What is specified by RFC 3261?

RFC 3261 is the main specification for the Session Initiation Protocol (SIP), which is a protocol used for initiating, maintaining, and terminating real-time sessions that include voice, video, and messaging applications. This document outlines the architecture, procedures, and signaling characteristics of SIP, making it foundational for understanding how SIP operates in various scenarios, including Voice over IP (VoIP) communications.

SIP is essential in establishing and managing communications over the internet, and RFC 3261 provides the necessary framework that developers and engineers utilize to implement SIP functionalities in networking equipment and software applications. This includes details on how SIP messages should be structured, how calls are set up and torn down, and how user agents interact with SIP servers.

The other options, while related to internet protocols and multimedia communications in some capacity, do not pertain specifically to SIP as defined in RFC 3261.