https://tools.ietf.org/html/rfc5763:
The endpoint MUST use the setup attribute defined in [RFC4145].
The endpoint that is the offerer MUST use the setup attribute
value of setup:actpass and be prepared to receive a client_hello
before it receives the answer. The answerer MUST use either a
setup attribute value of setup:active or setup:passive. Note that
if the answerer uses setup:passive, then the DTLS handshake will
not begin until the answerer is received, which adds additional
latency. setup:active allows the answer and the DTLS handshake to
occur in parallel. Thus, setup:active is RECOMMENDED. Whichever
party is active MUST initiate a DTLS handshake by sending a
ClientHello over each flow (host/port quartet).