This document specifies the Multicast Protocol for Low power and Lossy Networks (MPL) that provides IPv6 multicast forwarding in constrained networks. MPL avoids the need to construct or maintain any multicast forwarding topology, disseminating messages to all MPL Forwarders in an MPL Domain. MPL uses the Trickle algorithm to manage message transmissions for both control and data-plane messages. Different Trickle parameter configurations allow MPL to trade between dissemination latency and transmission efficiency.