# A Combinational Buffer Management Scheme in Mobile Opportunistic Network
## 3 Buffer Management
### Queuing Strategy  
The transmission is mainly dependent on the opportunistic communication of nodes in MON. The node is defined as mobile devices that are carried by individuals in general. Therefore, we suppose each message carries a relevant information package \(P=\{u_{i},v_{i},s_{i},t_{i},h_{i}\}\)[17] that contains the message of the source node \(u_{i}\), destination node \(v_{i}\), the size of the message \(s_{i}\), the time from the beginning of creation until now \(t_{i}\), and the number of hops from the source node to the current node \(h_{i}\). We set the TTL as the threshold of \(t_{i}\), and delete the message when \(t_{i}\) exceeds the TTL. We propose a comprehensive buffer management strategy in mobile opportunistic network. Figure 2 shows the process of the Combinational Buffer Management (CBM) queuing strategy. We decide that two nodes connect with each other and deliver the message when they enter the communication range of each other. We first sort the priority of the messages in the node before exchanging information. The node \(A\) determines whether it is congested when node \(B\) forwards message \(i\) to node \(A\). If node \(A\) has sufficient buffer space, node \(B\) directly sends message \(i\) to node \(A\), and the node \(A\) receives the message \(i\) and stores it in the appropriate location in the cache queue as shown in Figure 2a. When the size of the buffer space of node \(A\) is not enough to store message \(i\), we calculate the utility value of message \(i\) and the utility value of node \(A\) according to CBM strategy, where the utility value of node \(A\) is determined by the average utility value of all messages in the node. If the utility value of message \(i\) is less than the utility value of node \(A\), the node \(A\) does not store the message \(i\) and the message transfer process failed as shown in Figure 2b. If the message utility value is higher than the utility value of the node, node \(A\) receives the message. However, because the fact that node \(A\) does not have enough buffer size, we use the buffer replacement function in the CBM policy to transfer the message with the minimum utility value to the neighbor node \(C\) of the node \(A\) when the node \(C\) has extra available buffer space, (where \(q\) is the message with the lowest utility value in node \(A\)) until the buffer of node \(A\) is enough to contain the message [8], as shown in Figure 2c. Then, node \(A\) receives the message \(i\) from node \(B\). When the buffer occupancy rate of the neighbor node \(C\) reaches the threshold, we give up the migration and directly delete the least useful message until node \(A\) has enough buffer to store message \(i\). Here, the novelty of our strategy is that we migrate messages that need to delete the neighbor nodes with larger buffers. The proposed migration strategy can effectively assure that the delivery ratio does not decline due to directly deleting the message. The number of message copies is uncontrolled in the opportunistic network. Excessive storage of the same message will inevitably lead to the node buffer space being filled up in a short time; meanwhile, it also causes the waste of network resources, the waste of extensive network resources and the decline of buffer utilization [8]. In order to address this problem, we suppose messages in a node are not redundant. In other words, we determine whether the node keeps the same message when a new message arrives. If there are two messages with the same content, we compare the utility values of the two messages. The advantage of queuing strategy is that it not only avoids the repeated  
Figure 1: An example of node movement in Mobile Opportunistic Network. storage of messages, but also ensures that the node utility value is improved. The transmission process terminates when the message is forwarded to the destination node.