Which protocol is primarily used for network access control and has several server and client products available?

The protocol primarily used for network access control, and that has a variety of server and client products available, is RADIUS (Remote Authentication Dial-in User Service). RADIUS serves a critical role in authenticating users and providing accounting for network access services. It is particularly useful in scenarios where users connect to a network remotely, such as through dial-up services, VPNs, or wireless access points, and requires an authentication mechanism to grant or deny access based on user credentials.

RADIUS operates using two main components: the client, which is typically the network access server that users connect to, and the RADIUS server, which processes authentication requests and enforces access policies. Its ability to work with various client and server products makes it a flexible choice for organizations looking for centralized network access control.

In contrast, other options such as OAuth are primarily used for delegated authorization rather than direct access control, making them less suitable for this purpose. Diameter offers enhancements over RADIUS but is not as widely adopted for network access control scenarios in comparison to RADIUS. Similarly, LDAPS focuses on securely accessing directory information rather than managing network access control directly. Thus, RADIUS stands out as the most appropriate protocol for this context.