title: What is the default value of the apns-expiration field?
body: <p>The <code>apns-expiration</code> field governs how long Apple will hold on to an apns message before giving up on delivering it (for example, if the device is turned off).</p>
  
  <p>According to their <a href="https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html" rel="noreferrer">docs</a>, a value of zero means "no retention": meaning that if the message can't be delivered immediately, its discarded.</p>
  
  <p>But what happens if the header <strong>isn't</strong> specified? In other words, what is the default behavior?</p>