I am not so sure about that. I think I remember from a very very old memory that, actually, this specific case has been encountered or discussed and that the answer is no, you aren't allowed to do that. Basically, if you create your own custom protocol etc. You are just reinventing an inefficient wheel, but it's still a wheel. It's still linking.

The whole process could even be automated otherwise, nullifying the gpl effectively.

I am NOT a lawyer. And maybe it depends on the country (it seems USA has a very "what-the-law-say-exactly" while France (and maybe Europe) tend to look at the "spirit" of the law.

EDIT more about this question here ? https://news.ycombinator.com/item?id=15105920

EDIT 2:

(Rpc are listed)

https://www.gnu.org/licenses/gpl-faq.en.html

"Where's the line between two separate programs, and one program with two parts? This is a legal question, which ultimately judges will decide. We believe that a proper criterion depends both on the mechanism of communication (exec, pipes, rpc, function calls within a shared address space, etc.) and the semantics of the communication (what kinds of information are interchanged).

If the modules are included in the same executable file, they are definitely combined in one program. If modules are designed to run linked together in a shared address space, that almost surely means combining them into one program.

By contrast, pipes, sockets and command-line arguments are communication mechanisms normally used between two separate programs. So when they are used for communication, the modules normally are separate programs. But if the semantics of the communication are intimate enough, exchanging complex internal data structures, that too could be a basis to consider the two parts as combined into a larger program."