The Role of Agents in a Software Defined Network

An edited version of this story ran on Cisco.com.

In this article, I want to talk about network agents—what some in the industry refer to as the future of the software defined network. Agents are responsible for a great deal of flexibility and control, and the work they do can vastly impact how responsive and agile a software defined network can be. Let us dig in.

What are agents?

In almost all software defined networks, agents are the tool by which network applications expose their abilities for network controllers to modify their configuration at any given time. Network agents can live on the software defined network controller itself, providing a mechanism for the controller to talk to various applications through the controller’s northbound interface. (These northbound interfaces are generally output only—they only communicate outward to the individual SDN-aware applications.) Network agents can also live on different network elements, comprising a data path. All of these elements work together to forwarding traffic through the datapath’s external interfaces or process or terminate traffic within the element itself.

 

Where this all gets interesting is here: one or more of these datapaths (an agent and an engine) may live inside one physical network element—an “bucket” of welded-together communications resources, which the SDN sees and manages as a unit. But it goes further. A datapath may also be defined across multiple physical network elements. This logical datapath doesn’t care how datapaths map to actual physical assets, how those physical resources are managed or how they are abstracted away, how all of this is virtualized or how it all interrelates and interoperates, and more. It’s a completely different thing than physical wires, ports, and switches. The agent itself is responsible for communicating with the SDN controller about what the datapath is supposed to represent and how its engine is supposed to behave, and the datapath network agents do this communication across the network controller’s southbound interface.

How are network agents deployed in production SDNs?

In some networks, administrators install virtual network agents on every hypervisor host (in this scenario, each hypervisor host is a network element). The virtual network agent on each host then directs traffic toward the virtual switch. If that virtual switch recognizes that particular flow of traffic and is able to match it with a prescribed action, then it can adjust the configuration of the virtual machines on that host so that the flow of traffic exits through the host’s physical network interface card. If the virtual switch doesn’t recognize the flow, it sends a mapping request back to the agent, which queries a mapping service in the network controller so it can figure out where that traffic is supposed to flow.

 

In other scenarios, network agents are key to implementing virtual functions on the network. Workloads like load balancers, firewalls, threat monitors, intrusion detection, packet capture and packet and traffic forwarding, and more can all be implemented by various network agents residing in different network elements. The controller can deploy, activate, deactivate, and redeploy these agents as needed whenever traffic flows in one direction or another, and large scale SDNs can deploy multiple copies of these objects for multitenant scenarios or to respond to a breach, a distributed denial of service attack, an unusually large load during a spiky peak period, or other one time occurrence.

Putting it all together

The important element to remember when it comes to agents is actually what distinguishes a software defined network from a physical network: the fact that the physical components, the discrete switches and patch cables and ports and whatnot are completely abstracted away and the only networking we care about—and the networks to which these network agents serve and function—are the traffic “flows” created by applications.

 

Imagine how this would apply to human travel. SDNs in that context are essentially the difference between worrying about interstates and side roads versus the concept of traveling from Los Angeles to San Francisco. SDN cares out the route, the flow, the purpose of the traffic and its outcome, rather than concerning one’s self with which exits to take off the interstate, where to turn right, what to do if there is a traffic backup, and the like. By focusing on the flows, the need of packets and data to get from one application to another, we get the concepts of agents, the bits that carry out the instructions from the control plane and translate those into action on the data plane.

 

In conclusion, agents are key to the SDN experience and perform the vital role of translating instructions from the control plane to the data plane.

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