Which security strategy requires using several, varying methods to protect IT systems against attacks?

Defense in depth applies multiple independent security mechanisms - such as firewalls, intrusion detection systems, encryption, access controls, and physical security - across different layers of an IT environment. This strategy ensures an attacker must defeat several varied controls to succeed, directly addressing the concept of using 'several, varying methods' to protect IT systems as described in the question stem.

The three-way handshake is the TCP connection establishment process (SYN, SYN-ACK, ACK) and is a networking protocol mechanism, not a defensive security strategy.

Covert channels are hidden communication paths exploited to leak information in violation of security policy - they represent an attack vector, not a protective strategy.

Exponential backoff is a retry-timing algorithm used in networking and distributed systems to reduce congestion, not a security protection strategy.