Alessandro Floris (University of Cagliari, Italy) is Assistant Professor and member of the Multimedia & Communications Laboratory (MCLab).
Luigi Atzori (University of Cagliari, Italy) is Full Professor and coordinator of the Multimedia & Communications Laboratory (MCLab).
Editors: Tobias Hoßfeld (University of Würzburg, Germany), Christian Timmerer (Alpen-Adria-Universität (AAU) Klagenfurt and Bitmovin Inc., Austria)
The Software-Defined Networking (SDN) paradigm offers the flexibility and programmability in the deployment and management of network services by separating the Control plane from the Data plane. Being based on network abstractions and virtualization techniques, SDN allows for simplifying the implementation of traffic engineering techniques as well as the communication among different services providers, included Internet Service Providers (ISPs) and Over The Top (OTT) providers. For these reasons, the SDN architectures have been widely used in the last years for the QoE-aware management of multimedia services.
The paper  presents Timber, an open source SDN-based emulation platform to provide the research community with a tool for experimenting new QoE management approaches and algorithms, which may also rely on information exchange between ISP and OTT . We believe that the exchange of information between the OTT and the ISP is extremely important because:
- QoE models depend on different influence factors, i.e., network, application, system and context factors ;
- OTT and ISP have different information in their hands, i.e., network state and application Key Quality Indicators (KQIs), respectively;
- End-to-end encryption of the OTT services makes it difficult for ISP to have access to application KQIs to perform QoE-aware network management.
In the following we briefly describe Timber and the impact of collaborative QoE management.
Figure 1 represents the reference architecture, which is composed of four planes. The Service Management Plane is a cloud space owned by the OTT provider, which includes: a QoE Monitoring module to estimate the user’s QoE on the basis of service parameters acquired at the client side; a DB where QoE measurements are stored and can be shared with third parties; a Content Distribution service to deliver multimedia contents. Through the RESTful APIs, the OTTs give access to part of the information stored in the DB to the ISP, on the basis of appropriate agreements.
The Network Data Plane, Network Control Plane, and the Network Management Plane are the those in the hands of the ISP. The Network Data Plane includes all the SDN enabled data forwarding network devices; the Network Control Plane consists of the SDN controller which manages the network devices through Southbound APIs; and the Network Management Plane is the application layer of the SDN architecture controlled by the ISP to perform network-wide control operations which communicates with the OTT via RESTful APIs. The SDN application includes a QoS Monitoring module to monitor the performance of the network, a Management Policy module to take into account Service Level Agreements (SLA), and a Control Actions module that decides on the network control actions to be implemented by the SDN controller to optimize the network resources and improve the service’s quality.
Timber implements this architecture on top of the Mininet SDN emulator and the Ryu SDN controller, which provides the major functionalities of the traffic engineering abstractions. According to the depicted scenario, the OTT has the potential to monitor the level of QoE for the provided services as it has access to the needed application and network level KQIs (Key Quality Indicators). On the other hand, the ISP has the potential to control the network level quality by changing the allocated resources. This scenario is implemented in Timber and allows for setting the needed emulation network and application configuration to text QoE-aware service management algorithms.
Specifically, the OTT performs QoE monitoring of the delivered service by acquiring service information from the client side based on passive measurements of service-related KQIs obtained through probes installed in the user’s devices. Based on these measurements, specific QoE models can be used to predict the user experience. The QoE measurements of active clients’ sessions are also stored in the OTT DB, which can also be accessed by the ISP through mentioned RESTful APIs. The ISP’s SDN application periodically controls the OTT-reported QoE and, in case of observed QoE degradations, implements network-wide policies by communicating with the SDN controller through the Northbound APIs. Accordingly, the SDN controller performs network management operations such as link-aggregation, addition of new flows, network slicing, by controlling the network devices through Southbound APIs.
QoE management based on information exchange: video service use-case
The previously described scenario, which is implemented by Timber, portraits a collaborative scenario between the ISP and the OTT, where the first provides QoE-related data and the later takes care of controlling the resources allocated to the deployed services. Ahmad et al.  makes use of Timber to conduct experiments aimed at investigating the impact of the frequency of information exchange between an OTT providing a video streaming service and the ISP on the end-user QoE.
Figure 2 shows the experiments topology. Mininet in Timber is used to create the network topology, which in this case regards the streaming of video sequences from the media server to the User1 (U1) when web traffic is also transmitted on the same network towards User2 (U2). U1 and U2 are two virtual hosts sharing the same access network and act as the clients. U1 runs the client-side video player and the Apache server provides both web and HAS (HTTP Adaptive Streaming) video services.
In the considered collaboration scenario, QoE-related KQIs are extracted from the client-side and sent to the to the MongoDB database (managed by the OTT), as depicted by the red dashed arrows. This information is then retrieved by the SDN controller of the ISP at frequency f (see green dashed arrow). The aim is to provide different network level resources to video streaming and normal web traffic when QoE degradation is observed for the video service. These control actions on the network are needed because TCP-based web traffic sessions of 4 Mbps start randomly towards U2 during the HD video streaming sessions, causing network time varying bottlenecks in the S1−S2 link. In these cases, the SDN controller implements virtual network slicing at S1 and S2 OVS switches, which provides the minimum guaranteed throughput of 2.5 Mbps and 1 Mbps to video streaming and web traffic, respectively. The SDN controller application utilizes flow matching criteria to assign flows to the virtual slice. The objective of this emulations is to show the impact of f on the resulting QoE.
The Big Buck Bunny 60-second long video sequence in 1280 × 720 was streamed between the server and the U1 by considering 5 different sampling intervals T for information exchange between OTT and ISP, i.e., 2s, 4s, 8s, 16s, and 32s. The information exchanged in this case were the average length stalling duration and the number of stalling events measured by the probe at the client video player. Accordingly, the QoE for the video streaming service was measured in terms of predicted MOS using the QoE model defined in  for HTTP video streaming, as follows:
MOSp = α exp( -β(L)N ) + γ
where L and N are the average length stalling duration and the number of stalling events, respectively, whereas α=3.5, γ=1.5, and β(L)=0.15L+0.19.
Figure 3.a shows the average predicted MOS when information is exchanged at different sampling intervals (the inverse of f). The greatest MOSp is 4.34 obtained for T=2s, and T=4s. Exponential decay in MOSp is observed as the frequency of information exchange decreases. The lowest MOSp is 3.07 obtained for T=32s. This result shows that greater frequency of information exchange leads to low latency in the controller response to QoE degradation. The reason is that the buffer at the client player side keeps on starving for longer durations in case of higher T resulting into longer stalling durations until the SDN controller gets triggered to provide the guaranteed network resources to support the video streaming service.
Figure 3.b shows the video initial loading time, average stalling duration and latency in controller response to quality degradation w.r.t different sampling intervals. The latency in controller response to QoE degradation increases linearly as the frequency of information exchange decreases while the stalling duration grows exponentially as the frequency decrease. The initial loading time seems to be not relevantly affected by different sampling intervals.
Experiments are conducted on an SDN emulation environment to investigate the impact of the frequency of information exchange between OTT and ISP when a collaborative network management approach is considered. The QoE for a video streaming service is measured by considering 5 different sampling intervals for information exchange between OTT and ISP, i.e., 2s, 4s, 8s, 16s, and 32s. The information exchanged are the video average length stalling duration and the number of stalling events.
The experiment results showed that higher frequency of information exchange results in greater delivered QoE, but a sampling interval lower than 4s (frequency > ¼ Hz) may not further improve the delivered QoE. Clearly, this threshold depends on the variability of the network conditions. Further studies are needed to understand how frequently the ISP and OTT should collaboratively share data to have observable benefits in terms of QoE varying the network status and the deployed services.
 A. Ahmad, A. Floris and L. Atzori, “Timber: An SDN based emulation platform for QoE Management Experimental Research,” 2018 Tenth International Conference on Quality of Multimedia Experience (QoMEX), Cagliari, 2018, pp. 1-6.
 P. Le Callet, S. Möller, A. Perkis et al., “Qualinet White Paper on Definitions of Quality of Experience (2012),” in European Network on Quality of Experience in Multimedia Systems and Services (COST Action IC 1003), Lausanne, Switzerland, Version 1.2, March 2013.
 A. Ahmad, A. Floris and L. Atzori, “Towards Information-centric Collaborative QoE Management using SDN,” 2019 IEEE Wireless Communications and Networking Conference (WCNC), Marrakesh, Morocco, 2019, pp. 1-6.
 T. Hoßfeld, C. Moldovan, and C. Schwartz, “To each according to his needs: Dimensioning video buffer for specific user profiles and behavior,” in IFIP/IEEE Int. Symposium on Integrated Network Management (IM), 2015. IEEE, 2015, pp. 1249–1254.