@fanquake Sure, I’ll combine this with #13578. I did more digging. We already require 4.x or later in the configure script. That aligns with Debian and Ubuntu packages for still supported releases (see https://packages.ubuntu.com/search?keywords=libzmq3-dev and https://packages.debian.org/search?keywords=libzmq3-dev). The nice thing is that the ZMQ project has repos for common distros that would make it easier for us to raise the minimum version (see http://zeromq.org/area:download and https://build.opensuse.org/repositories/network:messaging:zeromq:release-stable). Although, from what I’ve found, the functions that I’m switching to were around before, or with `4.0.0`. We’re already covered there. It’s just a matter of whether to bump the minimum requirement, and if so, how far? I’m inclined to not mandate a _minimum_ version bump right now. I do encourage using the latest version.