寬頻多媒體應用之高效能傳輸系統

Abstract

寬頻多媒體應用(Broadband Multimedia Applications)將是現行及未來 網路服務中之主流。要使得這種型態之服務成為可行，必須要有高效能之 網路設備以及服務品質(Quality of Service)之保證。非同步傳輸模態 (Asynchronous Transfer Mode，ATM)網路嚐試從較低層的網路通訊協定 來解決前述之要件，我們則從傳輸層協定(Transport Layer Protocol)的 角度來解決同一個問題。傳統的傳輸層協定如TCP、ISO TP4已被公認為無 法滿足寬頻多媒體應用之需要，因為這些通訊協定在當初是為了配合低傳 輸速度及高傳輸錯誤率的通訊環境而設計的。新的傳輸層協定如VMTP及 NETBLT被提出來配合新發展出來的高傳輸速度及低傳輸錯誤率的網路環境 ；它們都採用了以速率為基的流量控制機制(Rate-based Flow Control) 。事實上，它們的流量控制機制在實作上不僅沒有說明如何給定速率的初 值，同時也缺乏管控資源的方法， 而這在許多高傳輸量的應用同時競爭 使用有限的傳輸資源時是不可或缺的。本論文提出了一個寬頻多媒體應用 的傳輸系統(Broadband Multimedia Applications Transport System， BMATS)的雛型，其中提出了可行的速率為基的流量控制機制及一套資源管 理的方法。在BMATS裡，各種應用被歸為三種類別；而針對不同之應用類 別，各有相對應的處理方法。BMATS的雛型架構兩種網路平臺(Network Platform)上：Network Interface Tap(NIT)及User Datagram Protocol( UDP)。NIT因為實作太差以致效能不理想，BMATS最後因此選擇架構在UDP 上。在論文中將顯示就傳輸包間的平均延遲時間及傳輸量而言，BMATS的 表現是優於TCP的。然而，所必須付出的代價是第二類及第三類應用之需 求有時會產生被回絕的情形。 Broadband miltimedia applications are becoming the main stream of current and future networking services. To make these services significant, a high-performance networking facility should be provided and the Quality of Service (QoS) should be guaranteed. While the introduction of ATM networks attempts to meet these requirements at lower layers of network protocol stacks, we tackle the same problem from a transport-layer perspective. Conventional transport protocols like TCP and ISO TP4 have revealed to be incapable of satisfying the requirements for broadband multimedia applications in that they were designed only for low-speed and high error-rate networking environment. New approaches, such as VMTP and NETBLT, have thus been proposed to confront the newly developed high-speed low error-rate network setting, particularly using the rate-based flow control mechanism . In truth, these new approaches lack not only for the selection of the initial rates in the implementation of the rate-based flow mechanism but also a resource management scheme owing to intensive rivalry of limited resources among high-throughput applications. This thesis hence proposes a Broadband Multimedia Applications Transport System (BMATS), aimed at the provision of a prototype of a high-performance transport system using a resource management scheme as well as a feasible rate-based flow control protocol. Essentially, applications are categorized as three classes for which various rates and resources are allocated . The BMATS prototype has been conducted on two underlying network platforms: stram-mode Network Interface Tap (NIT) and socket-mode User Datafram Protocol (UDP). Owing to the poor implementation of NIT, as will be shown, BMATS operates under the UDP environment. The thesis demonstrates the performance superiority of BMATS to TCP in terms of mean delay and throughput of types 2 and 3 applications. However, the price paid is the explicit blocking of types 2 and 3 applications.