# Development of a Social DTN for Message Communication between SNS Group Members
## 3 How to Build a Social DTN
### Service Overview  
Members of an SNS community can exchange messages over an ad hoc network, a social DTN built with the terminals of these members, without using the SNS. An example of using this network is shown in Figure 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Community Identification** & **Routing Table Table Table**Creation & **Criteria Used to Determine Flooding** & **Routing Method** \\ \hline \multirow{8}{*}{None} & \multirow{8}{*}{None} & None: flooding of messages to all adjacent nodes & Epidemic [4] \\ \cline{3-4}  & & \multicolumn{2}{c}{Flooding of messages to nodes up to a} & \multirow{2}{*}{Spray and Wait [5]} \\ \cline{3-4}  & & & certain number of hops (a certain number of copies) & \\ \cline{3-4}  & & & Distance to the destination node & Location-based [6] \\ \cline{3-4}  & & \multicolumn{2}{c}{Motion vector to the destination node} & \multicolumn{2}{c}{motion vector} \\ \cline{3-4}  & & & \\ \cline{3-4}  & & Fixed & \multirow{2}{*}{Routing table (includes encounter probability calculation type, social-context-awareness type)} & Message ferry [7], \\ \cline{3-4}  & & & & Routing based on traveling route information [8] \\ \cline{3-4}  & & & & \\ \cline{3-4}  & & & probability calculation type, social-context-awareness type) & Meta information exchange [9; 10], \\ \cline{3-4}  & & & & PRoPHET [11], \\ \cline{3-4}  & & & & HiBOp [14] \\ \hline Present & \multicolumn{2}{c}{Reactive} & \multicolumn{2}{c}{Proposed method} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of routing methods according to the criteria used to determine flooding and according to routing table creation methods. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Implementation** & **Detailed Method** & **Usage Conditions** & **Position** \\ \hline \multirow{8}{*}{OS modification} & Custom operating system (OS) + patches [15; 16] & \(\bullet\) & This cannot be used with the user privilege. \\ \cline{3-4}  & & \(\bullet\) & It is necessary to acquire the administrator privilege. \\ \cline{3-4}  & & \(\bullet\) & It is necessary to modify the OS of all terminals that use an ad hoc network. \\ \cline{3-4}  & & \(\bullet\) & Either Bluetooth or Wi-Fi can be used. \\ \hline \multirow{3}{*}{Application} & Google Nearby Connections API [13], which is independent of OS & \(\bullet\) & This can be used with the user privilege. \\ \cline{3-4}  & & \(\bullet\) & Bluetooth and Wi-Fi Direct can be used simultaneously. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Methods of building an ad hoc network with Android terminals. The authors proposed a method in which people (who are assumed to be visitors from abroad) use a community token by interacting with an SNS in [2]. The user creates a community token and uploads it to an SNS. Relay nodes that periodically search for community tokens download uploaded community tokens via the SNS and hold them. After that, users can use a community token to identify terminals that belong to the same SNS community. They can participate in an ad hoc network in the background and use DTN communication. In the proposed social DTN and message communication, ad hoc communication between terminals and mobile carrier communication do not always need to operate simultaneously. The current research assumes that the users are visitors from abroad and that they access only public wireless LANs. As shown in the left image in Figure 1, a user in an airport or a hotel can access the Internet and browse SNSs or post messages there. As shown in the right image in Figure 1, a user touring on the street cannot make message communication with other members of the group unless there happens to a public wireless LAN. The proposed social DTN is useful in such a case. People, such as students, who subscribe to a mobile carrier but want to reduce the amount of data communication to decrease communication costs can also be candidate users of the social DTN.