SDN: Modularity based on abstraction is the way things get done

???Motivation for SDN

Traditional IP networks posed a few challenges.

• Networks become hard to manage and evolve because of the increasing complexity and dynamic nature of traditional networks.
• Implementing new network policies involved change every network device, usually through manual configurations and vendor-specific commands. This required operators to put in a lot of effort.
• In the traditional IP network setup, the control plane (handles network traffic) and data plane (forwards traffic based on control plane's decision) were tightly coupled together with any network devices(switches or routers).
• Network designs lacked formal principles and were a collection of protocols with specific mechanisms.

Why we had these problems in networking?

"Building a system that scales and works, modularity based on abstractions is the way". If you can't evolve, you can't manage, and you can't understand the systems, chances you don't have abstractions right.

What abstractions do we need for networks?

Two planes, two completely different challenges.

• Data Plane process the packets with local forwarding state. Switch looks at the incoming packet header and local forwarding state that's sitting on the switch, figures out either to forward or to drop. This happens at nanoseconds time scales and is completely local.
• Control Plane computes the forwarding state. Uses distributed protocols and manual configurations. It is slower tens of milliseconds and non-local information comes from the outside world.

We have two problems: one is fast and local, while the other is slower and non-local, requiring different abstractions.

Data Plane, very well know abstractions or layers.

• Layers key to Internet's success: The IP protocol stack has a layered architecture that resembles an hourglass shape. Applications were built upon reliable transport, which depends on best effort global delivery, which in turn relies on best effort local packet delivery, all rooted in the local physical transfer of bits.
• Enabled innovation at every layer: Any new technology, like optical fiber, just new way to do local transfer of bits, everything else in stack remains the same. No need to re-write entire networking stack.

Control Plane no abstractions (Before SDN)

• Goals or tasks for Control Plane: Routing: Determine network path using distributed algorithms. Isolation: Do various kinds of isolation using VLANs, ACLs, Firewalls Traffic Engineering: Do not overload the links or routes.
• Each task has a set of mechanism but no modularity and needs to build from scratch. For example, every new routing algorithm starts from scratch.

What Abstractions for Control Plane?

• Figure out what the network looks (topology), before deciding how to route!.
• Figure out how to accomplish control plane tasks on given topology, how to do route and do isolation.
• Tell the switches what to do with a packet, by configuring or installing the forwarding state so a switch can do right things.

?? Control Plane: Two Layers or Abstraction

• Global Network view: Provide information about current networks by implementing "Network Operating System"
• Forwarding model: Standard way of defining and telling the forwarding state to switch, by "Open Flow"

?? Software Defined Networking is Layers for Control Plane

The key elements of SDN networks.

• SDN-controlled network elements, also known as the infrastructure layer, forward traffic in a network according to rules computed by the SDN control plane.
• The SDN controller is a logically centralized entity that acts as an interface between the network elements and the network-control applications.
• The network-control applications are programs that manage the underlying network by collecting information about the network elements with the help of SDN controller.
• Northbound APIs operate on a server-client model, where the server, as network control application, configures settings on the client controller.
• Southbound APIs involve the SDN controller acting as a server, providing configuration to the network device acting as the client.

??Key Takeaways:

• Disaggregation of the Control and Data Planes.
• NOS (Network Operating System), enables Control and Management of network using software application leading to better network connectivity, security and control.
• Open-flow is a protocol used for communication between SDN controllers and networking devices. It is an initial SDN standard and is managed by the Open Networking Foundation (ONF). The ONF is a user-led organization that is focused on promoting open standards and adoption of SDN.

Sources: "Software-Defined Networking at the Crossroads" -Scott Shenker, University of California, Berkeley