The AT Protocol (Authenticated Transfer) is the foundation backbone of the Atmosphere federated network. It's flagship application is Bluesky but lately more and more people are building their own stuff on top of the protocol.

It works by storing data in Personal Data Servers and events are aggregated into a global Firehose of events. Records are identified using decentralized identifiers where the most commonly used method is did:plc. This is the main point of criticism as this part of the network is highly centralized and while governance is moving to a nonprofit, its still not as resilient as the Fediverse.