The EAP RFC does not specify how messages should be passed around. It does not, for example, specify transport over the Internet using IP. In fact, EAP is not a LAN protocol at all because EAP was originally designed for use with dial-up authentication via a modem. So if we are going to get EAP messages passed around our network, we have to find a way to encapsulate the EAP messages during their journey. IEEE 802.1X defines a protocol called EAP over LAN to get EAP messages passed between the supplicant and the authenticator.
IEEE 802.1X provides the description for EAPOL. It describes frame formats for Ethernet (IEEE 802.3) and token ring LANs but not for IEEE 802.11. If you just wanted to encapsulate the EAP message, you could prepend an Ethernet MAC header on an EAP message and send it over the LAN. But the IEEE 802.1X committee decided to add a few more useful messages and fields while it was defining EAPOL. Not all EAPOL frames actually carry EAP messages; some are used to perform administrative tasks. The five types of EAPOL messages are:
We won't deal with the last message type here. It is connected with what we think is the rather dangerous idea of allowing an unauthorized device to send management alerts to the system. This message is not used by WPA/RSN.
When the supplicant first connects to the LAN, it does not know the MAC address of the authenticator. Actually it doesn't know whether there is an authenticator present at all. To help get things going, IEEE 802.1X defines a message called EAPOL-Start. By sending this message to a special group-multicast MAC address reserved for IEEE 802.1X authenticators, a supplicant can find out whether an authenticator is present and let it know that the supplicant is ready. In many cases the authenticator will already be notified that a new device has connected from some hardware notification. For example, a hub knows that a cable is plugged in before the device sends any data. In this case the authenticator may preempt the Start message with its own message. In either case the authenticator sends an EAP-Request Identity message using the EAPOL-Packet frame (perhaps twice, if both send the initial message at the same time).
Using this message type, the authenticator sends encryption (and other) keys to the supplicant once it has decided to admit it to the network. Of course it is necessary to encrypt the keys themselves before sending them, and IEEE 802.1X does not specify how this is done. In fact, IEEE 802.1X offers little help when it comes to combining encryption with the authentication process. This was a major obstacle that had to be overcome in the WPA/RSN network design. Chapter 10, "WPA and RSN Key Hierarchy," outlines how a slightly modified EAPOL-Key message is used to establish encryption keys and also to validate that both sides have correct keys before allowing access.
This EAPOL frame is used for sending the actual EAP messages. It is simply a container for transporting an EAP message across the LAN, which was the original objective of the EAPOL protocol.
This message type indicates that the supplicant wishes to be disconnected from the network.
For reference, the format of an EAPOL frame for use by Ethernet is shown in Figure 8.9. All of the packet types listed above fall into this format.
Figure 8.9. EAPOL Frame Format