Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/19077
標題: qtECN–一個改良的明確壅塞通知演算法
qtECN - An Improved Explicit Congestion Notification Algorithm
作者: 黃嘉佑 
Huang, Chia-Yu 
關鍵字: congestion control;佇列管理;queue management;Explicit Congestion Notification;壅塞控制;明確壅塞通知
出版社: 資訊科學與工程學系
摘要: 
ECN(Explicit Congestion Notification)是一種主動式的佇列管理機制(Active Queue Management, AQM),此協定的主要目的是為了減少路由器佇列中的壅塞現象,和目前使用最普遍的Drop-Tail機制相比,ECN能夠偵測早期的壅塞現象並利用明確的訊息通知TCP傳送端,使其降低傳送封包的速度以避免真正的壅塞發生。但在目前ECN的設計中,TCP傳送端是以封包遺失的方式來反應所收到的早期壅塞通知,使傳送速率過度的降低。另外,ECN機制在路由器中所使用的封包標記/丟棄規則會造成有效吞吐量、封包遺失率和傳送延遲在網路效能中相互的競爭,無法在獲得較高的有效吞吐量的同時也得到較低的傳送延遲。因此,在本篇論文中提出一個改良的明確壅塞通知演算法,稱為qtECN(Quantitatively Explicit Congestion Notification),qtECN以ECN的設計為基礎,修改封包在路由器被標記與丟棄的規則,同時在需要標記的封包中加上額外的資訊,使TCP傳送端在收到qtECN-ACK通知的同時,能夠知道壅塞路由器中的負載情況,藉以調整自身的壅塞視窗大小與封包的傳送速度。在本論文的實驗結果中顯示,qtECN不但能夠得到最佳的有效吞吐量、延遲抖動率與封包遺失率,同時具有不錯的封包傳送延遲,另外,使用qtECN的路由器因為佇列溢滿而將封包丟棄的數量趨近於零,代表qtECN比目前其它各種主動式佇列管理機制更適合用在有線無線並存的異質混合網路中。總結而論,當路由器發生無可避免的壅塞現象時,使用qtECN的設計可以使網路獲得更好的效能。

ECN (Explicit Congestion Notification) is an active queue management scheme for network routers. The purpose of this protocol is mainly aimed at solving the problem of queue management and relieving the congestion phenomenon on the network. Compared with the traditional method that drops packets when buffer is full, ECN could improve the network performance by noticing the TCP sender to reduce its sending rate in advance, i.e., before the congestion really happened on the network. However, in ECN algorithm, TCP sender could only invoke the same mechanism as that when it encounters a packet loss event and thus have the sending rate excessively reduced. In addition, the rule of ECN marking/dropping packets in routers will cause the transmission throughput and delay to compete with each other.
In this thesis, we propose an improved ECN algorithm, named qtECN (Quantitatively Explicit Congestion Notification). The qtECN algorithm is based on the original design of ECN but modifies the rule that ECN uses for packets marking and dropping. Besides, qtECN adds additional information in the randomly marked packets to inform the TCP sender of the loading of the congested router, so the TCP sender could adjust its sending rate accordingly. The experimental results show that qtECN has a better performance at goodput, packet loss rate and delay jitter with acceptable transmission delay. In addition, packets' dropping due to full queue is nearly zero when using qtECN. It means qtECN is more suitable for wired-wireless hybrid network than any other existing AQM schemes. In summary, applying the design of qtECN could make the performance of network further improved when unavoidable congestion happened in the router.
URI: http://hdl.handle.net/11455/19077
Appears in Collections:資訊科學與工程學系所

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