Which type of network attack involves randomly opening many Telnet requests to a router and results in a valid network administrator not being able to access the device?

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.