Content delivery network

A content delivery network or content distribution network (CDN) is a system of computers networked together across the Internet that cooperate transparently to deliver content most often for the purpose of improving performance, scalability, and cost efficiency, to end users.

CDN benefits

The capacity sum of strategically placed servers can be higher than the network backbone capacity. This can result in an impressive increase in the number of concurrent users. For instance, when there is a 10 Gbit/s network backbone and 100 Gbit/s central server capacity, only 10 Gbit/s can be delivered. But when 10 servers are moved to 10 edge locations, total capacity can be 10*10 Gbit/s.

Strategically placed edge servers decrease the load on interconnects, public peers, private peers and backbones, freeing up capacity and lowering delivery costs. It uses the same principle as above. Instead of loading all traffic on a backbone or peer link, a CDN can offload these by redirecting traffic to edge servers.

Since edge servers are usually placed near end users, assets are delivered via fewer hops, resulting in lower latency and increased delivery speed. End users will likely experience less jitter, fewer network peaks and surges, and improved stream quality - especially in remote areas. The increased reliability allows a CDN operator to deliver HD quality content with high QoS, low costs and low network load.

Modern CDNs can dynamically distribute assets to strategically placed redundant core, fallback and edge servers. Modern CDNs can have automatic server availability sensing with instant user redirection. A CDN can offer 100% availability, even with large power, network or hardware outages.

Modern CDN technologies give more control of asset delivery and network load. They can optimize capacity per customer, provide views of realtime load and statistics, reveal which assets are popular, show active regions and report exact viewing details to the customers.

ASP versus on-net

Most CDNs are operated as an ASP on the Internet, although an increasing number of internet network owners are building their own CDN to improve on-net content delivery and to generate revenues from content customers. Some develop internal CDN software, others use commercially available software.

Current market overview

As of 2008, the top 3 global Content Delivery ASP networks are Akamai, Limelight and CDNetworks, forecasted by Tier1 Research to account for roughly 79% of the U.S. market revenue for the year 2007..

Many content delivery networks have announced significant financing since Q3 of 2007 including CDNetworks who announced $96.5 million December 19, 2007 [1], Highwinds Network Group who announced $55 million equity financing on March 11, 2007, Move Networks who announced $46 million Series C on April 14, 2008, Velocix who announced $25 million fourth round on January 23, 2008, Panther Express who announced $15.75 million Series B on February 27, 2008, and EdgeCast Networkswho announced $6 million Series B on December 10, 2007.

Technology

Traditional CDNs focus on web acceleration. That is still their main revenue service. New CDNs have integrated media delivery services so they are optimized for live video streaming, high definition video and large asset delivery.

CDN nodes are deployed in multiple locations, often over multiple backbones. These nodes cooperate with each other to satisfy requests for content by end users, transparently moving content to optimize the delivery process. Optimization can take the form of reducing bandwidth costs, improving end-user performance, or increasing global availability of content.

The number of nodes and servers making up a CDN varies, depending on the architecture, some reaching thousands of nodes with tens of thousands of servers on many remote PoPs. Others build a global network and have a small number of geographical PoPs.

Requests for content are algorithmically directed to nodes that are optimal in some way. When optimizing for performance, locations that are best for serving content to the user may be chosen. This may be measured by choosing locations that are the fewest hops or fewest number of network seconds away from the requesting client, so as to optimize delivery across local networks. When optimizing for cost, locations that are least expensive may be chosen instead.

In a optimal scenario, these two goals tend to align, as servers that are close to the end user may have an advantage in serving costs, perhaps because they are located within the same network as the end user. However, when streaming to Asia for instance, it is cheaper to distribute from Europe or the US than from Asian located servers. It depends on the CDN operator if he chooses the best performing or the cheapest route.

Content networking techniques

The Internet was designed according to the end-to-end principle . This principle keeps the core network relatively simple and moves the intelligence as much as possible to the network end-points: the hosts and clients. As a result the core network is specialized, simplified, and optimized to only forward data packets.

Content Delivery Networks augment the end-to-end transport network by distributing on it a variety of intelligent applications employing techniques designed to optimize content delivery. The resulting tightly integrated overlay uses web caching, server-load balancing, request routing, and content services.. These techniques are briefly described below.

Because closer is better, web caches store popular content closer to the user. These shared network appliances reduce bandwidth requirements, reduce server load, and improve the client response times for content stored in the cache.

Server-load balancing uses one or more layer 4–7 switches, also known as a web switch, content switch, or multilayer switch to share traffic among a number of servers or web caches. Here the switch is assigned a single virtual IP address. Traffic arriving at the switch is then directed to one of the real web servers attached to the switch. This has the advantages of balancing load, increasing total capacity, improving scalability, and providing increased reliability by redistributing the load of a failed web server and providing server health checks.

A content cluster or service node can be formed using a layer 4–7 switch to balance load across a number of servers or a number of web caches within the network.

Request routing directs client requests to the content source best able to serve the request. This may involve directing a client request to the service node that is closest to the client, or to the one with the most capacity. A variety of algorithms are used to route the request. These include Global Server Load Balancing, DNS-based request routing, Dynamic metafile generation, HTML rewriting, and anycasting. Proximity—choosing the closest service node—is estimated using a variety of techniques including reactive probing, proactive probing, and connection monitoring.

Simple CDNs require manual asset copying. Earlier CDNs used active web caches and global hardware load balancers. Modern CDNs use cheap and simple edge servers and intelligent central CDN management technologies that distribute assets dynamically.

Content service protocols

Several protocols suites are designed to provide access to a wide variety of content services distributed throughout a content network. The Internet Content Adaptation Protocol (ICAP) was developed in the late 1990s to provide an open standard for connecting application servers. A more recently defined and robust solution is provided by the Open Pluggable Edge Services (OPES) protocol. This architecture defines OPES service applications that can reside on the OPES processor itself or be executed remotely on a Callout Server. Edge Side Includes or ESI is a small markup language for edge level dynamic web content assembly. It is fairly common for websites to have generated content. It could be because of changing content like catalogs or forums, or because of personalization. This creates a problem for caching systems. To overcome this problem a group of companies created ESI.

P2P CDNs

Although peer-to-peer (P2P) is not traditional CDN technology, it is increasingly used to deliver content to end users. P2P claims low cost and efficient distribution. There is however a downside to P2P usage, because the lower delivery costs only apply to the content owner. P2P actually uses much more traffic to deliver the same asset, increasing the delivery costs for the network owners. P2P can be very disruptive for internet access providers and backbone operators. There are more downsides of P2P, for instance the lack of QoS control and measurement.

Peer-to-peer networks also lack the ability to deliver in-order content of medium and high bandwidth. Because P2P clients are downloading from multiple random sources, there is no way to guarantee that the content is in order. Most P2P video streaming applications are limited to the typical upstream performance of most broadband consumers at roughly 300 kbit/s because that's the most bandwidth individual P2P nodes can watch and relay. For this reason, commercial grade on demand video services universally use private content distribution networks to deliver their content.

See also

Related technologies

• Web syndication
• Content delivery platform
• Application software

Commercial CDNs

• Akamai Technologies (stream, download)
• Amazon CloudFront(download)
• BitGravity (stream, download)
• CacheFly (download)
• EdgeCast Networks (stream, download)
• Highwinds Network Group (stream, download)
• Internap Network Services (stream, download, P2P hybrid, MIRO optimized)
• Level3 (download - stream launching Q4 2007)
• Limelight Networks (stream, download)
• Mirror Image (stream, download)
• Move Networks (stream, download)
• Panther Express (download)
• Qbrick (stream, download)
• SimpleCDN (stream, download)
• StreamZilla (stream, download)
• Swarmcast (streaming via HTTP)
• WINK Streaming - Streaming (RTMP + HTTP), Download, Site Caching, DDoS Protection

Commercial CDNs Using P2P for Delivery

• BitTorrent (P2P)
• Grid Networks (P2P)
• Itiva (P2P)
• Octoshape (P2P)
• Pando (P2P)
• Rawflow (P2P)
• Solid State Networks (P2P)
• VeriSign (P2P)
• pomeloTV (P2P)

Commercial CDN solutions

• PacketWarp by Vector Data
• ACNS by Cisco
• ProxySG by Blue Coat
• CDD by HP
• Aspera and Isilon
• VDO-X by Jet Stream

Academic CDNs

• Coral Content Distribution Network
• Codeen
• Globule
• FCAN
• NEXUS International Broadcasting Association WorldDirector

Source(s):

Parts of this page are taken from blog.streamingmedia.com and from VDO-X.

External links

• R. Buyya, M. Pathan and A. Vakali (eds.), Content Delivery Networks, ISBN 978-3-540-77886-8, Springer, Germany, 2008.
• Content Distribution Resources - selected academic papers
• Majumdar, Saugat; Kulkarni, Dhananjay; Ravishankar, Chinya (2007). "Addressing Click Fraud in Content Delivery Systems" (PDF). Infocom. IEEE. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help)CS1 maint: multiple names: authors list (link)