The Foundation of the Internet: TCP/IP Reference Model

Abstract

Spanning the world with its almighty presence, today’s Internet connects computers, telephones, entertainment electronics and, in a short time also the household devices and the goods we need for daily life. More and more it penetrates the surface of our lives. To enable these different devices to communicate smoothly and efficiently with each other their communication must follow defined rules—so-called communication protocols. These mold the individual layers of Internet communication determining the tasks, level of abstraction complexity and respective range of functions. By what means and way these specifications are put into practice is, however, not defined by the model but depends on the specific implementation. The TCP/IP reference model thus assumed a concrete form through practical application and builds today, as well as in the foreseeable future, a solid foundation for all of the communication tasks on the Internet.

"The borders of my language define the borders of my world."

– Ludwig Wittgenstein, (1889–1951)

Glossary

Authentication

Serves to give proof of a user’s identity. Certificates of a trustworthy authority are used to check identity in carrying out authentication. To verify the integrity of a message, digital signatures are created and transmitted with it

Broadcast

A broadcast transmission is a transmission conducted simultaneously from one point to all participants. Classical broadcast applications are radio and television

Circuit switching

Method of information exchange via a network. At the beginning of the information exchange an exclusive connection between the communicating terminals is established and remains in effect for the duration of the communication. Analog telephone networks, for example, work according to this principle

Client

Designates a program that contacts a server and requests information from it. The browser employed in the WWW is in this sense a client. But there are also other clients in the WWW that contact WWW servers and download information from it, e.g., search engines or agents

Client/Server-Architecture

An application is carried out as a collaborative effort over a network of connected, multiple computers. The server provides specific services, the client conversely requests services. Except for the actions of placing an order and responding to it, the components are independent of each other. Interfaces and the way of communicating in placing orders and responding are explicitly defined

Communication protocol

A communication protocol (also simply protocol) is a collection of rules and regulations that determines the data format of the information to be transferred as well as the mechanisms and procedures concerning their transmission. Protocols contain agreements about the establishment and termination of a connection between the communication partners and methods of data transmission

Computer network

A computer network (network) offers autonomic computing systems, which each have their own storage, periphery and computational ability, the infrastructure for data exchange. All of the subscribers are linked with each other via the computer network, therefore each has the possibility to get into contact with every other network participant

Connection-oriented/connectionless service

A basic distinction is made between connection-oriented and connectionless services on the Internet. Before the start of the actual data transmission, connection-oriented services must establish a connection over predetermined switching locations in the network. This specified connection route is used for the duration of the entire communication. Connectionless services do not choose a fixed connection route in advance. The transmitted data packets are each, independent of one another, transmitted in potentially different ways via the Internet

Cryptography

A branch of information technology and mathematics that is involved with the construction and evaluation of encryption. The objective of cryptography is to prevent unauthorized third parties from gaining access to confidential information

Diffusion network (broadcast network)

In a diffusion network the signal of a transmitter is received by all the computers connected in the network, with the respective time delay taken into account. Every receiver must itself determine if the message is intended for it and whether or to process it

Flow control

Method to ensure an even and as continuous as possible data transmission between network terminals that do not work synchronously. Flow control intercedes to regulate the transmission sequence of network terminals and to slow down transmission power. This takes effect when congestion situations occur along the path to the receiver in order to avoid potential data loss

Fragmentation/defragmentation

Because of technical restrictions, the length of the data packet sending a communication protocol in a packet-sending network is always limited below the application layer. If the message to be sent is larger than the respectively prescribed data packet length, then the message is broken down into single sub messages (fragments) corresponding to the required length restrictions. To enable the original message to be put back together again correctly at the receiver after transmission (defragmentation), the fragments are provided with sequence numbers. This is necessary as the transmission sequence in the Internet cannot always be guaranteed

Internetworking

The bridging of multiple, different networks that are separated from each other (LANs, WANs) into one internet. The appropriate switching computers (routers) are needed to do this. They mediate the path of a data packet through the network and ensure a secure delivery. The network appears as a homogeneous, virtual network (internet) to the user

Internet standard

Because there were many companies and organizations involved in the development of the Internet, it was necessary to create unified protocols and interfaces to simplify the development effort. These took the form of Internet standards and were ratified in a public standardization process. Every user was principally allowed to make suggestion for future standards (Request for Comment, RFC), and thereby to steer the course of the Internet

Internet Protocol (IP)

Protocol on the network layer of the TCP/IP reference model, more precisely called IPv4. As one of the pillars of the Internet, IP ensures that the global Internet, consisting of many heterogeneous, individual networks appears as a unified homogeneous network. A standardized addressing scheme (IP addresses) enables word wide unambiguous computer identification. IP additionally provides a connectionless, packet switching datagram service that cannot fulfill quality of service guarantees, but always works according to the best effort principle. For the communication of control information and error notification, the ICMP protocol is an integral component of the IP

ISO/OSI Reference Model

A specification of the ISO that was designed and made public as the basis for the development of communication standards. It is an international reference model for data transmission consisting of seven layers. The ISO/OSI reference model has the goal of enabling different computer and protocol worlds to communicate with each other. In contrast to the TCP/IP reference model, the protocol standard underlying the Internet, the ISO/OSI reference model has become increasingly less important

Layer model

Complex problems allow themselves be broken down hierarchically into subproblems, built one on top of the other. The resulting layering of individual subproblems makes the modeling of the problem as a whole easier. The abstraction level increases on each individual layer. Therefore, a layer located higher in the layer model is from detail 60 2 The Foundation of the Internet: TCP/IP Reference Model problems handled on a lower layer. Layer models play an important role in communication technology, but also in other areas of information technology. A further representation may be seen in the corresponding shell model. Instead of hierarchical layers its structure is composed of individual shells

Local Area Network (LAN)

A spatially limited computer network that can only accommodate a limited number of terminals (computers). A LAN enables an efficient and equal communication for all the connected end systems. As a rule, the connected computers share a common transmission medium

Multicasting

One source transmits simultaneously to a group of receivers in a multicast transmission. This is a 1: n-communication. Multicast is often used for the transmission of multimedia data

Network application

An application program whose process includes the access to resources that are not available locally on the exporting computer, but rather on a remote computer across the network

Overload (congestion)

With its means of operation (transmission media, router, and other intermediate systems) a network is able to manage a specific load (communication, data transmission). If the load created in the network nears 100 % of the available capacity, an overload (congestion) occurs. The network must react in an appropriate way to avoid data loss and the breakdown of communication

Packet header

In a packet switching network, the communication protocols implemented require the fragmentation of the information to be transmitted into individual data packets. In order to ensure that the data packets reach the designated receiver in the correct form, and can be reassembled into their original information, command and control information is added to the data packet in a so-called data packet header

Packet switching

The primary communication method in digital networks. The message is broken down into individual data packets of a fixed sized. The packets are then sent individually and independently of each other from the transmitter, over any existing switching centers, to the receiver. A distinction is made between connection-oriented and connectionless (datagram network) packet switching networks. In connection-oriented packet switching networks, a connection is established over a fixed packet switching center established in the network before the start of the actual data transmission. Conversely, in connectionless networks there is no predetermined connection path

Protocol stack

The various subproblems of network communication are each handled by special protocols. These must all work together smoothly to solve the problem of network communication as a whole. In order to guarantee the functioning of this interplay, the development of network protocol software is seen as a comprehensive task. To solve it, an accompanying family of protocols (protocol suite) was developed that addresses each subtask and integrates them efficiently with each other. The entire problem of network communication may be represented with the help of a layer model. As the individual protocols of a protocol family are each assigned to a specific layer, the term protocol stack is used. The most well-known protocol stacks are the TCP/IP protocol suites of the Internet and the ISO/OSI layer model, which often serves as an instructional example

Quality of service

Quantifies the performance of a service offered by a communication system. It is described by means of the performance of service, quality attributes, performance fluctuation, reliability and security, which in each case are specified via individual, quantifiable service quality parameters

Reference model

An abstract model that serves as the basis for deriving more specialized models or concrete implementations. Reference models are often used as general objects of comparison with other models describing the same technical concept. In the area of computer networks there exist two well-known reference models. The ISO/OSI reference model, which is primarily used today for didactic purposes, and the model actually implemented in the Internet: the TCP/IP reference model

Request for Comments (RFC)

New technologies pertaining to the Internet under discussion by experts are recorded in so-called RFCs. In the course of the Internet standardization process there evolved a collection of consecutively numbered documents where technologies, standards and miscellaneous information connected to the Internet were documented and standardized

Router

A switching computer that is capable of connecting two or more subnets with each other. Routers work in the transport layer (IP layer) of the network and are able to forward arriving data packets along the shortest route through the network based on their destination address

Routing

Along the path of a WAN there are often multiple switching elements between transmitter and receiver. These carry out mediation of the transmitted data to the respective receiver. The determination of the correct path from transmitter to receiver is called routing. The dedicated switching centers (routers) receive a transmitted data packet, evaluate its address information and forward it correspondingly to the designated receiver

Server

Describes a process that clients request in order to receive information or be provided with resources. The computer on which a server process runs is often known as the server

Security

In network technology the term security encompasses different security objectives (quality of service parameters), describing the degree of integrity and authenticity of the transmitted data. Among the most important goals of security are: confidentiality (no unauthorized third party being able to eavesdrop on data communication between the transmitter and the receiver), integrity (ensuring accuracy of the received data), authentication (guarantee of the identity of the communication partner), liability (legally binding proof of a completed communication) and availability (guarantee that an offer of service is in fact available)

Transmission Control Protocol (TCP)

Protocol standard on the transport layer of the TCP/IP Reference Model. TCP provides a reliable, connection-oriented transport service upon which many Internet applications are based

TCP/IP Reference Model

(also TCP/IP protocol suite TCP/IP communications model), Designates a communication layer model for the Internet. The TCP/IP reference model is divided into 5 protocol layers and enables different computers and protocol worlds to communicate with each other via standardized interfaces on the Internet

Topology

The topology of a computer network is understood as the geometric form of the distribution of individual computer nodes in the network. Widespread topologies for computer networks are the bus topology, ring topology and star topology

Wide Area Network (WAN)

A freely scalable computer network that is not limited by spatial or capacity restrictions. Individual subnets are connected with each other by switching systems (routers), which coordinate data transfer in the WAN. The WAN technology supplies the foundation for internetworking