Silent Push Notification in iOS 7 does not work

Question

In the WWDC 2013's "What's New with Multitasking" presentation, there is a section about Silent Push Notifications. It seems straight forward. According to the presentation, if you send the APS payload with just the content-available set to 1, users will not be notified of the notification.

// A. This doesn't work {    aps: {            content-available: 1         } } 

My testing shows that this does not work as no push is received. But if I include the sound attribute but exclude the alert attribute, it works (though not silent anymore).

// B. This works {    aps: {           content-available: 1,           sound: "default"        } } 

However, if I change the sound attribute to play a silent audio, I can mimic a silent push.

// C. This works too. {    aps: {           content-available: 1,           sound: "silence.wav"        } } 

Does anyone know:

1. If this a bug?
2. And if it is correct to assume that B or C is being treated as a Remote Notification (and not a bug with Silent Push where you need a sound attribute)? If so, this means it is not rate limited like Silent Pushes are... which Apple will likely fix. So I probably should not rely on it.
3. What the rate limit is (N pushes every X seconds, etc)?

Thanks in advance.

Edit with more information

For A, the state of the application does not matter. Notification is never received.

It seems like B and C only work if you enclose the attributes and values in quotes, like below.

{"aps":{"content-available": 1, "sound":"silent.wav"}} 

And the notification arrives in application:didReceiveRemoteNotification:fetchCompletionHandler: regardless of state.

Answer 1

This works also and does not play a sound when it arrives:

{     aps = {         "content-available" : 1,         sound : ""     }; } 

EDIT

People having this problem may want to check out this link. I have been participating in a thread on Apple's Developer forum that goes over all app states and when silent pushes are received and not received.

Answer 2

So I just came across this issue yesterday, and after trying sending a payload with a sound set to an empty string, it was still causing vibration/sound on the device. Eventually, I stumbled on a blog post from Urban Airship that suggested needing to send:

{ priority: 5 } 

in the push notification, which I had never seen. After perusing Apple's docs for push notifications, I stumbled on this page:

https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html

Which indicates that priority should be set as "5" or "10", and explains:

The notification’s priority. Provide one of the following values:

10 The push message is sent immediately.

The push notification must trigger an alert, sound, or badge on the device. It is an error to use this priority for a push that contains only the content-available key.

5 The push message is sent at a time that conserves power on the device receiving it.

Ultimately, we were able to get silent push notifications working with a badge count (and I suspect you could even do the same with an alert) with the following format:

    aps =     {         badge = 7;         "content-available" = 1;         priority = 5;     };