基于可扩展视频服务器集群的视频点播系统设计

随着计算机技术和高速网络技术的发展,视频点播系统已变成现实,并且具有巨大的潜在需求。利用视频对象简介能够给用户一个友好的互动收视环境。可扩展视频服务器集群可以适应未来的用户需求的快速增长。视频对象分段技术和前缀缓存技术使视频文件按照一定的缓存策略以分段方式分布在协作式的缓存服务器集群中,以利于服务器集群的负载平衡和减少对用户的启动延迟。系统还引入了IP组播技术来减少对网络带宽的开销。该文提出了混合式的协作缓存和IP组播的方式交付视频对象,并描述了它是如何工作的。     

On exploring aggregate effect for efficient cache replacement in transcoding proxies

Recent technology advances in mobile networking have ushered in a new era of personal communication. Users can ubiquitously access the Internet via many emerging mobile appliances, such as portable notebooks, personal digital assistants (PDAs), and WAP-enabled cellular phones. While the transcoding proxy is attracting an increasing amount of attention in this environment, it is noted that new caching strategies are required for these transcoding proxies. We propose an efficient cache replacement algorithm for transcoding proxies. Specifically, we formulate a generalized profit function to evaluate the profit from caching each version of an object. This generalized profit function explicitly considers several new emerging factors in the transcoding proxy and the aggregate effect of caching multiple versions of the same object. It is noted that the aggregate effect is not simply the sum of the costs of caching individual versions of an object, but rather, depends on the transcoding relationship among these versions. The notion of a weighted transcoding graph is devised to evaluate the corresponding aggregate effect efficiently. Utilizing the generalized profit function and the weighted transcoding graph, we propose, in this paper, an innovative cache replacement algorithm for transcoding proxies. In addition, an effective data structure is designed to facilitate the management of the multiple versions of different objects cached in the transcoding proxy. Using an event-driven simulation, it is shown that the algorithm proposed consistently outperforms companion schemes in terms of the delay saving ratios and cache hit ratios.     

Aggregate Profit-Based Caching Replacement Algorithms for Streaming Media Transcoding Proxy Systems

This work derives a generalized video object profit function from the extended weighted transcoding graph to calculate the individual cache profit of certain versions of a video object, and the aggregate profit from caching multiple versions of the same video object. This proposed function takes into account the popularity of certain versions of an object, the transcoding delay among versions, and the average duration of access of each version. Based on the profit function, cache-replacement algorithms are proposed to reduce the startup delay and network traffic by efficiently caching video objects with the most profits. Two kinds of simulations were conducted to evaluate the performance of the proposed algorithms. These simulations exploit partial viewing traces and complete viewing traces, separately. The results demonstrate that the proposed algorithms outperform the competing algorithms by 15%-39% in delay saving ratio and 5%-10% in byte-hit ratio     

Reducing bandwidth requirement for delivering video over wide area networks with proxy server

Due to the high bandwidth requirement and rate variability of compressed video, delivering video across wide area networks (WANs) is a challenging issue. Proxy servers have been used to reduce network congestion and improve client access time on the Internet by caching passing data. We investigate ways to store or stage partial video in proxy servers to reduce the network bandwidth requirement over WAN. A client needs to access a portion of the video from a proxy server over a local area network (LAN) and the rest from a central server across a WAN. Therefore, client buffer requirement and video synchronization are to be considered. We study the tradeoffs between client buffer, storage requirement on the proxy server, and bandwidth requirement over WAN. Given a video delivery rate for the WAN, we propose several frame staging selection algorithms to determine the video frames to be stored in the proxy server. A scheme called chunk algorithm, which partitions a video into different segments (chunks of frames) with alternating chunks stored in the proxy server, is shown to offer the best tradeoff. We also investigate an efficient way to utilize client buffer when the combination of video streams from WAN and LAN is considered.     

PTC: Proxies that Transcode and Cache in Heterogeneous Web Client Environments

Advances in computing and communication technologies have resulted in a wide variety of networked mobile devices that access data over the Internet. In this paper, we argue that servers by themselves may not be able to handle this diversity in client characteristics and so intermediaries, such as proxies, should be employed to handle the mismatch between the server-supplied data and the client capabilities. Since existing proxies are primarily designed to handle traditional wired hosts, such proxy architectures will need to be enhanced to handle mobile devices. We propose such an enhanced proxy architecture that is capable of handling the heterogeneity in client needs—specifically the variations in client bandwidth and display capabilities. Our architecture combines transcoding (which is used to match the fidelity of the requested object to client capabilities) and caching (which is used to reduce the latency for accessing popular objects). Proxies that Transcode and Cache, PTCs, intelligently adapt to prevailing system conditions using learning techniques to decide whether to transcode locally or fetch an appropriate version from the server. Our experimental results indicate that the use of PTCs produces significant improvements in the client response times. We show that such results hold true for a variety of data content types like images and video data. Further, we find that even simple learning techniques can lead to significant performance improvements.     

Network-aware partial caching for Internet streaming media

The delivery of multimedia over the Internet is affected by adverse network conditions such as high packet loss rate and long delay. This paper aims at mitigating such effects by leveraging client-side caching proxies. We present a novel cache architecture and associated cache management algorithms that turn edge caches into accelerators of streaming media delivery. This architecture allows partial caching of media objects and joint delivery from caches and origin servers. Most importantly, the caching algorithms are both network-aware and stream-aware; they take into account the popularity of streaming media objects, their bit rate requirements, and the available bandwidth between clients and servers. Using Internet bandwidth models derived from proxy cache logs and measured over real Internet paths, we have conducted extensive simulations to evaluate the performance of various cache management algorithms. Our experiments demonstrate that network-aware caching algorithms can significantly reduce startup delay and improve stream quality. Our experiments also show that partial caching is particularly effective when bandwidth variability is not very high.Shudong Jin: Corespondence to This research was supported in part by NSF (awards ANI-9986397, ANI-0095988, ANI-0205294 and EJA-0202067) and by IBM. Part of this work was done while the first author was at IBM Research in 2001.     

PopCap: popularity oriented proxy caching for peer-assisted Internet video-on-demand streaming services

With the success of Internet video-on-demand (VoD) streaming services, the bandwidth required and the financial cost incurred by the host of the video server becoming extremely large. Peer-to-peer (P2P) networks and proxies are two common ways for reducing the server workload. In this paper, we consider a peer-assisted Internet VoD system with proxies deployed at domain gateways. We formally present the video caching problem with the objectives of reducing the video server workload and avoiding inter-domain traffic, and we obtain its optimal solution. Inspired by theoretical analysis, we develop a practical protocol named PopCap for Internet VoD services. Compared with previous work, PopCap does not require additional infrastructure support, is inexpensive, and able to cope well with the characteristic workloads of Internet VoD services. From simulation-based experiments driven by real-world data sets from YouTube, we find that PopCap can effectively reduce the video server workload, therefore provides a superior performance regarding the video server’s traffic reduction.     

Caching video objects: layers vs versions?

Because Internet access rates are highly heterogeneous, many video content providers today make available different versions of the videos, with each version encoded at a different rate. Multiple video versions, however, require more server storage and may also dramatically impact cache performance in a traditional cache or in a CDN server. An alternative to versions is layered encoding, which can also provide multiple quality levels. Layered encoding requires less server storage capacity and may be more suitable for caching; but it typically increases transmission bandwidth due to encoding overhead. In this paper we compare video streaming of multiple versions with that of multiple layers in a caching environment. We examine caching and distribution strategies that use both versions and layers. We consider two cases: the request distribution for the videos is known a priori; and adaptive caching, for which the request distribution is unknown. Our analytical and simulation results indicate that mixed distribution/caching strategies provide the best overall performance.A shorter version of this work has appeared in Proc. of IEEE International Conference on Multimedia and Expo (ICME), Vol. 2, pages 45–48, Lausanne, Switzerland, August 2002     

PopCap: popularity oriented proxy caching for peer-assisted Internet video-on-demand streaming services

With the success of Internet video-on-demand (VoD) streaming services, the bandwidth required and the financial cost incurred by the host of the video server becoming extremely large. Peer-to-peer (P2P) networks and proxies are two common ways for reducing the server workload. In this paper, we consider a peer-assisted Internet VoD system with proxies deployed at domain gateways. We formally present the video caching problem with the objectives of reducing the video server workload and avoiding inter-domain traffic, and we obtain its optimal solution. Inspired by theoretical analysis, we develop a practical protocol named PopCap for Internet VoD services. Compared with previous work, PopCap does not require additional infrastructure support, is inexpensive, and able to cope well with the characteristic workloads of Internet VoD services. From simulation-based experiments driven by real-world data sets from YouTube, we find that PopCap can effectively reduce the video server workload, therefore provides a superior performance regarding the video server’s traffic reduction.     

A weighted segment-based caching algorithm for video streaming objects over heterogeneous networking environments

Traditional caching technology is not applicable to cache video streaming objects over heterogeneous networking environments. The popularity of mobile devices in the heterogeneous networking environments make the access of Internet content become a common phenomenon. To support different mobile devices in the heterogeneity networking environments, a transcoding proxy is used to transcode different versions of the streaming videos according to clients’ requests. In this paper, we propose a weighted caching replace strategy for video streaming objects over heterogeneous networking environments. A new caching algorithm with static weight transcoding graph and dynamic caching relation tree is introduced. The proposed algorithm is compared with LRU, LFU, CP and PF cache algorithms in three parts: hit ratio, byte hit ratio, and average transmission delay. Experimental results show that the proposed algorithm outperforms than traditional LRU, LFU, CP and PF cache algorithms.     

Design a progressive video caching policy for video proxy servers

Proxy servers have been used to cache web objects to alleviate the load of the web servers and to reduce network congestion on the Internet. In this paper, a central video server is connected to a proxy server via wide area networks (WANs) and the proxy server can reach many clients via local area networks (LANs). We assume a video can be either entirely or partially cached in the proxy to reduce WAN bandwidth consumption. Since the storage space and the sustained disk I/O bandwidth are limited resources in the proxy, how to efficiently utilize these resources to maximize the WAN bandwidth reduction is an important issue. We design a progressive video caching policy in which each video can be cached at several levels corresponding to cached data sizes and required WAN bandwidths. For a video, the proxy server determines to cache a smaller amount of data at a lower level or to gradually accumulate more data to reach a higher level. The proposed progressive caching policy allows the proxy to adjust caching amount for each video based on its resource condition and the user access pattern. We investigate the scenarios in which the access pattern is priorly known or unknown and the effectiveness of the caching policy is evaluated.     

网络相关媒体流缓存管理策略NRC

Internet网络上媒体流的应用受限于应用时的网络状况，如时延、包丢失率等．本文研究通过在网络边缘处设置缓存代理来减小这些影响，提出一种新的缓存管理算法NRC，即接入媒体流服务时，用户以两种方式获取媒体流对象：一部分对象内容从代理缓存中获取，而另一部分对象内容则直接从流媒体源服务器处传输而来；从而加速媒体流接入服务，提高媒体流服务质量,算法同网络特性和媒体流特性相关．最后仿真实验证实同网络和流媒体特性相关的缓存管理算法NRC可以很好地减少服务延迟和提高媒体流的总体服务质量．     

Coordinated multimedia object replacement in transcoding proxies

Finding replacement candidates for accommodating a new object is an important research issue in web caching. Due to the new emerging factors in the transcoding proxy and the aggregate effect of caching multiple versions of the same multimedia object, this problem becomes more important and complex as audio and video applications have proliferated over the Internet, especially in the environment of mobile computing systems. This paper addresses coordinated cache replacement in transcoding proxies. First, we propose an original model which determines cache replacement candidates on all candidate nodes in a coordinated fashion with the objective of minimizing the total cost loss for linear topology. We formulate this problem as an optimization problem and present a low-cost optimal solution for deciding cache replacement candidates. Second, we extend this problem to solve the same problem for tree networks. Finally, we conduct extensive simulations to evaluate the performance of our solutions by comparing with existing models.     

A multikey secure multimedia proxy using asymmetric reversible parametric sequences: theory, design, and implementation

Because of limited server and network capacities for streaming applications, multimedia proxies are commonly used to cache multimedia objects such that, by accessing nearby proxies, clients can enjoy a smaller start-up latency and receive a better quality-of-service (QoS) guarantee-for example, reduced packet loss and delay jitters for their requests. However, the use of multimedia proxies increases the risk that multimedia data are exposed to unauthorized access by intruders. In this paper, we present a framework for implementing a secure multimedia proxy system for audio and video streaming applications. The framework employs a notion of asymmetric reversible parametric sequence (ARPS) to provide the following security properties: i) data confidentiality during transmission, ii) end-to-end data confidentiality, iii) data confidentiality against proxy intruders, and iv) data confidentiality against member collusion. Our framework is grounded on a multikey RSA technique such that system resilience against attacks is provably strong given standard computability assumptions. One important feature of our proposed scheme is that clients only need to perform a single decryption operation to recover the original data even though the data packets may have been encrypted by multiple proxies along the delivery path. We also propose the use of a set of encryption configuration parameters (ECP) to trade off proxy encryption throughput against the presentation quality of audio/video obtained by unauthorized parties. Implementation results show that we can simultaneously achieve high encryption throughput and extremely low video quality (in terms of peak signal-to-noise ratio and visual quality of decoded video frames) for unauthorized access.     

一种流媒体CDN的内容部分推送策略

在流媒体CDN中,采用“推拉”结合的流媒体内容分发方式可进一步减少客户的平均启动时间,降低网络资源的消耗,根据流媒体文件的部分访问特性给出了一种流媒体内容的部分推送策略,并利用源服务器统计的历史访问信息,提出了目的代理缓存服务器的一种随机选择算法。实验表明该策略及算法可提高系统性能。     

Metadata-driven optimal transcoding in a multimedia proxy

An adaptive multimedia proxy is presented which provides (1) caching, (2) filtering, and (3) media gateway functionalities. The proxy can perform media adaptation on its own, either relying on layered coding or using transcoding mainly in the decompressed domain. A cost model is presented which incorporates user requirements, terminal capabilities, and video variations in one formula. Based on this model, the proxy acts as a general broker of different user requirements and of different video variations. This is a first step towards What You Need is What You Get (WYNIWYG) video services, which deliver videos to users in exactly the quality they need and are willing to pay for. The MPEG-7 and MPEG-21 standards enable this in an interoperable way. A detailed evaluation based on a series of simulation runs is provided.     

Efficient data distribution in a Web server farm

High-performance Web sites rely on Web server `farms', hundreds of computers serving the same content, for scalability, reliability, and low-latency access to Internet content. Deploying these scalable farms typically requires the power of distributed or clustered file systems. Building Web server farms on file systems complements hierarchical proxy caching. Proxy caching replicates Web content throughout the Internet, thereby reducing latency from network delays and off-loading traffic from the primary servers. Web server farms scale resources at a single site, reducing latency from queuing delays. Both technologies are essential when building a high-performance infrastructure for content delivery. The authors present a cache consistency model and locking protocol customized for file systems that are used as scalable infrastructure for Web server farms. The protocol takes advantage of the Web's relaxed consistency semantics to reduce latencies and network overhead. Our hybrid approach preserves strong consistency for concurrent write sharing with time-based consistency and push caching for readers (Web servers). Using simulation, we compare our approach to the Andrew file system and the sequential consistency file system protocols we propose to replace     

流媒体对象的缓存管理策略

基于流媒体服务的代理技术是流媒体研究领域中的重要课题.随着流媒体技术在Internet和无线网络环境中的高速发展,对流媒体代理服务器的研究也正在逐步深入.本文主要讨论通过代理技术改善媒体的服务质量,降低媒体的传输延迟以及减轻网络负载.在Internet环境下,对流媒体代理服务器的研究集中于流媒体的访问特性、缓存替换算法,构建和实现一个流媒体代理服务器是对流媒体代理技术研究的基础.     

An experimental analysis of digital video library servers

Much work on video servers has concentrated on movies on demand, in which a relatively small number of titles are viewed and users are given basic VCR-style controls. This paper concentrates on analyzing video server performance for non-linear access applications. In particular, we study two non-linear video applications: video libraries, in which users select from a large collection of videos and may be interested in viewing only a small part of the title; and video walk-throughs, in which users can move through an image-mapped representation of a space. We present a characterization of the workloads of these applications. Our simulation studies show that video server architectures developed for movies on demand can be adapted to video library usage, though caching is less effective and the server can support a smaller user population for non-linear video applications. We also show that video walk-throughs require extremely large amounts of RAM buffering to provide adequate performance for even a small number of users.     

Hybrid storage-based caching strategy for content delivery network services

This study proposes a novel caching scheme for content delivery network services. In general, video content users often watch the first part of video clips and then switch to other content. Therefore, a caching scheme is proposed, in which the first part of the frequently referenced content is stored on a solid state drive (SSD) while the remaining video content is stored on a hard disk drive (HDD),. The proposed hybrid (SSD/HDD) caching scheme offers several benefits, such as an improved average data output capacity due to the high average data rate and average hit capacity of the SSD. That is, performance can be significantly improved at a low extra cost with the cache server of a content delivery network (CDN).