On SDN

Many of these posts talk about network functions virtualization (NFV) rather than software-defined networking (SDN). NFV is a subset of SDN that is more specific, and it is applicable to a higher level of the application stack. Whereas SDN is aimed at the network layers, NFV is aimed at manipulating the data. The idea of NFV is to take the functions that traditionally would be a part of the network and move them into the compute stack. This move gives us many abilities that we wouldn’t have if the functions remained isolated from the compute. It also lets us move to a much simpler underlying network that is capable of moving traffic around much more quickly. This article aims to examine the different parts that NFV encompasses and to discuss what we gain.
The first, simplest, and most obvious function to virtualize is the switch. At the most basic level, we can’t virtualize servers without also virtualizing their connectivity. While we could in theory pass all of the traffic for all of the virtual machines to an external switch directly, we would not be able to differentiate between the traffic on the way back to the VM without something inspecting the traffic. In effect, we must have half of a virtual switch, so we may as well have a full one. The virtual switch, then, gives us the ability to avoid hairpinning traffic between VMs in the same host out to an external switch. We can move VLAN tagging and some QoS functions into the server, meaning that top-of-rack switches don’t need to do this grunt work. Virtual switching is an integral part of all hypervisors.