The idea of the CPP is to require all clients connecting to a server to correctly solve a mathematical puzzle before establishing a connection, if the server is under attack. After solving the puzzle, the client would return the solution to the server, which the server would quickly verify, or reject and drop the connection. The puzzle is made simple and easily solvable but requires at least a minimal amount of computation on the client side. Legitimate users would experience just a negligible computational cost, but abuse would be deterred: those clients that try to simultaneously establish a large numbers of connections would be unable to do so because of the computational cost (time delay). This method holds promise in fighting some types of spam as well as other attacks like Denial of Service.
Ari Juels and John Brainard, Client Puzzles: A Cryptographic Countermeasure Against Connection Depletion Attacks. In S. Kent, editor, Proceedings of NDSS '99 (Networks and Distributed Security Systems), pages 151-165, 1999.