Been working pretty heavily on APNS lately.
Learned some pretty interesting things about APNS
The flow of remote-notification data is one-way. The provider composes a notification package that includes the device token for a client application and the payload. The provider sends the notification to APNs which in turn pushes the notification to the device.
When it authenticates itself to APNs, a provider furnishes the service with its topic, which identifies the application for which it’s providing data. The topic is currently the bundle identifier of the target application on an iOS device.
Really good diagram.
Peters ramblings 