###### Abstract  
In opportunistic networks such as delay tolerant network, a message is delivered to a final destination node using the opportunistic routing protocol since there is no guaranteed routing path from a sending node to a receiving node and most of the connections between nodes are temporary. In opportunistic routing, a message is delivered using a'store-carry-forward' strategy, where a message is stored in the buffer of a node, a node carries the message while moving, and the message is forwarded to another node when a contact occurs. In this paper, we propose an efficient opportunistic routing protocol using the history of delivery predictability of mobile nodes. In the proposed routing protocol, if a node receives a message from another node, the value of the delivery predictability of the receiving node to the destination node for the message is managed, which is defined as the previous delivery predictability. Then, when two nodes contact, a message is forwarded only if the delivery predictability of the other node is higher than both the delivery predictability and previous delivery predictability of the sending node. Performance analysis results show that the proposed protocol performs best, in terms of delivery ratio, overhead ratio, and delivery latency for varying buffer size, message generation interval, and the number of nodes. delivery predictability; routing; PRoPHET protocol; delay tolerant networks; opportunistic routing protocol 2018 8 2215 2018  
in its buffer. Then it carries the message while moving and forwards it to node B when node A and the node B are within communication range. After that, node B stores the message in its buffer and moves while carrying it until it contacts node C. The message is finally delivered to node C from node B using opportunistic contact. Many works related to DTN protocols have been carried out [1; 2; 3; 4; 5; 6; 7; 8]. In epidemic protocols [6], a node forwards a message to other nodes whenever they are within the communication range. It is very simple but a high message overhead is a major drawback due to the limited buffer space of mobile nodes. In the spray and wait protocol [7], the number of total message copies for each newly generated message is limited by L to reduce the message overhead. Therefore, only L-1 copies of the messages can be disseminated to other nodes from the message generation node in the spray phase. In the wait phase, L nodes with have only one message copy delivers the message to the destination node of the message only. However, in the spray phase, message copies are disseminated blindly, without a predefined criterion. In the probabilistic routing protocol for intermittently connected networks (PRoPHET) [8], the delivery predictability between two nodes, which has a value between 0 and 1, is calculated based on contact history between them, and a message is forwarded based on the delivery predictability. The delivery predictability of a node A to a node B, P(A,B), estimates a likelihood of delivering a message to a node B. Therefore, it is highly likely that if a node has a higher delivery predictability, it is a better node to deliver the message. If two nodes contact, the delivery predictability between them increases, as defined in Equation (1) [8]:  
\[\mathrm{P(A,B)=P(A,B)old}+(1-\Delta-\mathrm{P(A,B)old})\times\mathrm{P_{ encounter}}, \tag{1}\]  
Figure 1: An example of a Delay-Tolerant-Network scenario. Figure 2: The Store-Carry-Forward strategy. where \(\rm P_{encounter}\) and \(\Delta\) are a scaling factor and parameter defining an upper bound of \(\rm P(A,B)\), respectively. As time passes on after the last contact, the delivery predictability between node A and B decreases, as defined in Equation (2) [8]:  
\[\rm P(A,B)=P(A,B)old\times\gamma^{K}, \tag{2}\]  
where \(\gamma\) and K are the aging constant an the number of passed time units after the last contact, respectively.