This document specifies the Remote Authentication Dial-In User Service (RADIUS) protocol, which is used for authentication, authorization, and configuration information between a Network Access Server (NAS) and an Authentication Server. The protocol uses UDP on port 1812 and supports various authentication mechanisms, including PAP, CHAP, and UNIX login. RADIUS packets are encapsulated in UDP Data fields and consist of a Code, Identifier, Length, Authenticator, and Attribute fields. Attributes carry specific authentication, authorization, and configuration details. The document outlines the packet types (Access-Request, Access-Accept, Access-Reject, Access-Challenge), their formats, and the attributes they can contain, such as User-Name, User-Password, CHAP-Password, NAS-IP-Address, and Service-Type. It also describes the security considerations and implementation notes, including the use of shared secrets and MD5 hashing for password protection.This document specifies the Remote Authentication Dial-In User Service (RADIUS) protocol, which is used for authentication, authorization, and configuration information between a Network Access Server (NAS) and an Authentication Server. The protocol uses UDP on port 1812 and supports various authentication mechanisms, including PAP, CHAP, and UNIX login. RADIUS packets are encapsulated in UDP Data fields and consist of a Code, Identifier, Length, Authenticator, and Attribute fields. Attributes carry specific authentication, authorization, and configuration details. The document outlines the packet types (Access-Request, Access-Accept, Access-Reject, Access-Challenge), their formats, and the attributes they can contain, such as User-Name, User-Password, CHAP-Password, NAS-IP-Address, and Service-Type. It also describes the security considerations and implementation notes, including the use of shared secrets and MD5 hashing for password protection.