Resilio Sync does its best to re-send as few packets as possible. It splits each files into pieces from 32KB (kilobytes) up to 2MB (megabytes),depending on overall file size, prior to transfer. Only changed pieces are transferred, not the whole file again. However, if a file changes in a way that all pieces shift , the whole file will be re-synced. Sync Business has diff delta sync feature, which allows avoid re-syncing even in this case. See here for more details.