Network Models PDF

CHAPTER 2 Network Models T he second chapter is a preparation for the rest of the book. The next five parts of the book is devoted to one of the layers in the TCP/IP protocol suite. In this chapter, we first discuss the idea of network mo...

CHAPTER 2 Network Models T he second chapter is a preparation for the rest of the book. The next five parts of the book is devoted to one of the layers in the TCP/IP protocol suite. In this chapter, we first discuss the idea of network models in general and the TCP/IP protocol suite in particular. Two models have been devised to define computer network operations: the TCP/IP protocol suite and the OSI model. In this chapter, we first discuss a general subject, protocol layering, which is used in both models. We then concentrate on the TCP/IP protocol suite, on which the book is based. The OSI model is briefly discuss for comparison with the TCP/IP protocol suite. ❑ The first section introduces the concept of protocol layering using two scenarios. The section also discusses the two principles upon which the protocol layering is based. The first principle dictates that each layer needs to have two opposite tasks. The second principle dictates that the corresponding layers should be identical. The section ends with a brief discussion of logical connection between two identi- cal layers in protocol layering. Throughout the book, we need to distinguish between logical and physical connections. ❑ The second section discusses the five layers of the TCP/IP protocol suite. We show how packets in each of the five layers (physical, data-link, network, transport, and application) are named. We also mention the addressing mechanism used in each layer. Each layer of the TCP/IP protocol suite is a subject of a part of the book. In other words, each layer is discussed in several chapters; this section is just an intro- duction and preparation. ❑ The third section gives a brief discussion of the OSI model. This model was never implemented in practice, but a brief discussion of the model and its comparison with the TCP/IP protocol suite may be useful to better understand the TCP/IP pro- tocol suite. In this section we also give a brief reason for the OSI model’s lack of success. 31 32 PART I OVERVIEW 2.1 PROTOCOL LAYERING We defined the term protocol in Chapter 1. In data communication and networking, a protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively. When communication is simple, we may need only one simple protocol; when the communication is complex, we may need to divide the task between different layers, in which case we need a protocol at each layer, or protocol layering. 2.1.1 Scenarios Let us develop two simple scenarios to better understand the need for protocol layering. First Scenario In the first scenario, communication is so simple that it can occur in only one layer. Assume Maria and Ann are neighbors with a lot of common ideas. Communication between Maria and Ann takes place in one layer, face to face, in the same language, as shown in Figure 2.1. Figure 2.1 A single-layer protocol Maria Ann Layer 1 Listen/Talk Listen/Talk Layer 1 Air Even in this simple scenario, we can see that a set of rules needs to be followed. First, Maria and Ann know that they should greet each other when they meet. Second, they know that they should confine their vocabulary to the level of their friendship. Third, each party knows that she should refrain from speaking when the other party is speaking. Fourth, each party knows that the conversation should be a dialog, not a monolog: both should have the opportunity to talk about the issue. Fifth, they should exchange some nice words when they leave. We can see that the protocol used by Maria and Ann is different from the commu- nication between a professor and the students in a lecture hall. The communication in the second case is mostly monolog; the professor talks most of the time unless a student has a question, a situation in which the protocol dictates that she should raise her hand and wait for permission to speak. In this case, the communication is normally very for- mal and limited to the subject being taught. Second Scenario In the second scenario, we assume that Ann is offered a higher-level position in her company, but needs to move to another branch located in a city very far from Maria. The two friends still want to continue their communication and exchange ideas because  CHAPTER 2 NETWORK MODELS 33 they have come up with an innovative project to start a new business when they both retire. They decide to continue their conversation using regular mail through the post office. However, they do not want their ideas to be revealed by other people if the let- ters are intercepted. They agree on an encryption/decryption technique. The sender of the letter encrypts it to make it unreadable by an intruder; the receiver of the letter decrypts it to get the original letter. We discuss the encryption/decryption methods in Chapter 31, but for the moment we assume that Maria and Ann use one technique that makes it hard to decrypt the letter if one does not have the key for doing so. Now we can say that the communication between Maria and Ann takes place in three layers, as shown in Figure 2.2. We assume that Ann and Maria each have three machines (or robots) that can perform the task at each layer. Figure 2.2 A three-layer protocol Maria Ann Layer 3 Listen/Talk Listen/Talk Layer 3 Identical objects Plaintext Plaintext Layer 2 Encrypt/Decrypt Encrypt/Decrypt Layer 2 Identical objects Ciphertext Ciphertext Send mail/ Send mail/ Layer 1 Layer 1 receive mail receive mail Identical objects Mail Mail US Post US Post Postal carrier facility Let us assume that Maria sends the first letter to Ann. Maria talks to the machine at the third layer as though the machine is Ann and is listening to her. The third layer machine listens to what Maria says and creates the plaintext (a letter in English), which is passed to the second layer machine. The second layer machine takes the plaintext, encrypts it, and creates the ciphertext, which is passed to the first layer machine. The first layer machine, presumably a robot, takes the ciphertext, puts it in an envelope, adds the sender and receiver addresses, and mails it. At Ann’s side, the first layer machine picks up the letter from Ann’s mail box, rec- ognizing the letter from Maria by the sender address. The machine takes out the cipher- text from the envelope and delivers it to the second layer machine. The second layer machine decrypts the message, creates the plaintext, and passes the plaintext to the third-layer machine. The third layer machine takes the plaintext and reads it as though Maria is speaking. 34 PART I OVERVIEW Protocol layering enables us to divide a complex task into several smaller and sim- pler tasks. For example, in Figure 2.2, we could have used only one machine to do the job of all three machines. However, if Maria and Ann decide that the encryption/ decryption done by the machine is not enough to protect their secrecy, they would have to change the whole machine. In the present situation, they need to change only the sec- ond layer machine; the other two can remain the same. This is referred to as modularity. Modularity in this case means independent layers. A layer (module) can be defined as a black box with inputs and outputs, without concern about how inputs are changed to outputs. If two machines provide the same outputs when given the same inputs, they can replace each other. For example, Ann and Maria can buy the second layer machine from two different manufacturers. As long as the two machines create the same cipher- text from the same plaintext and vice versa, they do the job. One of the advantages of protocol layering is that it allows us to separate the services from the implementation. A layer needs to be able to receive a set of ser- vices from the lower layer and to give the services to the upper layer; we don’t care about how the layer is implemented. For example, Maria may decide not to buy the machine (robot) for the first layer; she can do the job herself. As long as Maria can do the tasks provided by the first layer, in both directions, the communication system works. Another advantage of protocol layering, which cannot be seen in our simple exam- ples but reveals itself when we discuss protocol layering in the Internet, is that commu- nication does not always use only two end systems; there are intermediate systems that need only some layers, but not all layers. If we did not use protocol layering, we would have to make each intermediate system as complex as the end systems, which makes the whole system more expensive. Is there any disadvantage to protocol layering? One can argue that having a single layer makes the job easier. There is no need for each layer to provide a service to the upper layer and give service to the lower layer. For example, Ann and Maria could find or build one machine that could do all three tasks. However, as mentioned above, if one day they found that their code was broken, each would have to replace the whole machine with a new one instead of just changing the machine in the second layer. 2.1.2 Principles of Protocol Layering Let us discuss two principles of protocol layering. First Principle The first principle dictates that if we want bidirectional communication, we need to make each layer so that it is able to perform two opposite tasks, one in each direction. For example, the third layer task is to listen (in one direction) and talk (in the other direction). The second layer needs to be able to encrypt and decrypt. The first layer needs to send and receive mail. Second Principle The second principle that we need to follow in protocol layering is that the two objects under each layer at both sites should be identical. For example, the object under layer 3 at both sites should be a plaintext letter. The object under layer 2 at  CHAPTER 2 NETWORK MODELS 35 both sites should be a ciphertext letter. The object under layer 1 at both sites should be a piece of mail. 2.1.3 Logical Connections After following the above two principles, we can think about logical connection between each layer as shown in Figure 2.3. This means that we have layer-to-layer communication. Maria and Ann can think that there is a logical (imaginary) connection at each layer through which they can send the object created from that layer. We will see that the concept of logical connection will help us better understand the task of lay- ering we encounter in data communication and networking. Figure 2.3 Logical connection between peer layers Maria Ann Layer 3 Talk/Listen Listen/Talk Layer 3 Logical connection Plaintext Plaintext Layer 2 Encrypt/Decrypt Encrypt/Decrypt Layer 2 Logical connection Ciphertext Ciphertext Send mail/ Send mail/ Layer 1 Layer 1 receive mail receive mail Mail Logical connection Mail 2.2 TCP/IP PROTOCOL SUITE Now that we know about the concept of protocol layering and the logical communica- tion between layers in our second scenario, we can introduce the TCP/IP (Transmission Control Protocol/Internet Protocol). TCP/IP is a protocol suite (a set of protocols orga- nized in different layers) used in the Internet today. It is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality. The term hier- archical means that each upper level protocol is supported by the services provided by one or more lower level protocols. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model. Figure 2.4 shows both configurations. 2.2.1 Layered Architecture To show how the layers in the TCP/IP protocol suite are involved in communication between two hosts, we assume that we want to use the suite in a small internet made up of three LANs (links), each with a link-layer switch. We also assume that the links are connected by one router, as shown in Figure 2.5. 36 PART I OVERVIEW Figure 2.4 Layers in the TCP/IP protocol suite Application Application Layer 5 Transport Transport Layer 4 Internet Network Layer 3 Network Interface Data link Layer 2 Hardware Devices Physical Layer 1 a. Original layers b. Layers used in this book Figure 2.5 Communication through an internet Source (A) Destination (B) Application Application Transport Router Transport Network Switch Network Switch Network Data link Data link Data link Data link Data link Data link Physical Physical Physical Physical Physical Physical Communication from A to B Router A Link 1 Link 2 B Link 3 C Let us assume that computer A communicates with computer B. As the figure shows, we have five communicating devices in this communication: source host (computer A), the link-layer switch in link 1, the router, the link-layer switch in link 2, and the destination host (computer B). Each device is involved with a set of layers depending on the role of the device in the internet. The two hosts are involved in all five layers; the source host needs to create a message in the application layer and send it down the layers so that it is physically sent to the destination host. The destination host needs to receive the communication at the physical layer and then deliver it through the other layers to the application layer.  CHAPTER 2 NETWORK MODELS 37 The router is involved in only three layers; there is no transport or application layer in a router as long as the router is used only for routing. Although a router is always involved in one network layer, it is involved in n combinations of link and physical lay- ers in which n is the number of links the router is connected to. The reason is that each link may use its own data-link or physical protocol. For example, in the above figure, the router is involved in three links, but the message sent from source A to destination B is involved in two links. Each link may be using different link-layer and physical-layer protocols; the router needs to receive a packet from link 1 based on one pair of proto- cols and deliver it to link 2 based on another pair of protocols. A link-layer switch in a link, however, is involved only in two layers, data-link and physical. Although each switch in the above figure has two different connections, the connections are in the same link, which uses only one set of protocols. This means that, unlike a router, a link-layer switch is involved only in one data-link and one physical layer. 2.2.2 Layers in the TCP/IP Protocol Suite After the above introduction, we briefly discuss the functions and duties of layers in the TCP/IP protocol suite. Each layer is discussed in detail in the next five parts of the book. To better understand the duties of each layer, we need to think about the logical connections between layers. Figure 2.6 shows logical connections in our sim- ple internet. Figure 2.6 Logical connections between layers of the TCP/IP protocol suite Source Destination host host Logical connections Application Application Transport Transport Network Network Data link Data link Physical Physical Switch Router Switch LAN LAN Router Source Link 1 Link 2 Destination host To link 3 host Using logical connections makes it easier for us to think about the duty of each layer. As the figure shows, the duty of the application, transport, and network layers is end-to-end. However, the duty of the data-link and physical layers is hop-to-hop, in which a hop is a host or router. In other words, the domain of duty of the top three layers is the internet, and the domain of duty of the two lower layers is the link. Another way of thinking of the logical connections is to think about the data unit created from each layer. In the top three layers, the data unit (packets) should not be 38 PART I OVERVIEW changed by any router or link-layer switch. In the bottom two layers, the packet created by the host is changed only by the routers, not by the link-layer switches. Figure 2.7 shows the second principle discussed previously for protocol layering. We show the identical objects below each layer related to each device. Figure 2.7 Identical objects in the TCP/IP protocol suite Notes: We have not shown switches because they don’t change objects. Application Application Identical objects (messages) Transport Transport Identical objects (segments or user datagrams) Network Network Identical objects (datagrams) Identical objects (datagrams) Data link Data link Identical objects (frames) Identical objects (frames) Physical Physical Identical objects (bits) Identical objects (bits) Note that, although the logical connection at the network layer is between the two hosts, we can only say that identical objects exist between two hops in this case because a router may fragment the packet at the network layer and send more packets than received (see fragmentation in Chapter 19). Note that the link between two hops does not change the object. 2.2.3 Description of Each Layer After understanding the concept of logical communication, we are ready to briefly dis- cuss the duty of each layer. Our discussion in this chapter will be very brief, but we come back to the duty of each layer in next five parts of the book. Physical Layer We can say that the physical layer is responsible for carrying individual bits in a frame across the link. Although the physical layer is the lowest level in the TCP/IP protocol suite, the communication between two devices at the physical layer is still a logical communication because there is another, hidden layer, the transmission media, under the physical layer. Two devices are connected by a transmission medium (cable or air). We need to know that the transmission medium does not carry bits; it carries electrical or optical signals. So the bits received in a frame from the data-link layer are trans- formed and sent through the transmission media, but we can think that the logical unit between two physical layers in two devices is a bit. There are several protocols that transform a bit to a signal. We discuss them in Part II when we discuss the physical layer and the transmission media.  CHAPTER 2 NETWORK MODELS 39 Data-link Layer We have seen that an internet is made up of several links (LANs and WANs) connected by routers. There may be several overlapping sets of links that a datagram can travel from the host to the destination. The routers are responsible for choosing the best links. However, when the next link to travel is determined by the router, the data-link layer is responsible for taking the datagram and moving it across the link. The link can be a wired LAN with a link-layer switch, a wireless LAN, a wired WAN, or a wireless WAN. We can also have different protocols used with any link type. In each case, the data-link layer is responsible for moving the packet through the link. TCP/IP does not define any specific protocol for the data-link layer. It supports all the standard and proprietary protocols. Any protocol that can take the datagram and carry it through the link suffices for the network layer. The data-link layer takes a data- gram and encapsulates it in a packet called a frame. Each link-layer protocol may provide a different service. Some link-layer proto- cols provide complete error detection and correction, some provide only error correc- tion. We discuss wired links in Chapters 13 and 14 and wireless links in Chapters 15 and 16. Network Layer The network layer is responsible for creating a connection between the source computer and the destination computer. The communication at the network layer is host-to-host. However, since there can be several routers from the source to the destination, the routers in the path are responsible for choosing the best route for each packet. We can say that the network layer is responsible for host-to-host communication and routing the packet through possible routes. Again, we may ask ourselves why we need the network layer. We could have added the routing duty to the transport layer and dropped this layer. One reason, as we said before, is the separation of different tasks between different layers. The second reason is that the routers do not need the application and transport layers. Separating the tasks allows us to use fewer protocols on the routers. The network layer in the Internet includes the main protocol, Internet Protocol (IP), that defines the format of the packet, called a datagram at the network layer. IP also defines the format and the structure of addresses used in this layer. IP is also responsible for routing a packet from its source to its destination, which is achieved by each router forwarding the datagram to the next router in its path. IP is a connectionless protocol that provides no flow control, no error control, and no congestion control services. This means that if any of theses services is required for an application, the application should rely only on the transport-layer protocol. The net- work layer also includes unicast (one-to-one) and multicast (one-to-many) routing pro- tocols. A routing protocol does not take part in routing (it is the responsibility of IP), but it creates forwarding tables for routers to help them in the routing process. The network layer also has some auxiliary protocols that help IP in its delivery and routing tasks. The Internet Control Message Protocol (ICMP) helps IP to report some problems when routing a packet. The Internet Group Management Protocol (IGMP) is another protocol that helps IP in multitasking. The Dynamic Host Configuration Proto- col (DHCP) helps IP to get the network-layer address for a host. The Address Resolu- tion Protocol (ARP) is a protocol that helps IP to find the link-layer address of a host or 40 PART I OVERVIEW a router when its network-layer address is given. ARP is discussed in Chapter 9, ICMP in Chapter 19, and IGMP in Chapter 21. Transport Layer The logical connection at the transport layer is also end-to-end. The transport layer at the source host gets the message from the application layer, encapsulates it in a transport- layer packet (called a segment or a user datagram in different protocols) and sends it, through the logical (imaginary) connection, to the transport layer at the destination host. In other words, the transport layer is responsible for giving services to the application layer: to get a message from an application program running on the source host and deliver it to the corresponding application program on the destination host. We may ask why we need an end-to-end transport layer when we already have an end-to-end applica- tion layer. The reason is the separation of tasks and duties, which we discussed earlier. The transport layer should be independent of the application layer. In addition, we will see that we have more than one protocol in the transport layer, which means that each application program can use the protocol that best matches its requirement. As we said, there are a few transport-layer protocols in the Internet, each designed for some specific task. The main protocol, Transmission Control Protocol (TCP), is a connection-oriented protocol that first establishes a logical connection between trans- port layers at two hosts before transferring data. It creates a logical pipe between two TCPs for transferring a stream of bytes. TCP provides flow control (matching the send- ing data rate of the source host with the receiving data rate of the destination host to prevent overwhelming the destination), error control (to guarantee that the segments arrive at the destination without error and resending the corrupted ones), and conges- tion control to reduce the loss of segments due to congestion in the network. The other common protocol, User Datagram Protocol (UDP), is a connectionless protocol that transmits user datagrams without first creating a logical connection. In UDP, each user datagram is an independent entity without being related to the previous or the next one (the meaning of the term connectionless). UDP is a simple protocol that does not pro- vide flow, error, or congestion control. Its simplicity, which means small overhead, is attractive to an application program that needs to send short messages and cannot afford the retransmission of the packets involved in TCP, when a packet is corrupted or lost. A new protocol, Stream Control Transmission Protocol (SCTP) is designed to respond to new applications that are emerging in the multimedia. We will discuss UDP, TCP, and SCTP in Chapter 24. Application Layer As Figure 2.6 shows, the logical connection between the two application layers is end- to-end. The two application layers exchange messages between each other as though there were a bridge between the two layers. However, we should know that the commu- nication is done through all the layers. Communication at the application layer is between two processes (two programs running at this layer). To communicate, a process sends a request to the other process and receives a response. Process-to-process communication is the duty of the applica- tion layer. The application layer in the Internet includes many predefined protocols, but  CHAPTER 2 NETWORK MODELS 41 a user can also create a pair of processes to be run at the two hosts. In Chapter 25, we explore this situation. The Hypertext Transfer Protocol (HTTP) is a vehicle for accessing the World Wide Web (WWW). The Simple Mail Transfer Protocol (SMTP) is the main protocol used in electronic mail (e-mail) service. The File Transfer Protocol (FTP) is used for transferring files from one host to another. The Terminal Network (TELNET) and Secure Shell (SSH) are used for accessing a site remotely. The Simple Network Man- agement Protocol (SNMP) is used by an administrator to manage the Internet at global and local levels. The Domain Name System (DNS) is used by other protocols to find the network-layer address of a computer. The Internet Group Management Protocol (IGMP) is used to collect membership in a group. We discuss most of these protocols in Chapter 26 and some in other chapters. 2.2.4 Encapsulation and Decapsulation One of the important concepts in protocol layering in the Internet is encapsulation/ decapsulation. Figure 2.8 shows this concept for the small internet in Figure 2.5. Figure 2.8 Encapsulation/Decapsulation 4 Header at transport layer Encapsulate Legend 3 Header at network layer 2 Header at data-link layer Decapsulate Destination host Source host Message Application Router Application Message 4 Message Transport Transport 4 Message 3 4 Message Network 3 4 Message 3 4 Message Network 3 4 Message 2 3 4 Message Data link 2 3 4 Message 2 3 4 Message Data link 2 3 4 Message Physical Physical We have not shown the layers for the link-layer switches because no encapsulation/ decapsulation occurs in this device. In Figure 2.8, we show the encapsulation in the source host, decapsulation in the destination host, and encapsulation and decapsulation in the router. Encapsulation at the Source Host At the source, we have only encapsulation. 1. At the application layer, the data to be exchanged is referred to as a message. A message normally does not contain any header or trailer, but if it does, we refer to the whole as the message. The message is passed to the transport layer. 2. The transport layer takes the message as the payload, the load that the transport layer should take care of. It adds the transport layer header to the payload, which contains the identifiers of the source and destination application programs that 42 PART I OVERVIEW want to communicate plus some more information that is needed for the end-to- end delivery of the message, such as information needed for flow, error control, or congestion control. The result is the transport-layer packet, which is called the seg- ment (in TCP) and the user datagram (in UDP). The transport layer then passes the packet to the network layer. 3. The network layer takes the transport-layer packet as data or payload and adds its own header to the payload. The header contains the addresses of the source and destination hosts and some more information used for error checking of the header, fragmentation information, and so on. The result is the network-layer packet, called a datagram. The network layer then passes the packet to the data-link layer. 4. The data-link layer takes the network-layer packet as data or payload and adds its own header, which contains the link-layer addresses of the host or the next hop (the router). The result is the link-layer packet, which is called a frame. The frame is passed to the physical layer for transmission. Decapsulation and Encapsulation at the Router At the router, we have both decapsulation and encapsulation because the router is con- nected to two or more links. 1. After the set of bits are delivered to the data-link layer, this layer decapsulates the datagram from the frame and passes it to the network layer. 2. The network layer only inspects the source and destination addresses in the datagram header and consults its forwarding table to find the next hop to which the datagram is to be delivered. The contents of the datagram should not be changed by the network layer in the router unless there is a need to fragment the datagram if it is too big to be passed through the next link. The datagram is then passed to the data-link layer of the next link. 3. The data-link layer of the next link encapsulates the datagram in a frame and passes it to the physical layer for transmission. Decapsulation at the Destination Host At the destination host, each layer only decapsulates the packet received, removes the payload, and delivers the payload to the next-higher layer protocol until the message reaches the application layer. It is necessary to say that decapsulation in the host involves error checking. 2.2.5 Addressing It is worth mentioning another concept related to protocol layering in the Internet, addressing. As we discussed before, we have logical communication between pairs of layers in this model. Any communication that involves two parties needs two addresses: source address and destination address. Although it looks as if we need five pairs of addresses, one pair per layer, we normally have only four because the physical layer does not need addresses; the unit of data exchange at the physical layer is a bit, which defi- nitely cannot have an address. Figure 2.9 shows the addressing at each layer. As the figure shows, there is a relationship between the layer, the address used in that layer, and the packet name at that layer. At the application layer, we normally use names to define the site that provides services, such as someorg.com, or the e-mail  CHAPTER 2 NETWORK MODELS 43 Figure 2.9 Addressing in the TCP/IP protocol suite Packet names Layers Addresses Message Application layer Names Segment / User datagram Transport layer Port numbers Datagram Network layer Logical addresses Frame Data-link layer Link-layer addresses Bits Physical layer address, such as [email protected]. At the transport layer, addresses are called port numbers, and these define the application-layer programs at the source and destination. Port numbers are local addresses that distinguish between several programs network :IP, logical running at the same time. At the network-layer, the addresses are global, with the whole Internet as the scope. A network-layer address uniquely defines the connection of a device to the Internet. The link-layer addresses, sometimes called MAC addresses, are locally defined addresses, each of which defines a specific host or router in a network (LAN or WAN). We will come back to these addresses in future chapters. 2.2.6 Multiplexing and Demultiplexing Since the TCP/IP protocol suite uses several protocols at some layers, we can say that we have multiplexing at the source and demultiplexing at the destination. Multiplexing in this case means that a protocol at a layer can encapsulate a packet from several next-higher layer protocols (one at a time); demultiplexing means that a protocol can decapsulate and deliver a packet to several next-higher layer protocols (one at a time). Figure 2.10 shows the concept of multiplexing and demultiplexing at the three upper layers. Figure 2.10 Multiplexing and demultiplexing FTP HTTP DNS SNMP FTP HTTP DNS SNMP TCP UDP TCP UDP IP IP a. Multiplexing at source b. Demultiplexing at destination To be able to multiplex and demultiplex, a protocol needs to have a field in its header to identify to which protocol the encapsulated packets belong. At the transport 44 PART I OVERVIEW layer, either UDP or TCP can accept a message from several application-layer protocols. At the network layer, IP can accept a segment from TCP or a user datagram from UDP. IP can also accept a packet from other protocols such as ICMP, IGMP, and so on. At the data-link layer, a frame may carry the payload coming from IP or other protocols such as ARP (see Chapter 9). 2.3 THE OSI MODEL Although, when speaking of the Internet, everyone talks about the TCP/IP protocol suite, this suite is not the only suite of protocols defined. Established in 1947, the International Organization for Standardization (ISO) is a multinational body dedicated to worldwide agreement on international standards. Almost three-fourths of the countries in the world are represented in the ISO. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. ISO is the organization; OSI is the model. An open system is a set of protocols that allows any two different systems to com- municate regardless of their underlying architecture. The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable. The OSI model was intended to be the basis for the creation of the protocols in the OSI stack. The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven sep- arate but related layers, each of which defines a part of the process of moving information across a network (see Figure 2.11). Figure 2.11 The OSI model Layer 7 Application Layer 6 Presentation Layer 5 Session Layer 4 Transport Layer 3 Network Layer 2 Data link Layer 1 Physical  CHAPTER 2 NETWORK MODELS 45 2.3.1 OSI versus TCP/IP When we compare the two models, we find that two layers, session and presentation, are missing from the TCP/IP protocol suite. These two layers were not added to the TCP/IP protocol suite after the publication of the OSI model. The application layer in the suite is usually considered to be the combination of three layers in the OSI model, as shown in Figure 2.12. Figure 2.12 TCP/IP and OSI model Application Presentation Application Several application protocols Session Transport Transport Several transport protocols Internet Protocol Network Network and some helping protocols Data link Data link Underlying LAN and WAN Physical Physical technology OSI Model TCP/IP Protocol Suite Two reasons were mentioned for this decision. First, TCP/IP has more than one transport-layer protocol. Some of the functionalities of the session layer are available in some of the transport-layer protocols. Second, the application layer is not only one piece of software. Many applications can be developed at this layer. If some of the functionalities mentioned in the session and presentation layers are needed for a particular application, they can be included in the development of that piece of software. 2.3.2 Lack of OSI Model’s Success The OSI model appeared after the TCP/IP protocol suite. Most experts were at first excited and thought that the TCP/IP protocol would be fully replaced by the OSI model. This did not happen for several reasons, but we describe only three, which are agreed upon by all experts in the field. First, OSI was completed when TCP/IP was fully in place and a lot of time and money had been spent on the suite; changing it would cost a lot. Second, some layers in the OSI model were never fully defined. For example, although the services provided by the presentation and the session layers were listed in the document, actual protocols for these two layers were not fully defined, nor were they fully described, and the corresponding software was not fully 46 PART I OVERVIEW developed. Third, when OSI was implemented by an organization in a different application, it did not show a high enough level of performance to entice the Internet authority to switch from the TCP/IP protocol suite to the OSI model. 2.4 END-CHAPTER MATERIALS 2.4.1 Recommended Reading For more details about subjects discussed in this chapter, we recommend the following books, and RFCs. The items enclosed in brackets refer to the reference list at the end of the book. Books and Papers Several books and papers give a thorough coverage about the materials discussed in this chapter: [Seg 98], [Lei et al. 98], [Kle 04], [Cer 89], and [Jen et al. 86]. RFCs Two RFCs in particular discuss the TCP/IP suite: RFC 791 (IP) and RFC 817 (TCP). In future chapters we list different RFCs related to each protocol in each layer. 2.4.2 Key Terms International Organization for Standardization (ISO) Open Systems Interconnection (OSI) model protocol layering 2.4.3 Summary A protocol is a set of rules that governs communication. In protocol layering, we need to follow two principles to provide bidirectional communication. First, each layer needs to perform two opposite tasks. Second, two objects under each layer at both sides should be identical. In a protocol layering, we need to distinguish between a logical connection and a physical connection. Two protocols at the same layer can have a logi- cal connection; a physical connection is only possible through the physical layers. TCP/IP is a hierarchical protocol suite made of five layers: physical, data link, net- work, transport, and application. The physical layer coordinates the functions required to transmit a bit stream over a physical medium. The data-link layer is responsible for delivering data units from one station to the next without errors. The network layer is responsible for the source-to-destination delivery of a packet across multiple network links. The transport layer is responsible for the process-to-process delivery of the entire message. The application layer enables the users to access the network. Four levels of addresses are used in an internet following the TCP/IP protocols: phys- ical (link) addresses, logical (IP) addresses, port addresses, and specific addresses. The physical address, also known as the link address, is the address of a node as defined by its LAN or WAN. The IP address uniquely defines a host on the Internet. The port address identifies a process on a host. A specific address is a user-friendly address.