The Hierarchy Of Sns Sercers

Princeton S* Network Systems» Blog Archive » The Design of ...

In this post, I describe the architecture and mechanisms of CoralCDN at a high-level. This is meant to provide some of the background necessary for some of our experiences and lessons with operating the system.

System Overview

CoralCDN is composed of three main parts: (1) a network of cooperative HTTP proxies that handle users’ requests, (2) a network of DNS nameservers for .nyud.net that map clients to nearby CoralCDN HTTP proxies, and (3) the underlying Coral indexing infrastructure and clustering machinery on which the first two applications are built.  You’ll find that I refer to the entire system as “CoralCDN”, but the network of indexing nodes as “Coral” (although this clarification is not always shared by others, who refer to the entire system as either “Coral” or “Coral Cache”).

At a high-level, the following steps occur when a client issues a request to CoralCDN.

A client resolves a “Coralized” domain name (e.g., of the form sns.cs.princeton.edu.nyud.net) using CoralCDN nameservers, possibly starting at one of the 10–12 primary nameservers registered for .nyud.net under the .net domain. Upon receiving a query, a CoralCDN nameserver probes the client to determines its round-trip-time.  The nameserver uses this information to determine which nameservers (NS records) and CoralCDN proxies (authoritative A records) to return. The client sends an HTTP request for a Coralized URL to one of the returned proxy.  If the proxy is caching the file locally, it returns the file and stops.  Otherwise, this process continues. The proxy looks up the web object’s URL in Coral. If Coral returns the address of a node caching the object, the proxy fetches the object from this node.  Otherwise, the proxy downloads the object from the origin server (e.g., sns.cs.princeton.edu). The proxy stores the web object and returns it to the client browser.

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