As we will see later on, the TCP/IP model has become the model of reference on place of the OSI model. This is due to its story. Indeed, contrary to the OSI model, the TCP/IP model was first implemented before being specified. This particular story makes TCP/IP’s characteristics, its advantages and drawbacks.

TCP/IP dates from the the ARPANET network. ARPANET is a telecommunication network developed by the ARPA (Advanced Research Projects Agency), the research agency of the American ministry of defence (the DOD: Department Of Defense). Besides the possibility to interconnect heterogeneous networks, this network was supposed to resist to a possible nuclear war, contrary to the telephon network usually used for telecommunication but considered as too vulnerable. It was then decided that ARPANET would use a standing out and promising new technology: packet switching (datagram mode). It is to meet this context that the TCP and IP protocols were invented in 1974. The ARPA then signed several agreements with manufacturers (especially BBN) and the Berkeley University, where a Unix system was under development, to impose this standard, and that was done.

Description

A 4-layer model

The TCP/IP model can indeed be described as a 4-layer network architecture:

The OSI model was added to ease the comparison between these two major models.

The host-network layer

It is a pretty strange layer. Indeed, it seems to comprise both physical and data link layers of the OSI model. Actually, this layer has not been really specified; the only constraint of this layer, is to allow a host to send IP packets of a network. The implementation of this layer is free. In more concrete terms, this implementation is typical of the technology used to build the local network (LAN). For example, LANs use Ethernet; Ethernet is an implementation of the host-network layer.

The internet layer

This layer is the key of the architecture. It realizes the interconnection of remote (heterogeneous) networks without establishing a connection. Its role is to inject packets into any network and deliver them to the destination independently to one another. As no connection is established first, packets may not be received in order; the delivery order control process is the responsability of the upper layers.

Because of the major role of this layer in the packet delivery process, the critical point of this layer is routing. For this reason, we may compare this layer to the network layer of the OSI model.

This internet layer has an offical implementation: the IP protocol (Internet Protocol).

We may notice that the name of this layer (“internet”) is written with a small i, for the simple reason that we consider here internet as an interconnection of networks, even if the Internet (with a big I) uses this layer.

The transport layer

It has the same role as the transport layer of the OSI model: it is used to make peer entities dialog with one another.

Officialy, this layer only has two possible implementations: the TCP protocol (Transmission Control Protocol) and the UDP protocol (User Datagram Protocol). TCP is a reliable and connection-oriented protocol that delivers packets without error from a machine of an internet to another machine of the same internet. Its role is to split up the message to be transmitted into a form the internet layer can handle. Conversely, on the receiving machine, TCP places fragments into order to reconstruct the initial message. TCP is also in charge of the flow control of the connection.

On the other hand, UDP is a very simple protocol: it is a non-reliable and connectionless protocol. Using it presupposes that we do not need flow control, either preserving the order of packets. For instance, we use it when the application layer is supposed to control the order of delivery. I remind you that in the OSI model, several layers were in charge of this order delivery control. It is an advantage of the TCP/IP model, but we will see this later on. We also use UDP to transmit voice. Indeed, inverting two sounds does not compromise the comprehension of the final message. In the more general way, we use UDP whenever it is more important to deliver packets on time.

The application layer

Contrary to the OSI model, this layer is immediately bound to the transport layer, simply because the session and presentation layers are useless. Indeed, with use, we have discover that network software hardely use these layers, and finally, the OSI model without these 2 layers is really close to the TCP/IP model

This layer holds all high level protocols, such as Telnet, TFTP (trivial File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), HTTP (HyperText Transfer Protocol). The main issue for this layer is that it can use both TCP or UDP protocols. For example, TFTP (mainly used over LANs) uses UDP, because we consider that physical links on a LAN are rather reliable and transmission times are short enough to avoid packets to be inverted. This makes TFTP faster than FTP, which uses TCP. On the contrary, SMTP uses TCP, because we want mails to be perfectly delivered, without any error.

Comparaison with the OSI model and criticism

Comparaison with the OSI model

First of all, let’s start with the common points. The TCP/IP and OSI models are both based on the concept of independent protocols stacks. Then, taken as a whole, functions are the same.

About the differences, we can notice the following issue: the OSI model makes clearly the difference between 3 main concepts, although TCP/IP does not make this distinction. These 3 concepts are services, interfaces and protocole. Indeed, TCP/IP does not make a clear difference between these concepts, in spite of the efforts of designers to bring the TCP/IP model closer to the OSI model. This is due to the fact that, for the TCP/IP model, protocols appear first, before specifications. The model is finally a theoretical justification of protocols, without making them independent to each other.

finally, the last important difference comes from the mode of connection. Indeed, connection-oriented modes and connectionless modes are available in both models, but not at the same layer: in the OSI model, these modes are only available at the network layer (at the transport layer, only the connection-oriented mode is available), though they are available at the transport layer in the TCP/IP model (the internet layer only offers the connectionless mode). Therefore, the TCP/IP has an advantage, compared to the OSI model: applications (that directly use the transport layer) have the choice between both modes.

Criticism

First, we can say that the TCP/IP model does not make the distinction between specifications and implementation: IP is a protocl that is an integral part of the specifications of the model.

We can also speak about the host-network layer: Indeed, it is not a real abstraction layer, insofar as its specification is not accurate enough. Manufacturers are then obliged to propose solutions to fill in the lacks. Finally, we notice that the physical and data link layers are as important as the transport layer. From this fact, we may propose a hybrid model with 5 layers, which would gather the good points of every model:

Hybrid model of reference Finaly, this model is the real reference in the Internet world. We have kept most layers of the OSI model (all but the session and presentation layers) because they are well specified. On the other hand, OSI’s protocols have no success and finally, we have kept those from the TCP/IP model.

The presentation layer then transforms this message and adds (or not) a new header (possibly empty). The presentation layer does not know and does not have to know the possible existence of AH; actually, for the presentation layer, AH is part of the user data. Once the data processing is finished, the presentation layer sends the new “message” to the session layer and the same process starts again.

The data then reach the physical layer which will indeed transmit the data to the recipient. Once received, the message will go up the layers and the headers are gradually removed until it reaches the receiving process:

The most important concept is as follows: it should be considered that each layer is programmed as if it were really horizontal, i.e. as if it dialogued directly with its receiving peer layer. When dialoguing with its peer layer, each layer adds a header and sends it (virtually, thanks to the subjacent layer) to its peer layer.

Criticism of the OSI model

The most striking issue concerning the OSI model is that it is perhaps the most studied and most unanimously accepted network structure and yet it is not the model which it is really implemented and used. The specialists, who analyzed this failure, determined 4 main reasons.

It was not the right moment

David Clark from the MIT has developed the following theory regarding the manner to publish a standard t the right time. To him, in the cycle of life of a standard, there are 2 principal peaks of activity: the research carried out in the field covered by the standard, and the industrial investments for the implementation and deployment of the standard. These two peaks are separated by a off-peak of activity that actually appears to be the ideal moment for the publication of the standard: it is neither too early compared to research so that we control the technology, nor too late for the investments and manufacturers are ready to spend capital to implement it.

The OSI model was perfectly released regarding research, but alas, the TCP/IP model was already receiving huge investments (when the OSI model was released, the American universities were already successfully using TCP/IP) and the manufacturers did not feel like investing on it.

It was not the right technology

The OSI model is maybe to complete and too complex. The gap between the concrete use (implementation) and the model is sometimes significant. Indeed, few programs can use or wrongly use the 7 layers of the model: the session and presentation layers are hardly used and on contrary the data link and network layers are often split into several sub-layers, since they are pretty complex.

The OSI model is in fact too complex to be effectively and properly implemented. The committee that wrote the standard even had to leave aside some technical points, like security and coding, so much it was delicate to preserve a well defined role to each layer completed with these extra technical points. This model is also redundant (the flow control and the error control appear in most layers). At the implementation level, TCP/IP is much more optimized and effective.

The most significant criticism that one can make against the OSI model is that it is not adapted at all to telecommunication applications on computer! Some choices are in disagreement with the way computers and software communicate. The standard actually uses “system interruptions” to report events, and with high level programming languages, that is not very realizable.

It was not the right implementation

This simply comes from the fact that the model is relatively complex, and therefore the first implementations were pretty heavy and slow. Conversely, the first implementation of TCP/IP in the Unix system of the Berkeley University (BSD) was free and relatively effective. Historically, people thus had a natural tendency to use TCP/IP.

It was not the right policy

Actually, the OSI model suffered from its too strong standardization. The efforts of implementation of the model were above all “bureaucratic” and therefore people might have discredited the model.

The future of the OSI model

Regarding its use and implementation, and in spite of an update of the model in 1994, the OSI model has clearly lost the war against TCP/IP. Only few dominating manufacturers keep the model but it is likely to disappear, all the more quickly since the Internet (and thus TCP/IP) is developing.

However, the OSI model will still remain for a while in memories for several reasons. First, it is one of the first main efforts as regards standardization in the area of networks. Manufacturers now tend to do with TCP/IP, but also WAP, UMTS etc. what they were supposed to do with the OSI model, namely to propose standardizations from the beginning. The OSI model will also remain memories for another reason: even if TCP/IP is the model concretely used, people have tendency and use OSI like the current network model of reference. In fact, TCP/IP and OSI have very close structures, and it is especially the effort of standardization of OSI which imposed this general “confusion” between the 2 models. One commonly tends to consider TCP/IP as the real implementation of OSI.

When designing this model, taking the heterogeneity of the equipment into account was a fundamental issue. Indeed, this model was designed to allow the interconnection of heterogeneous systems for historical and economic reasons. Besides, it should not support a particular provider. Lastly, it should make it possible to adapt to the evolution of data to process without calling into question the investments. Thus, all this led the adoption of common communication and co-operation rules between the equipment, i.e. this model should logically carry out to an international standardization of protocols.

The OSI model is not a real network architecture, because it does not really specify the services and protocols each layer should use. It rather describes what the layers must do. Nevertheless, the ISO has developed its own standards for each layer, and this independently of the OSI model, i.e. as does any manufacturer.

The first works related to the OSI model date from 1977. They were based on the experience gained in the area of wide area networks and local private networks; the OSI model was indeed supposed to be valid for any type of network. In 1978, the ISO proposed this model as the standard ISO IS7498. In 1984, 12 European manufacturers, joined in 1985 by the main American manufacturers, adopted this standard.

The different layers of the OSI model

The 7 layers

The OSI model is composed of 7 layers:

The principles that led to these 7 layers were the following:

• A layer must be created every time a new level of abstraction is necessary,
• every layer has well defined functions,
• the functions of each layer must be chosen in the objective of the international standardization of protocols,
• boundaries between layers must be chosen so as to minimize the flows of data through interfaces,
• the number of layers must be such as there is no cohabitation of completely different functions within the same layer and such as it is not too difficult to control the architecture.

The low layers (1, 2, 3 and 4) are necessary to the routing of information between the two concerned ends and depend on the physical medium. The higher layers (5, 6 and 7) are responsible for the data processing relative to the management of exchanges between information processing systems. In addition, layers 1 to 3 intervene between close machines, but not between ending machines that can be separated by several routers. On the contrary, layers 4 to 7 intervene only between distant hosts.

The physical layer

This layer is in charge of the raw transmission of bits over a communication channel. This layer must guarantee the perfect transmission of the data (a bit set to 1 must be received as a bit set to 1). Concretely, this layer must standardize the electrical characteristics (for instance, a bit set to 1 is represented by a voltage of 5V), the mechanical characteristics (the shape of the connectors, topology…), the functional characteristics of the circuits of data and the procedures of establishment, maintenance and release of the circuit of data.

The typical information unit for this layer is the bit, represented by a given voltage.

The data link layer

Its has a role of “binder”: it will transform the physical layer into a connection a priori freefrom transmission errors for the network layer. It splits the input data of the sender into frames, sends these frames in sequence and manages the acknowledgement frames sent back by the receiver. To remind, for the physical layer, the data do not have any particular meaning. The data link layer must therefore be able to recognize the limits of frames. This can actually pose problems, since the sequences of bits used to identify boundaries may also appear in the data.

The data link layer must be able to signal a transmission problem by sending an appropriate frame. In a general way, an important role of this layer consists in detecting and correcting errors that occured on the physical layer. This layer integrates also a flow control function to avoid the blocking of the receiver.

The information unit for this layer is the frame made up of a few hundreds to a few thousands of bytes maximum.

The network layer

This layer is in charge of the sub-network, i.e. the routing packets over the sub-networks and the interconnection of the various sub-networks. When designing it, it is very important to determine the routing mechanism and calculation of the routing tables (static or dynamic tables…).

The network layer also controls sub-network congestions. It is also possible to complete it with accounting functions for invoicing on volume, but this may be delicate.

The information unit for this layer is the packet.

The transport layer

This layer is responsible for the good delivery of messages to the recipient. Its main role is to take the messages of the session layer, split them into smaller units and give them to the network layer, while checking pieces arrive correctly. Therefore, this layer also re-assembles the initial message when it receives the pieces.

This layer is also responsible for the optimization of the network resources: normally, the transport layer should create a network connection for every transport connection required by the session layer, but it is able to create several network connections by session layer’s process, for example to improve the bit rate. Conversely, this layer can use one network connection to transport several messages at the same time, using multiplexing. In any case, all this must transparent for the session layer.

This layer is also responsible for the type of service to provide to the session layer, any finaly to the users of the network: connection-oriented or connectionless service, with or without guarantee of the delivery order, broadcast… Thus, this layer is also responsible for opening and closing network connections.

One of its latest role is flow control.

It is one of the most important layers, because it provides the basic service to the user and controls the whole connection process, with all the related constraints.

The information unit for this layer is the message.

The session layer

This layer sets up and synchronizes the exchanges between distant processes. It binds logical addresses to physical addresses for distributed tasks. It also binds two application programs that must cooperate control their dialogue (which one should speak, which is currently speaking…). In this latter case, the service of set up is called the token management. The session layer also makes it possible to insert recovery points in the data flow in order to resume dialogue after a failure.

The presentation layer

This layer deals with the syntax and semantics of the transmitted data: it processes the data so as to make it compatible between communicating tasks. It will ensure the independance between the user and the data transport.

Typically, this layer can convert, format, crypt and compress the data.

The application layer

This layer is the point of contact between the user and the network. Therefore, it brings the basic network services to the user, such as file transfer, electronic mail…

A characteristic of these technologies is that these technologies have progressively converged. Gradually we passed from centralized systems with generally a single central computer used by several computer scientists, to a global network of devices and distributed systems that allow users to share out calculation and storage capabilities.

Some landmarks

• Beginning of the 60′s:
  • flight ticket booking systems
  • distribution of banking operations
• 1969: the American ARPA network is built to connect the main American research centres. It is the first architecture set with abstraction layers.
• 1974: IBM invents SNA (system Network Architecture) that allows the communication between a central system and remote peripherals
• 1975: Intel and Xerox write the Ethernet standard
• 1980: Beginning of the Internet
• 1982: IBM sells the Token Ring standard
• 1985: The IEEE publishes standards for the 802.X protocols (especially Ethernet and Token Ring)
• 1989-1991: Beginning of the Web, the main Internet application
• 1990: Beginning of the ISDN (Integrated Services Digital Network)

From the beginning of the 80′s, networks world-wide have been progressively connected together to build the Internet. Internet means “interconnected networks”; it is a world-wide network that provides a set of services, set that is absolutely not limited to the Web, contrary to what we could commonly think.

Definitions

A network of computers is a set of independant computers (called nodes), sites or hosts connected together and that are able to exchange data with communication lines:

The autonomy of computers excludes any master/slave relation; therefore, any system made up of a central controling unit and several slave units (e.g. a computer with remote printers and terminals) are excluded from this definition.

We call network system or network software a set of software modules that manage communications between sites. The software is installed on every node of the network.

We distinguish computer networks and distributed systems: a distributed system is a set of independant computers connected together that host a software so that the distribution into autonomous computers is transparent and not detectable by users. In concrete terms, when a user wants to launch a program, the operating system manages to select the best processor, find and convey input files to this processor and put results at the right place.

In the case of a network, everything is explicitely performed (we first have to connect to a given machine, send and execute a given command on this given machine, we must explicitely move files…); although in the case of a distributed system, everything is automatically performed by the operating system. The difference is located at the operating system level, and not at the hardware level (type of computer, type of connectors…). We can sum up all this on the following outline:

Goals and use of networks

Networks have been developed for certain reasons. There are mainly 4 of them.

Resource sharing

The first reason is resource sharing: networks give access to resources (software, databases, printers…) in an way independent from the geographical location of users. For instance, we use this to access shared trading data in a company: each employee of a global company can access balance sheets.

Increasing of reliability and performances

This is the second reason. Networks can be used to duplicate vital data onto several servers; in case of problem the backup version is immediately available. The increasing of performances comes from the fact that it is easy to increase performances of a system by simply adding one or two extra computers. This latter point, associated with an economical fact (see the next goal), makes mainframes useless.

Costs reduction

The third goal of networks is cost reduction. Personal computers are indeed less expensive than mainframes (about 1.000 times less expensive), and this only for performances only 10 times less efficient. Networks also allow people to reacte faster to some events (e.g. invitations to tender) and therefore to earn (or save) money.

Access information

This is also a main goal of networks. With networks and especially the Internet, it is very easy to be informed about any kind of subject. This latter goal is a very crucial one in the way people use networks. Nowadays it is even the main goal.

Other uses

Beyond these 4 points, networks have a couple of other goals, but these appear recently in the same time as liberalization of networks and especially with the emergence of the Internet. What is remarkable is that these new goals are not needs for companies. For instance, networks can be the medium for interative games and other entertainments, as well as the medium for communication. These reasons have rather important social consequences because they influence a lot people’s behaviors.

We.. (Nag & Srini)..working as network administrators in CMMI Level 3 company. After working for 3 years and faced lot many problems and googled for the solutions, with the knowledge acquired all these days we thought to share with others by writing all about Networks (mostly Cisco), Operating systems (Windows, Linux, Solaris) and other softwares which we use in day to day work or if you suggest a problem to us we will try to help by providing better solution.

we hope to see you all here.