隨著近年來互聯網使用人口與流量快速增長，網路設備不斷加入互聯網以滿足使用者對於網路傳輸的需求。現今的網路設備，因其控制和數據轉發功能整合在一起，同時網路協定採取分散式協作模式，因此網路設備必須一直保持開啟狀態以使整體網路運作順暢，導致無法有效節能，使其能源消耗佔全球能源消耗相當程度的比重，在全球日益暖化的今天，如何有效降低網路耗能並能保持網路通訊效能是一個極為重要且亟待解決的問題。
軟體定義網路是近年來新興的網路技術，因其能使資料網路更靈活、更易於操作和管理、及能夠更好地應對應用程式發展和網路條件不斷變化的需求、以及成本控制，預期將成為下一世代的主流網路技術。由於其將原本同一網路設備內控制部分與封包轉發部分分離，將同一網路中所有網路設備的控制部分全部整合到單一設備稱為控制器，利用其中央集中式的控制模式可以實現即時監控流量，並且非常快速地重新調整網路以達成特定目標（例如，最大化網絡利用率或平衡鏈路利用率）。因此，它將能有效改善網路的整體性能，並且降低網路運作的耗能，提高能源效率。然而過去的研究只著重在單一目標的最佳化，如負載平衡或節能，並未一起考慮網路節能與避免網路壅塞的問題，而且也未考慮日常網絡流量其實變動相當劇烈，在流量高峰與低峰時，應該如何採取不同的策略以同時達到有效的節能與避免網路壅塞。因此，本論文針對此一問題，我們提出一套機制使軟體定義網路甚至是混合式的軟體定義網路在網路流量變動下，有效地降低耗能同時避免網路壅塞。另外，我們也針對傳統網路如何逐漸改變成軟體定義網路提出一項遷移機制，使網路在轉換的過程中依然讓網路耗能和網路壅塞機率最小化。本研究透過廣泛的模擬，顯示我們提出的機制有良好的效能，在避免網路壅塞的前提下，平均可以節省大約40％的能源消耗。另外，我們提出的遷移機制也能在最小化網路壅塞機率下，有效降低能源消耗，由模擬的結果顯示，當網絡中存在約60％的軟體定義網路節點時，平均可節約40％的能源消耗。

With the rapid growth of population and traffic in the Internet in recent years, network devices join the Internet almost continuously in order to meet users' needs for network transmission. As a result, today’s networking hardware systems are required to run almost around the clock in order to keep the network running smoothly. Consequently, the global energy consumption of communication networks is extremely high. The problem is exacerbated by the fact that the control plane and data forwarding plane are often embedded in the same device in current networks, while network protocols are usually designed to operate in a distributed manner. Thus, with today's increasing focus on environmental issues, devising the means to effectively reduce network energy consumption while simultaneously maintaining the performance of network communications is an extremely important but challenging problem.
Software Defined Networking is a newly-introduced networking paradigm, in which the control plane is separated from the forwarding plane and moved to a globally-aware software controller. By exploiting a centralized control mode, the network traffic can be monitored in real time and rapidly rerouted as required to satisfy different objectives, such as maximizing the network utilization, achieving load balance, and improving the energy efficiency. However, previous researches have focused only on the optimization of a single goal, such as load balancing or energy saving. Moreover, they have generally ignored the dynamic characteristics of the traffic load in most networks. Consequently, a need still exists for effective strategies capable of simultaneously maximizing the network energy efficiency and preventing congestion avoidance under both light and heavy traffic load conditions.
Accordingly, this thesis proposes a mechanism for reducing the energy consumption and avoiding network congestion in software-defined networks with a variable traffic load. In addition, a migration mechanism is proposed to minimize the energy consumption and congestion probability for the case where a traditional network is gradually upgraded to a software-defined network. Through extensive simulations, it is shown that the proposed mechanism has a good performance and reduces the energy consumption by around 40% on average under the premise of congestion avoidance. Moreover, the proposed migration mechanism also reduces the energy consumption by 40% on average while minimizing the probability of network congestion given around 60% of SDN nodes in the hybrid network.

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