Payment channels have become an increasingly popular solution to address the scalability issues of current decentralized cryptocurrencies. They allow for multiple payments between users connected through a path of intermediate payment channels, while only requiring interaction with the blockchain during the opening and closing of the channels. However, these payment channels have limitations, such as a lack of support for more complex smart contract functionalities, and a reliance on intermediaries which can lead to issues with reliability, speed, cost, and privacy.
In recent years, research has been focused on addressing these limitations through the development of virtual channels. These virtual channels allow two endpoints of a path to create a direct channel over intermediaries without any interaction with the blockchain. This allows for more applications other than payments and eliminates the need for intermediaries to be involved in updates.
One of the key challenges in the development of virtual channels is the potential for attacks on the payment channel network (PCN) itself. In a recent study, Aumayr et al. (2021) introduced the "Domino attack", a new type of denial-of-service (DoS) attack that leverages virtual channels to destruct the PCN itself. This attack is inherent to the design of existing Bitcoin-compatible virtual channels. The study also discussed other serious drawbacks of existing virtual channel designs, such as limitations to a single intermediary, linear latency and blockchain overhead, and non-constant storage overhead per user.
To address these issues, the study proposed "Donner", a new virtual channel construction that relies on a novel design paradigm. Donner is designed to be more efficient, reducing the number of on-chain transactions for disputes from linear in the path length to a single one, and reducing the storage overhead from logarithmic in the path length to constant. Donner is also Bitcoin-compatible and can be easily integrated into existing payment channel networks.
Other notable research in this field includes "Duplex Micropayment Channels" by Decker and Wattenhofer (2013), which introduced the concept of using a series of simple payment channels to create a larger, more complex channel, and "The Bitcoin Lightning Network: Scalable Off-Chain Instant Payments" by Poon and Dryja (2016), which proposed the use of a network of payment channels to enable instant, off-chain transactions.
In summary, research on payment channels and virtual channels has been focused on addressing the limitations of current payment channel networks, such as scalability, support for complex smart contract functionalities, and reliance on intermediaries. The introduction of the "Domino attack" and the proposed solution of "Donner" have highlighted the importance of addressing these challenges in the design of virtual channels. Further research is needed to continue to improve the efficiency, scalability, and security of payment channel networks.