How should ZeroMQ replace TCP? Internet runs on TCP/IP (last time I checked). In private networks many problems of public networks don't appear. I'm aware that organisations use it for their internal systems. As soon as one bridges to the outside world, one is going to hit with a problem.

ZeroMQ is a messaging layer that runs over numerous network protocols (TCP, UDP, PGM, TIPC,...), as well as inter-process and inter-thread. It provides abstractions (like pub-sub) that work over all these protocols, and it shows the developer the abstraction using a socket-like API.

In practice you build services which are internally composed and scaled using threads and processes, and which talk to each other over plain (unencrypted) protocols, e.g. TCP between boxes, IPC between processes on the same box. And when you need to talk to the outside world you build bridges that can speak any protocol you like, such as HTTP, or ZeroMQ's encrypted TCP protocol (CurveZMQ). From the developer's point of view, it's mostly transparent. Obviously you need security infrastructure, e.g. key/certificate management.

You can build entire ZeroMQ applications using only encrypted TCP and they will run on public infrastructure. You can develop and test the same apps on a laptop. You might run the same code using IPC on a laptop, and CurveZMQ over the Internet.

So though ZeroMQ definitely uses TCP as one of its transport protocols, it's not a replacement for it.

Which means that doing things like asking for the IP address of a peer make no sense. What's the IP address of a thread, or another process on the same box? If you need identity information, you should not use the transport protocol for that.