The assured forwarding per hop behavior standardized by the IETF Differentiated Services working group provides four class-based differentiated IP services. In this service, however, unexpected service degradation may occur and differentiation among classes may be disordered if the network is designed to minimize over-provisioning or is under-provisioned. We therefore developed a packet scheduling scheme that dynamically allocates bandwidth to each class queue to guarantee the differentiation among classes under any traffic conditions. The scheme estimates the sum of CIRs (committed information rates), i.e. rate of the packets having lowest drop preference, of active flows in each class and initially allocates the link bandwidth according to the sum of CIRs. It allocates the excess bandwidth by using a combination of CIR-proportional allocation and equal-share allocation. The equal share part enables that the flows in best effort class or the flows having zero CIRs can utilize minimum share of the bandwidth. Our scheme also introduces a scalable scheduling technique to improve fairness among flows in the same class. We evaluate the proposed scheme and show that it makes DiffServ operations fairer under any traffic conditions.