In short, Forge Networking is a free and open source multiplayer game (multi-user) networking system that has a very good integration with the Unity game engine. You wanna make a multiplayer game or real time multi-user application? This is the library for you.
An easy-to-learn, high performance network io library written in modern cpp (c++11), It borrows concepts from Netty, and with well defined internal modules, It enables you to write rpc, http/https,websocket application with just few lines。
Details about Android App and server connection using Retrofit Network library is given in my blog site. Please be mentioned the blog is written in Bengali. If you know Bengali, you are most welcome. I described here Client side (Android) as well as server side (by PHP).