Which of the following protocols open back connections on another port to that which the initial
connection is made as part of the normal progression of the connection? (Choose all that apply)

A.
FTP

B.
RSH

C.
Telnet

D.
SMTP

Explanation:

: A clear case of this behavior is the FTP protocol, because it uses 2 ports in parallel, it uses port
21 for the connection control on port TCP 21 and it use port TCP 20 for the actual data transfer.
With FTP we begin the connection in port 20 TCP and with the progression of it, the port 20 TCP
is also open as a back connection. This behavior is also true with the “RSH” protocol, obviously
using other ports than FTP.
With telnet and SMTP, we only use 2 ports for the whole protocol functionality at all times. They
are port 23 TCP for telnet and port 25 TCP for SMTP.