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.     

Frame Selection for Dynamic Caching Adjustment in Video Proxy Servers

By caching video data, a video proxy server close to the clients can be used to assist video delivery and alleviate the load of video servers. We assume a video can be partially cached and a certain number of video frames are stored in the proxy server. In our setting, the proxy server is allowed to cache the passing data from the video server. A video provides several options (levels) in terms of bandwidth requirement over the server-proxy path. For each video, the proxy server decides to cache a smaller amount of data at a lower level or to accumulate more data to reach a higher level. The proxy server can dynamically adjust the cached video data by choosing an appropriate level based on the network condition or the popularity of the video. We propose a frame selection scheme, Dynamic Chunk Algorithm, to determine which frames are to be cached in the proxy server for the dynamic caching adjustment scenario. The algorithm guarantees the rate constraint over the server-proxy path to be satisfied for each level. This approach also maintains the set of cached frames at a higher level as a superset of the cached frames at a lower level. Hence, it enforces the proxy server to simply cache more data without dropping frames when it intends to reduce network bandwidth consumption for a video and vice versa.     

A case study of Web server benchmarking using parallel WAN emulation

This paper describes the use of a parallel discrete-event network emulator called the Internet Protocol Traffic and Network Emulator (IP-TNE) for Web server benchmarking. The experiments in this paper demonstrate the feasibility of high-performance wide area network (WAN) emulation using parallel discrete-event simulation (PDES) techniques on a single shared-memory multiprocessor. Our experiments with an Apache Web server achieve up to 8000 HTTP/1.1 transactions/s for static document retrieval across emulated WAN topologies with up to 4096 concurrent Web/TCP clients. The results show that WAN characteristics, including round-trip delays, packet losses, and bandwidth asymmetry, all have significant impacts on Web server performance, as do client protocol behaviors. WAN emulation using the IP-TNE enables stress testing and benchmarking of Web servers in ways that may not be possible in simple local area network (LAN) test scenarios.     

Loopback: exploiting collaborative caches for large-scale streaming

In this paper, we propose a Loopback approach in a two-level streaming architecture to exploit collaborative client/proxy buffers for improving the quality and efficiency of large-scale streaming applications. At the upper level we use a content delivery network (CDN) to deliver video from a central server to proxy servers. At the lower level a proxy server delivers video with the help of collaborative client caches. In particular, a proxy server and its clients in a local domain cache different portions of a video and form delivery loops. In each loop, a single video stream originates at the proxy, passes through a number of clients, and finally is passed back to the proxy. As a result, with limited bandwidth and storage space contributed by collaborative clients, we are able to significantly reduce the required network bandwidth, I/O bandwidth, and cache space of a proxy. Furthermore, we develop a local repair scheme to address the client failure issue for enhancing service quality and eliminating most required repairing load at the central server. For popular videos, our local repair scheme is able to handle most of single-client failures without service disruption and retransmissions from the central server. Our analysis and simulations have shown the effectiveness of the proposed scheme.     

Performance analysis of a pull-based parallel video server

In conventional video-on-demand systems, video data are stored in a video server for delivery to multiple receivers over a communications network. The video server's hardware limits the maximum storage capacity as well as the maximum number of video sessions that can concurrently be delivered. Clearly, these limits will eventually be exceeded by the growing need for better video quality and larger user population. This paper studies a parallel video server architecture that exploits server parallelism to achieve incremental scalability. First, unlike data partition and replication, the architecture employs data striping at the server level to achieve fine-grain load balancing across multiple servers. Second, a client-pull service model is employed to eliminate the need for interserver synchronization. Third, an admission-scheduling algorithm is proposed to further control the instantaneous load at each server so that linear scalability can be achieved. This paper analyzes the performance of the architecture by deriving bounds for server service delay, client buffer requirement, prefetch delay, and scheduling delay. These performance metrics and design tradeoffs are further evaluated using numerical examples. Our results show that the proposed parallel video server architecture can be linearly scaled up to more concurrent users simply by adding more servers and redistributing the video data among the servers     

Dynamic proxy-assisted scalable broadcasting of videos for heterogeneous environments

Periodic broadcasting (PB) is a scalable technique for providing video-on-demand services. It significantly reduces server I/O and backbone network bandwidth requirements at the expense of high storage space and high network bandwidth requirements for clients. Traditional protocols assume homogeneous clients with identical resources. Unfortunately, in practice clients have very different bandwidths, and these are usually insufficient to provide video-on-demand (VoD) service from a PB server. Existing work on heterogeneous clients has focused on devising broadcast schedules to cater to low-bandwidth clients, which inevitably requires an extra backbone network bandwidth between the server and the clients. In our previous work, we proposed to use proxies residing at the edge of backbone network to accommodate low bandwidth clients for PB-based VoD services. The server broadcasts a video using a PB protocol while the proxy receives and stores the data in its local buffer and broadcasts the stored data to the clients in its local network. It significantly reduces the waiting time of low-bandwidth clients without requiring any extra backbone bandwidth by using a proxy buffer and channels. However, although lots of PB protocols have been proposed, the scheme can be applied only to some old PB protocols based on a pyramid protocol. In this paper, we propose a proxy-assisted PB system that can be generally applied to almost all the existing PB protocols, by dynamically managing buffer space and channels in proxy servers. Thus, with our proposed system, PB VoD system can be optimized in terms of the resource usages in backbone networks, proxy servers, and clients, by adopting more suitable PB protocols.     

基于P2P协作代理的广域网组播

提出了在广域网内以P2P协作的代理服务器群的VOD点播系统实现视频流的传输,主要设计目的是通过相互协作的代理服务器群以减轻服务器的压力,各代理服务器间实行P2P传输,实现资源共享.代理服务器在各局域网内实现组播,减少局域网内冗余数据量.     

Presentation planning for distributed VoD systems

A distributed video-on-demand (VoD) system is one where a collection of video data is located at dispersed sites across a computer network. In a single site environment, a local video server retrieves video data from its local storage device. However, in distributed VoD systems, when a customer requests a movie from the local server, the server may need to interact with other servers located across the network. In this paper, we present different types of presentation plans that a local server can construct in order to satisfy a customer request. Informally speaking, a presentation plan is a temporally synchronized sequence of steps that the local server must perform in order to present the requested movie to the customer. This involves obtaining commitments from other video servers, obtaining commitments from the network service provider, as well as making commitments of local resources, while keeping within the limitations of available bandwidth, available buffer, and customer data consumption rates. Furthermore, in order to evaluate the quality of a presentation plan, we introduce two measures of optimality for presentation plans: minimizing wait time for a customer and minimizing access bandwidth which, informally speaking, specifies how much network/disk bandwidth is used. We develop algorithms to compute three different optimal presentation plans that work at a block level, or at a segment level, or with a hybrid mix of the two, and compare their performance through simulation experiments. We have also mathematically proven effects of increased buffer or bandwidth and data replications for presentation plans which had previously been verified experimentally in the literature.     

A distributed admission control model for QoS assurance in large-scale media delivery systems

Conventional admission control models incur some performance penalty. First, admission control computation can overload a server that is already heavily loaded. Also, in large-scale media systems with geographically distributed server clusters, performing admission control on each cluster can result in long response latency, if the client request is denied at one site and has to be forwarded to another site. Furthermore, in prefix caching, initial frames cached at the proxy are delivered to the client before the admission decisions are made. If the media server is heavily loaded and, finally, has to deny the client request, forwarding a large number of initial frames is a waste of critical network resources. In this paper, a novel distributed admission control model is presented. We make use of proxy servers to perform the admission control tasks. Each proxy hosts an agent to coordinate the effort. Agents reserve media server's disk bandwidth and make admission decisions autonomously based on the allocated disk bandwidth. We develop an effective game theoretic framework to achieve fairness in the bandwidth allocation among the agents. To improve the overall bandwidth utilization, we also consider an aggressive admission control policy where each agent may admit more requests than its allocated bandwidth allows. The distributed admission control approach provides the solution to the stated problems incurred in conventional admission control models. Experimental studies show that our algorithms significantly reduce the response latency and the media server load.     

Proxy-assisted scalable periodic broadcasting of videos for heterogeneous clients

Periodic broadcasting (PB) is a scalable technique for providing video-on-demand services. It significantly reduces server input and output (I/O) and backbone network bandwidth requirements, but increases the clients’ need for storage space and network bandwidth. Traditional protocols assume homogeneous clients with identical resources. In practice, however, clients have very different bandwidths, which are usually not sufficient for video-on-demand service from a PB server. Existing work on heterogeneous clients has focused on devising broadcast schedules that cater to low-bandwidth clients; these schedules inevitably require additional backbone network bandwidth between the server and the clients. In this paper, we propose a scheme to significantly reduce the waiting time of all heterogeneous clients, without the need for any additional backbone bandwidth. This scheme uses a proxy buffer within video-on-demand systems using PB. In the proposed system, the server broadcasts a video using one of the traditional PB protocols. Simultaneously, the proxy receives the stream from the server and stores it in its local buffer, then broadcasts the stored data to the clients in its local network. Because the proxy provides extra, transparent channels to the server, clients are likely to reduce their reception bandwidth requirements through the use of efficient reception schedules using the extra channels.     

图像转换代理服务器及其在Internet中的应用

在Web服务器和客户机之间设置代理服务器，可以使不同性能的客户机浏览Internet的内容。通过分析图像在网页源文件中的表示方法，可以推断出图像在网页中的意义。根据图像在网页中的意义、客户机的性能、网络带宽和用户设置，代理服务器采用不同的转换方法和策略进行图像的转换，对图像重新编码，使转换后的图像适应于不同的客户机。     

Performance evaluation of smoothing algorithms for transmittingprerecorded variable-bit-rate video

The transfer of prerecorded, compressed variable-bit-rate video requires multimedia services to support large fluctuations in bandwidth requirements on multiple time scales. Bandwidth smoothing techniques can reduce the burstiness of a variable-bit-rate stream by transmitting data at a series of fixed rates, simplifying the allocation of resources in video servers and the communication network. This paper compares the transmission schedules generated by the various smoothing algorithms, based on a collection of metrics that relate directly to the server, network, and client resources necessary for the transmission, transport, and playback of prerecorded video. Using MPEG-1 and MJPEG video data and a range of client buffer sizes, we investigate the interplay between the performance metrics and the smoothing algorithms. The results highlight the unique strengths and weaknesses of each bandwidth smoothing algorithm, as well as the characteristics of a diverse set of video clips     

Throughput optimization for video streaming proxy servers based on video staging

A video streaming proxy server needs to handle hundreds of simultaneous connections between media servers and clients. Inside, every video arrived at the server and delivered from it follows a specific arrival and delivery schedule. While arrival schedules compete for incoming network bandwidth, delivery schedules compete for outgoing network bandwidth. As a result, a proxy server has to provide sufficient buffer and disk cache for storage, together with memory space, disk space and disk bandwidth. In order to optimize the throughput, a proxy server has to govern the usage of these resources. In this paper, we first analyze the property of a traditional smoothing algorithm and a video staging algorithm. Then we develop, based on the smoothing algorithm, a video staging algorithm for video streaming proxy servers. This algorithm allows us to devise an arrival schedule based on the delivery schedule. Under this arrival and delivery schedule pair, we can achieve a better resource utilization rate gracefully between different parameter sets. It is also interesting to note that the usage of the resources such as network bandwidth, disk bandwidth and memory space becomes interchangeable. It provides the basis for inter-resource scheduling to further improve the throughput of a video streaming proxy server system.

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.     

Fast proxy delivery of multiple streaming sessions in shared running buffers

With the falling price of memory, an increasing number of multimedia servers and proxies are now equipped with a large memory space. Caching media objects in the memory of a proxy helps to reduce the network traffic, the disk I/O bandwidth requirement, and the data delivery latency. The running buffer approach and its alternatives are representative techniques to caching streaming data in the memory. There are two limits in the existing techniques. First, although multiple running buffers for the same media object co-exist in a given processing period, data sharing among multiple buffers is not considered. Second, user access patterns are not insightfully considered in the buffer management. In this paper, we propose two techniques based on shared running buffers in the proxy to address these limits. Considering user access patterns and characteristics of the requested media objects, our techniques adaptively allocate memory buffers to fully utilize the currently buffered data of streaming sessions, with the aim to reduce both the server load and the network traffic. Experimentally comparing with several existing techniques, we show that the proposed techniques achieve significant performance improvement by effectively using the shared running buffers.     

一种基于Sever和Proxy流媒体流行性的Caching策略

1 概述经过了2000、2001两年的社区宽带网建设的高速发展后,摆在中国ISP们面前的任务是如何在已建成的宽带网上开展增值服务,许多ISP尝试在宽带网上开展流媒体(Streaming Media)服务,如视频点播VOD(Video On-Demand)系统。然而,流媒体对网络带宽和实时性的要求使得流服务器必须能够进行端对端(End-to-End)的拥塞控制和质量调整,由于     

种基于Sever和Proxy流媒体流行性的Caching策略

It is expected that by 2003, continuous media will account for more than 50% of the data available on origin servers. This will provoke a significant change in Internet workload, due to the high bandwidth requirements and the long-lived nature of digital video, streaming server loads and network bandwidths are proving to be major limiting factors. Aiming at the characteristic of broadband network in a residential area, we propose a popularitybased on server-proxy caching strategy for streaming media. According to a streaming media popularity on streaming server and proxy, this strategy caches the content of this streaming media partially or completely, and plays an important role in decreasing server load, reducing the traffic from streaming server to proxy, and improving the startup latency of the client.     

Predictive buffer control in delivering remotely stored video using proxy servers

We study the problem of using proxy servers to stream video stored at a geographically separate location. The separation of the server and the storage introduces a non-negligible delay in retrieving video frames in real time. We assume an additive-increase/multiplicative-decrease transport protocol to support the streaming process and develop an effective scheme to achieve consistent, high streaming quality. The heart of the scheme is the control of buffer occupancy at the proxy server. We model the buffer as a bilinear dynamical system disturbed by a point process with stochastic state-dependent intensity. We first develop a buffer controller that does not exploit this model. Then, using the buffer model, we construct a second controller based on an optimal-control analysis for the case without retrieval delay. Extending these two controllers, we subsequently synthesize two controllers based on prediction of future system states using the model, taking into account both the delay and the state-dependent disturbance intensity. Our empirical study illustrates the effectiveness of the streaming scheme. We further find that the controllers exploiting the buffer model demonstrate performance significantly superior to that of the model-free controller in overcoming the adverse impact of the retrieval delay. We also conduct limited experiments to study the impact of variable retrieval delays.     

Efficient Selective Frame Discard Algorithms for Stored Video Delivery Across Resource Constrained Networks

Video delivery from a server to a client across a network is an important component of many multimedia applications. While delivering a video stream across a resource constrained network, loss of frames may be unavoidable. Under such circumstances, it is desirable to find a server transmission schedule that can efficiently utilize the network resources while maximizing the perceived quality-of-service (QoS) at the client. To address this issue, in this paper we introduce the notion ofselective frame discard at the server and formulate the optimal selective frame discard problem using a QoS-based cost function. Given network bandwidth and client buffer constraints, we develop an O (N log N) algorithm to find the minimum number of frames that must be discarded in order to meet these constraints. The correctness of the algorithm is also formally established. We present a dynamic programming based algorithm for solving the problem of optimal selective frame discard. Since the computational complexity of the optimal algorithm is prohibitively high in general, we also develop several efficient heuristic algorithms for selective frame discard. These algorithms are evaluated using JPEG and MPEG video traces.     

Providing Controlled Quality Assurance for Streaming Stored-Videos Across the Internet Using VPNs

Video Streaming across wide-area networks is one of the most important applications on the Internet. In this paper we focus on the quality assurance issue on best-effort networks and propose a practical technique, named staggered two-flow video streaming. We deliver a stored video through two separate flows in a staggered fashion via a VPN pipe from a central server to a proxy server. One flow containing the essential portion of the video is delivered using a novel controlled TCP (cTCP), and the other flow containing the enhanced portion of the video is transmitted using a rate-controlled RTP/UDP (rUDP). To provide video-quality assurance in such a system, we design several application-aware flow-control and adaptation approaches to control bandwidth sharing and interactions among flows by exploiting the inherent priority structure in videos, the storage space on proxy servers and the coarse-grain bandwidth assurance of VPN. Our experiments using FreeBSD and simulations on NS2 both have demonstrated the efficacy of the proposed technique in protecting essential data and significantly reducing the numbers of packets retransmitted/lost in transmission and the sizes of video prefixes required on proxy servers. In summary, our application-aware approach provides stable and predictable performance in streaming videos across wide-area best-effort networks. In addition, another salient feature of our approach is that it requires no changes on the client-receiving side and minimal changes on the server-sending side.