Network services let networked computers send and receive messages, share files, and print. A network service uses one or more protocols or ports to pass through a specific type of traffic. For example, the HTTP service uses ports 80 and 443 in the TCP protocol. You can create a firewall rule that allows or blocks network services. A network service trigger identifies one or more network protocols that are significant in relation to the described network traffic.
When you define TCP-based or UDP-based service triggers, you identify the ports on both sides of the described network connection. Traditionally, ports are referred to as being either the source or the destination of a network connection.