點對點式影音串流系統之開發與實作

Abstract

近年來，影音串流系統引起廣泛的注意與討論，許多相關研究也陸續開展。目前網路上所提供之影音串流服務皆以主從式架構為主，但此種架構卻有無法容納過多使用者之缺陷。為了解決此問題本研究提出PAM，PAM 是一個利用點對點式架構提供影音串流服務的系統，以解決前述無法容納過多使用者之問題。 本篇論文所開發的系統PAM，乃是一專為即時影音串流設計，建構在無特定形狀之網路拓樸上的分散式系統。本論文引入BitTorrent 之概念，意即將影音檔案切散為許多零碎封包，傳送至不同觀賞者，達到讓觀賞者之間交互傳送檔案以分享頻寬之目的。本論文與傳統固定樹狀結構系統的不同之處，在於使用者並無固定檔案供應來源、亦無特定檔案傳輸對象。使用者能根據當下網路之狀況，動態調整其檔案傳輸之途徑。此外，由於無特定檔案來源，當少量檔案來源離開系統時，對系統整體並不會有太大的影響。 為了達到容納眾多使用者的目的，PAM 將使用者劃分為數個層級。每層的使用者都有義務將檔案封包傳送給下一層的使用者。透過建立此種樹狀結構、並使每層播放檔案的順序由上而下地延遲，一個檔案封包才有足夠餘裕，將檔案從最上層的提供者傳送至最下層的使用者。 根據我們的實驗結果，PAM 可以動態地根據網路狀況調整期傳輸路徑、對於網路發生大量錯誤的情況有極佳的抵抗能力，並能容納眾多使用者。

Media streaming has gained increasing attention in recent years, and many researchers have tried to improve its quality. In general, many of the existing services are using client/server models, which are hard to achieve scalability. In this paper, we propose a P2P-based protocol called P2P-based Application- Level Multicast (PAM), which is scalable. PAM uses a network graph overlay and is designed for live broadcasting. We introduced the concepts of BitTorrent into our design, which means the source file is break into many small blocks, and each peers is obligated to distribute parts of file it owns to other peers. In contrary to traditional tree-based streaming services, a peer has neither a fixed source neighbor nor a fixed download peer. It dynamically adjusts its sources according to the network conditions. We achieves scalability by arranging the peers in a tree-shape overlay. Participants in PAM are divided into several levels, and peers in higher level are responsible to distribute file blocks to peers in lower level. By maintaining a tree-shape overlay and delaying the display sequence in lower levels, a file block is then able to transmitted from the top source to all the participants before it is played. As the simulation shows, PAM can adapt its overlay to the physical net- work, has strong failure resistance, and achieves scalability.