As explained in the TCP/IP Guide, it is possible to configure TCP/IP to operate in simplex mode and this mode is utilized, among other things, in communication from satellites.
One consideration with TCP/IP is however that, in itself, it "lacks even the most basic mechanisms for security, such as authentication or encryption", as explained in this article.
Neither mutual authentication nor an exchange of keys should be possible in fully simplex mode (or maybe it is but I am missing something?).
Anyway, the question I wanted to ask is: if TCP/IP is actually ever operated in simplex mode, what are the options for encrypting the data?