## 4 Delay-Tolerant Networks for V2V Communication  
DTN is the routing method by the store-carry-forward type of protocol for poor network connectivity [16]. It was originally proposed for interspace communication [3], but it is also considered to use it for the case of the ultra-large-scale disasters or the local areas where 3G/4G networks are not available. In the methods, the vehicle firstly stores the data if there is no vehicle where they can be transmitted. Then, when other vehicles come into the transmittable radio range, the data are duplicated and sent to the others. However, previous papers such as [17; 18] pointed out that epidemic DTN routing has difficulties in itslower delivery rate or the high latency depending on the various factors such as hardware resources, the node's movements, or the queuing order of the stored data. Thus, various modifications to epidemic DTN routing have been proposed, such as spray and wait [18] and MaxProp [19].  
This paper introduces DTN with the data triage methods [10] for improvements of network efficiency of V2V communication in local mountain areas. The data triage methods is a queue ordered type of DTN routing such as Maxprop, and it was initially considered for the improvements of the network efficiency after a disaster in the local areas. In the methods, the queuing data are rearranged by the priority of the user policy such as life safety or rescue, as shown in Figure 4.  
The right figure shows the first-in first-out (FIFO) method in which the most recent message is duplicated to others, and which most DTN routing applied in their queuing methods. On the other hand, in the data triage method, the order of the stored messages is rearranged based on priority values in accordance with the user policy after the message comes to the cache in the stored nodes [17; 18]. Then, the most significant and recent message is duplicated to others when a vehicle that can receive a transmission is shown. The paper [17] also pointed out that the ordered queue in the node cache and the radio range is more important for the effectivity of DTN routing although the previous DTN approaches focus on hardware resources. Therefore, the data triage method focuses on reducing the overhead of the queued messages in the proposed methods.  
Moreover, according to the paper [20], the structure of the priority ID in each message consists of the 3-digit priority values (PPP), a 14-digit date (YYYYMMDDhhmms), and a 64-digit android ID(NNNNNNN... ), and the income messages are rearranged by the order of the priority ID in the stored data. Therefore, in this paper, the priority value in the priority ID is classified into several levels by the QEF sensors on the vehicles, and the significant data are firstly duplicated to others in the proposed road surveillance system.  
Figure 4: Queue rearrangements of the data triage. The left figure shows the FIFO (first-in first-out) type of queuing method that most DTN routing introduced. On the other hand, the right figure shows the proposed queuing methods that are rearranged based on the priority of the messages.  
Further, in the data triage method, the node connections are carried by the MAC address and the Android ID to establish a quick connection [20] since the DHCP configuration takes a couple of seconds to set the local IP address. Additionally, the Global IP address for every device is difficult to set in the actual fields because of the limited number of Global IPs.