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Warp Speed Web Content
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April 29, 2002
By Sean Doherty
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Executive Summary: Enterprise CDNs
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Content-delivery networks combine cache servers, content distribution and redirection to bring Web content closer to end users. Although outsourcing content delivery is one option, several vendors have recently introduced eCDNs (enterprise content-delivery networks) to serve up Webcasting, videoconferencing, e-learning and collaboration applications quickly, reliably and inexpensively.
Rich media, with its large files, puts a strain on most networks. ECDNs cut latency by reducing the number of router hops content must make to reach end users. Implementing such a solution can save costs by decreasing bandwidth requirements and eliminating the need to increase pipe size.
We tested three eCDN solutions: Volera's Velocity CDN (vCDN), CacheFlow's Security Gateway Appliance and cIQ Director, and F5 Networks' Edge-FX 540 and Global-Site Controller. Volera's solution excelled in every test, providing up to 99 percent WAN-bandwidth reduction, dramatically increasing cache hits and blowing the competition away in hits per second. Although Volera's solution is more expensive than the others once you factor in the cost of hardware you'll need to provide (vCDN's Excelerator cache server ships on a CD, not in an appliance), its performance improvements are worth the extra capital.
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Online Only: Proxy Primer
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Proxy caches can be deployed in three ways:
• Reverse proxy cache
• Forward proxy cache, non-transparent mode
• Forward proxy cache, transparent mode
Reverse proxy caches, or server-side proxy caches, are deployed in the data center close to the original content servers. These caches resolve to the Web address of the original content server and offload most HTTP transactions and TCP connections from Web servers. Although this deployment offloads central server processing, it does little to reduce the bandwidth requirements for content requests over slow WAN links.
Forward proxy caches are deployed close to end users, in branch offices, for example. These caches can work in a non-transparent or transparent mode with intermediate devices such as routers and switches.
In non-transparent mode, browsers and media players need to be configured to point directly to the caches. This is a manual process that's prone to errors. Non-transparent forward caches proxy requests for original content from the configured clients. If the requested content is not in memory or on disk, the cache retrieves it from the original content server for delivery. On subsequent requests, the cache serves the content directly to end users. This obviates the need to traverse a slow WAN link to the original content server. Forward proxy caches aim to minimize redundant traffic over network links, thus reducing bandwidth requirements and costs. This also reduces latency in end-user content requests and, like reverse proxy caches, minimizes the loads on origin servers.
Forward proxy caches can also be deployed in a transparent manner. In transparent forward caching, traffic is routed to cache servers without configuring desktop applications. This transparency comes from other intermediate devices. Content requests are routed to caches from Web-Cache-Control-Protocol-enabled (WCCP-enabled) routers or content layer switches (Layer 4-7 switch).
The WCCP v.2.0 protocol specifies interactions between routers and Web caches. The interactions establish transparent redirection of configured traffic types (e.g., HTTP, RTSP, MMS) from routers to caches. WCCP is a component of Cisco Systems IOS and has not been widely deployed in other switches. Cache servers also need to support WCCP. CacheFlow, F5 Networks and InfoLibria currently support WCCP.
Transparency can also be achieved with a Layer 4 (L4) switch. Filters can be configured on an L4 switch to redirect original requests for content to cache devices. For example, an L4 switch can redirect user requests for TCP or UDP services based on the port number of the request. All requests for port 80 or 1755 can be routed to a specific cache server.
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Web Links
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• "CacheFlow Security Gateway Ain't Your Pappy's Cache" (Network Computing, April 1, 2002)
• "InfoLibria Rich Media Starter Kit" (Network Computing, Feb. 4, 2002)
• "Emerging ESI: Lower Costs, Better Performance" (Network Computing, Jan. 7, 2002)
• "Tutorial: The Wizardry of Multicast" (Network Computing, Feb. 29, 2001)
• "CDNs: Stand and Deliver" (Network Computing, Dec. 17, 2001)
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