Operating Models

Are there any downsides? Well of course! A well-known challenge with centralization is making it safe. What happens if the control plane fails? What are the necessary high availability concepts? If multiple control instances are involved, does this immediately raise the question of how many there are? What's the situation with scalability? Very critical voices also ask about the additional overhead of the communication between control logic and infrastructure. But one thing at a time: there are different approaches for operating SDN.

In the symmetric model, the control body is centralized as much as possible. This article has already described the advantages, but the central susceptibility is a disadvantage. The asymmetric model, on the other hand, gives the elements of the data plane enough information for the immediate operation. The individual systems are familiar with the relevant configurations and can therefore work even if the control logic fails.

This approach often works well with the work methods of administrators who organize the network into autonomous cells. However, the greater independence from the central control plane also has its price. The asymmetric method leads to redundant information and more decentralized management.

In highly virtualized environments, it makes sense to let the hypervisor or the host do the necessary mental work. The other approach is network-centered, involving dedicated network devices performing the SDN job. A combination of the two options is also conceivable but reduces the profit.

The third dimension of the SDN operating models uses the distribution of information as a differentiator. This approach can be either proactive or reactive. In a reactive environment, the network device receives a data package and asks the control plane how to proceed. The advantage is that only the required information migrates across the network. However, problems will result in delivery delays.

Alternatively, the control logic can proactively supply the data plane with all the information. This technique typically depends on well-known broad and multicast mechanisms. The advantages and disadvantages are similar to those of the asymmetric operating model.

What Remains in the End

A lot has happened in the SDN environment over the past five years. The ONF product page [11] is now rather long and doesn't just list the usual suspects from the network area. OpenFlow may have introduced standardization, but the implementation by the manufacturer was, and unfortunately still is, a painful experience. SDN has appeared in tandem with Network Function Virtualization (NFV) [12] for some time. Both technologies are high on the list of topics for telecommunications giants. The OpenStack community showed great interest in the recent polls.

SDN is here to stay. However, classic environments are still finding it difficult to part from the traditional approach. The development of technology is happening much faster than the necessary change in thinking. Anyone planning their network for the future will need to give some thought to SDN.