In a method of network communication that mitigates denial of service attacks, a server broadcasts cryptographic puzzles with certain time intervals, where each puzzle is only valid for the given time interval. A client receives the puzzle, generates a solution for the puzzle, and sends a network request to the server along with the solution of the puzzle. The server verifies the puzzle solution. If the puzzle solution is valid and received within a designated validity time period, then the server processes the request of the client. The server generates the puzzle and transmits the puzzle to the client before the client generates a request for services from the server.