Protocol Boundary Network (PBN): A Security Layer for AI Agent Communication

Protocol Boundary Network (PBN) is a proposed security layer for AI-agent tool communication. It places a mandatory, fail-closed gateway between agents and tool servers to enforce identity, policy, tokenization, lineage, destination scope, and audit. The core problem is that authorization can answer which agent may call which tool, but it does not by itself answer whether data read from one authorized tool may be sent to another authorized tool later in the same session. PBN’s narrow security claim is that, under mandatory gateway mediation, protected values tokenized at ingress cannot be directly released to tools whose destination scope does not permit that classification. This technical draft describes the PBN architecture, threat model, MCP deployment model, protocol and policy surface, prototype behavior, red-team outcomes, prior-art positioning, limitations, and adoption path. This release is a technical draft and reference-prototype description, not production security infrastructure.