Client puzzles and amplification Denial-of-service (DoS) attacks attempt to overwhelm a server with a huge volume of requests. Researchers have proposed a defense against DoS attacks called client puzzles: For each request, the server sends the client a freshly generated random challenge r and a difficulty parameter n, and the client has to produce a solution s such that the last n bits of HMACr(s) are all 0. Clients must present a valid solution to receive service.

Somedenial-of-serviceattacksattempttoexhaustthevictimsnetworkbandwidthrather that its CPU resources, and amplification plays an important role in such attacks too. Attackers recently began exploiting the Network Time Protocol (NTP) for this purpose, as described in this article: https://goo.gl/oVXH7V. What two features of NTP make it an ideal DoS tool?