When people need to synchronize, backup or frequently share files/data between a variety of devices (including computers and mobile phones), any traditional ways of file transfer are relatively inadequate. For example, a lot of cloud storage services like Dropbox have more and more and increasingly obvious limitations or restrictions. These include: files have to be uploaded to their servers for transfer, very limited disk space capacity, pretty high requirements on Internet connection speed and stability, limitation of transmission speed and bandwidth usage, very few acceptable file types, not high enough security, easy to be blocked by some countries, expensive cost, and so on. Thereupon, Resilio Sync (formerly called BTsync, full name BitTorrent Sync, now Sync for short) came out.