A flood of packets with invalid source IP addresses requests a connection on the network. The server busily tries to respond, resulting in valid requests being ignored. What type of attack has occurred?

The TCP SYN Flood attack exploits the TCP three-way handshake. The threat actor continually sends TCP SYN session request packets with a randomly spoofed source IP address to an intended target. The target device replies with a TCP SYN-ACK packet to the spoofed IP address and waits for a TCP ACK packet. Those responses never arrive. Eventually the target host is overwhelmed with half-open TCP connections and denies TCP services.