This Recommendation ∣ International Standard specifies the Enhanced Communications Transport Protocol (ECTP), which is a transport protocol to support Internet multicast applications over the multicast-capable IP networks. This Recommendation ∣ International Standard specifies the ECTP part 3 (ECTP-3) for the duplex multicast transport connection in which the participants are classified into one TC-Owner and many TS-users. The duplex multicast transport connection supports two kinds of data transport: the multicast data transport from TC-owner to all the other TS-users and the unicast data transport from TS-users to TC-Owner. A TS-user is allowed to send unicast data to TC-Owner, only if it gets a token from TC-Owner. This Specification describes the protocol for supporting the duplex multicast transport, which includes the connection management (establishment, termination, pause, resumption, user join and leave) and the reliability control mechanisms for the multicast and unicast data transport. In particular, the protocol operations for the multicast data transport from TC-Owner to the TS-users will be designed with the congruency of the simplex multicast transport protocol (ECTP-1), as specified in ITU-T Rec. X.606 ∣ ISO/IEC 14476-1.