## 1 Introduction  
The original design of the Internet (the underlying protocol) was decentralized, which greatly ensured the stability, fault tolerance and security of the network [1, 2]. Despite the initial design principles, as the Internet evolved, there was a gradual move towards a single centralized point of access. This is because a centralized approach is the easiest way to achieve the target functionality and at the same time facilitate management (e.g., Twitter, Facebook, WhatsAPP, Google Drive), but some of the problems associated with centralization, single point of failure [3], certificate leaks [4, 5], leaked files [6], centralized social networking privacy disclosure [7], smart camera leaks privacy [8] etc. cannot be well solved. This has led many researchers to explore decentralized networks, for example, the early emergence of P2P networks such as Bittorrent [9] and Napster [10] represent this series of attempts to decentralise about file sharing. The emergence of cryptocurrencies such as Bitcoin [11] and Ethereum [12], using blockchain technology that allows peer-to-peer communication to be de-trusted. These have contributed to a process of \"re-decentralization\",which has seen the emergence of many decentralized applications, such as decentralized data trading [13; 14; 15] and decentralized social networking [16; 17]. They have benefits that are difficult to achieve with centralized networks, such as no single point of failure, no centralized authority, and privacy protection. However, with the development of the network, the shortage of IPv4 addresses [18; 19], the slow transition to IPv6 and other issues [20; 21], there are many nodes that are either actively or passively behind NAT or firewall and become externally unavailable for an active connection. Because NAT protocol [22] was originally designed to solve the IPv4 address reuse problem under the traditional centralized network represented by the C/S model, they could only be initiated by clients who could not connect directly from the outside. This causes a lot of unreachable nodes in decentralized networks, for example 52.2% of unreachable nodes in IPFS [23; 24] and 86.8% of unreachable nodes in the Bitcoin network [25]. In decentralized applications, they have the same communication needs as centralized applications, such as decentralized social [16; 17]. They no longer communicate only with the central server as in centralization, as shown in Figure 1a, but may communicate with any node, as shown in Figure 1b, which will prevent communication from being established directly when unreachable nodes exist. Thus, the communication of unreachable nodes in decentralized networks is a matter of concern. In this case, some of the unreachable nodes in the decentralized network will not work properly and the unreachable nodes become a limitation to the development of the decentralized network to enable direct end-to-end communication. Early P2P networks such as Napster [10] and Bittorrent [9] had their resources indexed centrally, so they could not be called decentralized networks yet, but only P2P networks. Although unreachable nodes can establish connections to resource fragmentation index nodes, they cannot establish connections to unreachable nodes to access data resources to solve this problem. Skype [26] divides nodes into super nodes and normal nodes according to their different capabilities, with the goal of trying to relay unreachable nodes through super nodes, solving the communication between unreachable nodes to some extent, but essentially not being able to solve the problem well using a decentralized approach. The Bitcoin network's network layer protocol uses Gossip [11; 27], an unstructured P2P network that also has no mechanism for dealing with unreachable nodes. However, because the blockchain shares a distributed ledger that is consistent across the network, there is no need to deliberately find any node, so this problem is not addressed in the blockchain network layer represented by Bitcoin either.