The advantage of pushing is that the administrators have more control over wide-area network utilization, but this is to be balanced against the previously mentioned storage costs. Running a reliable webcast Computer equipment and software is not known for 100 percent reliability. Fiber cables can get severed. So if you are planning to host important webcasts, it is worth considering what would happen if the webcast went down. Even if you have taken great care in selection of equipment with a long mean time before failure ratings, the unexpected can always happen. So for a highly available system, it is best to assume that things will go wrong, and accordingly plan systems that are fault tolerant. As an enterprise uses streaming more and more, then the requirements for high availability increase. Once streaming is core to corporate communications, the expectations of the systems approach those of the phone system. Conventional splitters are single points-of-failure in the delivery network. Real s Helix servers lets you design in redundancy to improve the QoS of the corporate network. Helix platform RealNetworks RealSystem iQ has been replaced by the Helix Universal Server and Gateway. One of the features of the system is support for redundant paths from the encoder or origin server to the media player. This means that if any part of the network is no longer available, the content delivery will fail-over to another path, thus ensuring the continuous delivery of the stream. Where the redundant streams reconverge at a Helix edge server, the server accepts the first packet and ignores any other packets that duplicate the same media data. A peer-to-peer network can be set up so that servers can act as transmitters or receivers. This means that live content can be injected anywhere in the network. The distribution can be push or pull. The push system transmits content to all the receivers that it has been configured to feed. Pull awaits a request from the player for content. The receiver feeding the player then requests content from the origin server. The push distribution can use forward error correction to correct for packet loss. This allows a unidirectional stream to be transmitted to edge servers, with no requirement for a back-channel. This can be very useful for satellite distribution by multicast. The level of error correction is configurable to suit the network conditions. Different paths can be set up separately, so, for example, a satellite link that already may use error correction could be set to a Content distribution 303
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the link, clearly not a viable option. One way around this is to place a proxy server at the remote site. This can be preloaded with the presentation overnight, and then viewed from the cache during office hours. So during the day the T-1 line is left free for the regular data traffic. A second solution would be to segregate traffic. Existing transaction processing, e-mail, and other data traffic can use the T-1 line, and media traffic can utilize a VPN (virtual private network) over the Internet via a POP close to the remote office. The solution with the proxy server is pushing content. Most CDNs rely on a request from a player before content is delivered. This is called pull delivery. It has advantages in that the cache does not waste disk space by storing unwanted material. The disadvantage is that the first user to request a clip has to wait for delivery. Later users of the same clip will get a prompt response to their request. 302 The Technology of Video and Audio Streaming border router interior router Proxy server corporate LAN office clients office clients DMZ Internet Figure 15.10 Corporate intranet with proxy server.
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area circuits (to lower costs). Many companies rely on streaming for financial news and other corporate information. So controlled access to the Internet is becoming a necessity. Firewall The corporate network already will have a firewall to protect the internal networks from unauthorized access. The firewall listens to network traffic passing between the corporate network and the external Internet. Most firewalls are set up to block all incoming traffic and to restrict outgoing traffic. The outgoing traffic allowed is most likely TCP and HTTP. All UDP traffic is blocked, but that is the optimum network transport layer for streaming video. Streaming can pass through if it is cloaked as HTTP traffic or streamed over TCP. Neither provides the best streaming experience, especially if you want interactivity and the best control over the streaming delivery rate. The firewall has two ports: one on the corporate side, the other in a socalled demilitarized zone (DMZ). The DMZ is where equipment that has to be accessed from the Internet is located. This will include Internet and extranet web servers. If you want to provide staff with access to streaming media from the Internet, rather than passing the files through the firewall, a separate proxy streaming server can be used. The proxy allows your network administrators to set up access control rules, but does not allow external access to the internal network. There is a potential issue with cached content it may be that you need authorization to view a file. If the file is sitting on a cache it easily could be copied. RealServer Proxy gets around this by encrypting the file and the file name. The only way to access the file is to stream it. Material is retrieved from the cache and decrypted for streaming after an accounting connection is established to the origin server. This way, full usage reporting is maintained. The Real player can be configured to point at the proxy server rather than trying to connect directly to the origin server. This would be the normal set-up in a corporate application. The CDN and managing bandwidth The proxy server can be used as a cache to sensibly manage the delivery of large files over external links. Suppose that you have a remote office connected by a T-1 line (1.5 Mbit/s). The link originally was leased for transaction data to the company databases and/or e-mail traffic. Both applications use relatively small files, so a T-1 has the capacity for a mid-sized office. Now you want to start streaming internal presentations across the link. A highquality encode (at say, 1 Mbit/s) is going to use almost the entire capacity of Content distribution 301
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Speedera Speedera uses a network of edge servers to deliver content close to the viewer s POP. The viewer is directed to the best edge server by the Speedera DNS Server. This provides intelligent routing, derived from a network of probes that collect data about the QoS of the network and the load status of the Speedera server network. Content distributors can view viewer statistics and monitor live events with the SpeedEye interface. The Speedera network extends across North America, Europe, and the Asia Pacific region. Speedera supports Windows Media and Real architectures and, using the open-source Darwin server, QuickTime and MPEG-4. Corporate intranets The corporate network has a different set of requirements from the public media viewer. An enterprise may want to use streaming for corporate communications, sales presentations, and distance learning. Planning Within an enterprise there will be conflicting demands. The different user departments training and marketing will want the flexibility to schedule webcasts at times to suit themselves. They will want a high QoS whatever the network load. Conversely the IT department will want to maximize utilization of the network by the proper scheduling of high network demands. IT will have to keep important services like transaction processing and mail running reliably, whatever the streaming load. The solution here is for the network to be content-aware, so that the service provision can be controlled for the different applications. Return of investment You must have a top-down strategy to fund the network resources properly to provide for the agreed levels of use for streaming media. When scoping the ROI for the additional network cabling, router, and switches, remember the legacy costs. These include duplication costs for VHS tapes and CD-ROMs, plus their distribution costs. Another cost is travel time and expenses for meetings with regional and branch offices. The corporation probably wants restrictions on the content that can be accessed. This is partly to stop the viewing of inappropriate content in working time; the other reason is to manage the bandwidth requirements for the wide- 300 The Technology of Video and Audio Streaming
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The Globix network has continuous monitoring to ensure that media players are directed to the optimum connect points to receive their stream. The routing control keeps content on the Globix network where possible, rather than packets roaming around peered networks. This direct routing gives the high QoS that streaming demands. Content distribution 299 origin server content CDN Hosting Provider cache engine CDN Operator regional data center Internet service provider Internet data center Internet backbone Internet backbone Acess Gateway Point of presence Content creator media players Internet users Edge Server Figure 15.9 Content network.
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America and Western Europe, especially for live events. Globix uses caching for streaming media as well as web content. Their EarthCache CDN uses a minimum of 1 Tbyte at each cache node with wide-band connections to the Internet backbone. The caching enables media servers to cope with peaky traffic demands, without unduly stressing the origin servers. 298 The Technology of Video and Audio Streaming Figure 15.8 Redirecting the player to the edge server.
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The alternative is to provide a quick response to a request for streaming content. The wait for the first frame of video is the crucial period; that is when you can lose the impatient viewer. If the first 30 seconds of a clip could be cached, the remainder can be streamed on-demand from the origin server to the edge server. To the viewer, this process can be made transparent by assembling the two halves in a playlist. Reporting The cache server provides full reporting data back to the origin server. This means that the origin server can collect the same usage statistics as if the content were being served directly to the client, and not via a proxy server. Network controller The core of the CDN is the network controller. This can intercept requests for streams and redirect the player to the nearest cache server. So the DNS server, rather than using a single IP address for an URL, will return the IP address of the correct edge server for the current conditions. This redirection can take into account network congestion and server utilization. Referring to Figure 15.8, when the media player receives the metafile from the web server (4), the URL addresses the CDN controller (5). The controller then redirects the request to the optimum edge server (6) and the content is streamed to the player (7). The redirection can load-balance requests across a number of server clusters. Akamai Akamai has one of the most extensive CDNs with their FreeFlow streaming network. Akamai has become very popular for the rapid delivery of web content from distributed caches dispersed around POPs. Web sites use Akamai for widely accessed content and banner advertisements. If you have your content akamized, the reference tag uses an ARL or Akamai Resource Locator instead of an URL. The Akamai CDN uses this ARL to divert the browser request for a streaming file to the optimum edge server to retrieve the content. Globix Globix has tier-one fiber network linking major data centers in Santa Clara, New York, Atlanta, and London. This gives high performance coverage of North Content distribution 297
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originally were developed for the speedy serving of graphic content for web sites. But the file sizes are usually under 1 Mbyte. Although caching can be used for streaming, it does not promise the same return as conventional web page caching. Streaming files are just too large to cache anything but the most frequently accessed files. 296 The Technology of Video and Audio Streaming origin server proxy server Only one stream multiple players Internet Figure 15.6 Edge splitting server. origin server cache server disk array Only one clip download 1st user of clip 2nd user of clip clip cached to disk Internet Figure 15.7 Edge cache server.
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ing experience. It also makes interactive applications much more responsive to the user. The CDN feeds the content to the edge server through content accelerators. Edge splitting The local proxy offers another opportunity to make more efficient use of the network. If more than one client logs in to a live webcast at the same point-ofpresence, a single stream is hauled to the POP. Cache servers then split the content and serve directly to the multiple clients. This has two advantages. The first is the reduction in network traffic; only one stream crosses the backbone. The second is that the clients are receiving the stream from a local server, rather than a long-distance negotiation through 20 or so routers. Local caching If a client requests content on-demand, that content is cached at the POP as it is streamed. If a second user requests the same file, then it is served from the cache, not the origin server. Caching has long been used for static web content. It greatly improves performance for popular pages, and also saves capacity on the main Internet backbone by serving content locally to the user. One potential issue with a cache is stale material. Some content is very dynamic, for example news sites, and changes from hour to hour. It is important that the CDN keeps the cache synchronized with the origin server. Cache servers Content distribution 295 origin server proxy server client content transmitter CDN overlay network Internet Figure 15.5 Proxy edge server.
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to approach the large CDNs like Akamai, Speedera, Globix, and Mirror Image. In 2004 Akamai had over 15,000 servers deployed around the Internet. These global networks have the resources to handle very large numbers of media streams as well as conventional HTML files. Overlay network To provide high-quality delivery some intelligence is needed to control the transmission of multimedia streams. The Content Delivery Networks use a control layer overlaid on the Internet. This layer can capture requests for files, and then manage the delivery by the most efficient way to give a high quality of service at the media player. The CDNs aim to improve the streaming experience by three main processes: splitting, caching, and proxy serving. Local proxy This is the basic edge server. The delivery from origin to user is split into two hops. The first is from the origin to the edge, and the second is from the edge to the client. If streams are delivered locally to the client, the RTSP and RTCP control messages are exchanged locally rather than across the network. This cuts down control latencies and the network traffic to provide a better stream- 294 The Technology of Video and Audio Streaming Figure 15.4 The overlay control network.
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