1. As said in the original comment, it doesn't necessarily have to be protobuf, but just a data serialization system.
2. Protobuf per se is just a serialization system, it does not push RPC "Protocol buffers are Google's language-neutral, platform-neutral, extensible mechanism for serializing structured data – think XML, but smaller, faster, and simpler." [0]. It can be used in combination with RPC systems, but does not have to.