A control plane is a key enabling technique for dynamic and intelligent end-to-end path provisioning in optical networks. In this paper, we present an OpenFlow-based control plane for spectrum sliced elastic optical path networks, called OpenSlice, for dynamic end-to-end path provisioning and IP traffic offloading. Experimental demonstration and numerical evaluation show its overall feasibility and efficiency.
© 2013 OSA
The spectrum sliced elastic optical path network, which is also known as the Elastic Optical Network (EON) or the flexible grid optical network, has been recently proposed to more efficiently utilize network spectrum resources . In an EON, optical spectrum ranges are adaptively allocated to an optical path according to the client (e.g. IP) traffic demand, modulation format and path attributes (e.g., physical length or optical impairments) . For dynamic and intelligent end-to-end optical path provisioning and IP traffic offloading in an EON, a control plane is a key enabling technique.
Therefore, a lot of studies have started to design a Generalized Multi-Protocol Label Switching (GMPLS)-based control plane for an EON [3–5]. Despite massive progress, it should be noted that such studies mainly focused on the control of the optical layer. The recent studies  have validated the application of a GMPLS-based unified control plane for controlling a multi-layer network composed of both packet (Multi-Protocol Label Switching Transport Profile (MPLS-TP)) and optical switching (Wavelength Switched Optical Network (WSON)) technologies. However, to the best of our knowledge, the specific GMPLS-based unified control for both IP and EON layers has not been addressed yet. More importantly, although more mature and intelligent, a GMPLS-based control plane may not be an ideal solution for the deployment in a real operational scenario due to its distributed nature and high complexity, especially for a unified control functionality in IP and optical multi-layer networks [7–9].
On the other hand, the OpenFlow protocol , which has been recently proposed as a unified control plane , provides the maximum flexibility for operators to control a network and arguably matches carriers’ preference given its simplicity and manageability [12, 13]. In light of this, for the first time, our previous work in  experimentally presented an OpenFlow-based control plane, referred to as OpenSlice, to achieve dynamically optical path provisioning and IP traffic offloading in an EON. In this paper, we propose a more transparent and detailed description of OpenSlice and the main points of this study. In addition, more experimental results and discussions, such as the control of real optical switching node through OpenSlice, are added in this paper to verify the overall feasibility of OpenSlice.
The rest of this paper is organized as follows. Section 2 proposes the technical details for OpenSlice, including network architecture, OpenFlow protocol extensions, and the procedure for end-to-end path provisioning by using the OpenSlice. Section 3 presents the experimental demonstration and performance evaluations of the OpenSlice. Section 4 concludes this paper by summarizing our contributions and discussing directions for our future works.
2. OpenSlice: architecture and protocols
2.1 Network architecture
Figure 1 and Fig. 2 show the proposed network architecture. In order to connect IP routers to an EON with IP offloading capability, a Multi-flow Optical Transponder (MOTP) has been proposed  and demonstrated  recently. In a MOTP, a flow classifier at the transmitter side is deployed, to identify the packets of an incoming IP flow according to their destination addresses, virtual local area network tags, etc, and to split such flow into several sub-flows . Before mapping each sub-flow to an appropriate optical transport unit, the flow classifier generates a Path Setup Request (PSR) for each sub-flow, containing not only the source/destination addresses, but also the bit rate of each sub-flow. This requires that the flow classifier is able to monitor or detect the bit rate for a flow, and the approach for this detection is beyond the scope of this paper.
The EON layer is configured with the Bandwidth Variable Wavelength Cross-Connects (BV-WXCs) , which are implemented by using bandwidth variable wavelength selective switches (BV-WSS) . Both the MOTP and BV-WXC are extended with the OpenSlice functionality, which are referred to as OpenSlice-enabled MOTP (OF-MOTP) and BV-WXC (OF-BV-WXC), as shown in Fig. 1 and Fig. 2 respectively. A centralized controller is introduced to control all the IP routers through the standard OpenFlow protocol, and control all the OF-MOTPs and OF-BV-WXCs through the OpenSlice protocol. The controller implemented in this paper is based on NOX . For simplicity, we will still use NOX to represent our controller, but note that it is an extended controller from the original NOX enabled to support the OpenSlice protocol.
Specifically, in an OF-MOTP, an OpenSlice protocol converter is deployed, which is able to convert a PSR into an extended OpenFlow Packet In message (as detailed next) for further processing by the NOX. In addition, it can convert a Slice Mod message (as detailed next) into a vendor-specific command (e.g. Transaction Language 1) to control each Tx/Rx pair in a MOTP for a suitable central frequency (CF), slot width (SW) and modulation format.
In an OF-BV-WXC, an OpenSlice module with a cross-connection table (CT) is introduced. The CT maintains all the cross-connection information within an OF-BV-WXC, including input/output ports, CF, SW, and modulation format (as shown in Fig. 3 ). Note that, the calculation of CF and SW presented in Fig. 3 is based on IETF work-in-progress drafts . Conceptually, the CT is similar to the flow table in standard OpenFlow terminology. The Slice Mod message can add/delete a cross-connection entry into the CT, and thus control the OF-BV-WXC, allocating a cross-connection with the spectrum bandwidth to create an appropriately-sized optical path.
2.2 OpenFlow protocol extensions
The key extensions to OpenFlow protocol to support an EON are briefly summarized as follows: (a) The Feature Reply message is extended to report the new features of an EON (e.g., flexi-grid switching capability, available spectrum ranges, etc.) to the NOX controller; (b) The OpenFlow Packet In message is extended to carry the bit rate of each incoming sub-flow. (c) The NOX is extended to perform a routing and spectrum assignment (RSA) algorithm, allocating suitable frequency slots and the selected modulation format, according to the source/destination addresses and the bit rate of the flow, which are obtained from the extended Packet In messages. (d) A new message referred to as Slice Mod is introduced, which is based on the OpenFlow Flow Mod message. This new message carries the RSA results from the NOX, including actions (i.e., add a new cross-connection, delete a matching cross-connection, modify an existing cross-connect), input and output ports, central frequency, slot width, and modulation format.
2.3 Procedure for path provisioning in the EON
Firstly, a handshake procedure between the NOX and each OpenFlow-enabled network element (NE), e.g., an OF-BV-WXC, is required for the NOX to know the feature/capability of each NE, as the procedure shown in Fig. 4(a) . Once a new NE is introduced to the network, Hello messages are changed between the NOX and the new NE. Then the NOX sends a Feature Request message to the NE, and the NE replies with Feature Reply message specifies the features and capabilities supported by this NE. Here, we extended the Feature Reply message for each OF-BV-WXC to report its new features/capabilities to the NOX, including its datapath ID, port number, supported switching type, available spectrum ranges for each port, etc. to the NOX controller. In addition, each NE also uses this extended Feature Reply message to report its peering connectivity, including the port ID and data path ID of its neighboring NEs. In order to guarantee the liveness of a connection between a NE and the NOX, Echo Request and Echo Reply messages are used, which can be sent from either the NE or the NOX. Figure 4(b) shows the procedure of path provisioning for one sub-flow from the ingress OF-MOTP. When the NOX receives a Packet In message from the ingress OF-MOTP, it performs the RSA computation according to the source and destination addresses and the bit rate information of this sub-flow, and then configures each OF-MOTP and OF-BV-WXC along the computed path, by using the aforementioned Slice Mod messages.
3. Experimental setup, results and discussions
To evaluate the overall feasibility and efficiency of OpenSlice, we set up a testbed as shown in Fig. 5 , which models the Japan core network topology. The testbed is deployed only with the OpenSlice-based control plane, and the data plane is emulated. All the nodes are connected to a dedicated NOX controller, which is located at node 12. The value close to each node indicates the message propagation latency from the NOX to each node. The DWDM links are characterized by 128 individual slots of 6.25 GHz each. The network topology and resource information is statically configured in the NOX, and is dynamically updated by the NOX, notably when cross-connections are set up or released. In this experiment, we consider three different bit rates 100Gb/s, 200Gb/s and 400Gb/s (112Gb/s, 224Gb/s and 448Gb/s including overhead), and three modulation formats Dual-Polarization 64-ary Quadrature Amplitude Modulation (DP-64QAM), DP-16QAM and Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK). The RSA is based on the algorithm presented in , where a route list with necessary slot number and modulation format is pre-computed. The modulation format is selected based on the path distance . An efficient format such as DP-64QAM is selected for short paths and a more robust one such as DP-QPSK is selected for long paths. We also assume that an OF-MOTP is attached to each OF-BV-WXC (Fig. 5), and the Tx/Rx in the OF-MOTP is based on Nyquist-WDM  and coherent detection. The use of aggressive optical prefiltering with spectrum shape approaching that of a Nyquist filter, together with a square spectrum, minimizes the required bandwidth to a value equal to the channel baud rate. For example, the required bandwidth for a 400Gb/s flow with the modulation format DP-QPSK is 112GHz (i.e., 448/4) . Therefore, the required SW is 9x12.5GHz.
Figure 6 shows the Wireshark capture of an extended Feature Reply message during the handshake phase. It can be seen that, when a Feature Request message is received, the OF-BV-WXC automatically replied a Feature Reply message. The processing latency between Feature Request/Reply messages was around 1.3 ms, as shown Fig. 6. By using the Feature Reply message, each OF-BV-WXC reported its feature to the NOX controller, including its datapath ID, port number, supported switch type, neighbor information, available spectrum ranges for each port, etc. Note that the packet format for the extended Feature Reply message presented in Fig. 6 is based on the OpenFlow circuit switch addendum v0.3 . The only difference is that we newly defined bit map information to represent the flexible grid switching capability and resource availabilities. More details and the meaning of each field can be referred to .
In the experiment, we set up six paths with different hop count, as shown in Table 1 . Figure 7 shows the Wireshark capture of OpenSlice messages for creating the path (1), including the message sequence (Fig. 7(a)), the extended Packet In message (Fig. 7(b)), and the Slice Mod message (Fig. 7(c)). It can be seen that the flow bit rate is encapsulated in the extended Packet In message, and the information for a new cross-connection entry is carried within the Slice Mod message. Table 1 also shows the OpenSlice message average latencies, obtained by repeating the experiment 100 times. In our tested scenario, the OpenSlice message latency for creating a path with 1~5 hops is around 32~36 ms. The path release procedure is also evaluated, by setting the Action type (Fig. 7(c)) in the Slice Mod message to “delete a matching cross-connection”. The results show that the latency for releasing a path with 1~5 hops is around 14~17 ms.
We also compare the performance of OpenSlice with the GMPLS-based control plane in terms of path provisioning latency for an EON. The results related to GMPLS are measured on the ADRENALINE testbed  with a same network topology shown in Fig. 5, and the GMPLS extensions for an EON are based on [3, 5]. The experimental results are summarized in Fig. 8 and Table 2 . These results show that, for creating a path with more than 3 hops, the OpenSlice outperforms GMPLS-based control plane in terms of the average path provisioning latency. It is because in GMPLS, PATH/RESV messages are processed hop-by-hop. With longer paths (increased hop count), the GMPLS signaling latency increases proportionaly. On the other hand, the path provisioning latency in an EON is less sensitive to the hop count since the centralized controllers controls all the nodes almost simultaneously.
Finally, we verify the feasibility of the real hardware control by using the proposed OpenSlice control plane. Due to the hardware limitation, we set up a simple test scenario, as shown in Fig. 9(a) . In the data plane, we deployed a BV-WSS with real hardware, which was controlled by the NOX controller through the OpenSlice protocol. An amplified spontaneous emission (ASE) broadband light source and an optical spectrum analyzer were attached at this BV-WSS. In this experiment, the operator directly sent extended Packet In messages to the NOX controller, and according to the flow bit rate information in the Packet In messages, the NOX controller automatically controlled the BV-WSS to allocate a suitable spectrum bandwidth through Slice Mod messages. Figure 9(b) and Fig. 9(c) show the filter profiles of the BV-WSS by allocating 16 continuous slots and 4 continuous slots (12.5GHz per slot) for Packet In messages with different flow bit rate information. This test verified that the proposed OpenSlice control plane can be deployed in a real operational scenario to control real optical switching node. We observed that the latency for controlling a real BV-WSS was around 120 ms.
We successfully demonstrated OpenSlice, an OpenFlow-based control plane for spectrum sliced elastic optical path networks. We experimentally verified its overall feasibility for dynamic end-to-end path provisioning and IP traffic offloading through OpenFlow-based protocol extensions and seamless interworking operations. We also quantitatively evaluated its performance in terms of path provisioning latency, and compared it with the GMPLS-based control plane. The results indicate that, in our tested scenario, the OpenSlice outperforms the GMPLS-based control plane when creating a elastic optical path with more than 3 hops.
Our future works and open issues for OpenSlice include actual testbed demonstration with intelligent interworking between the OpenSlice and MOTPs, mitigation of the scalability issue for the OpenSlice architecture, OpenSlice-based control for multi-domain EON, as well as multi-vendor interoperability tests/field trials. So far, some preliminary studies have been carried out to address these issues [22–24]. We hope the work presented in this paper will be beneficial for industrial deployment of EON with an intelligent unified control plane, and shed light on future researches in this area.
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