User:Dan Nessett/Sandboxes/Sandbox 1

The Internet is a term with many meanings, depending on the context of its use. To the general public, the term is often used synonymously with the World Wide Web, its best-known application. But the internet supports many other applications, such as electronic mail, streaming media, such as internet radio and video, a large percentage of telephone traffic, system monitoring and real-time control applications, to name a few. In one respect the Internet is similar to an iceberg. The vast majority of it is out of sight. While distributed applications allow users to utilize internet services, they require a large suite of technologies visible only to the enterprises that provide them. To Internet Service Providers, the Internet identifies these underlying services. In addition, there are internet services that are accessible to the general public, while these same services are provided in restricted environments, such as those in an enterprise intranet, in military and government private internets and in local home networks. Further complicating the notion of an Internet is is the frequent interconnection of public and private networks in ways that allow limited interaction. This article and the subgroup it describes uses the term Internet in the broadest sense. That is, it identifies the applications that provide an interface between users and communications services, those services themselves, public and private instances of application and communications services and the aggregation of private and public networks into a global communications and application resource.

In order to engineer the internet, internet designers and engineers place its services into one of several layers, which in total comprise the internet protocol architecture. While there have been several different protocol architecture schemes, the one with the strongest support consists of 5 layers: 1) the application layer, 2) the transport layer, 3) the network layer, 4) the link-layer, and 5) the physical layer. Each protocol layer utilizes the services of the next lower layer (except the lowest, the physical layer) to provide a value-added service to the layer above it (except for the application layer, which provides services to users). Utilizing this protocol architecture, it is possible to describe how the Internet works.

Web browsers are the most common user interface in the Internet. Such browsers translate human requests to the [[Hypertext Transfer Protocol], which actually moves data between the browser and a Web server. Consequently, measured solely in terms of percentage of use, web applications are the most frequently used application. The communications services provided by the Internet have no direct human interfaces; every user-visible function must go through a program resident on a client or server computer. There are literally hundreds of different protocols, applications and services that run over the Internet. Virtual private networks interconnecting the parts of individual enterprises, or sets of cooperating enterprises, overlay the Internet. As mentioned previously a wide range of interconnected networks using the same protocols as the public Internet, but are isolated from it, provide services ranging from passing orders to launch nuclear weapons, authorizing credit card purchases, collecting intelligence information, controlling the electric power grid (see System Control And Data Acquisition), telemedicine such as transferring medical images and even allowing remote surgery, etc.]] Many of these applications utilize custom application interfaces that do not involve a web browser. Consequently, internet distributed applications comprise a much larger set than those experienced by the general public.

In addition to applications that are directly experienced by Internet customers, there are a wide-range of internet applications that exist to provide infrastructure services to the internet. Examples of infrastructure services are the Doman Name System (DNS), which associates computers connected to the Internet with human friendly names. The movement of data through the internet requires that it visit intermediate systems called routers. The activity of directing the data through the internet, called routing, utilizes an infrastructure application that distributes routing data to routers. The secure identification of users to applications requires the use of authentication servers, such as RADIUS and Kerberos, each of which is a distributed application in and of itself. These are just a few of the internet infrastructure applications that support the provision of internet service.

Internet applications are distributed. That is, they normally are comprised of pieces that reside at different locations. That means they must exchange data through communications equipment that is subject to various failure modes. Furthermore, one element may have the capability to send data faster than the receiver can process. The next layer in the protocol architecture, the transport layer, provides services that address these issues. Transport layer protocols, like the Transmission Control Protocol (TCP) provide end-to-end error management and flow-control services that ensure application elements can exchange data in an error-tolerant and synchronized manner.

The next layer of internet service, the network layer moves data between end-systems through an interconnected set of systems, routers, which are mentioned above. Routers come in all shapes and sizes. Some, normally located at the periphery of the internet such as those in a home or small business, are known as edge routers. Others are service provider equipment with varying capabilities, from modest performance border routers to high performance core routers. These routers are interconnected, moving the data between them in a way that increases the probability of successful transit. There are two types of routing schemes. Virtual circuit routing reserves resources over a fixed path between two computers. Packet routing operates in a way whereby individual packets of data may take different paths through the systems that interconnect two computers. The network layer also supports specialized data services, such as multicast and anycast routing.

Routers and end systems (normally customer computers, but in some cases infrastructure systems) directly connect to each other through physical channels (addressed below) that introduce communications errors and that are themselves not flow-controlled. Each of these systems is called an intermediate system. It is the function of the link-layer to provide services that correct most of the errors that occur on physical channels and to provide the two directly communicating intermediate systems with flow-controlled data exchange. The characteristics of the physical channel may vary widely from the fairly reliable ethernet, the less reliable wireless channels, to the very unreliable deep space radio channels. Each type of physical channel may require a different link-layer [[protocol] to accommodate its characteristics.

Physical channels, which populate the physical layer, encode data utilizing various techniques, thereby providing the basic data transmission service between directly connected equipment. There are a wide variety of physical channels, each utilizing its own