Nankai University Nankai University Nankai University Nankai University The Agent Transfer Protocol (ATP) is a communication protocol designed for autonomous agents to exchange messages, requests, and events in a secure, structured manner. ATP supports the emerging Internet of Agents (IoA) paradigm, where autonomous agents operate across four deployment scenarios: household (small scale), service (medium scale), enterprise (large scale), cloud provider (huge scale), and etc. ATP employs a two-tier architecture where agents connect to the global Internet through ATP servers, enabling server-mediated communication for proper routing, security enforcement, and resource management. The protocol provides DNS-based service discovery using SVCB records, mandatory authentication via Agent Transfer Sender (ATS) policies and Agent Transfer Keys (ATK), and support for multiple interaction patterns including asynchronous messaging, synchronous request/response, and event-driven streaming. This specification defines the discovery mechanism, identity model, authentication framework, transport layer, and message semantics for ATP.