xAP

xAP is a lightweight, open protocol designed for message-based communication among devices and software in automation, monitoring, and control systems on local networks. It aims to enable interoperability across vendors and platforms with a simple, extensible data model and a text-based wire format.

Protocol characteristics include a publish/subscribe style of messaging. Devices publish messages to the local network, and interested clients subscribe to specific classes and types. Messages are typically transported over User Datagram Protocol (UDP), often using a multicast address so multiple listeners can receive updates without direct connections. The wire format is human-readable ASCII; an xAP message generally starts with a header and is followed by a payload. The header includes fields such as version, source address, destination address, class, and type, while the payload carries data as key-value pairs or simple fields.

Architecture and implementations commonly feature a hub or bridge component that routes messages between networks or segments, enabling cross-vendor communication. A variety of open-source libraries and commercial products provide xAP support, allowing sensors, controllers, and software applications to generate or consume xAP messages.

History and status indicate that xAP originated in the open electronics and home-automation communities in the early 2000s to create a simple, interoperable protocol. While its use has declined in mainstream products, it remains in legacy installations and in some hobbyist projects with community-maintained implementations.

Security and limitations: xAP emphasizes simplicity over built-in security, with messages on local networks often unencrypted. When used in sensitive environments, network segmentation or additional protective measures are advisable.

See also: Home automation, IoT protocol families, UDP multicast.