The Nearby Messages API is a publish-subscribe API that lets you pass small binary payloads between internet-connected Android and iOS devices. The devices don't have to be on the same network, but they do have to be connected to the Internet.
Nearby uses Bluetooth Low Energy to communicate a unique-in-time pairing code between devices. The server facilitates message exchange between devices that detect the same pairing code. When a device detects a pairing code from a nearby device, it sends the pairing code to the Nearby Messages server for validation, and to check whether there are any messages to deliver for the application’s current set of subscriptions.
Nearby Messages is unauthenticated and does not require a Google Account.
The exact mechanism for data exchange may vary from release to release. The following sequence shows the events leading to message exchange:
A publishing app makes a request to associate a binary payload (the message) with a unique-in-time pairing code (token). The server makes a temporary association between the message payload and the token.
The publishing device uses Bluetooth Low Energy to make the token detectable by nearby devices. The publishing device also uses Bluetooth Low Energy to scan for tokens from other devices.
A subscribing app associates its subscription with a token and uses Bluetooth Low Energy to send its token to the publisher, and to detect the publisher's token.
When either side detects the other's token, it reports it to the server.
The server facilitates message exchange between two devices when both are associated with a common token, and the API keys used by the calling apps are associated with the same project in the Google Developers Console.