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.
According to this other question, IPSec can be used to provide encryption for IP, but as can be read at Wikipedia, IPSec is used to establish mutual authentication and exchange of encryption keys.
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?