An FTP Proxy System to Assure Providing the Latest Version of Replicated Files

摘要

As so many software titles are now being distributed via the Internet, the number of accesses to file servers, such as FTP servers, is rapidly increasing. To prevent the concentration of accesses to the original file server, mirror servers are being introduced that contain the same directories and files as held by the original server. However, inconsistency among the mirror servers and the original server is often observed because of delivery latency, traffic congestion on the network, and management policies of the mirror servers. This inconsistency degrades the value of the mirror servers. Accordingly, we have developed an intermediate FTP proxy server system that guarantees the freshness of the files as well as preventing access concentration on the original FTP server. The system adopts per-file selection of the replicated files; most existing methods are based on per-host or per-directory selection. Therefore it can assure users of a quick, stable, and up-to-date FTP mirroring service even in the face of frequent content updates, which tend to degrade the homogeneity of services. Moreover, it can forward the retrieved files with little overhead. Tests confirmed that our system is comparable to existing systems from the viewpoint of actual retrieval time, required traffic, and load endurance. This technology can assure clients that they will receive the latest version of the file(s) desired. It well supports heterogeneous network environments such as the Internet.