Andy Myers

Resilient multicast support for continuous-media applications

The IP multicast delivery mechanism provides a popular basis for delivery of continuous media to many participants in a conferencing application. However, the best-effort nature of multicast delivery results in poor playback quality in the presence of network congestion and packet loss. Contrary to widespread belief that the real-time nature of continuous media applications precludes the possibility of recovery of lost packets using retransmissions, we have found that these applications offer an interesting tradeoff between the desired playback quality and the desired degree of interactivity. In particular, we propose a new model of multicast delivery called resilient multicast in which each receiver in a multicast group can decide its own tradeoff between reliability and real-time requirements. To be effective, error recovery mechanisms in such a model need to be both fast (due to the real-time constraint) and have a low overhead (due to high volume of continuous media data). We have designed a resilient multicast protocol called STORM (STructure-Oriented Resilient Multicast) in which senders and receivers collaborate to recover from lost packets using two key ideas. First, group participants self-organize themselves into a distribution structure and use the structure to recover lost packets from adjacent nodes. Second, the distribution structure is dynamic and a lightweight algorithm is used to adapt the structure to changing network traffic conditions and group membership. We have implemented STORM in both VAT and a packet level simulator. Experimental results using both the MBONE and a simulation model demonstrate the effectiveness of our approach.

Performance characteristics of mirror servers on the Internet

As a growing number of Web sites introduce mirrors to increase throughput, the challenge for clients is determining which mirror will offer the best performance when a document is to be retrieved. We present findings from measuring 9 clients scattered throughout the United States retrieving over 490,000 documents from 47 production Web servers which mirror three different Web sites. We have several interesting findings that may aid in the design of protocols for choosing among mirror servers. Though server performance varies widely, we have observed that a servers performance relative to other servers is more stable and is independent of time scale. In addition, a change in an individual servers transfer time is not a strong indicator that its performance relative to other servers has changed. Finally, we have found that clients wishing to achieve near-optimal performance may only need to consider a small number of servers rather than all mirrors of a particular site.

A third-party value-added network service approach to reliable multicast

We propose a third-party value-added services framework for enhancing the performance of reliable multicast protocols. In this framework, a value-added service provider will place servers called waypoints throughout ISPs’ networks. Waypoints run a fully distributed, dynamic algorithm to determine which multicast groups to join. Having joined a group, a waypoint participates in the error recovery protocol, supplying repairs to receivers. From the application’s perspective, waypoints appear to be additional application endpoints in the network. Waypoints seamlessly interoperate with current reliable multicast algorithms with only a minor change to receivers and no changes to routers. In our implementation, receivers and waypoints use STORM, a structure-based error recovery protocol. The waypoint recovery service is not limited to one error recovery protocol, and can be extended to enhance other reliable multicast protocols. Results from simulation experiments are presented to evaluate the potential benefits of theproposed scheme. We find that when multicast group members are isolated from each other, a waypoint recovery service can significantly enhance receivers’ performance.

UK Overseas Territories in the Northeast Caribbean: Anguilla, British Virgin Islands, Montserrat

Abstract Anguilla, the British Virgin Islands (BVI), and Montserrat are three United Kingdom Overseas Territories (UKOTs) located in the north-eastern Caribbean region. Their local economies shifted from agriculture and artisanal fishing to tourism after the advent of international travel to the Caribbean in the 1950s and 1960s, but this has not been without adverse environmental impacts. Ensuring the protection of the very assets that attract visitors to each Territory is critical, but is increasingly more difficult due to both preexisting issues and due to the increasingly prevalent impacts of climate change. What immediate actions each Territory takes now will ultimately determine the future sustainability of their marine and coastal resources.