# A Design of a Time Synchronization Protocol Based on Dynamic Route and Forwarding Certification
## 1 Introduction  
Time synchronization is a fundamental technology in wireless sensor networks (WSN) [1, 2]. Due to the collaboration features [3] of WSN nodes, it is important for each node to synchronize accurately [4]. Considering the particular demands [5, 6, 7] compared with the internet in terms of energy consumption limits and synchronization precision, traditional time synchronization protocols are not suitable for WSN. A variety of typical protocols have been proposed since the time synchronization subject of WSN was defined, such as the timing-sync protocol for sensor networks (IPSN) [8], delay measurement time synchronization (DMPS) [9], flooding time synchronization protocol (FTSP) [10], reference broadcast synchronization (RBS) [11], and pairwise broadcast synchronization (PBS) [12]. These protocols directly synchronize nodes' clocks or trigger the process according to reference clock time information.  
These typical protocols can be perfectly applied in a one-hop situation, but they cannot satisfy the synchronization needs in large-scale WSN. Meanwhile, the clock frequency skew swinging [13] that occurs in the long-running is another problem that is currently ignored. Accumulation error will increase severely if repeating one hop synchronization process between neighboring nodes in the network.  
Therefore, based on typical protocols, several time synchronization protocols for large-scale WSN were put forward. For example, the dynamic route list based timing-sync protocol (DRL-TSP) [14] isa strategy that chooses a parent node in the last hop as the synchronization reference for each node, which provides the minimal hops from the root node by dividing the network into a hierarchical structure. Efficient and simple algorithm for time synchronization (E-SATS) [15] is proposed for reducing computations and energy consumption. However, for the WSN with a limited energy, a fixed hierarchical structure or a settled clustered structure results in a strong dependency on topology, which will consume extra energy in the process of building and fixing the established structure. Furthermore, it cannot react rapidly if the topology changes [16; 17; 18].  
On the basis of the current time synchronization research, in order to tackle the problem of multi-hop error accumulation, the clock frequency skew influence, and the network topology changing, a time synchronization protocol named the time synchronization protocol based on dynamic routing and forwarding certification (DRFC-TSP) for large-scale WSN is proposed. In this protocol, by acquiring optimal reference time information in each hop instead of employing a fixed pre-established structure, every node can obtain an optimum route from the root node, which will ensure that the synchronization precision is good and allow adaptations to topology changes. The certification of time information forwarding is evaluated by a forwarding certification strategy. Therefore, the bad performance nodes do not forward their packets, and network traffic and energy consumption can be decreased.  
The rest of the paper is organized as follows: Section 2 gives an introduction to the typical time synchronization models in large-scale WSN; Section 3 describes details on the algorithm of DRFC-TSP; Section 4 provides the design principles of realization on an embedded Linux platform; Section 5 illustrates the results of the simulation and field experiment; and the final section presents the conclusion.