The discovery of scaling behavior in Internet traffic has provided hope that mathematical models can be found to describe the nature of the traffic. In this paper, a novel joint multifractal (JMF) generator for network traffic is proposed, which focuses on both the scaling function and moment factor as the joint key for revealing the multifractal characteristics of the network traffic. The JMF generator is a combination of a multiplicative cascade with an independent K distributed process. This paper demonstrates that the JMF generator is flexible enough to capture the complete multifractal characteristics by considering both the scaling function and moment factor. It points out that the generator has all the important properties observed in data traffic including LRD, multifractality and K distributed properties by theoretical analysis and computer simulations.