臺灣博碩士論文加值系統

在傳統的有線網路環境中，傳輸控制協定(TCP)是一種可信賴的傳輸協定，因為有線網路的誤碼率(BER)較低。在TCP的傳統假設中，所有的封包遺失(packet loss)皆是因為網路壅塞而遭受轉傳的路由器將封包丟棄所致。但TCP可惜的是，當TCP被使用於無線網路時，無線網路將使TCP的效能嚴重地下降。
TCP是設計給許多重要應用的傳輸協定，例如：網頁瀏覽、檔案傳輸等。而TCP的壅塞控制主要是基於additive increase multiplicative decrease (AIMD)來對TCP傳送端的壅塞視窗做調整。AIMD可使傳送端接近目前網路環境的可用頻寬。TCP的滑動視窗流量控制機制時常會導致瞬間的大量封包流量，而產生網路的壅塞。目前的網路壅塞通常會造成連續的封包遺失。一個傑出的TCP機制，在發生封包遺失之前應該能夠明智的設定壅塞視窗的大小(cwnd)與慢速啟動的門檻(ssthresh)，這些設定將顯著的影響TCP之傳輸效能。
在此篇論文中，我們將提出一個方法來動態的調整慢速啟動中的ssthresh。我們的方法將把原本的ssthresh調整成一個更適當的ssthresh。一個較佳的ssthresh將會明顯地提升TCP的傳輸效能。另外我們提出一個方法來探測壅塞避免階段的額外可用頻寬。在壅塞避免的階段裡，我們藉由連續的觀察round trip time (RTT)的變化進而適當地調整cwnd，並且在fast retransmission與timeout之後重新評估一個合適的ssthresh。而後TCP的傳送端即可藉由ssthresh的評估與連續的觀察RTT進而提升效能。我們的方法可以有效地提升傳輸的效率，並且使傳送端達到較其它TCP版本更高的頻寬使用率。模擬的結果證實我們的方法能夠有效率地提升TCP的效能。當網路的瓶頸頻寬接近整體網路頻寬的30%時，我們的方法最少能夠提升10%的效能。

Transmission control protocol (TCP) is a reliable transport protocol in traditional networks which consist of wired links with low bit error rates (BER). In the traditional assumption for TCP, the packet loss was caused by the congestion at the relaying routers. The congestion at the relaying routers is the major reason of packet loss. Unfortunately, when TCP is used over wireless links, the performance of TCP would be significantly degraded.
TCP is designed for many important applications such as web browsing and file transferring. The congestion control of TCP is based on an additive increase multiplicative decrease (AIMD) that adjusts the congestion window of TCP sender. The AIMD makes the sender to utilize the available bandwidth of current networks.
TCP’s sliding window flow control mechanism often leads to burst packet traffic in the Internet. In the presence of network congestion, multiple packet losses would occur. It would degrade the performance of TCP and the TCP sender perceived the limit of networks. An excellent congestion control can intelligently set the congestion window (cwnd) and slow-start threshold (ssthresh) before a packet loss that affects the throughput of transmission significantly.
In this thesis, we proposed a mechanism to dynamic adjust the slow-start threshold. The ssthresh estimation would set an appropriate ssthresh. The better ssthresh would improve the transmission performance of TCP distinctly. In the congestion avoidance state, we presented a mechanism to probe the available bandwidth. We would adjust the cwnd appropriately by consecutive observation round trip time (RTT) and reset the ssthresh after the fast retransmission or the timeout by the ssthresh estimation. Then, the TCP sender can enhance their performance by ssthresh estimation and consecutive observation round trip time. Our mechanism would define an efficient transmission rate and it could achieve higher utilization than other TCP versions.
Simulation results show that our scheme can effectively improve the TCP performance. For example, when the average bottleneck bandwidth is close to 30% of the whole networks bandwidth, our mechanism improves the TCP performance by 10% at least.

Chapter 1 Introduction 1
1.1 Traditional TCP Networks 1
1.2 TCP in the Wireless Networks 2
1.3 The slow-start threshold of TCP 2
1.4 Thesis Organization 5
Chapter 2 Related Work 6
2.1 TCP Reno 6
2.2 TCP Westwood 8
2.3 TCP Vegas 10
2.4 Other TCP Versions 11
Chapter 3 Preliminaries 15
3.1 TCP in Conventional Networks 15
3.2 TCP in Heterogeneous Networks 15
3.2.1 The Characteristic of Wireless links 17
3.2.2 The Local Retransmission at the Wireless Link 17
3.2.3 The Compression of TCP Acknowledgement 18
Chapter 4 The Proposed Scheme for Dynamic Bandwidth Environment 19
4.1 Slow-start Threshold Estimation 19
4.2 Appropriate Congestion Window 22
4.2.1 The Consecutive Decrease of RTT 23
4.2.2 The Consecutive Increase of RTT 24
4.3 The Operations of Our Scheme 25
Chapter 5 Performance Evaluation 29
5.1 Simulation Environment 29
5.2 Simulation Results 31
5.3 The Summary of Simulation 35
Chapter 6 Conclusions 36
References 37

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