I really recommend against using protobuffs. There are long standing bugs that Google just refuses to fix in the public version. I can't remember what they are off the top of my head, but I know a semi-prominent YC company that uses them and they pull their hair out all the time.

Just use zerorpc. It's more reliable than zeromq + protobuffs and it comes with a bunch of freebies, like built in heartbeats, streamed responses, etc.