NTP has rate limiting built into the protocol (https://www.eecis.udel.edu/~mills/ntp/html/accopt.html https://www.eecis.udel.edu/~mills/ntp/html/accopt.html), b

NTP has rate limiting built into the protocol (https://www.eecis.udel.edu/~mills/ntp/html/accopt.html https://www.eecis.udel.edu/~mills/ntp/html/accopt.html), but Snapchat appear to be using a large list of servers. Also, NTP runs on UDP rather than TCP, and it only requires one packet from each party to exchange time, so it's harder to rate-limit it without making your server unreliable.