Which open protocol was designed to replace RADIUS, providing extensible commands but lacking backward compatibility?

The correct answer is Diameter, which was specifically designed to enhance and extend the capabilities of RADIUS (Remote Authentication Dial-In User Service). Diameter addresses some of the limitations of RADIUS by offering more robust, extensible commands and a richer command set, allowing for greater flexibility and additional features. Unlike RADIUS, Diameter is capable of supporting more complex applications and environments, accommodating services beyond just authentication, such as authorization and accounting.

One key aspect of Diameter is that it was not designed to be backward compatible with RADIUS. This means that while Diameter can fulfill the roles traditionally covered by RADIUS, it operates on different principles and protocols, requiring implementations to be entirely adapted to Diameter without any facility for RADIUS compatibility. As a result, transitioning to Diameter may require substantial changes to infrastructure that previously relied on RADIUS.

In summary, Diameter's development focused on providing a more powerful framework for identity and access management protocols, which highlights its utility as a successor to RADIUS despite the lack of backward compatibility.