  Cross-chain technology facilitates the interoperability among isolated
blockchains on which users can freely communicate and transfer values. Existing
cross-chain protocols suffer from the scalability problem when processing
on-chain transactions. Off-chain channels, as a promising blockchain scaling
technique, can enable micro-payment transactions without involving on-chain
transaction settlement. However, existing channel schemes can only be applied
to operations within a single blockchain, failing to support cross-chain
services. Therefore in this paper, we propose Cross-Channel, the first
off-chain channel to support cross-chain services. We introduce a novel
hierarchical channel structure, a new hierarchical settlement protocol, and a
smart general fair exchange protocol, to ensure scalability, fairness, and
atomicity of cross-chain interactions. Besides, Cross-Channel provides strong
security and practicality by avoiding high latency in asynchronous networks.
Through a 50-instance deployment of Cross-Channel on AliCloud, we demonstrate
that Cross-Channel is well-suited for processing cross-chain transactions in
high-frequency and large-scale, and brings a significantly enhanced throughput
with a small amount of gas and delay overhead.
