relationship between two or more devices that describe how the devices will use IPSec's security services to securely communicate with each other. An SA is unidirectional. Hence, to secure bi-directional communication channels between two nodes in an interconnected computer network, two SAs are required, one for each direction. These individual secure communication channels are generally referred to as an “inbound tunnel” and an “outbound tunnel,” where one device's inbound tunnel is the other device's outbound tunnel, and vice-versa.