The goal of CBR is to reach the target bitrate on average (at the level of one or a few GOPs) and to comply with the HRD model, that is, avoiding decoder buffer overflows and underflows. In CBR mode, a single bitrate value defines both the target stream bitrate and the output/transmission (leaky bucket) bitrate. The reference decoder buffer parameters are cpb-size and initial-delay. The CBR rate control mode tries to keep the bit rate constant while avoiding buffer overflows and underflows. If a buffer underflow happens, the QP is increased (up to max-qp) to lower the size in bits of the next frames. If a buffer overflow occurs, the QP is decreased (down to min-qp) to increase the size in bits.