BT Mesh secure beacon propagation speed during IV update

Question

I have a question about propagation speed of the secure network beacon over a bluetooth mesh networks. 
 Last year we installed a mesh network consisting of 1750 light fixture each with a NRF52840 bluetooth mesh node. These lights are dimmable, and arranged in a total of 60+ groups. There is an edge device with a nrf52840 dongle that controls the dimming setting of all the groups, this one transmits somewhere between 20.000 and 100.000 messages per day. This is a higly dynamic installation where our lights compensate for fluctuations in natural daylight, so the dim levels throughout the facility fluctuate all the time. 
 This has been running flawless for many months, however the transmitting node is close to reaching the sequence limit and it will start the IV update procedure. This will involve propagating a secure network beacon throughout the network signaling the update procedure state should be entered. Once this has been done, as I understand it, one of the nodes in the network will increment its IV and switch to normal mode. This node will transmit a secure beacon with the incremented IV and "normal mode", triggering immediate increment of the IV index by all the nodes receiving that secure beacon. 
 I was wondering, how soon will those nodes also start relaying the secure network beacon? And will they be competing with surrounding nodes transmitting the old IV index beacon? As I understood each node strives towards making sure there is a beacon approximately every 10 seconds, so if it does not see one for more than that, it will transmit it's own. Will a node that has already increased its IV index, see an old beacon and transmit the new beacon faster to make sure it will "win"? Or is it a matter of change which nodes win, irrespective if they have an old or new beacon, and will it just be a matter of luck before all nodes have finally seen the new beacon?

Hieu · Accepted Answer

Hi tmennink, 
 tmennink said: Luckily there is the rule of no more than 600 seconds between beacon transmissions for any node, and this will eventually force every node to send out a beacon. The only "bad" thing that can happen is, if you turn on the entire network all at once, all those nodes will reach their 10 minute limit at approximately the same time. This might cause a large burst of beacons coming all at the same time. 
 [...] 
 But anyhow, it seems we don't have to worry that an IV update gets stuck in a small part of the network, it will be forced to propagate everywhere. I expect this to happen over the course of about 4 hours 
 Yes, I share the same conclusion as you. If you calculate that it will be done in about 4 hours, then that would be all right. As noted in Section 3.10.5, still in the Mesh Profile Specification v1.0.1, a node must go into IV Update In Progress at least 96 hours before the sequence numbers are exhausted. The update thus will complete in time. 
 tmennink said: What I was fearing, and what seems to be confirmed by the simulation, is that quite quickly a sort of oscillating pattern arises where only a handful of nodes are sending out beacons approximately every 10 seconds, while all other nodes never take a turn because the cadence of the beacons they receive from that handful of nodes will never trigger them to send out a beacon themselves. 
 Is there still a concern about this? Are you worried that the update will not propagate across the network in a short enough time, or is it about something else?

BT Mesh secure beacon propagation speed during IV update

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