The Secure Real-time Transport Protocol (SRTP) is a Real-time Transport Protocol (RTP) profile, intended to provide encryption, message authentication and integrity, and replay attack protection to the RTP data in both unicast and multicast applications. It was developed by a small team of Internet Protocol and cryptographic experts from Cisco and Ericsson. It was first published by the IETF in March 2004 as RFC 3711.
Since RTP is closely related to RTP Control Protocol (RTCP) which can be used to control the RTP session, SRTP also has a sister protocol, called Secure RTCP (SRTCP); SRTCP securely provides the same features to RTCP, as the ones provided by SRTP to RTP.
Utilization of SRTP or SRTCP is optional in RTP or RTCP applications; but even if SRTP or SRTCP are used, all provided features (such as encryption and authentication) are optional and can be separately enabled or disabled. The only exception is the message authentication feature which is indispensably required when using SRTCP.