TCP option 7, Echo reply

Protocol suite:	TCP/IP.
Protocol type:	Transport layer protocol.
Option length:	6 bytes
Host implementation:	Obsolete.
Router implementation:	Obsolete.
Links:	IANA: TCP option numbers.

RFC 1072, pg 11 - 13:

A simple method for measuring the round-trip delay (RTT in seconds); of a segment would be: the sender places a timestamp in the segment and the receiver returns that timestamp in the corresponding ACK segment. When the ACK segment arrives at the sender, the difference between the current time and the timestamp is the RTT. To implement this timing method, the receiver must simply reflect or echo selected data (the timestamp) from the sender's segments. This idea is the basis of the "TCP Echo" and "TCP Echo reply" options.

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.

MAC header

IP header

TCP header

TCP option 7

Data

TCP Option 7: