# Routing Protocols for Low Power and Lossy Networks in Internet of Things Applications
## 4 Enhanced Routing Solutions for Internet of Things Networks
### Multicast Communication Support
Note that, due to a possible disturbance in the communication of other nodes, the transmission of acknowledgment packets is not used in ContikiMAC broadcast transmission. A receiver node cannot immediately forward the received packet due to possible collisions with the sender that may still be transmitting. For that reason, SMRF proposes the use of a short delay between the message reception and its forwarding for allowing the use of ContikiMAC broadcast and avoiding possible collisions (Figure 8). This behavior prevents the use of multiple ContikiMAC unicasts to the task of multicast transmission, commonly used by multicast RPL, and contributing to the reduction of energy consumption. Furthermore, multicast RPL does not specify any control mechanism to avoidthe reception and transmission of duplicated multicast packets. Thus, SMRF constrains that a node must only process multicast packets received from its preferred parents. SMRF, although showing several benefits when compared with standard RPL multicast, still presents some limitations. One of the main drawbacks of SMRF is the lack of support for upward multicast. Due to its mechanism for avoiding the processing of duplicated packets, as mentioned above, SMRF only allows nodes to process packets sent from its preferred parent. Thus, messages received from children nodes are neither processed nor routed. Therefore, to allow the nodes to transmit multicast messages both upward and downward, the Enhanced Stateless Multicast RPL Forwarding for IPv6-based low-lower and lossy networks (ESMRF) was proposed [71]. ESMRF proposes that multicast messages are first sent to the root node that, afterwards, should realize the message forwarding to its final destination. Thus, when a node wants to send a multicast message, it should encapsulate its content and destinations in an ICMPv6 (Internet Control Message Protocol Version 6) packet. This packet, also named a delegation packet, is sent in unicast to the root node (using the default RPL upward mechanism), which is in the top of the tree structure and can communicate with all network nodes. After receiving the delegation packet, the root node must extract the multicast content and send it to the destinations indicated. The process of forwarding multicast messages performed by the root node is similar to that proposed by SMRF. Thus, by using the same principle of SMRF, ESMRF also avoids the processing of duplicated packets. Figure 8: SMRF mechanism for multicast packet transmission. Figure 6: ContikiMAC unicast packet transmission. Figure 7: ContikiMAC broadcast packet transmission. ESMRF allows, using a simple approach, both upward and downward multicast communication, solving the gap of SMRF. However, the proposed solution suggests the use of the root node to forward all multicast messages. This approach becomes too expensive in a large routing tree, provoking communication overhead and high end-to-end latency. Thus, to mitigate the problem found in SMRF with a more robust method than proposed by ESMRF, [PERSON] et al. (2017) introduced a new multicast routing protocol named Bidirectional Multicast RPL Forwarding (BMRF). BMRF seeks to merge the best features of RPL and SMRF. Thus, the approach offers three different modes for packet sending. In unicast mode, the sender node verifies the nodes interested in the multicast message according to the routing table, and then, the message is sent in unicast to each one, as proposed by RPL multicast. In broadcast mode, the sender verifies the group of nodes that want to receive the multicast message, and then, the communication is performed as in the broadcast proposed by SMRF. In the last mode, mixed-Tmode, the sender performs the transmission according to a \(T\) threshold that decides between multiple unicasts or broadcasts. \(T\) is defined according to the number of children interested in the message and the RDC rate.