FCM - Difference between Downstream Message and Upstream Message
<p>I have been studying materials related to Web push notification from FCM Messenger. While I came across downstream message and upstream message, apparently HTTP server only allows downstream message while XMPP server allows upstream message.</p>

<p>I have trouble understanding the difference between downstream and upstream message and the details are not well explained on FCM. I tried to search online but I have no luck. Please do help me as I am new to coding. </p>