## 3 System Model
### Communication Channels
#### 4.3.2 Connection by Third-Node Relay  
This section will describe the direct connection mode for two unreachable nodes. The direct connection method may be unsuccessful due to the NAT type of the network device behind the unreachable node accessing the network. In case of unsuccessful use of the scheme described in this section, as shown in Figure 13, is the same as the direct connection scheme, where in the previous steps node \(u_{A}\) has already been contacted by node \(u_{B}\) through a route lookup in the overlay network. Later it is necessary to establish the forwarding transmission channel of the reachable node \(r\) with the assistance of the reachable node \(r\), as shown in Figure 14. A transmission channel is established between unreachable node \(u_{A}\) and reachable node \(r\), unreachable node \(u_{B}\) and reachable node \(r\), respectively to forward data. Unreachable nodes \(u_{A}\) and \(u_{B}\) at both ends are authenticated and encrypted using the methods shown in the previous section. Reachable node \(r\) only forwards data and does not participate in authentication and encryption. The detailed steps for establishing a communication channel between the unreachable nodes \(u_{A}\) and \(u_{B}\) are as follows.  
Figure 14: Flow chart of unreachable nodes \(u_{A}\) and \(u_{B}\) establishing data transmission channels via reachable node \(r\) relays.  
Figure 13: Packet flow diagram of unreachable nodes \(u_{A}\) and \(u_{B}\) in the overlay and underlay planes establishing connections via reachable node \(r\) relays.  
1. First, assume that node \(u_{A}\) has already determined that node \(u_{B}\) is unreachable by the method described previously, and has found a reachable node \(r\) that maintains a long connection with node \(u_{B}\), and has already established an encrypted transmission channel. When the \(u_{A}\) node and the \(u_{B}\) node have the intention to establish a communication channel, an initialisation message to help establish the connection is sent to the reachable node \(r\).
2. Reachable node \(r\) receives the message and finds the long connection session of node \(u_{B}\) in its own table of unreachable nodes and sends a message to node \(u_{B}\) about the wish of node \(u_{A}\) to connect.
3. Once received by the \(u_{B}\) node, a transmission channel to the reachable node is established using public message transmission. At this time messages can be viewed by a third party, but are tamper-proof.
4. After the establishment of the transmission channel is complete, the reachable node \(r\) establishes a transmission channel with the \(u_{A}\) node.
5. After the establishment is complete, the authentication between unreachable nodes \(u_{A}\) and \(u_{B}\) is started.  
Up to this point, any node in the overlay network can be discovered and a secure transmission channel can be established and data can be transmitted. We use a decentralized scheme to implement data communication between unreachable nodes in the system that does not rely on trusted third parties and achieves the establishment of data communication channels between unreachable nodes while retaining the advantages of a decentralized system.