How the internet works Essay

Although the details of routing and software are complex, the operation of the internet from the users’ perspective is fairly straight forward. As an example of what happens when the Internet is used, consider that you type the URL www. helpmegetoutofthis. com into the Netscape browser. The browser contacts a DNS server to get the IP address. A DNS server would start its search for an IP address. If it finds the IP address for the site, then it returns the IP address to the browser, which then contacts the server for www.helpmegetoutofthis. com, which then transmits the web page to your computer and browser so you can view it. The user is not aware that of the operation of an infrastructure of routers and transmission lines behind this action of retrieving a web page and transmitting the data from one computer to another. The infrastructure of the internet can be seen as a massive array of data relay nodes (routers) interconnected by data transmission lines, where each node can service multiple transmission lines. In the general case where information must be sent across several nodes before being received, there will be many possible pathways over which this transmission might occur. The routers serve to find a path for the data transmission to occur. The routing of a file or data packets of a file is either be done by the technique of source routing or the technique of destination routing. In source routing, the path the data transmission will follow id specified at the source of the transmission, while destination routing is controlled by the routers along the path. In the modern internet, almost all routing is done by destination routing because of security issues associated with source routing. Thus, the routers must be programmed with protocols that allow a reasonable, perhaps optimum, path choice for each data packet. For the routers to choose an optimum path also requires that the interconnected routers communicate information concerning local transmission line metrics. Router communication is thus itself a massive information transfer process, given that there is more than 100,000 networks and millions of hosts on the Internet. When viewing the enormity of the problem, it is perhaps easier to understand why engineers have accepted a sub-optimal solution to the problem of efficiency in data transfer on the Internet. When initially confronting a problem, the practical engineering approach is to simplify the problem to the point where a working solution can be obtained and then refine that solution once the system is functional. Some of the simplifying assumptions used by engineers for the current internet data transmission system include. 1) A transmission line is never over capacity and is always available as a path choice. 2) The performance of the router and transmission line does not depend on the amount of traffic. These two assumptions do simplify the problem of path choice considerably because now all the transmission lines and nodes may be considered equal in capacity and performance completely independent of traffic. As such, it is a much simpler optimization problem consisting of finding the route with the shortest path length. To simplify the problem even further, another assumption is made: 3) Consider that an â€Å"Autonomous System† (AS), is a small internet inside the Internet. An AS is generally considered to be a sub-network of an Internet with a common administrative authority and is regulated by a specific set of administrative guidelines. It is assumed that every AS is the same and provides the same performance. The problem of Internet routing can now be broken down into the simpler problem of selecting optimum paths inside the AS and then considering the optimum paths between the AS. Since there are ‘only’ around 15,000 active AS’s on the Internet, the overall problem is reduced to finding the best route over 15,000 AS nodes, and then the much simpler problem of finding the best route through each AS. There is an important (to this thesis) set of protocols which control the exchange of routing information between the AS’s. The sort of routers in an AS which communicates with the rest of the internet and other AS’s are called border routers. Border routers are controlled by a set of programming instructions known as Border Gateway Protocol, BGP. A more detailed discussion of computer networking principals and the Internet facts can be found in e. g. [7]. An Introduction to Router Protocols. Routers are computers connected to multiple networks and programmed to control the data transmission between the networks. Usually, there are multiple paths that are possible for transmission of data between two points on the Internet. The routers involved in the transmission between two points can be programmed to choose the ‘best path’ based on some metric. The ‘protocols’ used to determine the path for data transmission are routing algorithms. Typical metrics used by routing algorithms include path length, bandwidth, load, reliability, delay (or latency) and communication cost. Path length. Path length is a geometric measure of how long the transmission lines are. The routers can be programmed to assign weights to each transmission line proportional to the length of the line or each network node. The path length is then the sum of the weights of the nodes, lines or lines plus nodes along the possible transmission path. Bandwidth. Bandwidth is used to describe the available transmission rate (bps) of a given section the possible transmission path. An open 64 kbps line would not generally be chosen as the pathway for data transmission if an open 10 Mbps Ethernet link is also open, assuming everything else is equal. However, sometimes the higher bandwidth path is very busy and the time required for transmission on a busy, high bandwidth line is actually longer than on a path with a lower bandwidth. Load. This data packet transmission per unit time or the percent of CPU utilization of a router on a given path is referred to as the load on this path. Reliability. The reliability of a data transmission path can be quantitatively described as the bit error rate and results in the assignment of numeric reliability metrics for the possible data transmission pathways. Delay. The delay in data transmission along a certain path is due to a combination of the metrics that have already been discussed, including geometric length of the transmission lines, bandwidth, and data traffic congestion. Because of the hybrid nature of the communications delay metric, it is commonly used in routing algorithms.