In this paper, we have described a scheme for a peer-to-peer Internet backup system that appears to be one to two orders of magnitude cheaper than existing Internet backup services. We believe the cost savings stem largely from savings on administrative expenses and the use of cheaper resources, much of whose operating cost is paid for by other uses. Preliminary experiments with a prototype show that the scheme's performance is acceptable in practice.
The most difficult part of the scheme design was guarding against the inevitable free rider and disrupter attacks to which a cooperative system is vulnerable. We came up with several novel mechanisms--periodic random-block challenges, disk-space wasting, and limiting reads to mutually-chosen random blocks--to address these attacks.