  Full nodes in a blockchain network store and verify a copy of the whole
blockchain. Unlike full nodes, light clients are low-capacity devices that want
to validate certain data on a blockchain. They query the data they want from a
full node. If light clients do not verify the data they receive, full nodes
might deceive them. SPV, introduced in the Bitcoin paper, is a practical
solution to this problem currently used in many PoW blockchains. In SPV, the
resources needed to verify a full node's response grow linearly with the
blockchain size, making it inefficient over the long run. Another issue with
SPV is that the full nodes do not get compensated for the services they
provide.
  In this work, we introduce LightSync, a simple and cost-effective solution
for light clients to verify the inclusion of certain data in a PoW blockchain.
The resources needed for running LightSync remain constant no matter what the
size of the blockchain is. LightSync uses an incentive mechanism that
encourages full nodes to participate in the protocol. We perform a thorough
analysis of the security of LightSync and discuss the details of deploying it
in a real-world environment.
