Subject: [Roll] timer-based counters

From: Philip Levis <[email-masked]>
Date: Mon, 21 Jun 2010 09:16:09 -0700

One issue that has come up in RPL security is the use of timer-based counters. This mechanism is similar to that used in 802.15.4e, where nodes have an approximate global clock and can use that clock to detect and reject delayed packets.

Because we don't want to enforce the presence of such a clock in RPL, this means that nodes with clocks and those without need to be able to interoperate. This is what calls for the 'T' flag in the security section (different than the soon-to-be defunct 'T' flag in a DIO). There are also implications to counter compression.

The question I'd like to ask the WG is: Is the benefit of supporting clocks directly in RPL worth the added complexity of clock/no-clock interoperability?

Phil

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From: Alexandru Petrescu <[email-masked]>
Date: Mon, 21 Jun 2010 19:55:21 +0200

Le 21/06/2010 18:16, Philip Levis a écrit :
> One issue that has come up in RPL security is the use of timer-based
>  counters. This mechanism is similar to that used in 802.15.4e, where
>  nodes have an approximate global clock and can use that clock to
> detect and reject delayed packets.
>
> Because we don't want to enforce the presence of such a clock in RPL,
> this means that nodes with clocks and those without need to be able
> to interoperate. This is what calls for the 'T' flag in the security
> section (different than the soon-to-be defunct 'T' flag in a DIO).
> There are also implications to counter compression.
>
> The question I'd like to ask the WG is: Is the benefit of supporting
>  clocks directly in RPL worth the added complexity of clock/no-clock
>  interoperability?

Clock is not my specialty but I'd say: no, the benefit of supporting
clocks directly in RPL is not worth the added complexity of
clock/no-clock interop.

One wouldn't want an IP path to break because the clock is not synched
enough.

Clock is good for many things.  Precise clock availability has evolved
dramatically in recent years, witness the miniaturization of GPS, GSM,
LF (wristwatches) and NTP devices.

However, I am generally reluctant to rely the protocol inner function to
depend on clock.  Because clock sync is difficult to achieve.  It may be
easier between computers communicating on same link-layer but very hard
over various links.

I once did a test for clock synch : NTP, GPS, GSM - the time indicated
on same computer (and watch) varied as much as 1second.  The time
shown by a TV broadcaster was even larger.

If all RPL computers had NTP then the precision is guaranteed in the
order of hundreds of milliseconds.  Is this enough, not enough - hard to
say.  Even harder to say in the low-bandwidth constrained devices: how
would one synch the clock when the time to communicate a timestamp is
the one of 300baud speed.

Clock is not my specialty but I have a reluctance towards relying
protocol inner workings on clock.

Alex

>
> Phil _______________________________________________ Roll mailing
> list [email-masked] https://www.ietf.org/mailman/listinfo/roll
>

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From: Richard Kelsey <[email-masked]>
Date: Tue, 22 Jun 2010 14:04:24 -0400

> From: Philip Levis <[email-masked]>
> Date: Mon, 21 Jun 2010 09:16:09 -0700
> 

Using this for security requires that the clock synchronization
is robust in the face of attackers.  Does this require Byzantine
fault tolerance?  It seems likely to get complicated.

                                      -Richard Kelsey

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From: JP Vasseur <[email-masked]>
Date: Wed, 23 Jun 2010 15:56:38 +0200

> fault tolerance?  It seems likely to get complicated.

Same feeling.

JP.

>
>                                      -Richard Kelsey
> _______________________________________________
> Roll mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/roll

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From: Henning Rogge <[email-masked]>
Date: Wed, 23 Jun 2010 16:44:54 +0200

> > fault tolerance?  It seems likely to get complicated.
Global clock synchronization is a horror in MANETs... even without an 
attacker.

Henning

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