## 2 Related Work  
In DTN, routing protocols aim to achieve a high message delivery rate and optimal delay, while maintaining low overhead. There have been numerous routing protocols proposed for DTN, with their own characteristics and optimal goals, and they are used in corresponding scenarios. Epidemic [5] is a traditional routing protocol based on flooding messages throughout the network. Its basic principle is to exchange messages with all nodes, so called epidemic, but it always causes a waste of resources. A more efficient way to perform routing in DTN is PROPHET [7]. [PERSON] et al. designed a probabilistic routing protocol, which select intermediate nodes based on historical encounters and transmission. The challenge of a routing protocol is always an active research area in DTN. Recently, there are two key factors considered in designing routing protocols, which are encountering opportunity and node mobility. Some researchers studied on the node movement environment. They described that the area of the nodes' movement and encountering opportunity are strongly related to their locationsat a particular time, and improve performance [8; 9]. In a similar way, Refs. [10; 11] select nodes with physical location and mobility behavior. Some routing protocols calculate transmission reliability by using history information such as encounters [7; 12; 13]. [PERSON] et al. calculates the average encountering time to select the nodes [14]. Social-based approaches attempt to exploit the social behaviors of DTN nodes to make better routing decisions. [PERSON] et al. in Ref. [15] propose a social-energy-based routing protocol that considers the social energy of encountering nodes and is in favor of the node with a higher social energy in its or the destination's social community. [PERSON] et al. in Ref. [16] focus on the DTN routing without a priori mobility knowledge and provide approximation algorithms with theoretical guarantees that can be applied to cases where the number of hops allowed in the routing process is arbitrary. Some other routing protocols are about simply-copy in transmission [17; 18]. The GTDM (Game Theory Based Decision Making) routing protocol improves performance based on game theory [19]. [PERSON] et al. in Ref. [20] design an abstract of anonymous routing protocols for DTNs and augment the existing solution with multi-copy message forwarding. They construct simplified mathematical models, which can be used to understand the fundamental performance and security guarantees of onion-based anonymous routing in DTNs. [PERSON] et al. in Ref. [21] present an adaptive and efficient buffer management scheme called size-aware drop that strives to improve buffer utilization and avoid unnecessary message drops. Those protocols suffer from the problem of a continuous spread of messages even though they are already delivered to a destination. This drawback leads to bandwidth overhead and high resource usage on the entire network. Generally, existing routing protocols in DTN can be classified into some categories: flooding-based protocol, quota-based protocol, copy-based routing and so on. Some of them achieve high delivery rates, but expend excessive network resources, while some of them reduce the overhead, but they are not able to obtain comparable delivery rates. Our proposed protocol is a type of message ferrying routing protocol based on PROPHET. FIFO (First Input First Output) queuing is the nature of PROPHET. When a new message enters the full buffer of node, messages with the longest storage time will be dropped consistently. Messages may be lost due to network failures or forwarding to a few concentrated nodes. During the above situations, transmission may be delayed. Therefore, there is a variety of different routing protocols proposed to improve the performance of PROPHET. Similar to our study, Refs. [22; 23; 24; 25; 26] modified or appended some parameters to achieve their goals based on PROPHET.