[6tisch] Time Synchronization

[Tero Kivinen <[e-mail]>]

Oliver Hahm writes:
> Probably I miss something obvious here, but I just stumbled about two things
> in RFC7554 and Minimal that seem to be contradicting at a first glance:
> RFC7554 in A8 states: "Nodes can keep synchronization exclusively by
> exchanging EBs." However, draft-ietf-6tisch-minimal-14 in section 6 states:
> "EBs MUST NOT be used for time synchronization."
> 
> Can anyone enlighten me how to interpret this? May EBs be used for time
> synchronization or not?

In the TSCH there is two ways to do time synchronization:
acknowledgment-based and frame-based (see section 6.5.3.2 of
802.15.4-2015).

Enhanced beacons are broadcast messages, thus they are not
acknowledged, so acknowledgment-bsaed synchronization cannot be used.

On the other hand Frame based could be used for the Enhanced beacons,
but the problem is that if Enhanced beacons are not authenticated with
proper secret key, the attacker could perhaps send beacons in wrong
time.

I.e., it is better to use the acknowledgment-based node syncronization
using properly generated link keys, or if using Frame-based
authentication, use frames which use some other key than K1, and thats
why the minimal draft says that Enhanced Becaons are not used for time
syncronization. 
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