This document specifies the Session Initiation Protocol (SIP), an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. SIP uses proxy servers to route requests to the user's current location, authenticate and authorize users, implement provider call-routing policies, and provide features to users. It also includes a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols. The document covers various aspects of SIP, including message structure, user agent behavior, transaction processing, dialog management, session initiation, modification, and termination. It also discusses security considerations, transport mechanisms, and common message components. SIP is designed to be flexible and can be used with other protocols to build a complete multimedia architecture. The document provides detailed examples and explanations of SIP operations, such as location discovery, session negotiation, and session termination.This document specifies the Session Initiation Protocol (SIP), an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. SIP uses proxy servers to route requests to the user's current location, authenticate and authorize users, implement provider call-routing policies, and provide features to users. It also includes a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols. The document covers various aspects of SIP, including message structure, user agent behavior, transaction processing, dialog management, session initiation, modification, and termination. It also discusses security considerations, transport mechanisms, and common message components. SIP is designed to be flexible and can be used with other protocols to build a complete multimedia architecture. The document provides detailed examples and explanations of SIP operations, such as location discovery, session negotiation, and session termination.