Systems for on-demand delivery of large, widely-shared data can use several techniques to improve cost/performance, including: multicast data delivery, segmented data delivery, and regional (or proxy) servers that cache some of the data close to the clients. This paper makes three contributions to the state-of-the-art design of such systems. First, we show how segmented multicast delivery techniques, in particular the recently proposed high-performance dynamic skyscraper scheme, can be modified to allow each object to be partially or fully cached at regional servers. The new partitioned delivery architecture supports shared delivery between the regional and remote servers and improves performance even if one server delivers the entire object. The second contribution is an analytic model that can be solved to determine the full/partial object caching strategy that minimizes delivery cost in the context of a system that has homogeneous regional servers. Finally, results in the paper illu...

An important question in providing on-demand access to large widely shared data files, such as popular video files, is how to effectively use regional (proxy) servers that can store some of the data close to the clients. The proxy caching problem is more complex in the context of continuous media files because of the need to consider bandwidth as well as storage constraints at the proxy servers, and because of the bandwidth sharing possibilities provided by recently proposed multicast delivery techniques. This paper develops new highly efficient analytic models for determining optimal proxy cache content in such environments. Specifically, the new models apply to heterogeneous systems in which the proxy servers have different client workloads and server capabilities. Results from the models provide general insights into caching strategies for such systems, and suggest that it may be useful to employ efficient cost models in actual systems to determine what content should be cached in r...

The proliferation of the Internet is leading to high expectation on the fast turnaround time. Clients abandoning their connections due to excessive downloading delays translates directly to profit losses. Hence, minimizing the latency perceived by end-users has become the primary performance objective compared to more traditional issues, such as server utilization. The two promising techniques to improve the Internet responsiveness are caching and replication. In this paper we present an overview of recent research in replication. We begin by arguing on the important role of replication in decreasing client perceived response time and proceed by illustrating the main topics that affect its successful deployment on the Internet. We analyze and characterize existing research, providing taxonomies and classifications whenever possible. Our discussion reveals several open problems and research directions. 1

The Replica Placement Problem (RPP) aims at creating a set of duplicated data objects across the nodes of a distributed system in order to optimize certain criteria. Typically, RPP formulations fall into two categories: static and dynamic. The first assumes that access statistics are estimated in advance and remain static, and, therefore, a one-time replica distribution is sufficient (1RPP). In contrast, dynamic methods change the replicas in the network potentially upon every request. This paper proposes an alternative technique, named Continuous Replica Placement Problem (CRPP), which falls between the two extreme approaches. CRPP can be defined as: Given an already implemented replication scheme and estimated access statistics for the next time period, define a new replication scheme, subject to optimization criteria and constraints. As we show in the problem formulation, CRPP is different in that the existing heuristics in the literature cannot be used either statically or dynamically to solve the problem. In fact, even with the most careful design, their performance will be inferior since CRPP embeds a scheduling problem to facilitate the proposed mechanism. We provide insight on the intricacies of CRPP and propose various heuristics.

. The growing number of digital audio and video repositories has resulted in a desperate need for effective techniques to deliver data to users in a timely manner. Due to geographical distribution of users, it is not cost effective to have a centralized media server. In this paper, we investigate issues involved in the design of a distributed video server (DVS) to support movie-on-demand (MOD) application. We propose a redundant hierarchical (RedHi) architecture for DVS where the nodes are continuous media servers and the edges are dedicated network lines. With RedHi, each node has two or more parents. We show that the redundant links in RedHi yield a more reliable and efficient system. Our simulation results demonstrate that RedHi can tolerate a single link failure with no degradation in performance while with pure hierarchy almost 2:5% of requests are rejected due to the failure. In normal mode of operation, RedHi outperforms pure hierarchy significantly (160% improvement on the aver...

A geographically distributed web server (GDWS) system, consisting of multiple server nodes interconnected by a MAN or a WAN, can achieve better efficiency in handling the ever-increasing web requests than centralized web servers because of the proximity of server nodes to clients. It is also more scalable since the throughput will not be limited by available bandwidth connecting to a central server. The key research issue in the design of GDWS is how to replicate and distribute the documents of a web site among the server nodes. This paper proposes a density-based replication scheme and applies it to our proposed Extensible GDWS (EGDWS) architecture. Its document distribution scheme supports partial replication targeting only at hot objects among the documents. To distribute the replicas generated via the density-based replication scheme, we propose four different document distribution algorithms: Greedy-cost, Maximal-density, Greedypenalty, and Proximity-aware. A proximity-based routing mechanism is designed to incorporate these algorithms for achieving better web server performance in a WAN environment. Simulation results show that our document replication and distribution algorithms achieve better response times and load balancing than existing dynamic schemes. To further reduce user's response time, we propose two document grouping algorithms that can cut down on the request redirection overheads.

Self-certified public keys are proposed to eliminate the burden of verifying the public key before using. To alleviate reliance on system authority and strengthen the security of system, Chang et al propose a new digital signature schemes, no redundancy is needed to be embedded in the signed messages in this scheme. Moreover, Chang et al claimed that the schemes are still secure even without the trustworthy system authority, and only the specified recipient can recover the message in his authentication encryption schemes. Unfortunately, In this work, we analyze the security of Chang et al scheme and show that if the system authority is trustless, the scheme is insecure, namely, the system authority can recover the message without the private key of the recipient in Chang ’ authentication encryption schemes. Finally, we propose an improved scheme to overcome the weakness of Chang et al scheme. Povzetek: Predstavljena je tehnika digitalnega certifikata z javnim ključem. 1

Caching and replication have emerged as the two primary techniques for reducing the delay experienced by end-users when downloading web pages. Even though these techniques may benefit from each other, previous research work tends to focus on either one of them separately. In particular, caching has been studied mostly in the context of proxy server systems, while replication is the technology behind Content Distribution Networks (CDNs). In this paper we investigate the potential performance gain by using a CDN server both as a replicator and as a proxy server. We develop an analytical model to quantify the benefit of each technique, under various system parameters, and propose a greedy algorithm to solve the combined caching and replica placement problem. Our simulation results indicate that a simple LRU caching scheme can improve significantly the response time of HTTP requests, when utilized over a replication-based infrastructure. Moreover, due to its simplicity, this hybrid approach does not affect the administrative overhead of the CDN architecture. q 2005 Elsevier B.V. All rights reserved.

The problem of minimizing the access time of a requested multimedia (MM) document on a network based environment is addressed. A generalized version of this problem is formulated and retrieval strategies that minimize the access time of the user-requested MM document from a pool of MM servers are proposed. To this end, we design single-installment and multi-installment MM document retrieval strategies, through which the minimization of access time can be carried out. The main idea is to utilize more than one MM server in downloading the requested document. Each server assumes the responsibility of uploading a predetermined portion of the entire document in a particular order. Single- and multi-installment strategies differ in the number of disjoint document pieces each server sends to the client. We first introduce a directed flow graph (DFG) model to represent the retrieval process and generate a set of recursive equations using this DFG. Then, we derive closed-form solutions for the portions of the MM document downloaded from the various servers and the corresponding access time. We present rigorous analysis for these two strategies and show their performance under MPEG-I and MPEG-II video streams playback rates. Their behavior under different network bandwidths is also examined, revealing in-depth information about their expected performance. We also show that in the case of a multi-installment strategy, the access time can be completely controlled by fine tuning the number of installments. Since the number of installments is software tunable, the adaptive nature of the strategies to different channel bandwidths is also demonstrated. Important trade-off studies with respect to the number of servers involved in the retrieval process and the number of installments are present...

Caching and replication have emerged as the two primary techniques for reducing the delay experienced by end users when downloading web pages. Even though these techniques may benefit from each other, previous research work tends to focus on either one of them separately. In this paper we investigate the potential performance gains by using a CDN server both as a replicator and as a proxy server. We assume a common storage space for both techniques, and develop an analytical model that characterizes caching performance under various system parameters. Based on the models predictions, we can reason whether it is beneficial to reduce the caching space in order to allocate extra replicas. The resulting problem of finding which object replicas should be created where, given that any free space will be used for caching, is NP-complete. Therefore, we propose a hybrid heuristic algorithm (based on the greedy paradigm), in order to solve the combined replica placement and storage allocation problem. Our simulation results indicate that a simple LRU caching scheme can considerably improve the response time of HTTP requests, when utilized over a replication-based infrastructure. 1.