Network measurement is the basis of researching on network performance monitoring, traffic management, and fault diagnosis. In-band network telemetry has become a hot issue in current network measurement research due to its real-time, accuracy, and scalability. With the emergence and development of Programmable Data Planes, many practical in-band network telemetry solutions have been proposed thanks to its rich information feedback and flexible function deployment. First, we analyze the principles and deployment challenges of typical in-band network telemetry solutions INT and AM-PM. Second, according to the optimization measures and extension of in-band network telemetry, we analyze the characteristics of the optimization mechanism from the aspects of data collection process and multi-task orchestration, and analyze the feasibility of technology extension from the aspects of wireless network, optical network and hybrid network. Third, we compare and analyze the latest applications from the aspects of in-network performance sensing, network-level telemetry system, traffic scheduling and fault diagnosis. Finally, we summarize research work of in-band network telemetry and highlight the future research directions.