Multimedia Networking

7.5 Distributing Multimedia: Content Distribution Networks
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Introduction
7.1 Multimedia Networking Applications
7.1.1 Examples of Multimedia Applications
7.1.2 Hurdles for Multimedia in Today's Internet
7.1.3 How Should the Internet Evolve to Support Multimedia Better?
7.1.4 Audio and Video Compression
7.2 Streamimg Stored Audio and Video
7.2.1 Accessing Audio and Video Through a Web Server
7.2.2 Sending Multimedia from a Streaming Server to a Helper Application
7.2.3 Real-Time Streaming Protocol (RTSP)
7.3 Making the Best of the Best-Effort Service: An Internet Phone Example
7.3.1 The Limitations of a Best-Effort Service
7.3.2 Removing Jitter at the Receiver for Audio
7.3.3 Recovering from Packet Loss
7.4 Protocols for Real-Time Interactive Applications
7.4.1 RTP
7.4.2 RTP Control Protocol (RTCP)
7.4.3 SIP
7.4.4 H.323
7.5 Distributing Multimedia: Content Distribution Networks
7.6 Beyond Best Effort
7.6.1 Scenario 1: A 1 Mbps Audio Application and an FTP
7.6.2 Scenario 2: A 1 Mbps Audio Application and a High-Priority FTP Transfer
7.6.3 Scenario 3: A Misbehaving Audio Application and an FTP Transfer
7.6.4 Scenario 4: Two 1 Mbps Audio Applications over an Overload 1.5 Mbps Link
7.7 Scheduling and Policing Mechanisms
7.7.1 Scheduling Mechanisms
7.7.2 Policing: The Leaky Bucket
7.8 Intergrated Services and Differentiated Services
7.8.1 Intserv
7.8.2 Diffserv
7.9 RSVP
7.9.1 The Essence of RSVP
7.9.2 A Few Simple Examples
Distributing Multimedia: Content Distribution Networks

Content distribution networks (CDNs) provide an alternative apporach to distributing stored multimedia content.
 
CDNs are based on the philosophy that if the client can't come to the content, the content should be brought to the client.  CNDs thus use a different model than Web caching.  For a CDN, the paying customers are no longer the ISPs but the content providers.  A content provider with a video to distribute pays a CDN company to get tis viedo to requesting users with the shortest possbile delays.
 
A CDN company typically provides its content distribution service as follows:
  • The CND company installs hundreds of CDN servers throughout the Internet.The CDN company typically places the CDN servers in a data center.  A data center is typically a building filled with server hosts.  These data centers are often in lower-tier ISPs, close to ISP access networks and the clients.
  • The CDN replicates its customers' content in the CDN servers.  Whenever a customer updates its content, the CDN redistributes the fresh content to the CDN servers.
  • The CDN company provides a mechanism so that when a client requests content, the content is provided by the CDN server that can best deliver the content to the specific client.

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The requesting host first goes to the orgin Web server to get the base HTML object, then to the CDN's authoritativeDNS server to get the IP address of the best CDN server, and finally to that CDN server to get the video.  Note that no changes need be made to HTTP, DNS, or the browser to implement this distribution scheme.

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CDNs are also often deployed by enterprises that have a large number of regional offices.  An enterprise's regional offices are typically connected to the Internet via relatively low-speed links.  Many such enterprises today need to distribute video e-learning or other corporate video material to their employees.  To accomplish this task, an enterprise purchases CDN servers and installs them in tis regional offices.  When the enterprise has a new video to distribute, it pushes the video to all of its regional offices, and the employees are then served the video by their regional CDN server.