Re: [6tisch] Handling Inconsistent Allocation in 6P

[Xavi Vilajosana Guillen <[e-mail]>]

Hi Yatch,
my 2 cents inline


> I've been thinking about how to handle inconsistencies. I know the
> current draft has an inconsistency detection mechanism with generation
> management; just wondering if there is another way or a supplemental
> mechanism to deal with such a situation.
>
> We decided at the IETF meeting last week to reduce the number of
generation counters from 2 to 1 (2bits field) as now 6P commands can add
different types of cells so we need to account for transactions now. I
state that here to outline that the proposed mechanism is very simple. At
every transaction we increment a generation counter. It cannot happen that
the two sides of the transaction have inconsistent counters. If this
happens, then the schedules are reset. I agree that this is detected after
the error has occurred.


> I thought that the 2-phase commit (2PC) protocol could be useful
> here. Then, I found the nice idea by Nicola in the ML archive. In
> terms of the 2PC protocol, 6P ACK is Commit. 6P NACK (mentioned in
> another email by Nicola) is Abort or Rollback.
> # We may need another type of message to acknowledge Commit or Abort.
>
> An advantage of this approach is that 6P can resolve an inconsistency
> when it occurs at the least cost, by cancelling the concerned
> operation alone. An apparent disadvantage is adding further complexity
> to 6P.
>

it adds complexity and more messages over the air, which are costly and can
also fail (e.g external interference). What happens if we loose the 6P
NACK? How the NACK sender know that the NACK has been received?

>
> What others think...?
>

I like to answer with another question. What causes less overhead, 2 bits
per each 6P command or 1 or 2 extra packets per transaction (assuming only
write/state modification transactions). For me the former is way simpler.

regards,
X



>
> Best,
> Yatch
>
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-- 
Dr. Xavier Vilajosana Guillén­
Research Professor
Wireless Networks Research Group
Internet Interdisciplinary Institute (IN3)
Universitat Oberta de Catalunya­

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