the ER reboots without state - the next NR message will refresh
the entries. It could be nice if the LoWPAN somehow knew that this
happened (how?) to avoid waiting up to NR_PERIOD. At least its default
route will stop working, that is an indication.
2. If the ER reboots and kept its whiteboard state (recommended) - then
of course the ER can't forward on behalf of the node while down. Same
for any router, so this isn't a whiteboard problem.
3. If the ER goes down permanently, then the node will detect this
because its default route no longer works or the NR can no longer be
sent to the ER. It either starts using an alternative ER or continues
operating until a new ER comes on-line.
In any case the LoWPAN can continue to operate while an ER is down
(routing within the LoWPAN). In an extended LoWPAN nodes will start
using alternative ERs automatically. Several techniques could be used by
an ER vendor to automatically switch state to an alternative ER, use
non-volatile memory for Whiteboard state, use secondary bindings etc.
Section 7.9 of nd-04 deals with this partially:
http://tools.ietf.org/html/draft-ietf-6lowpan-nd-04#section-7.9
A related question: how frequent you believe broadcast NS messages
(i.e. incoming
sessions) will be in a 6lowpan network? Or, sessions will be mostly initiated by
6lowpan nodes?
We aren't using NS messages at all inside the LoWPAN. Node Registration
messages are always node initiated.
- Zach
Regards,
pars
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