Instead of using tcpflow, you should maybe consider using nc instead like this:
nc -l 2003
You are now listening on port 2003. To send data to port 2003, you can open a separate terminal and type:
nc 127.0.0.1 2003
This opens a connection to the listening terminal. Anything you type in that terminal thereafter will be sent to localhost:2003 and will output on the listening terminal. Furthermore, anything typed in the listening port will be sent to the terminal output of the other.
Also, instead of using the second command you can use the same command you tried earlier instead:
echo "hello" | nc 127.0.0.1 2003
The listening terminal should output hello before closing the connection.
For more information, type the following command in a terminal to view the manual: