Blockchain has received increasing attention in academia and industry. However, the increasing transaction volumes and limited on-chain storage underscore scalability as a key challenge hindering the widespread adoption of blockchain. Fortunately, off-chain networks that enable transactions outside the blockchain show promising potential to mitigate the scalability challenge. Off-chain solutions that address blockchain scalability hurdles, such as payment channel networks, facilitate secure and fast off-chain transactions, thus relieving the main chain's strain. In this article, we provide a comprehensive review of key technologies, solutions, and challenges of off-chain networks. First, we introduce the background of off-chain networks encompassing design motivation, framework, overview, and application scenarios. We then review the key issues and technologies associated with off-chain networks. Subsequently, we summarize the mainstream solutions for the corresponding key issues. Finally, we discuss some research challenges and open issues in this area.
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