[[alternative]]Improving the performance of TCP protocol Over Heterogeneous Networks

碩士資訊工程學系[[abstract]]現存的TCP是被設計用於封包遺失率較低並且封包遺失最主要的原因是由於壅塞的關係。換言之，TCP假設封包遺失是網路壅塞所造成，故降低其傳送速率。但如果是由無線端所造成的bit error (random loss)，傳送端應該嘗試著增大它的傳送速率。擴大逾時值或滑動視窗大小會使得事態更嚴重因為它產生額外的頻寬浪費。如果傳送端沒有任何的機制來區分封包遺失的原因，會使得TCP在無線網路的效能大打折扣。在本篇論文當中，我們提出一個新興並且簡單的方法，稱之為MSnoop-DTO(Modified Snoop-Decrease TimeOut)，來改善TCP於異質性網路環境下之效能。主要構想為藉由減少逾時值來讓TCP快速反應遺失的封包。它不但能區分無線端所造成的bit error和有線網路壅塞所造成的遺失，還能分辨無線網路中因collision所產生的封包遺失。除此之外，它仍然維持TCP端點對端點的概念且相容於目前無線端所使用的TCP協定。為了驗證我們提出的機制確實可以改善TCP之效能，我們使用NS2這套模擬軟體來做實驗。在不同的網路拓撲架構下，MSnoop-DTO都有較佳的效能及公平性。 關鍵字：傳輸層協定，異質性網路，窺探法[[abstract]]Traditional transmission control protocol (TCP) is designed for wired networks where the packet loss rate (PLR) is low and the congestion is the main cause of packet loss. That is to say, TCP supposes that every packet loss is an indication of the network congestion and consequentially reduces its transmission rate. In effect, when a packet is dropped by routers because of network congestion, the sender should slow down, but when one is lost because of noisy channel, the sender should try harder. Extending the time-out value or sliding window size just makes matters worse because it brings bandwidth wastage. When the sender has no mechanism to discriminate the cause of packet loss, TCP demonstrates poor performance over wireless networks. In this paper, we propose a novel and simple method called MSnoop-DTO (Modified Snoop-Decrease TimeOut) to improve the performance for TCP in heterogeneous networks. The main idea is to make TCP reacts quickly to packet loss by reducing time-out value. It not only distinguishes random loss from congestion loss but also discriminates packet loss from collision. In addition, MSnoop-DTO maintains the end-to-end TCP semantics and is compatible with existing TCP protocol. The performance of the proposed MSnoop-DTO is evaluated by NS2. We found that under various traffic and system configurations, MSnoop-DTO will have better performance when packet loss rate is high. Key words: TCP, Heterogeneous Networks, Snoo