In multi-tenant datacenters, the hardware may be homogeneous but thetraffic often is not. For instance, customers who pay an equal amount ofmoney can get an unequal share of the bottleneck capacity when they do notopen the same number of TCP connections. To address this problem, severalrecent proposals try to manipulate the traffic that TCP sends from the VMs.VCC and AC/DC are two new mechanisms that let the hypervisor controltraffic by influencing the TCP receiver window (rwnd). This avoids changingthe guest OS, but has limitations (it is not possible to make TCP increaseits rate faster than it normally would). Seawall, on the other hand,completely rewrites TCP’s congestion control, achieving fairness butrequiring significant changes to both the hypervisor and the guest OS.There seems to be a need for a middle ground: a method to control TCP’ssending rate without requiring a complete redesign of its congestioncontrol. We introduce a minimally-invasive solution that is flexible enoughto cater for needs ranging from weighted fairness in multi-tenantdatacenters to potentially offering Internet-wide benefits from reducedinter-flow competition.