poll should return with an exceptional condition if the socket dies. If it doesn't this is a problem the OS should solve; it's trivial to do if you can see the connection go down/up again.
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Connections die pretty quickly if you aren't sending anything through them on mobile networks and you don't receive reliable notice of that. OS doesn't know about it either and the device is generally asleep when this happens. Need to decide how often to wake to keep them going.
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Is this NAT-hell only? In principle it shouldn't happen on a real IPv6 network, which we'll hopefully be able to assume in a few years...
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An IPv6 network without stateful firewalls shouldn't have this issue, but stateful firewalls will still end up dropping old / inactive connections. There's also just a whole lot of broken stuff. I think there are a lot of pure IPv6 mobile networks already and it still happens.
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Why would you put stateful firewalls on an IPv6 network with millions of nodes? Do they just *want* to burn money and spew CO2?
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I don't know what they are doing or why, but carriers love dropping connections. *shrug*
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en.wikipedia.org/wiki/Carrier-g is definitely the main issue, but IPv6 definitely isn't 100% reliable either. The code really needs to be designed to actively figure this out.
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Shouldn't TCP keepalive at the kernel level be able to handle it?
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The device goes to sleep though and that's not going to wake it up. The only way it could work is if it was offloaded to hardware that kept running. It's totally possible that exists, I don't know.
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But kernel can and *should* schedule periodic wakeups just to do stuff like keepalive. Unlike gigantic userspace apps it could do it very efficiently.
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Apps aren't allowed to keep the device awake or trigger waking it up whenever they want though, so it can't apply to every usage of stuff like TCP keepalive. It's also really best if every single app needing push notifications isn't trusted to do it right themselves.
Well now we're getting into the domain of Android policy (and why it's idiotic). Mobile OS should just force-suspend any app that consumes more than X% of cpu while in background, letting them use standard APIs like TCP keepalive but suffer consequences if they do it wrong.
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Waking up the device frequently is a bigger battery drain than doing it very infrequently but using a lot of CPU. It gets to sleep for long periods of 30+ minutes at a time on a reliable network. It always tries to get back to sleep very fast but apps will stop that.
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