Fair bandwidth allocation is one of the most challenging research issues in the context of Assured Forwarding (AF) in the Differentiated Services (DiffServ) networks. There exist many works that tried to assure the fairness of bandwidth allocation. However, these works only focused on studying the simple case, in which multiple AF flows share a single bottleneck link, and they also lacked a solid theoretical analysis to validate themselves. In this paper, we propose a Network-assist Packet Marking (NPM) scheme to offer fair bandwidth allocation among multiple aggregates. By both theoretical analysis and experimental evaluation, we demonstrate that NPM can fairly distribute bandwidth among these aggregates in both single and multiple bottleneck link networks.