Networking in a virtualized environment: The TCP case

Virtualization is heavily used in modern private and public data centers. By enabling the consolidation of many virtual machines (VMs) in a physical server, virtualization blurs the frontier between networking and system. We have investigated in a previous work the various prices due to virtualization when a packet is moved from one VM to the physical NIC, by focusing on a pure UDP scenario and multiple competing VMs. We estimated how the inter-packet delay moves away from the optimal point as the number of VMs increases. In the present article we report the impact of virtualization over a whole data stream controlled by the TCP protocol. Our findings reveal that the impact of the additional inter-packet delay is negligible on the performance of TCP. Indeed, virtualized systems have similar goodput levels to the ones of natives systems. Moreover, a less expected result show that virtualized systems improve the fairness between flows, even when such flows are streamed from a same VM. The bad consequences of the virtualization concern mainly bugs in the implementation of the networking function, which can potentially introduce spurious retransmissions and information leaks.