IP Multicast is facing a slow take-off although it is a hotly debated topic since more than a decade. Many reasons are responsible for this status. Hence, the Internet is likely to be organized with both unicast and multicast enabled networks. Thus, it is of utmost importance to design protocols that allow the progressive deployment of the multicast service by supporting unicast clouds. This paper proposes HBH (Hop-By-Hop multicast routing protocol). HBH adopts the source-specific channel abstraction to simplify address allocation and implements data distribution using recursive unicast trees, which allow the transparent support of unicast-only routers. Additionally, HBH is original because its tree construction algorithm takes into account the unicast routing asymmetries. As most multicast routing protocols rely on the unicast infrastructure, these asymmetries impact the structure of the multicast trees. We show through simulation that HBH outperforms other multicast routing protocols in terms of the delay experienced by the receivers and the bandwidth consumption of the multicast trees.