The Kiss-of-Death Packet

Ordinarily, packets denied service are simply dropped with no further action except incrementing statistics counters. Sometimes a more proactive response is needed to cause the client to slow down. A special packet has been created for this purpose called the kiss-o'-death (KoD) packet. KoD packets have leap indicator 3, stratum 0 and the reference identifier set to a four-octet ASCII code. At present, only one code RATE is sent by the server if the limited and kod flags of the restrict command are present and either the guard time or MAH time are violated.

A client receiving a KoD packet is expected to slow down; however, no explicit mechanism is specified in the protocol to do this. In the reference implementation, the server sets the poll field of the KoD packet to the greater of (a) the server MAH and (b) client packet poll field. In response to the KoD packet, the client sets the peer poll interval to the maximum of (a) the client MAH and (b) the server packet poll field. This automatically increases the headway for following client packets.

In order to make sure the client notices the KoD packet, the server sets the receive and transmit timestamps to the transmit timestamp of the client packet. Thus, even if the client ignores all except the timestamps, it cannot do any useful time computations. KoD packets themselves are rate limited to no more than one packet per guard time, in order to defend against flood attacks. http://www.eecis.udel.edu/~mills/ntp/html/rate.html