Timestamp option
http://www.networksorcery.com/enp/protocol/tcp/option008.htm
Timestamp Value (TSval). 32 bits.
This field contains the current value of the timestamp clock of the TCP sending the option.
Timestamp Echo Reply (TSecr). 32 bits.
This field is only valid if the ACK bit is set in the TCP header. If it is valid, it echos a timestamp value that was sent by the remote TCP in the TSval field of a Timestamps option. When TSecr is not valid, its value must be zero. The TSecr value will generally be from the most recent Timestamp option that was received; however, there are exceptions that are explained below. A TCP may send the Timestamp option in an initial SYN segment (i.e., segment containing a SYN bit and no ACK bit), and may send a TSopt in other segments only if it received a TSopt in the initial SYN segment for the connection.