A binary file needed to be studied for its logic

I would like to study the behavior of an old binary files in order to understand its logic since the source is lost. The binary file is likely compiled by c/c++ as .dll for windows or .so for linux. It exposes like around 10 blocking API functions for user program to communicate with a back-end server. Most of the functions basically takes parameters pointing to some data buffer, do some processing or wrapper, and then send to server side via TCP and return when server feedback is received. The goal of the project is to study and document the data handling logic of the API functions and the protocol how it talks to the server so eventually I could connect to the server with my own code (without replying on the DLLs provided). You will be provided with the binaries and the API spec. but the server end will not be available for you to test things out. FYI, the binary file size is somewhere more than a hundred KB. You may *study* the binary files by your own method and skill. Say, testing or calling the API with your own simulated TCP server or making use of various tools you see fit. Please note that you are NOT asked to *reverse engineer* the binary since *I was told* this is violating the rule of this site.