TCP壅塞控制技術之研究與設計

Abstract

隨著網際網路訊務流量的快速成長，如何以有效率的方式使用網路資源是一個成功的壅塞控制機制所要面對的根本問題。TCP身為網際網路上一個被廣為使用的端對端傳輸層協定，它被創造出許多不同的版本，用來改進網路的使用效能。在目前的TCP版本中有兩個特別值得留意的方案，一個是現今網際網路上廣為使用的Reno，另一個則是宣稱相較於Reno可增進百分之三十七到七十一傳輸效能的Vegas。 TCP Vegas能夠偵測出初期的網路壅塞而且可以成功的避免在TCP Reno上時常發生的週期性封包遺失的現象，很多研究報告已經指出，Vegas在很多方面都要優於Reno，例如整體網路的使用率、穩定性、公平性、以及傳輸速率等。很可惜的是Vegas並不完美，仍然有一些缺點存在於它的壅塞控制機制中，這些問題或問題的起因包含了重新繞路、永久壅塞、競爭連線間的公平性、非對稱網路傳輸、高頻寬-延遲乘積網路、以及無線傳輸下的非壅塞封包遺失等。在這份論文中，我們提出了四個改進Vegas的機制，為Vegas移除邁向成功的障礙。這些新提出的機制有些是單純的端對端方法，有些則利用了路由器所提供的資訊以改善連線的傳輸效能。 第一個提出的方案RoVegas是一個使用路由器訊息回饋的改進方案，藉由封包路徑上的路由器執行特定的機制，RoVegas可以解決重新繞路時所引起的問題，可以解決永久壅塞問題，也可以增進競爭連線間的公平性，以及改善在非對稱網路傳輸時，TCP可能的效能損失。 Enhanced Vegas是一個純粹端點對端點的改進機制，不用路由器的協助，它可以量測出發生在前送路徑和返回路徑上的網路壅塞程度，因此它能精準而合宜的調整封包的傳送速率，有效提高當壅塞發生在返回路徑時的連線效能。 由於在擁有大壅塞窗口時的反應速度過於緩慢，TCP在高頻寬-延遲乘積網路中顯得效能不彰，因此第三個改進機制Quick Vegas被提出來。Quick Vegas利用連線壅塞窗口的調整紀錄以及連線估測堆積在佇列中的封包數量為依據，對TCP壅塞控制演算法做出調整，這個改變使得一個連線的送端在調整壅塞窗口大小時採取更有效和積極的態度，因此讓連線在高頻寬-延遲乘積網路中能有較好的表現。 TCP壅塞控制的一個眾所週知的問題是它沒有辦法分辨出封包遺失的原因，傳統的TCP把所有封包遺失的原因都歸咎於網路的壅塞，這種推測在異質性日益顯著的網際網路中並不合宜。錯把傳輸失誤所造成的封包遺失當成網路壅塞的訊號將導致TCP不必要的效能損失。最後一個提出的改進方案RedVegas利用TCP Vegas原有的特性以及路由器在封包上的壅塞標記，可以準確的判斷出傳輸失誤所造成的封包遺失，透過封包遺失原因的分類，RedVegas可以適切的對不同原因的封包遺失做出不同的反應，因此改善了在異質網路傳輸中的TCP效能。

With the fast growth of Internet tra?c, how to e?ciently utilize network resources is essential to a successful congestion control. Transmission Control Protocol (TCP) is a widely used end-to-end transport protocol in the Internet, it has several implementation versions (i.e., Tahoe, Reno, Vegas...) which intend to improve network utilization. Among these TCP variants, there are two notable approaches. One is Reno which has been widely deployed on the Internet; the other is Vegas with a claim of 37 to 71 percent throughput improvement over Reno was achieved. TCP Vegas detects network congestion in the early stage and successfully prevents periodic packet loss that usually occurs in TCP Reno. It has been demonstrated that TCP Vegas outperforms TCP Reno in the aspects of overall network utilization, stability, fairness, and throughput. However, TCP Vegas still suffers problems that inhere in its congestion control algorithm, these include issues of rerouting, persistent congestion, fairness, network asymmetry, high bandwidth-delay product (BDP) networks, and internetworking of wired and wireless networks. In this dissertation, we propose four enhanced mechanisms to remove the obstacles of TCP Vegas for achieving a real success. These mechanisms not only adopt end-to-end approaches but also utilize the information that provided by routers to improve the performance of connections. The first proposed mechanism, RoVegas, uses a router-assisted approach. By performing the proposed scheme in routers along the round-trip path, RoVegas can solve the problems of rerouting and persistent congestion, enhance the fairness among the competitive connections, and improve the throughput when congestion occurs on the backward path. An end-to-end scheme, Enhanced Vegas, is also presented to improve the performance degradation of TCP Vegas in asymmetric networks. Through distinguishing whether congestion occurs in the forward path or not, Enhanced Vegas signifcantly advances the connection throughput when the backward path is congested. TCP congestion control may function poorly in high BDP networks because of its slow response with large congestion window size. In the third mechanism, we propose an improved version of TCP Vegas called Quick Vegas, in which we present an effcient congestion window control algorithm for a TCP source. The modification allows TCP connections to react faster and better to high BDP networks and therefore improves the overall performance. A well-known problem in providing TCP congestion control over wired and wireless networks is that it may encounter both congestion loss and random loss. Traditional TCP interprets every packet loss as caused by congestion which may not be the case in the current Internet. In the last proposed mechanism, RedVegas, we utilize the innate nature of TCP Vegas and congestion indications marked by routers to detect random packet losses precisely. Through the packet loss differentiation, RedVegas reacts appropriately to the losses, and therefore the throughput of connection over heterogeneous networks can be signi?cantly improved.