# A Comprehensive Survey on Knowledge-Defined Networking
## 6 Data Plane
### Forwarding Devices
#### 6.2.4 Optical Switches  
Optical networks, as opposed to packet switching networks, rely on circuit switching, so KDN switches belonging to an optical network rely on optical circuit switching technology. Unlike packet switching, which operates at the network layer of the Open System Interconnected (OSI) model, circuit switching operates at the physical layer, using light paths to establish connections between the switches. This approach has lower latency, higher bandwidth, and improved efficiency compared to packet switching forwarding elements. Micro-Electro-Mechanical Systems (MEMS)-based optical switches have been used for large data center networks, which have enabled dynamic topology reconfiguration and centralized control for traffic engineering in optical networks [421]. There have been attempts to unify circuit switching in optical networks with packet switching in IP networks using a unified controller [422]. However, such unification needs to be accompanied by upgrading each physical optical switch with a virtual switch that converts control messages received from the controller to commands acceptable by the physical optical switch. Some have attempted to unify packet switching with circuit switching in optical networks by integrating a Generalized Multi Protocol Label Switching (GMPLS) control plane that  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Protocol** & **Architecture** & **Forwarding Model** & **Message Types** & **Open-Source** & **Version** \\ \hline OpenFlow [291] & Control plane physically, logically decoupled from data plane & Flow table & Feature request, packet-in, FlowMod, packet-out & Yes & v1.5 \\ \hline ForCES [292] & Control and data planes logically decoupled, physically together & Logical functioning block & Configuration, notification, query, response & Yes & v1.4 \\ \hline OpFlex [293] & Part of control plane redistributed in data plane & Policy based forwarding & Policy requests, responses, updates, withdrawals & Yes & v1.0 \\ \hline POF [294] & Forwarding plane oblivious of the protocol & Flow instruction sets & Configuration, packet-in, packet-out, query & Yes & v1.0 \\ \hline PCEP [295] & Logically decoupled control and data planes & Computed paths & Open, request, close, reply & Yes & v1.0 \\ \hline OpenState [296] & Decentralize some control for forwarding plane using FSM & Stateful Flow table & Packet-in, FlowMod, packet-out, StateMod & Yes & v1.0 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary of protocol details that implement different switch forwarding models.  
manages circuit switching with an SDN controller that manages packet switching [423]. GMPLS has been proposed as the control plane for wavelength-switched optical networks. However, a recent survey suggests that OpenFlow shows better performance in terms of less probability of blocking and average time of establishing a light path than GMPLS architecture in large multi-domain networks [424]. A photonic integrated wavelength selective switch aiming at providing flexible bandwidth for optical data center networks that can be reconfigured dynamically using the controller has been presented in [425]. In such a scheme, based on variable traffic patterns, the controller can effectively control the bandwidth per link in optical networks. Similar works suggest how photonic integrated circuits in optical switches (photonic switches) can be automatically controlled and managed using a centralized controller [426].