| Developer(s) | Michael Freedman |
|---|---|
| Initial release | 2004 |
| Development status | Active |
| Operating system | Cross-platform (web-based application) |
| Type | P2P Web cache |
| Website | www.coralcdn.org |
The Coral Content Distribution Network, sometimes called Coral Cache or Coral, is a free peer-to-peer content distribution network designed and operated by Michael Freedman. Coral uses the bandwidth of a world-wide network of web proxies and nameservers to mirror web content, often to avoid the Slashdot Effect or to reduce the general load on websites servers in general.
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One of Coral's key goals is to avoid ever creating 'hot spots' of very high traffic, as these might dissuade volunteers from running the software out of a fear that spikes in server load may occur. It achieves this through an indexing abstraction called a distributed sloppy hash table (DSHT); DSHTs create self-organizing clusters of nodes that fetch information from each other to avoid communicating with more distant or heavily-loaded servers.
The sloppy hash table refers to the fact that coral is made up of concentric rings of distributed hash tables (DHTs), each ring representing a wider and wider geographic range (or rather, ping range). The DHTs are composed of nodes all within some latency of each other (for example, a ring of nodes within 20 milliseconds of each other). It avoids hot spots (the 'sloppy' part) by only continuing to query progressively larger sized rings if they are not overburdened. In other words, if the two top-most rings are experiencing too much traffic, a node will just ping closer ones: when a node that is overloaded is reached, upward progression stops. This minimises the occurrence of hot spots, with the disadvantage that knowledge of the system as a whole is reduced.
Requests from users are directed to a relatively close node, which then finds the file on the coral DSHT and forwards it to the user.
A website can be accessed through the Coral Cache by adding .nyud.net to the hostname in the site's URL, resulting in what is known as a 'coralized link'. So, for example,
http://example.combecomes
http://example.com.nyud.netAny additional address component after the hostname remains after .nyud.net; hence
http://example.com/folder/page.htmlbecomes
http://example.com.nyud.net/folder/page.htmlFor websites that use a non-standard port, for example,
http://example.com:8080becomes
http://example.com.8080.nyud.netThe project has been deployed since March 2004, during which it has been hosted on PlanetLab, a large-scale distributed research network of several hundred servers deployed at universities world wide. It has not, as originally intended, been deployed by third-party volunteer systems. About 300-400 PlanetLab servers are currently running CoralCDN. The source code is freely available under the terms of the GNU GPL.
Coral Cache gained widespread recognition in the aftermath of the 2004 Indian Ocean earthquake, when it was used to allow access to otherwise inaccessible videos of the resulting tsunami.
Some web filtering tools, such as Websense and OpenDNS, block access to the Coral Cache as it is seen as a form of proxy avoidance. Many anti-virus and Internet Security software packages, such as Trend Micro Internet Security, also block access to the Coral Cache as it is seen as a mask for dangerous URLs.