Classic STUN is defined under which RFC?

The correct identification of Classic STUN (Session Traversal Utilities for NAT) is defined in RFC 5389. This RFC outlines the specification for how STUN operates, which is crucial for establishing communication across network address translators (NATs) in VoIP communications. STUN is used in scenarios where clients need to discover their public address and determine the type of NAT they are behind.

RFC 5321 pertains to the Simple Mail Transfer Protocol (SMTP), which is unrelated to STUN. RFC 791 deals with Internet Protocol (IP), providing foundational networking standards but not specifically addressing NAT or STUN. RFC 3261 describes the Session Initiation Protocol (SIP), detailing how to initiate, maintain, and terminate multimedia sessions, but it doesn't define STUN either. Therefore, the clear distinction and definition provided by RFC 5389 is essential for understanding how STUN functions in NAT traversal scenarios.