Which protocol is primarily used for establishing multimedia communication sessions?

SIP, which stands for Session Initiation Protocol, is specifically designed for establishing, modifying, and terminating multimedia communication sessions over the internet. This includes voice and video calls and is particularly vital for VoIP (Voice over Internet Protocol) communications. SIP facilitates the signaling aspect of these communications by enabling endpoints (such as phones or computers) to discover one another and set up their session parameters, making it a crucial protocol for real-time communications.

In contrast, the other protocols listed serve different purposes. HTTP (Hypertext Transfer Protocol) is primarily used for transmitting web pages and files on the internet, FTP (File Transfer Protocol) is focused on transferring files between clients and servers, and SMTP (Simple Mail Transfer Protocol) is utilized for sending and receiving email messages. These protocols do not handle the establishment or management of multimedia sessions like SIP does.