A success Low-rate Distributed Denial of Service (LDDoS) attack is composed of many single sources of Low-rate Denial of Service (LDoS) attacks, which are well-organized in time synchronization to aggregate the attack flow at the end of target router. This paper addresses time synchronization and flow aggregation in LDDoS attacks for the purpose of exploring the attack performance. A cross-correlation algorithm is proposed to guarantee all network-wide distributed attack pulses aggregate at the victim end with a strict time limitation to form a powerful attack pulse. Experiments on the attack performance of LDoS and LDDoS attack are performed in NS-2 simulation platform. Experiment results show that the LDDoS attack effects can be improved significantly by using cross-correlation algorithm to coordinate attack pulses, which obtain better attack performance than many single dependent attack pulses.