SMTP, FTP, and SSH: The Older Protocols

SMTP, FTP, and SSH were designed in different eras and solve different problems:

They remain valuable to study because their designs expose important networking ideas. SMTP shows store-and-forward delivery and asynchronous failure. FTP shows how a protocol can coordinate separate control and data connections. SSH shows how key exchange, authentication, encryption, and multiplexing can be layered over one TCP connection.

Their age also matters. SMTP and FTP were originally designed for trusted networks and did not protect traffic by default. Security was added later through extensions and deployment practices. SSH was designed specifically to operate over an untrusted network, although it is only secure when host verification, authentication, and authorization are configured correctly.

The conventional TCP ports are:

These are registered defaults rather than guarantees. Administrators can run a service on another port, and a port number alone does not prove which protocol or security mode is present.