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Yes, this seems to be one of the rather clever aspects of the design - this appears to make it possible to discover iOS devices that are in the proprietary "iOS-only" mode of discovery. It seems that once one of these "overflow" advertisements is discovered, it attempts a connect that is enough to wake the device and carry out normal tracing. I left an iPhone unplugged, stationary, idle and asleep, running the app for about 5 hours. Then introduced an Android device and it was detected immediately. That seems to suggest this aspect works well. There's also a clever "ping-pong" type keepalive system at play - there are multiple characteristics broadcast, and one of them is for keep-alive. It seems devices also use this keepalive to keep the app active over time. That should work with even 2 devices - if I understand it correctly, device A will K/A device B after a period of time, and this will cause B to act. That means B will then K/A device A after a period of time, and the cycle can continue. I see the concerns about "2 sleeping iOS devices might not find each other", but wonder to what extent this will be a realistic scenario, versus an academic one - given the extent to which people are on their phones these days, and the size of that population, the issue likely fades in significance. Especially considering no app will get full adoption, no system will ever be perfect, and it's very much all about probabilities - the app can't determine if your 30 second exposure was higher risk than 30 minutes sitting 5m apart. Hopefully the testing phase currently underway will give some insight as to if this is a problem. |