## 2 Related Work
### Multicast Architectures
#### 2.1.5 IoT-Based Multicast  
Lots of IoT applications such as service discovery [47], network management and information dissemination [48] will benefit from multicast. Different to the previous multicast routing, IoT multicast cannot maintain the multicast routing topology due to the limited memory and computation capability, which prevents IP multicast being applied to IoT.  
To satisfy the requirements of IoT, IETF have published the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) [49] which supports built-in multicast by introducing the Modes of Operations (MOPs) 3 to allows messages to carry a multicast address as a destination address. However, RPL has to maintain the topology which is unaccepted for IoT devices, and it also lacks of providing detailed multicast forwarding mechanism. The following research on Multicast protocol for Low-Power and Lossy networks (MPL) [50] aims to provide IPv6 multicast forwarding in constrained networks without constructing or maintaining any multicast forwarding topology. For this purpose, MPL introduces two operation modes. The first mode uses the Trickle algorithm [51] for low density multicast, and the other adopts classic flooding for high density multicast. The most important trait of MPL is the parametrization of Trickle algorithm which can evolve into different dissemination techniques such as flooding and gossip. In this way, MPL can be used in various multicast scenarios and makes the trade-off between latency and efficiency. As an variation of MPL, Stateless Multicast RPL Forwarding algorithm (SMRF) [52] forwards multicast packets in downward direction in the RPL tree. However, SMRF has high end-to-end delay. After that, the Enhanced Stateless Multicast RPL Forwarding (ESMRF) [53] is proposed, which forwards multicast packets up and down the RPL tree. The following work is Bidirectional multicast RPL Forwarding (BMRF) [54], which explores the MOP 3 of RPL and supports bi-directional forwarding and un-subscribe group mechanism. Most recently, a REliable and secure Multicast routing protocol for IoT networks (REMI) [55] is proposed, which supports the cluster creation in RPL tree.  
The above multicast schemes aim to provide IPv6 multicast for IoT by considering the constrained network conditions, which inherit the shortcoming of IP multicast. Besides, some application multicast schemes are proposed such as group communication for Constrained Application Protocol (CoAP) [56]; however, application communication loses the advantages of IP multicast in term of bandwidth consumption. Compared with these IoT multicast schemes, our scheme adopts a central control method which separates the multicast routing computation function and multicast data forwarding function from the router, and is easy to realize the multicast routing in resource-limited applications.  
From the aspect of multicast architectures, we can find that each multicast architecture has its own emphasis. For example, SDN-based multicast focuses on control capability, and HIP-based multicast focuses on mobility, multi-homing and security support. LISP-based multicast and ILNP-based multicast focus on identifier and location separation, and provide the IP-compatible multicast. IoT-based multicast puts more emphasis on resource-limited. Therefore, in this paper we adopt the UIN as the basic network architecture to study the multicast routing problem, and use the advantages of UIN in terms of mobility, security to provide the secure and controllable multicast services.