This document describes how a multi-tenant (or multi-department) data centre operator can isolate tenants from network performance degradation due to each other's usage, but without losing the multiplexing benefits of a LAN-style network where anyone can use any amount of any resource. Zero per-tenant configuration and no implementation change is required on network equipment. Instead the solution is implemented with a simple change to the hypervisor (or container) beneath the tenant's virtual machines on every physical server connected to the network. These collectively enforce a very simple distributed contract - a single network allowance that each tenant can allocate among their virtual machines, even if distributed around the network. The solution uses layer-3 switches that support explicit congestion notification (ECN). It is best if the sending operating system supports congestion exposure (ConEx). Nonetheless, the operator can unilaterally deploy a complete solution while operating systems are being incrementally upgraded to support ConEx and ECN.