The existing credit-based mechanisms mainly focus on stimulating the rational packet droppers to relay other nodes' packets, but they cannot identify the irrational packet droppers such as compromised or broken nodes, which has negative impact on the network performance. In this paper, we propose a creditbased mechanism that uses credits to stimulate the rational packet droppers to cooperate, and uses reputation system to identify the irrational ones. Payment receipts are processed to reward the cooperative nodes, and to detect the broken links so that a reputation system can be built to identify the irrational packet droppers. Our evaluations demonstrate that our mechanism can secure the payment, and precisely identify the irrational packet droppers.