SRTP (Secure RTP, RFC 3711) is the security layer that wraps RTP and RTCP with encryption and message authentication, so media can't be eavesdropped or tampered with on the wire. It encrypts the payload and authenticates each packet while leaving the headers usable, so routers and the jitter buffer can still do their jobs. In WebRTC, SRTP is mandatory and its keys are negotiated via DTLS, which means every browser-based call is encrypted between the peers by default — there is no unencrypted mode. For an engineer, SRTP is mostly invisible until a key-exchange or certificate problem causes a call to connect but carry no audio.
