Android app is an important class of today’s Internet applications that generate roughly 40-50% of mobile Web and app traffic. More than 1.4 million Android apps are offered through Google’s open market and tens of thousands of new apps are added every month. However, very little information is known about their application protocol behaviors because they predominantly use proprietary protocols on top of HTTP. The problem is further exacerbated by the popular use of common data representation, such as JSON (JavaScript Object Notation) and XML (eXtensible Markup Language), due to the popularity of REST-ful web services.