The emerging trend to softwarize networks based on concepts such as Network Virtualization, Software Defined Networking (SDN) and Network Functions Virtualization promises to increase flexibility in networking. So far, flexibility is used rather as a qualitative argument for a network design choice. Furthermore, the meaning of flexibility behind such qualitative arguments is highly varying in the state of the art as a common understanding of flexibility is missing.
We present an approach towards evaluating network flexibility through a definition of a flexibility measure, which provides a quantitative analysis and a comparison of different network designs. Further, it can illustrate the trade-off between flexibility and cost needed to provide flexibility.
This web page intends to collect discussions and results about network flexibility including definitions, guidelines, benchmarks, use cases and tools. Stay tuned!
Network flexibility refers to the ability to support adaptation requests (=challenges) that can be, for example, changes in the requirements or new traffic distributions. We define the flexibility $\rho$ of a system $S$ as the fraction of requests that can be supported from a given set and sequence of requests and within a given time threshold $T$ and cost budget $C$ for each adaptation request:
\begin{equation} \rho_{T} = \frac{\mid \text{supported requests within } T \text{ and } C\mid}{\mid \text{total number of requests}\mid} \end{equation}
As with other network measures like Quality of Service, Flexibility as a measure is not a singular measure, but used with a specific objective in mind. To reflect such objectives, we come up with flexibility aspects. A flexibility aspect describes a concrete ability in which a network can adapt, e.g., change the flow routes or change of the allocated resources.
Category | Aspect |
---|---|
Adapt Configuration |
Flow Configuration Function Configuration Parameter Configuration |
Locate Functions | Function Placement |
Scale |
Resource and Function Scaling Topology Adaptation |
A dynamic SDN control plane can adapt its configuration to changing traffic flows to achieve an optimal con- trol performance, i.e., minimal average flow setup time. The flexibility of the dynamic SDN control plane represents its ability to respond to changing traffic flows under a migration time threshold $T$.
Intuitively, one would think a 4 controller system is more flexible than a 1 controller system. However, as we show in [3] for a very tight adaptation time threshold $T$, a 1 controller system exhibits higher flexibility.
Objective | Aspect | Request | Measure | Time threshold | Cost |
---|---|---|---|---|---|
Control Performance of dynamic SDN control plane | Function placement | Flow arrival (from distribution) | Fraction of successful controller placements | Varied | Average flow setup time |
Failure recovery of SDN system | Flow configuration | All possible single and dual link failures | Fraction of recovered failures | Varied | resource overhead for reservation |
The flexibility of survivable networks can be defined as the ability to recover from failures in a timely manner. Network operators can choose either proactive recovery approaches (i.e., protection) where the network is prepared for failures in advance at the cost of redundant resources; or reactive approaches (i.e., restoration).
As we show in [1], networks with restoration are less flexible for very tight time thresholds $T$ but can supersede networks with protection for larger $T$.