Synchronize Co-Simulation with Simulink using Sockets and MEX S-Functions

Dear Community,

I would like to setup a Co-Simulation between different Simulink instances running either on the same or different PCs. The communication is established using TCP sockets and MEX S-Functions. A promising approach using ZeroMQ sockets has been published here:

https://blogs.mathworks.com/simulink/2018/05/01/communicating-with-an-external-application-for-co-simulation/

An exemplary setup is show in the image below, where a regular (Simulink) model with 3 systems is exchanging messages (signals) between one another is shown on the left and a distributed Co-simulation of the same overall system on the right. The orange rectangles represent end-to-end connections, e.g. sockets.

My basic question is: How to implement a kind of blocking mode, so that e.g. System 1 stops the simulation until msg A and msg C reach System 2 and System 3 respectively, are processed there, and the reponse from System 3 (msg D) has been received back? I would assume that something very similar to the "blocking mode" from Instrument Control Toolbox' "TCP/IP Receive" block would do the job:

https://de.mathworks.com/help/instrument/tcpipreceive.html

Is this going in the right direction? And if so, how can this "blocking", e.g. waiting until the next full message from e.g. a socket is received, be implemented in a MEX S-Function?

Any help / input would be appreciated!

PS: The assumption here is that the Instrument Control Toolbox is not available and / or a "self-made" implementation, e.g. with the above mentioned ZeroMQ library, is prefered!