In an SNA network, different classes of service can be specified based upon the needs of the end users in the network. SNA's class of service (CoS) is similar to TCP's Type of Service (TOS). The two methods handle the prioritization of messages in the network.

A class of service designates the characteristics of a session. It includes such characteristics as security, transmission priority, and bandwidth. The process of defining a class of service is an activity that must take place before a route for LU-LU session is selected.

During session initiation, the class of service for the session is obtained from the session-initiation request, or derived from a mode name specified in the session-initiation request. The route then selected for the session depends on the class of service for the session and available routes.