Why is SIP considered a signaling protocol?

SIP, or Session Initiation Protocol, is considered a signaling protocol primarily because it is responsible for establishing, modifying, and terminating communication sessions in a network. This function is essential for real-time communication applications such as Voice over IP (VoIP) and video conferencing. By enabling the setup of sessions between different endpoints, SIP facilitates the negotiation of parameters for the session, such as which codecs to use, among other details.

The other options, while related to the overall communication process, do not accurately describe the role of SIP. For instance, while media streams are an integral part of communication, SIP does not carry the media itself; this is typically handled by separate protocols such as RTP (Real-Time Protocol). Handling data encryption is also not the primary function of SIP; encryption is usually managed by protocols specifically designed for that purpose, like TLS. Similarly, compression of audio streams is a function of codecs, not SIP. Thus, the primary role of SIP as a signaling protocol is clearly defined by its capability to establish and manage communication sessions.