On the use of Reliable Multicast for Content Distribution Vassilis Hatzigiannakis,
[email protected] Dimitrios Kalogeras,
[email protected] Panagiotis Astithas,
[email protected] Basileios Maglaris,
[email protected] Network Management Center (NOC) and Network Management & Optimal Design Laboratory (NETMODE), National Technical University of Athens, Iroon Polytexneiou 9, 15780 Zografou, Greece Keywords: Content Distribution, Reliable multicast, DNS redirection Short Abstract Content Distribution Networks (CDNs) are used to deliver content inexpensively in terms of network bandwidth and computational resources. The typical CDN architecture consists of an origin server and at least one level of content replication or caching servers. When content is replicated, CDN operation breaks up into two different stages: cache server synchronization and content delivery to the client. Our goal is to optimize the server synchronization process in terms of network bandwidth and computational resources. The protocol is based on multicast for the control and signaling, and on reliable multicast for the transmission of data. Content delivery to the clients is optimized through a DNS redirection mechanism. The design proposed was implemented within the Greek School Network (EDUNet). Extended Abstract Content Distribution Networks (CDNs) are used to deliver content inexpensively in terms of network bandwidth and computational resources. Content includes any form of data served via web: static web pages, streaming media and even file collections such as an OS distribution. The typical CDN consists of an origin server and one or more levels of content replication/caching servers. The way content propagates through the CDN depends on whether replication or caching is used. If replication is chosen, new content is pushed from the root of the hierarchy to the final nodes. In the second case, the update process is not launched from the origin server; every node pulls content on-demand from its parent and caches it. When clients request data, they are transparently redirected to their adjacent server, thus optimizing network performance and response time. We propose a design for reliable distribution of content within a large community of users based on a synchronization protocol that uses multicast for both signaling and transmission of content. As in every CDN, there are two levels in our approach, the server and the end-user synchronization. The content is uploaded to one of the distributed peer content servers, i.e. any server may play the role of the origin point. The peer servers are automatically informed for the new material and the content propagates through reliable multicast. Delivery to the end-user is performed through DNS redirection. Each client is transparently redirected to an adjacent server for faster downloading speed and load balancing. The mechanism relies on DNS Views introduced in BIND 9 [10] . Notifications for new content (advertisements) are propagated from the origin servers to their peers as XML messages via multicast. Content propagation is also performed through multicast, but it employs an acknowledgement mechanism and transport level
reliability (FEC) [12]. The acknowledgement messages provide a progress indication and a means for the origin server to be aware of all the receiving peers, so that content transmission is terminated when the propagation is complete throughout the CDN. The proposed design does not require a master server; any of the peer servers can play this role. This has the advantage of avoiding the single point of failure vulnerability, and allows the parallel uploading of new content to multiple origin servers (faster content propagation within the CDN). There is also a mechanism that prevents conflicts in the cache of the servers produced by uploading the same file in more than one server. We developed a prototype of the peer server that implements the proposed server synchronization design. Our prototype is implemented in JAVA in order to be platform independent, using the standard SUN libraries for XML parsing (JAXP) and multicast (java.net package). The module that handles file transfer is based on MCLv2.99[15], a software library based on ALC [13] and FEC that provides the API for reliable multicast bulk data transfers. We chose this library because of its stability and the fact that ALC does not require feedback from the receivers, thus providing good scalability. We enhanced and deployed the prototype within the Greek school network (EDUNet) [16] for providing a content delivery service. EDUNet needs a CDN for distributing content, mainly documents and patches, to all schools throughout Greece. To this purpose we deployed the prototype on fourteen core servers of the EDUNet infrastructure. The location of these peer servers was defined according to organizational structure and load balancing issues. We also implemented the mechanism for uploading content to any of the servers through a web application (JAVA-based application deployed on Tomcat that uses SSL encryption and certificate-based authentication). We are currently examining the possibility of extending multicast communication to end-user delivery. This would require availability of multicast throughout the network and client software for receiving notifications and FEC-coded data. Multicast delivery to the client would automate the process (“push model”), and protect against request network flooding and server bottlenecks caused by large numbers of client unicast connections.
Acknowledgements We would like to thank people from EDUNeT for giving us the opportunity to implement, test and use our design for the Greek school network. This work was founded by the Greek Ministry of Education and the European Union. References [1] Balachander
Krishnamurthy, Craig Wills, Yin Zhang, “On the Use and Performance of Content Distribution Networks”, ACM SIGCOMM INTERNET MEASUREMENT WORKSHOP 2001 [2] M.
Baentsch et al., “Enhancing the Web’s Infrastructure:From Caching to Replication,” IEEE Internet Computing,vol. 1, no. 2, 1997, pp. 18–27. [3] Athena
Vakali and George Palis, “Content Delivery Networks: Status and Trends”, Ieee Internet Computing Magazine November-December 2003 Jung, Balachander Krishnamurthy and Michael Rabinovich, “ Flash Crowds and Denial of Service Attacks: Characterization and Implications for CDNs and Web Sites” [4] Jaeyeon
[5] Yan
Chen, Randy H. Katz and John D. Kubiatowicz, “Dynamic Replica Placement for Scalable Content Delivery” Qiu, V.N. Padmanabhan and G.M. Voelker, “ On the placement of web server replicas”, Proceedings of IEEE Infocom April 2001. [6] L.
[7] Jussi
Kangasharju, James Roberts and Keith W. Ross, “Object Replication Strategies in Content Distribution Networks” W. Byers, Michael Luby Michael Mitzenmacher, “Accesing Multiple Mirror Sites in Parallel: Using Tornado codes to Speed Up Downloads” [8] John
[9] Stefan
Saroiu, Krishna P. Gummadi, Richard J. Dunn, Steven D. Gribble, and Henry M. Levy, “An Analysis of Internet Content Delivery Systems” [10]
Internet Software
Consortium,
Berkeley
Internet
Name
Domain
(BIND),
http://www.isc.org/products/BIND/. [11]
Christian Maihofer and Kurt Rothermel, A Delay Analysis of Reliable Multicast Protocols
[12]
RFC 3453 - The Use of Forward Error Correction (FEC) in Reliable Multicast
[13]
RFC 3450 - Asynchronous Layered Coding (ALC) Protocol Instantiation
[14]
Internet draft, TESLA: Multicast Source Authentication Transform Introduction
[15]
MCLv3 Home Page www.inrialpes.fr/planete/people/roca/mcl/mcl.html
[16]
EduNet, The greek School Network, http://www.sch.gr/
Author Biographies: Professor Basileios Maglaris was born in Athens in 1952 and is the founder and director of the Network Management and Optimal Design Laboratory at NTUA since 1989. He received the Diploma in Mechanical & Electrical Engineering from the National Technical University of Athens (NTUA), Greece in 1974, the M.Sc. in Electrical Engineering from the Polytechnic Institute of Brooklyn, New York in 1975 and the Ph.D. degree in Electrical Engineering & Computer Science from Columbia University, New York in 1979. In 1981 he joined the Department of Electrical Engineering at Columbia Polytechnic University, where he was promoted to Associate (tenured) Professor, and in 1989 he joined the faculty of the Electrical & Computer Engineering Department of the NTUA, where he is Professor of Computer Science. He has supervised many post-graduate students who obtained Ph.D. Degrees both in the United States and Greece. He delivered many lectures to industrial and academic audiences, authored more than fifty research papers and participates in Greek and European strategic planning committees. Since 1998 he is the Chairman of GRNET, a state owned corporation founded by the Greek Ministry of Development. Dr. Dimitrios Kalogeras was born in Athens in 1967. He graduated from the Department of Electrical and Computer Engineering of the National Technical University of Athens (NTUA) in 1991 and in 1996 he acquired the Doctoral diploma from the same department. Dr Kalogeras has participated in numerous research programs of the EC and the General Secretariat of Research and Technology in Greece. He has pioneered in the design and development of the NTUA and the GRNET data networks and is a member of the technical and scientific committee of the Greek School Network. Dr Kalogeras is a consultant on issues on networking and
video signal processing. He is also the author and coauthor of publications in international magazines and proceedings of numerous conferences. From 2000 to 2002 he has served as a member of the Terena Technical Committee. Dr. Panagiotis Astithas was born in 1972 in Athens, Greece. He obtained his Diploma in Electrical and Computer Engineering from the Electrical & Computer Engineering Department of the National Technical University of Athens (NTUA) in 1997. In 2001 he obtained his PhD from the Computer Science Division of the same department. He is currently affiliated with the Network Management Center at NTUA and serves as a member of the technical committee of the Greek School Network. He has extensive experience in computer security, middleware, network management, network services, operating systems, distributed multi-tier applications and computer languages. Vassilis Chatzigiannakis is a PhD Student at the National Technical University of Athens (NTUA). He works in the Network Operations Center in NTUA, is a researcher in the NETwork Management and Optimal DEsign laboratory (NETMODE) of the National Technical University of Athens. His research interests include network management, network security and distributed systems. He has a Diploma degree in Electrical and Computer Engineering from National Technical University of Athens.
