I don't think it's reliable transport per se that makes simplified communication possible, but direct transport. The TCP complexity is because the packets travel through a complex network. They get switched around here and there and compete with streams between other independent nodes for resources.
So I think friend sockets would be appropriate for any two sockets connnected via a dedicated link. As well as an in-kernel link or an SSH-based TCP link, that could be a PPP link or an ethernet with only two nodes on it.
In all those cases, a socket friendship protocol like this would be useful.
In fact, even without the ability to use existing TCP/IP applications, it would be nice to have stream sockets that exploit such direct connection, but I've never seen them.
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