Servers with custom binary protocol?

Question

C++ / C#

First of all, I'm interested in how this is implemented in C. That is, I would like to set the packet formats and, possibly, the interaction logic on some DSL. As I understand it, you can form packages using asn1c, or protobuf, or Thrift (there are other tools, but the mainstream, as it seemed to me, is these), the server can be written on it. In other cases, you can use, for example, ØMQ. And what other options are possible?
I can’t say yet about the tasks of a particular server, I’m interested, rather, for research purposes.

Answer 1

another option is to just take it and silently write

Answer 2

What about the client side?