Konstantinos Stamos

CDNsim: A simulation tool for content distribution networks

Content distribution networks (CDNs) have gained considerable attention in the past few years. Hence there is need for developing frameworks for carrying out CDN simulations. In this article we present a modeling and simulation framework for CDNs, called CDNsim. CDNsim has been designated to provide a realistic simulation for CDNs, simulating the surrogate servers, the TCP/IP protocol, and the main CDN functions. The main advantages of this tool are its high performance, its extensibility, and its user interface, which is used to configure its parameters. CDNsim provides an automated environment for conducting experiments and extracting client, server, and network statistics. The purpose of CDNsim is to be used as a testbed for CDN evaluation and experimentation. This is quite useful to both the research community (to experiment with new CDN data management techniques), and for CDN developers (to evaluate profits on prior certain CDN installations).

Prefetching in Content Distribution Networks via Web Communities Identification and Outsourcing

Content distribution networks (CDNs) improve scalability and reliability, by replicating content to the “edge” of the Internet. Apart from the pure networking issues of the CDNs relevant to the establishment of the infrastructure, some very crucial data management issues must be resolved to exploit the full potential of CDNs to reduce the “last mile” latencies. A very important issue is the selection of the content to be prefetched to the CDN servers. All the approaches developed so far, assume the existence of adequate content popularity statistics to drive the prefetch decisions. Such information though, is not always available, or it is extremely volatile, turning such methods problematic. To address this issue, we develop self-adaptive techniques to select the outsourced content in a CDN infrastructure, which requires no apriori knowledge of request statistics. We identify clusters of “correlated” Web pages in a site, called Web site communities, and make these communities the basic outsourcing unit. Through a detailed simulation environment, using both real and synthetic data, we show that the proposed techniques are very robust and effective in reducing the user-perceived latency, performing very close to an unfeasible, off-line policy, which has full knowledge of the content popularity.

A latency-based object placement approach in content distribution networks

Content distribution networks (CDNs) are increasingly being used to disseminate data in todays Internet. The growing interest in CDNs is motivated by a common problem across disciplines: how does one reduce the load on the origin server and the traffic on the Internet, and ultimately improve response time to users? In this direction, crucial data management issues should be addressed. A very important issue is the optimal placement of the outsourced content to CDNs servers. Taking into account that this problem is NP complete, a heuristic method should be developed. All the approaches developed so far assume the existence of adequate popularity statistics. Such information though, is not always available, or it is extremely volatile, turning such methods problematic. This paper develops a network-adaptive, non-parameterized technique to place the outsourced content to CDNs servers, which requires no a-priori knowledge of request statistics. We place the outsourced objects to these servers with respect to the network latency that each object produces. Through a detailed simulation environment, using both real and synthetic data, we show that the proposed technique can yield up to 25% reduction in user-perceived latency, compared with other heuristic schemes which have knowledge of the content popularity.

Evaluating the utility of content delivery networks

Content Delivery Networks (CDNs) balance costs and quality in services related to content delivery. This has urged many Web entrepreneurs to make contracts with CDNs. In the literature, a wide range of techniques has been developed, implemented and standardized for improving the performance of CDNs. The ultimate goal of all the approaches is to improve the utility of CDN surrogate servers. In this paper we define a metric which measures the utility of CDN surrogate servers, called CDN utility. This metric captures the traffic activity in a CDN, expressing the usefulness of surrogate servers in terms of data circulation in the network. Through an extensive simulation testbed, we identify the parameters that affect the CDN utility in such infrastructures. We evaluate the utility of surrogate servers under various parameters and provide insightful comments.

Replication Based on Objects Load under a Content Distribution Network

Users tend to use the Internet for “resource-hungry” applications (which involve content such as video, audio on-demand and distributed data) and at the same time, more and more applications (such as e-commerce, elearning etc.) are relying on the Web. In this framework, Content Distribution Networks (CDNs) are increasingly being used to disseminate data in todays Internet, providing a delicate balance between costs for Web content providers and quality of services for Web customers. The growing interest in CDNs is motivated by a common problem across disciplines: how does one reduce the load on the origin server and the traffic on the Internet, and ultimately improve response time to users? In this direction, crucial data management issues should be addressed. A very important issue is the optimal placement of the outsourced content to CDN’s servers. Taking into account that this problem is NP complete, an heuristic method should be developed. All the approaches developed so far either take as criterion the network’s latency or the workload. This paper develops a novel technique to place the outsourced content to CDN’s servers, integrating both the latency and the load. Through a detailed simulation environment, using both real and synthetic data, we show that the proposed method can improve significantly the response time of requests while keeping the CDNs’ servers’ load at a very low level.

A similarity based approach for integrated Web caching and content replication in CDNs

Web caching and content replication techniques emerged to solve performance problems related to the Web. We propose a generic non-parametric heuristic method that integrates both techniques under a CDN. We provide experimentation showing that our method outperforms the so far separate implementations of Web caching and content replication. Moreover, we show that the performance improvement compared with an existing algorithm is significant. We test all these techniques in a simulation environment under a flash crowd event and a workload of a typical light-weighted CDN operation

Caching Techniques on CDN Simulated Frameworks

It is evident that in the new Web era, content volume and services availability play a major role, leaving behind typical static pages which have solely text and images. The majority of the business oriented service providers are concerned for the Quality of Services (QoS), in terms of content delivery. In this context, proxy servers and Content Delivery Networks (CDNs) have been prosposed as different technologies, dealing with this concern. Their common goal is to bring content close to the users, reducing the response time. Both technologies demonstrate different advantages and disadvantages. CDNs are characterized by robustness in serving huge amounts of requests and content volumes. However, their main shortcoming is that due to replication and distribution cost, replica placements should be static for a large amount of time. This leads to unoptimized storage capacity usage since the surrogate servers would contain redundant, possibly outdated, or unwanted content. On the other hand, proxy servers adapt content caching according to varying access patterns, using cache replacement algorithms. However, proxy servers do not scale well for serving large volumes of data or user populations. In an effort to combine the advantages of both, earlier recent work [2, 20, 29, 30] investigated different approaches that enable Web caching in CDNs, taking proxy servers’ characteristics into account. As new caching ideas emerge, the need for a CDN testbed, suitable for performance evaluation and stress testing, becomes evident. Such a testbed should provide a

Utilization-aware redirection policy in CDN: a case for energy conservation

Due to the gradual and rapid increase in Information and Communication Technology (ICT) industry, it is very important to introduce energy efficient techniques and infrastructures in large scale distributed systems. Content Distribution Networks (CDNs) are one of these popular systems which try to make the contents closer to the widely dispersed Internet users. A Content Distribution Network provides its services by using a number of surrogate servers geographically distributed in the web. Surrogate servers have the copies of the original contents belonging to the origin server, depending on their storage capacity. When a client requests for some particular contents from a surrogate server, either this request can be fulfilled directly by it or in case of absence of the requested contents, surrogate servers cooperate with each other or with the origin server. In this paper, our focus is on the surrogate servers utilization and using it as a parameter to conserve energy in CDNs while trying to maintain an acceptable Quality of Experience (QoE).

Mani-Web: Large-Scale Web Graph Embedding via Laplacian Eigenmap Approximation

The Web as a graph can be embedded in a low-dimensional space where its geometry can be visualized and studied in order to mine interesting patterns such as web communities. The existing algorithms operate on small-to-medium-scale graphs; thus, we propose a close to linear time algorithm called Mani-Web suitable for large-scale graphs. The result is similar to the one produced by the manifold-learning technique Laplacian eigenmap that is tested on artificial manifolds and real web-graphs. Mani-Web can also be used as a general-purpose manifold-learning/dimensionality-reduction technique as long as the data can be represented as a graph.

Dynamic Code Generation for Cultural Content Management

Digital repositories are popular means for preserving, restoring, and indexing archaeological and cultural content. They provide the base for development of a fauna of related applications including virtual tours and data management. Common difficulties such as the ever changing software specifications from domain experts make this task challenging as the alterations of the database schema lead to massive code rewrites. Within this context we propose and implement in practice a model for automated code generation building essentially a content management application by traversing a custom tree-based ERschema.