Книга: Iptables Tutorial 1.2.2
The timestamp type is obsolete these days, but we bring it up briefly here. Both the reply and the request has a single code (0). The request is type 13 while the reply is type 14. The timestamp packets contains 3 32-bit timestamps counting the milliseconds since midnight UT (Universal Time).
The first timestamp is the Originate timestamp, which contains the last time the sender touched the packet. The receive timestamp is the time that the echoing host first touched the packet and the transmit timestamp is the last timestamp set just previous to sending the packet.
Each timestamp message also contains the same identifiers and sequence numbers as the ICMP echo packets.