Subject: [6lowpan] Edge router crash, reboot or new edge router

From: Pars Mutaf <[email-masked]>
Date: Thu, 16 Jul 2009 19:33:02 +0300

Hello,

I have some doubts about the 6lowpan ND protocol, concerning its reliablity.

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?

Is there any countermeasure for this scenario? Note that standard ND is costly
but perfectly handles such a situation.

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?

Regards,

pars

---

From: Zach Shelby <[email-masked]>
Date: Mon, 27 Jul 2009 15:01:58 +0300

Hi Pars,

> 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

> sessions) will be in a 6lowpan network? Or, sessions will be mostly initiated by

We aren't using NS messages at all inside the LoWPAN. Node Registration 
messages are always node initiated.

- Zach

> Regards,
> 
> pars
> _______________________________________________
> 6lowpan mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/6lowpan

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Zach Shelby
Head of Research
Sensinode Ltd.
Kidekuja 2
88610 Vuokatti, FINLAND

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