With wide application of virtualization technology, tenants are able to access isolated cloud services by renting the shared resources in datacenters. Unlike resources such as CPU and memory, datacenter network, which relies on traditional transport-layer protocols, suffers unfairness due to a lack of VM-level network isolation. In this paper, we propose Falloc, a new bandwidth allocation protocol, towards VM-based fairness across the datacenter with two main objectives: (i) guarantee bandwidth for VMs based on their base bandwidth requirements, and (ii) share residual bandwidth in proportion to weights of VMs. To design Falloc, we model the datacenter bandwidth allocation as a bargaining game and propose a distributed algorithm to achieve the asymmetric Nash bargaining solution (NBS). We apply the theory to practice by implementing Falloc with OpenFlow in experiments under diversed scenarios, which shows that Falloc can achieve fairness by adapting to different network requirements of VMs, and balance the tradeoff between bandwidth guarantee and proportional bandwidth share. By carrying out large scale trace-driven simulations using real-world Mapreduce workload, we show that Falloc achieves high utilization and maintains fairness among VMs in datacenters.