3.2 TCP Timestamps Option

TCP is a symmetric protocol, allowing data to be sent at any time in either direction, and therefore timestamp echoing may occur in either direction. For simplicity and symmetry, we specify that timestamps always be sent and echoed in both directions. For efficiency, we combine the timestamp and timestamp reply fields into a single TCP Timestamps Option.

      TCP Timestamps Option (TSopt):

         Kind: 8

         Length: 10 bytes

          +-------+-------+---------------------+---------------------+
          |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
          +-------+-------+---------------------+---------------------+
              1       1              4                     4

The Timestamps option carries two four-byte timestamp fields. The Timestamp Value field (TSval) contains the current value of the timestamp clock of the TCP sending the option.

The Timestamp Echo Reply field (TSecr) is only valid if the ACK bit is set in the TCP header; if it is valid, it echos a times- tamp 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 Timestamps option (TSopt) 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 re- ceived a TSopt in the initial <SYN> segment for the connection.