# Mail Man Ferry: A Novel Routing Protocol in Intermittently Connected Networks
## 4 Mail Man Ferry in ICNs  
The concept of the protocol proposed in this paper is inspired by the traditional Mail Man Ferry model. In such a model, a traditional mail man takes a regular path to collect all the mails in this particular path to the mail office. In the mail office, the letters are classified and routed in groups to their destinations. For example, 10 letters could be directed to country A, whereas another 10 letters are directed to country B. The question here is if the Mail Man traditional model has been successful for decades, can the same concept be applied as a routing method in ICN. Therefore, this work examines the benefits of the Mail Man model toward improving the two main metrics in any ICN routing method: delivery probability and delay. The importance of the Mail Man Ferry method is not just to propose a new ICN routing method, but to ensure node cooperation, which is essential to increase delivery probability. In the Mail Man Ferry method, node cooperation is guaranteed because a credit concept for cooperating nodes is employed.  
A few paths are predefined in the Mail Man Ferry method to be known as ferry paths. The predefined ferry paths can be chosen based on the deployed environment. For real mapping of city, as an example, the city highways and major roads could be chosen to be the ferry paths. A node taking these paths is a candidate to carry other messages from nodes in the network. To encourage node cooperation, whenever a node passes through these predefined ferry routes, it receives a credit, and its credits increase as it passes via these routes every time. Note that the node credit can be interpreted as allowing a node to disseminate more messages compared to a node with no credit. A node with high credit is capable of disseminating multiple copies of its own messages to its encountered ferry nodes, whereas a message with low credit is not allowed to disseminate its messages to an encountered ferry node; i.e., nodes without credits have to deliver their messages by themselves. Note that one of the expected costs in the Mail Man Ferry protocol is the need for a dedicated buffer space to identify the predefined ferry paths because nodes are encouraged to travel through to raise their credit as cooperating nodes. This cost can be separately studied in future work.  
Figure 1 presents the message exchange when the Mail Man Ferry method is not employed and shows that node S can exchange messages with its encountered nodes L and K although only L is on the ferry route, whereas K does not take a ferry path, i.e., Epidemic message dissemination. Figure 2 shows the message exchange process when the Mail Man Ferry method is employed without taking into consideration the credit concept for node cooperation. Note that S only disseminates messages to L where it takes a Mail Man Ferry path, whereas K does not receive any messages from S because it takes a random path, not a predefined as ferry path. Figure 2 also shows that both R and T are able to disseminate messages to E without consideration of their credit history in cooperation. Finally, Figure 3 presents the message exchanges when both the Mail Man Ferry method and the credit concept of node cooperation are employed. Figure 3 shows that S is able to forward messages only to L because it is on the Mail Man Ferry path. Furthermore, only R is able to disseminate messages to E because it has credit in cooperation, whereas T needs to keep its copies until it reaches its destination because it did not show any cooperation in the past.  
Figure 1: Message forwarding mechanism without Mail Man Ferry method is employed.