When does a client utilize a TGT?

A Ticket Granting Ticket (TGT) is used when a client initially authenticates with a Key Distribution Center (KDC) in a Kerberos authentication system. When a user logs in, they are granted a TGT, which allows them to request additional service tickets for accessing specific network services without having to provide their credentials repeatedly. The TGT essentially serves as a proof of authentication for the client against the KDC and is vital for obtaining service tickets for different resources or services later on.

This process is key to Kerberos as it provides a secure and streamlined authentication mechanism, eliminating the need for the client to re-enter their username and password each time they want to access a new service. The client will use the TGT when it needs to request these service tickets, simplifying and enhancing security during the user’s session. The other options do not correctly reflect the function of a TGT in the authentication process. For instance, the establishment of a session or accessing network resources would typically involve the already secured identity assertion through service tickets rather than the TGT itself.