ICMP echo (what ping actually asks)

The pair of messages ping sends and waits for: an ICMP echo request, then an echo reply. That is the whole protocol involved — no port, no TCP connection, no process belonging to your service. Which is why 3 packets transmitted, 0 received, 100% packet loss is not a verdict on the server: providers and host firewalls drop echo as a matter of routine, and the same address can answer {"status":"ok"} over HTTP a minute later. A live ping proves no more: an address replied in ICMP, and nothing was learned about your port or the process meant to be listening. It keeps one narrow use: when a TCP test to your port times out, a ping that answers shows packets reach the host, so the filter is dropping the port rather than the whole address.