In Internet of Things, wireless access networks need to support a large number of user equipments (UEs) in real time, where UEs frequently arrive and leave and plenty of packet collisions occur. In this paper, we propose a protocol termed adaptive framing with successive decoding (AFSD), that not only supports vast connectivity but also involves little signalling overhead. By using AFSD, the frame length can be adaptively adjusted based on the present UE number to deal with the fluidity of UEs, and successive decoding is adopted to recover signal from the mixed received signal to alleviate the packet collision. We explicitly analyze the average delay of our protocol. Numerical results verify the accuracy of our analysis, as well as the effectiveness of the proposed protocol on handling massive access.