## 2 Literature Review
Specifically, the root only saves downward route information of leaf nodes, while other middle nodes store route information of non-leaf nodes. In addition, a modified hop extension header is designed to provide the last one hop from routers to leaf nodes. This mechanism reduces the storage burden of the root. However, the large number of middle nodes still suffer memory limitations, which leads to the child-unreachable problem when a network size is large. [PERSON] et al. [28] have proposed an adaptive RPL (ARPL). The authors utilized the situation where the required number of route entries of a node depends on the relative position to the root. The closer to the root, the greater number of entries are required. Therefore, a mechanism was proposed to switch MOPs adaptively based on whether a node's memory space is full of route entries. Initially, all the nodes work in storing mode. However, the nodes in the storing mode perform like the root in the non-storing mode to perform source routing to destinations. When the number of route entries is full, the MOP is switched to the non-storing mode. At the same time, the entries of mode-switched nodes are removed. Other schemes are kept the same with operations in RFC 6550. [PERSON] et al. [29] have proposed the H-RPL, which is like the design in [28]. However, it considered the residual battery condition of nodes. Generally, the authors provide a better method to utilize the memory space of each node by switching MOPs on certain conditions. However, the switch is unidirectional and introduces hybrid-overhead by flushing the previously established route entries during the MOP switch process. In addition, [PERSON] et al. (2018) have proposed a new hybrid MOP scheme for P2P communication. There are two cases defined, where ranks and number of children are considered to decide MOPs in case one. In case two, the probabilities of transmission are included as a condition and performed in cellular schemes. However, it is similar to the other hybrid-MOPs works in that extra control overheads are introduced along with the more complex data control and information collection mechanisms. All the previous related works can be viewed as to attempts to implement hybrid-MOPs in downward routing of RPL. Generally, three categories could be classified based on the specific hybrid methods: (1) interoperation; (2) switch; (3) modification of storing mode. Furthermore, the term \"interoperation\" means that the two MOPs are existing at the same time and fixed, and the concern is how to foster interaction between the two different modes. The term \"switch\" indicates that the MOPs of nodes could be changed on some conditions. Specifically, the storing mode changes to the non-storing mode or vice versa. The last one is the modification of storing mode. In this category, compared to the standard way, the mechanism of the storing mode is modified to meet some specific functions in order to obtain hybrid-MOPs. The summary of related works is shown in Table 1, based on previous classification. Given all the related works, TB-RPL in this article constructs a new single type of MOP, which is a functional fusion of the storing mode and the non-storing mode intended to produce a new single one. It is different from the previous three hybrid-MOP types. There is no distinction between different modes. All nodes perform in the same single TB-RPL mode that includes the consistent logic and the common working context, which achieves the advantages of the storing mode and the non-storing mode simultaneously. Based on the review of previous works, and to the best of our knowledge, it is a breakthrough to achieve MOPs fusion and unification. The proposed TB-RPL aims to achieve better network performance by solving the long-path problem, overcoming the children-unreachable problem, and avoiding the hybrid-overhead problem. In the following sections, we describe, explain, and verify the specification of TB-RPL in detail.