Smart City constructions are mostly closed-loop applications creating Information Silo in specific industries or areas. Practical interconnection and interoperability among systems and applications have not been extensively realized. In fact, the essential problem is the lack of robust/efficient resolution mechanism for identifier services breadth of areas and industries. This Recommendation explores the set of requirements for identifier services used in Smart City. An identifier service for Smart City must be scalable and secure, and not only promote interoperability among different Smart City applications, but also compatible with any existing practices in the application domain. Purpose: This Recommendation proposes to specify a set of requirements for identifier services in Smart City, and the following aspects should be studied. Contents: - Application scenarios of interconnection and interworking in Smart City; - General requirements for identifier services in Smart City; - Reference models of identifier services in Smart City.