  Payment channel networks are a highly discussed approach for improving
scalability of cryptocurrencies such as Bitcoin. As they allow processing
transactions off-chain, payment channel networks are referred to as second
layer technology, while the blockchain is the first layer. We uncouple payment
channel networks from blockchains and look at them as first-class citizens.
This brings up the question what model payment channel networks require as
first layer. In response, we formalize a model (called RFL Model) for a first
layer below a payment channel network. While transactions are globally made
available by a blockchain, the RFL Model only provides the reduced property
that a transaction is delivered to the users being affected by a transaction.
We show that the reduced model's properties still suffice to implement payment
channels. By showing that the RFL Model can not only be instantiated by the
Bitcoin blockchain but also by trusted third parties like banks, we show that
the reduction widens the design space for the first layer. Further, we show
that the stronger property provided by blockchains allows for optimizations
that can be used to reduce the time for locking collateral during payments over
multiple hops in a payment channel network.
