In July 2003, IEEE 802.11f, a recommended practice for multivendor interoperability by the inter-access point protocol (IAPP), got standardized. The recommended practice describes a service access point (SAP), service primitives, a set of functions, and a protocol for APs to interoperate on a common distribution system (DS), using Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) to carry IAPP packets between APs as well as describing the use of the Remote Authentication Dial-in User Service (RADIUS) Protocol, so APs may obtain information about one another. A proactive caching method is also described for faster roaming. This recommended practice also affects layer-2 devices like switches and bridges. An AP architecture is shown in Figure 6.5.
In the figure the AP management entity (APME) refers to functions external to IAPP but implemented in an AP. The gray areas indicate areas where there is an absence of connection between blocks. The IAPP services are accessed by the APME through the IAPP SAP. Some IAPP service primitives require RADIUS protocol; in particular the AP should be able to find and use a RADIUS server to look up the IP addresses of other APs in the ESS when given the basic service set Identifier (BSSIDs) of those other APs (if a local capability to perform such a translation is not present), and to obtain security information to protect the content of certain IAPP packets.
There are three main steps in inter access point protocol (IAPP), these are:
- IAPP ADD procedure: This is when a new STA associates with the AP and is added in the list;
- IAPP Move procedure: This procedure comes in action as soon as the STA re-associates with a new AP;
- IAPP Cache: This procedure happens when context is used when proactive caching is used. Proactive caching means sending and caching of the context information of the STAs at the APs in the neighborhood graph. The neighborhood graph contains the APs in the near vicinity of the AP with which the STA is currently associated and the AP has a secure connection with.