## 1 Introduction
The IOTA message channel [28] can transmit messages to a node in a directed manner, but it is implemented by broadcasting and cannot transmit large data streams, which does not solve the problem at the root. The IPFS [23] dependency library LibP2P [29] has to specify a relay node for unreachable nodes, which wastes network resources, so it does not solve the problem well in terms of efficiency, centralization and ease of use. To fix the connection establishment problems caused by NAT protocols, protocols such as STUN [30], TURN [31], PCP [32] and UPNP [33] have been proposed, but the disadvantage of these protocols is that they require a series of centralized servers to help peers establish connections, pass the necessary information or relay packets. Protocols such as PCP [32] and UPNP [33], which rely on actively opening ports, also require the support of ISP equipment. This problem has been solved to some extent in traditional centralized  
Figure 1: Two modes of network communication. (**a**) Center model. (**b**) Peer-to-peer model. networks. However, in a decentralized network a series of servers implies the presence of a central node, and more seriously unreachable nodes have to trust these servers when communicating, which can undermine the benefits of decentralization. This is contrary to the principles of decentralized design and so does not fit well into a decentralized network. [PERSON] et al. [34] to overcome this effect, decentralized PCP protocols have been proposed, but this is not a generic solution, requires protocols with NAT device support on both sides, is not applicable for multi-layer NAT, and is less efficient for establishing connections via blockchain for consensus data. Moreover, decentralized naming systems [35; 36], decentralized social [16; 17] etc. are brought in to solve the problem of too much centralized power brought about by centralization, and the benefits of decentralization will be lost if traditional centralized solutions are used. There are some decentralized applications (e.g., data trading scenarios [13; 14; 15]) that use smart contracts, IPFS, etc. as a medium for data exchange or control in order to avoid communication between unreachable nodes. Although there is no communication between unreachable nodes, this can lead to low resource utilization and communication bottlenecks. Therefore, finding a universal, structured solution that does not rely on trusted centralized servers, and that can transmit a large number of data streams continuously when communicating without consuming additional network resources, thus allowing unreachable nodes in a decentralized network to communicate with each other peer-to-peer, is currently the biggest challenge. For the above background, the current status of the problem, and the challenges, we propose a decentralized, unreachable node communication scheme in a P2P network that does not depend on a centralized node. The proposed scheme uses a peer-to-peer overlay network as the underlying scheme for routing, where addressing between nodes does not depend on IP addresses and does not require a central server, thus maintaining the characteristics of a decentralized network. The unreachable nodes communicate with each other by finding a certain reachable node that is connected to the unreachable node in the index distributed throughout the network. Depending on the network conditions at both ends, a direct connection or a relay is selected to establish a data transmission connection between unreachable nodes. Once established, they can continuously transmit data without relying on other facilities. In terms of security, the scheme proposed in this paper does not rely on centralized trusted CA institutions, and the security of P2P overlay network and data transmission is ensured by the implicit relationship between node addresses and asymmetric encryption system.