UDP/TCP "ping"
When ICMP is blocked — use TCP-SYN or UDP packet with an unusual port to measure reachability + RTT. Tools: hping3, nping, tcping, Python's scapy.
Many modern networks block ICMP (unfortunately) — so regular ping gives no info. TCP-ping to port 80/443: send SYN, measure time to SYN-ACK or RST. UDP-ping against closed port: time to ICMP "port unreachable" (if not blocked). Practical: nc -zv host port for simple reachability check.