I am going to try something new. I am just going to blog something random and stupid every week just to get my thoughts out there. It has been almost a year since I did any significant blogging. I just picked the first topic that came to mind: overlays.
In data networking what is an overlay? It is pretty simple, you are adding a layer of abstraction in the header space. You take a data packet + the original header and you turn all of that into data and slap on a new header. Over simplified, but there.
What does this do for you. Well, it adds more data to the header space and gives us more information to work with for one. Normally you have an ‘intelligent’ (IE more complex) edge set of network devices that are doing a sort of translation between the more traditional header + data format and the new header + header + data overlay. What this allows you to do is to hide information that other components of the network like a data center spine or perhaps service provider core devices do not need to know. They simply need to move traffic as quickly and efficiently as possible between ingress and egress.
They are not new. The concept of an overlay has been around for years. As far as I am concerned the concept really gained traction with Ethernet VLANs. Yah, yah some will say ATM or what-not but for most Engineers they saw this first with Ethernet VLANs (dot1 or ISL tagging). While many will debate if VLANs are an overlay, MPLS is probably the most well known and less debated ‘overlay’ in use today.
Why does it matter? In modern data center networks there is a significant disconnect between the application and the network. As you have heard, networks are rather static in nature and take too much effort to provision and modify. A modern layer 3 overlay, such as VxLAN, which can bridge layer 2 and layer 3 as it were can abstract the complexity of functions such as segmentation for multi-tenancy from the core building blocks of the network, packet forwarding.
At the end of the day the network needs to move packs efficiently but it also needs to provide many services for security, service agreements, business needs, service assurance, etc. In the data center this gets to be very complex very fast using traditional protocols and standards. An overlay can be the abstraction between the underlying network and the intelligence that is needed above.