How does FusionVM detect that a machine is “active”?
FusionVM uses several methods for determining that an IP address is “active”. Initially, a “Ping Sweep” using ICMP messages is sent to each address. Then a “TCP Ping” is sent to commonly used ports (21, 22, 23, 25, 80, 443, etc.). TCP “Pings” use a deviation of the TCP standard three-way handshake to determine if a machine responds. This method sends an unsolicited TCP “Acknowledge” (TCP ACK) to the specified port. If an active machine is listening on this port, it should send back a reset to the unsolicited request. Another method involves sending a TCP “Synchronize” (TCP SYN) message (similar to the TCP ACK) to the commonly used ports and looking for a response. Also, while this allows for a high success in detecting active hosts, only machines with open ports apply to licensing for billing. This prevents blank network address space, such as NAT pools, from inflating IP usage numbers.
