台灣學術網路上因果次序之群組通訊協定實作與測試

Abstract

近來由於網路技術與電腦科技的發展，電腦系統的環境逐漸地由單機單工的系統架構朝向多機多工的分散式系統架構發展；藉由分散式系統的架構，能夠整合多方資源，甚至改變人們的生活習性。分散式的系統雖帶來了許多的好處，但仍伴隨著一些技術上的問題，其中以同步各分散點的工作為最主要的課題。端點同步問題的產生，主要是因為各端點間的訊息傳送延遲不同，而造成網路非同步現象。為了克服上述因網路非同步的現象，許多相關的多端點（ 群體） 通訊同步協定被提出[3][4][5][6][14]；其中，最著名的是因果次序協定（causal order protocol [2][7]）。之後所提Δ—causal order protocol [10]同時考慮因果次序及即時性，符合新一代網際網路應用服務的需求，但之前的研究並未分析即時性與因果次序效能上的平衡（trade-off），本文實作Δ—causal order protocol，並在台灣學術網路上測試。實驗中我們發現Δ值的取捨必須在分散式即時應用對端點的最低處理訊息的通透率（Throughput）要求下力求盡可能的小。另一方面，我們也指出當Δ大時，適當的可靠傳輸機制將可以減輕某些端點的同步負擔。我們的成果可供設計群組軟體效能上的參考。

The revolutions of networking and computing technology motivate the deployment of widely distributed system that integrates resources and make people convenient. People take advantage of distributed system but there still have many technical issues to be addressed such as synchronization or coordination of the distributed processes. The distributed processes suffer from asynchronous phenomenon because of the delay variance of messages through the heterogeneous networks. This situation becomes worse when the end-to-end delays of links in the distributed system are asynchronous due to different bandwidth, different routing or switching technology, physical location of stations, etc.. To overcome the above problem, many coordination protocols for group-ware applications are proposed. In these protocols, delta-causal order protocol considers both requirements of causal order, guarantees cause and effect between messages, and real-time, guarantees messages are significant for the receiver. Nevertheless, previous research does not study the performance trade-off between real-time and causal order. In this paper, we implement the protocol and analyze its performance on Internet. From our experiments, we find that the value must be as possible as small, subject to the lowest throughput requirement of the distributed real-time system. If the value is large, we find that retransmission may help some sites to decrease the overhead of synchronization. We believe our research will be beneficial to the improvement of the group communication in the future and can provide some helpful reference for the designer.