DTLS-SRTP is a key exchange mechanism that is mandated for use in WebRTC.

DTLS-SRTP uses DTLS to exchange keys for the SRTP media transport.

SRTP requires an external key exchange mechanism for sharing its session keys, and DTLS-SRTP does that by multiplexing the DTLS-SRTP protocol within the same session as the SRTP media itself.

This method is considered to be more secure than the SDES mechanism that was first used in WebRTC but later on banned from use altogether.