Wi-Fi Protected Setup - Technical Architecture

Technical Architecture

The WPS protocol defines three types of devices in a network:

• Registrar: A device with the authority to issue and revoke credentials to a network. A registrar may be integrated into a wireless access point (AP), or it may be separate from the AP.
• Enrollee: A device seeking to join a wireless network.
• AP: An AP functioning as a proxy between a registrar and an enrollee.

The WPS standard defines three basic scenarios that involve these components:

1. AP with internal registrar capabilities configures an Enrollee Station (STA). In this case, the session will run on the wireless medium as a series of EAP request/response messages, ending with the AP disassociating from the STA and waiting for the STA to reconnect with its new configuration (handed to it by the AP just before).
2. Registrar STA configures the AP as an enrollee. This case is subdivided in two aspects: first the session could occur on both a wired or wireless medium, and second the AP could already be configured by the time the registrar found it. In the case of a wired connection between the devices, the protocol runs over Universal Plug and Play (UPnP), and both devices will have to support UPnP for that purpose. When running over UPnP, a shortened version of the protocol is run (only 2 messages) as no authentication is required other than that of the joined wired medium. In the case of a wireless medium, the session of the protocol is very similar to the internal registrar scenario, just with opposite roles. As to the configuration state of the AP, the registrar is expected to ask the user whether to reconfigure the AP or keep its current settings, and can decide to reconfigure it even if the AP describes itself as configured. Multiple registrars should have the ability to connect to the AP. UPnP is intended to apply only to a wired medium, while actually it applies to any interface to which an IP connection can be set up. Thus having manually set up a wireless connection, the UPnP can be used over it in the same manner as with the wired.
3. Registrar STA configures enrollee STA. In this case the AP stands in the middle and acts as an authenticator, meaning it only proxies the relevant messages from side to side.