Given the number of authentication methods that could be used with RSN, the question arises, which one is correct? There is no simple answer. If you are starting from scratch to implement security, you should choose the method that is most widely supported in the available products. Today a leading candidate is TLS. However, if you have an existing system such as Kerberos V5 in operation, perhaps used with your wired network, it makes sense to try to apply that existing system to RSN. RSN is intended to provide this flexibility. In the interests of interoperability, the Wi-Fi Alliance has mandated that all WPA products should, at least, support TLS.
The Wi-Fi Alliance was free to choose which upper-layer authentication methods should be supported. However, the IEEE 802 working group is more restricted in specifying such things because, by virtue of being "upper-layer," the authentication method falls outside the scope of LAN protocol standards.
As such, IEEE 802.11 cannot and does not define the upper-layer authentication method, and instead leaves it to the implementers of the systems to decide. This was an issue of much rancor during the early days of the IEEE 802.11i standards work. Some people pointed out that it would be very hard to guarantee interoperability between different vendors' systems unless all the details of the authentication methods were specified. However, other people pointed out that, because of the range of different applications for Wi-Fi LAN, a single authentication method could not be suitable in all cases. This problem has been reduced by WPA, which does specify the method (TLS). It seems very likely that the method that is deployed for WPA will also be the most popular one when the transition to IEEE 802.11i RSN occurs.
This chapter presents solutions for several choices, including TLS, Kerberos V5, Protected EAP (PEAP), and the use of cellular phone authentication for wireless LAN devices (GSM-SIM). While the use of TLS is well defined through WPA, different vendors may implement other methods differently and interoperability cannot always be guaranteed. For example, the RFCs for Kerberos as defined by the IETF do not specify how to implement over IEEE 802.1X, let alone RSN. If you are not using WPA with TLS, you need to check carefully whether a vendor supports the authentication method you want, and whether they do so in the same way as any other vendor whose products you have purchased.