What is 802.1x?

Understanding what the IEEE 802.1x standard is and why you should care means understanding three separate concepts: PPP, EAP and 802.1x itself.

Most people are familiar with PPP - Point-to-Point Protocol. PPP is most commonly used for dial-up Internet access. PPP is also used by some ISPs for DSL and cable modem authentication, in the form of PPP over Ethernet. PPP is part of Layer 2 Tunneling Protocol, a core part of Microsoft's secure remote access solution for Windows 2000 and beyond.

PPP evolved beyond its original use as a dial-up access method and iis now used all over the Internet. One piece of PPP defines an authentication mechanism. With dial-up Internet access, that's the username and password you're used to using. PPP authentication is used to identify the user at the other end of the PPP line before giving them access.

Most enterprises want to do more for security than simply employing usernames and passwords for access, so a new authentication protocol, called the Extensible Authentication Protocol (EAP), was designed. EAP sits inside of PPP's authentication protocol and provides a generalized framework for several different authentication methods. EAP is supposed to head off proprietary authentication systems and let everything from passwords to challenge-response tokens and public-key infrastructure certificates all work smoothly.

With a standardized EAP, interoperability and compatibility of authentication methods becomes simpler. For example, when you dial a remote-access server and use EAP as part of your PPP connection, the RAS doesn't need to know any of the details about your authentication system. Only you and the authentication server have to be coordinated. By supporting EAP authentication a RAS server gets out of the business of acting as middle man, and just packages and repackages EAP packets to hand off to a RADIUS server that will do the actual authentication.

This brings us to the IEEE 802.1x standard, which is simply a standard for passing EAP over a wired or wireless LAN. With 802.1x, you package EAP messages in Ethernet frames and don't use PPP. It's authentication and nothing more. That's desirable in situations in which the rest of PPP isn't needed, where you're using protocols other than TCP/IP, or where the overhead and complexity of using PPP is undesirable.

802.1x uses three terms that you need to know. The user or client that wants to be authenticated is called a supplicant. The actual server doing the authentication, typically a RADIUS server, is called the authentication server. And the device in between, such as a wireless access point, is called the authenticator. One of the key points of 802.1x is that the authenticator can be simple and dumb - all of the brains have to be in the supplicant and the authentication server. This makes 802.1x ideal for wireless access points, which are typically small and have little memory and processing power.

The protocol in 802.1x is called EAP encapsulation over LANs (EAPOL). It is currently defined for Ethernet-like LANs including 802.11 wireless, as well as token ring LANs such as FDDI. EAPOL is not particularly sophisticated. There are a number of modes of operation, but the most common case would look something like this:

The authenticator sends an "EAP-Request/Identity" packet to the supplicant as soon as it detects that the link is active (e.g., the supplicant system has associated with the access point).

1. The supplicant sends an "EAP-Response/Identity" packet to the authenticator, which is then passed on to the authentication (RADIUS) server.
   
2. The authentication server sends back a challenge to the authenticator, such as with a token password system. The authenticator unpacks this from IP and repackages it into EAPOL and sends it to the supplicant. Different authentication methods will vary this message and the total number of messages. EAP supports client-only authentication and strong mutual authentication. Only strong mutual authentication is considered appropriate for the wireless case.
   
3. The supplicant responds to the challenge via the authenticator and passes the response onto the authentication server.
   
4. If the supplicant provides proper identity, the authentication server responds with a success message, which is then passed onto the supplicant. The authenticator now allows access to the LAN- - possibly restricted based on attributes that came back from the authentication server. For example, the authenticator might switch the supplicant to a particular virtual LAN or install a set of firewall rules.