TCP Echo Option:
Kind: 6
Length: 6
+--------+--------+--------+--------+--------+--------+
| Kind=6 | Length | 4 bytes of info to be echoed |
+--------+--------+--------+--------+--------+--------+
This option carries four bytes of information that the receiving TCP may send back in a subsequent TCP Echo Reply option (see below). A TCP may send the TCP Echo option in any segment, but only if a TCP Echo option was received in a SYN segment for the connection.
When the TCP echo option is used for RTT measurement, it will be included in data segments, and the four information bytes will define the time at which the data segment was transmitted in any format convenient to the sender.
TCP Echo Reply Option:
Kind: 7
Length: 6
+--------+--------+--------+--------+--------+--------+
| Kind=7 | Length | 4 bytes of echoed info |
+--------+--------+--------+--------+--------+--------+
A TCP that receives a TCP Echo option containing four information bytes will return these same bytes in a TCP Echo Reply option.
This TCP Echo Reply option must be returned in the next segment (e.g., an ACK segment) that is sent. If more than one Echo option is received before a reply segment is sent, the TCP must choose only one of the options to echo, ignoring the others; specifically, it must choose the newest segment with the oldest sequence number (see next section.)
To use the TCP Echo and Echo Reply options, a TCP must send a TCP Echo option in its own SYN segment and receive a TCP Echo option in a SYN segment from the other TCP. A TCP that does not implement the TCP Echo or Echo Reply options must simply ignore any TCP Echo options it receives. However, a TCP should not receive one of these options in a non-SYN segment unless it included a TCP Echo option in its own SYN segment.