Proxy caching for media streaming over the Internet

Streaming media has contributed to a significant amount of today's Internet traffic. Like conventional Web objects (e.g., HTML pages and images), media objects can benefit from proxy caching; but their unique features such as huge size and high bandwidth demand imply that conventional proxy caching strategies have to be substantially revised. This article discusses the critical issues and challenges of cache management for proxy-assisted media streaming. We survey, classify, and compare the state-of-the-art solutions. We also investigate advanced issues of combining multicast with caching, cooperating among proxies, and leveraging proxy caching in overlay networks.     

Scalable proxy caching of video under storage constraints

Proxy caching has been used to speed up Web browsing and reduce networking costs. In this paper, we study the extension of proxy caching techniques to streaming video applications. A trivial extension consists of storing complete video sequences in the cache. However, this may not be applicable in situations where the video objects are very large and proxy cache space is limited. We show that the approaches proposed in this paper (referred to as selective caching), where only a few frames are cached, can also contribute to significant improvements in the overall performance. In particular, we discuss two network environments for streaming video, namely, quality-of-service (QoS) networks and best-effort networks (Internet). For QoS networks, the video caching goal is to reduce the network bandwidth costs; for best-effort networks, the goal is to increase the robustness of continuous playback against poor network conditions (such as congestion, delay, and loss). Two different selective caching algorithms (SCQ and SCB) are proposed, one for each network scenario, to increase the relevant overall performance metric in each case, while requiring only a fraction of the video stream to be cached. The main contribution of our work is to provide algorithms that are efficient even when the buffer memory available at the client is limited. These algorithms are also scalable so that when changes in the environment occur it is possible, with low complexity, to modify the allocation of cache space to different video sequences.     

A bird's eye view of Web caching research [book review]

The article is a review of the book: Web Caching and Its Applications, written by S.V. Nagaraj and published by Springer, 2004. Web caching technology improves client download times and reduces network traffic by caching frequently accessed copies of Web objects close to the clients. The primary research issues in Web caching are where to cache copies of objects (cache placement), how to keep the cached copies consistent (cache consistency), and how to redirect clients to the optimal cache server (client redirection). Web caching systems' design space is huge, and building a good caching system involves several issues. Over the past decade, researchers have carried out a tremendous amount of work in addressing these issues. In Web Caching and Its Applications, S.V. Nagaraj aims to provide a bird's eye view of this research. He has exhaustively surveyed the literature and summarized the results of several research publications. The author concludes that the book can serve as a reference tool for researchers and for graduate students working on Web systems. However, its approach isn't suitable for Web administrators or students who are new to the field.     

An overview of web caching replacement algorithms

The increasing demand for World Wide Web (WWW) services has made document caching a necessity to decrease download times and reduce Internet traffic. To make effective use of caching, an informative decision has to be made as to which documents are to be evicted from the cache in case of cache saturation. This is particularly important in a wireless network, where the size of the client cache at the mobile terminal (MT) is small. Several types of caching are used over the Internet, including client caching, server caching, and more recently, proxy caching. In this article we review some of the well known proxy-caching policies for the Web. We describe these policies, show how they operate, and discuss the main traffic properties they incorporate in their design. We argue that a good caching policy adapts itself to changes in Web workload characteristics. We make a qualitative comparison between these policies after classifying them according to the traffic properties they consider in their designs. Furthermore, we compare a selected subset of these policies using trace-driven simulations.     

Limitations and benefits of cooperative proxy caching

Cooperating proxy caches are groups of HTTP proxy servers that organize to share cached objects. This paper develops analytical models for proxy cooperation which use speedup in user response time as the performance metric. Speedup expressions are derived for the cooperation upper bound, a proxy mesh, and a three-level proxy hierarchy. The equations compare fundamental design approaches by separating the proxy organization for object delivery from the mechanism for object discovery. Discovery mechanisms analyzed for the mesh and hierarchy models include ideal discovery, Internet cache protocol (ICP) query, and distributed metadata directories. Equations are evaluated using parameter estimates from experiments and from analysis of cache trace logs. Results indicate that proxy cooperation is marginally viable from the standpoint of average user response time, and that the miss penalty for the hierarchy renders it less viable than the mesh. Proxy cooperation can, however, reduce the variability in user response time and the number of long delays. A trace-driven simulation shows that caching constraints have little effect on cooperation performance due to request filtering by lower level caches.     

Traffic analysis of a Web proxy caching hierarchy

Understanding Web traffic characteristics is key to improving the performance and scalability of the Web. In this article Web proxy workloads from different levels of a caching hierarchy are used to understand how the workload characteristics change across different levels of a caching hierarchy. The main observations of this study are that HTML and image documents account for 95 percent of the documents seen in the workload; the distribution of transfer sizes of documents is heavy-tailed, with the tails becoming heavier as one moves up the caching hierarchy; the popularity profile of documents does not precisely follow the Zipf distribution; one-timers account for approximately 70 percent of the documents referenced; concentration of references is less at proxy caches than at servers, and concentration of references diminishes as one moves up the caching hierarchy; and the modification rate is higher at higher-level proxies     

A Novel Dynamic and Scalable Caching Algorithm of Proxy Server for Multimedia Objects

This paper presents a novel dynamic and scalable caching algorithm of proxy server with a finite storage size for multimedia objects. Among the multimedia such as text, image, audio and video, video is a dominant component in terms of the performance of proxy server due to its traffic characteristics. For the fast caching process, caching sequences for videos are obtained to decrease both the buffer size and the required bandwidth and saved into metafiles in advance. Then, we present a novel caching and replacing algorithms for multimedia objects based on the metafiles. Finally, experimental results are provided to show the superior performance of the proposed algorithm.     

Architecture of the Mobile Environment for Intelligent Genetic Search and Proxy Caching

As the number of documents and servers on Internet grows with the enormous speed, it becomes necessary to design efficient algorithms and tools for search and retrieval of documents. Also, the number of accesses to servers on Internet constantly grows. Congestion of servers and links can be alleviated using proxy caches. Latency on Web can be reduced using prefetching and caching. Efficient search of documents can be done with improved genetic algorithm that exploits the principles of temporal and spatial locality. Mobile agents can be used to optimize network traffic for distributed applications. This paper describes several existing solutions to the problem and discusses the implementation of Java distributed object application for experimenting with genetic search and proxy caching algorithms for Internet.     

Segment-based proxy caching for Internet streaming media delivery

The proliferation of multimedia content on the Internet poses challenges to existing content delivery networks. While proxy caching can successfully deliver traditional text-based static objects, it faces difficulty delivering streaming media objects because of the objects' sizes as well as clients' rigorous continuous delivery demands. We present two techniques supporting segment based proxy caching of streaming media. We evaluated these techniques in simulations and real systems.     

基于媒体用户访问行为偏好模型的代理缓存算法

目前,代理缓存技术广泛应用于改善流媒体传输的服务质量.文章从实际用户日志文件的分析出发,利用发现的用户浏览流媒体对象时的行为分布模型,提出了一种新的视频流媒体缓存算法.仿真结果证明,该算法可以通过记录很少的用户访问信息获取较高的性能表现.     

Cache Optimization on Hot‐Point Proxy Caching Using Weighted‐Rank Cache Replacement Policy

The development of proxy caching is essential in the area of video‐on‐demand (VoD) to meet users' expectations. VoD requires high bandwidth and creates high traffic due to the nature of media. Many researchers have developed proxy caching models to reduce bandwidth consumption and traffic. Proxy caching keeps part of a media object to meet the viewing expectations of users without delay and provides interactive playback. If the caching is done continuously, the entire cache space will be exhausted at one stage. Hence, the proxy server must apply cache replacement policies to replace existing objects and allocate the cache space for the incoming objects. Researchers have developed many cache replacement policies by considering several parameters, such as recency, access frequency, cost of retrieval, and size of the object. In this paper, the Weighted‐Rank Cache replacement Policy (WRCP) is proposed. This policy uses such parameters as access frequency, aging, and mean access gap ratio and such functions as size and cost of retrieval. The WRCP applies our previously developed proxy caching model, Hot‐Point Proxy, at four levels of replacement, depending on the cache requirement. Simulation results show that the WRCP outperforms our earlier model, the Dual Cache Replacement Policy.     

一种有资源适应性的transcoding代理缓存机制

在基于transcoding代理的流媒体服务系统中,CPU和网络是两种潜在的瓶颈资源.本文提出了一种有资源适应性的transcoding代理缓存机制,统一考虑CPU和网络的资源需求,以提高系统的服务能力.首先推导了多版本缓存策略下网络收益和CPU收益的计算方法.通过引入一个时变的影响因子 α (t),给出了缓存系统聚合资源收益的表达.在此基础上给出了单个对象的缓存价值函数,并设计了RAC替换算法.实验表明RAC具有较好的资源适应性和系统吞吐率.     

针对3G上网用户的无线网络加速系统

随着3G上网业务的不断发展,越来越多的用户使用3G网络移动上网,这样会导致无线资源紧张,影响用户的使用体验,同时也给移动网络带来了沉重的负担。针对这一问题开发了针对3G移动上网用户Web应用的无线网络加速系统,该系统使用缓存和数据压缩技术,可以通过单向透明或者双向透明2种无客户端的方式部署在移动数据网到互联网的出口位置,以便于产品的推广使用。对该系统进行的现网测试包括单用户测试和现网流量测试两个部分,测试结果表明,使用无线网络加速系统大幅度减少移动上网用户的Web页面打开时间,从而提高了用户体验;同时使用该系统后,对于移动用户的Web应用产生的流量,在网络出口一侧和无线接入网一侧,能实现16%以上的流量节省。     

Implications of proxy caching for provisioning networks and servers

In this paper, we examine the potential benefits of Web proxy caches in improving the effective capacity of servers and networks. Since networks and servers are typically provisioned based on a high percentile of the load, we focus on the effects of proxy caching on the tail of the load distribution. We find that, unlike their substantial impact on the average load, proxies have a diminished impact on the tail of the load distribution. The exact reduction in the tail and the corresponding capacity savings depend on the nature of the workload and the percentile of the load distribution chosen for provisioning networks and servers-the higher the percentile, the smaller the savings. For workloads considered in this study, compared with over a 50% reduction in the average load, the savings in network and server capacity was only 20%-35% for the 99th percentile of the load distribution. We also find that while proxies can be somewhat useful in smoothing out some of the burstiness in Web workloads; the resulting workload continues, however, to exhibit substantial burstiness and a heavy-tailed nature. We identify one-time requests for large objects to be the limiting factor that diminishes the impact of proxies on the tail of load distribution. We conclude that, while proxies are immensely useful to users due to the reduction in the average response time, they are less effective in improving the capacities of networks and servers.     

Optimizing Web delivery over wireless links: design, implementation, and experiences

World over wide-area wireless Global System for Mobile Communication (GSM) networks have been upgraded to support the general packet radio service (GPRS). GPRS brings "always-on" wireless data connectivity at bandwidths comparable to that of conventional fixed-line telephone modems. Unfortunately many users have found the reality to be rather different, experiencing very disappointing performance when, for example, browsing the Web over GPRS. In This work, we show what causes the web and its underlying transport protocol TCP to underperform in a GPRS wide-area wireless environment. We examine why certain GPRS network characteristics interact badly with TCP to yield problems such as: link underutilization for short-lived flows, excess queueing for long-lived flows, ACK compression, poor loss recovery, and gross unfairness between competing flows. We also show that many Web browsers tend to be overly aggressive, and by opening too many simultaneous TCP connections can aggravate matters. We present the design and implementation of a web optimizing proxy system called GPRSWeb that mitigates many of the GPRS link-related performance problems with a simple software update to a mobile device. The update is a link-aware middleware (a local "client proxy") that sits in the mobile device, and communicates with a "server proxy" located at the other end of the wireless link, close to the wired-wireless border. The dual-proxy architecture collectively implements a number of key enhancements-an aggressive caching scheme that employs content-based hash keying to improve hit rates for dynamic content, a preemptive push of Web page support resources to mobile clients, resource adaptation to suit client capabilities, delta encoded data transfer of modified pages, DNS lookup migration, and a UDP-based reliable transport protocol that is specifically optimized for use over GPRS. We show that these enhancements results in significant improvement in web performance over GPRS links.     

Performance Analysis of Proxy Caching for VOD Services With Heterogeneous Clients

Proxy-caching strategies, especially prefix caching and interval caching, are commonly used in video-on-demand (VOD) systems to improve both the system performance and the playback experience of users. However, because these caching strategies are designed for homogeneous clients, they do not perform well in the real world where clients are heterogeneous (i.e., different available network bandwidths and different sizes of client-side buffers). This paper investigates the problems caused by heterogeneous client-side buffers. We analyze the theoretical performance of these caching strategies, and then, derive cost functions to measure the corresponding performance gains. Based on these analytical results, we develop a caching strategy that employs both prefix caching and interval caching to minimize the input bandwidth of a proxy. The simulation results demonstrate that the bandwidth requirements of a proxy implementing our caching strategy are significantly lower compared to adopting prefix caching or interval caching alone.     

A Learning-Based Approach for Web Cache Management

Web caching has been widely used to alleviate Internet traffic congestion in World Wide Web (WWW) services. To reduce download throughput, an effective strategy on web cache management is needed to exploit web usage information in order to make a decision on evicting the document stored in case of cache saturation. This paper presents a so-called Learning Based Replacement algorithm (LBR), a hybrid approach towards an efficient replacement model for web caching by incorporating a machine learning technique (naive Bayes) into the LRU replacement method to improve prediction of possibility that an existing page will be revised by a succeeding request, from access history in a web log. The learned knowledge includes information on which URL objects in cache should be kept or evicted. The learning-based model is acquired to represent the hidden aspect of user request pattern for predicting the re-reference possibility. By a number of experiments, the LBR gains potential improvement of prediction on revisit probability, hit rate and byte hit rate overtraditional methods; LRU, LFU, and GDSF, respectively.     

Considering User Behavior and Multiple QoS Supports in Multimedia Streaming Caching

With the prevalence of video-on-demand (VOD) services as well as the diffusion of various multimedia devices, caching in a multimedia streaming server is becoming increasingly important. However, due to some peculiar characteristics of multimedia objects and user activities in streaming services, design of an efficient caching system becomes a more challenging problem compared to the traditional caching systems. This paper discusses some important issues that are of interest in the domain of multimedia streaming caching and presents a new cache management scheme for multimedia streaming servers. Our new scheme considers different streaming rates of multimedia objects as well as the inter-arrival time between two consecutive requests on an identical object. It also considers user activities in requesting and playing multimedia contents. Trace-driven simulations with real world VOD traces show that the proposed scheme improves the performance of multimedia streaming systems significantly.     

基于TTL的Web缓存过滤效果

文章研究了基于TTL的Web缓存层次过滤效果，业务量性质对基于TTL的动态Web缓存系统的性能有重要影响。在层次缓存中，由于只有错失的请求才会被转发给下一级缓存，因而逐级对业务量存在过滤作用，业务量性质随之改变。文章利用仿真研究了基于TTL的动态Web缓存层次过滤对业务量的影响。重点考察了请求到达间隔模型及对象流行度分布的变化。     

World Wide Web caching: trends and techniques

Academic and corporate communities have been dedicating considerable effort to World Wide Web caching. When correctly deployed, Web caching systems can lead to significant bandwidth savings, server load balancing, perceived network latency reduction, and higher content availability. We survey the state of the art in caching designs, presenting a taxonomy of architectures and describing a variety of specific trends and techniques