A network session is an exchange of messages that occurs between two or more communication devices, lasting for some period of time. A session is established and is torn down when the session ends. Different types of sessions occur at three layers of the OSI model: the Transport layer, the Session layer, and the Application layer.

The Transport Layer operates at Layer 4 of the OSI model, providing reliable or unreliable, end-to-end delivery and flow control of data. Internet protocols that implement sessions at the Transport layer include the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

This section describes the global settings that affect TCP, UDP, and ICMPv6 sessions, in addition to IPv6, NAT64, NAT oversubscription, jumbo frame size, MTU, accelerated aging, and Captive Portal authentication. There is also a setting (Rematch Sessions) that enables you to apply newly configured security policy rules to sessions that are already in progress.