The TCP/IP and OSI Networking Models

computer & technologyThe term networking model, or networking architecture, refers to an organized description of all the functions needed for useful communications to occur. Individual protocols and hardware specifications then are used to implement the functions described in the networking model. When multiple computers and other networking devices implement these protocols, which, in turn, implement the functions described by the networking model, the computers can successfully communicate.

You can think of a networking model like you think of a set of architectural plans for building a house. Sure, you can build a house without the architectural plans, but it will work better if you follow the plans. And because you probably have a lot of different people working on building your house, such as framers, electricians, bricklayers, painters, and so on, it helps if they can all reference the same plan. Similarly, you could build your own network, write your own software, build your own networking cards, and create a network without using any existing networking model. However, it is much easier to simply buy and use products that already conform to some well-known networking model. And because the products from different vendors conform to the same networking architectural model, the products should work well together.

The CCNA exams include detailed coverage of one networking model-the Transmission Control Protocol/Internet Protocol, or TCP/IP. TCP/IP is the most pervasive networking model in the history of data networking. You can find support for TCP/IP on practically every computer operating system in existence today, from mobile phones to mainframe computers. Almost every network built using Cisco products today supports TCP/IP.

Not surprisingly, the CCNA exams focus on TCP/IP. The INTRO exam, and the ICND exam to a small extent, also covers a second networking model, called the Open Systems Interconnection (OSI) model. Historically, OSI was the first large effort to create a vendor-neutral networking model that could be added to any and every computer in the world. Ironically, OSI might be the least- pervasive networking model deployed today. However, because OSI was the first major effort to create a vendor-neutral networking architectural model, many of the terms used in networking today come from the OSI model.

Foundation Topics

It is practically impossible to find a computer today that does not support the set of networking protocols called TCP/IP. Every Microsoft, Linux, and UNIX operating system includes support for TCP/IP. Hand-held digital assistants and cell phones support TCP/IP. Even IBM Mainframe operating systems support TCP/IP. And because Cisco sells products that create the infrastructure that allows all these computers to talk with each other using TCP/IP, Cisco products also include extensive support for TCP/IP.

The world has not always been so simple. Once upon a time, there were no networking protocols, including TCP/IP. Vendors created the first networking protocols; these protocols supported only that vendor’s computers, and the details were not even published to the public. As time went on, vendors formalized and published their networking protocols, enabling other vendors to create products that could communicate with their computers. For instance, IBM published its Systems Network Architecture (SNA) networking model in 1974. After SNA was published, you could buy computers from other vendors as well as IBM, and they could communicate—as long as they supported IBM’s proprietary SNA.

Using only vendor-proprietary networking models allowed a business to successfully communicate between computers from multiple vendors. However, to talk to a computer using the hardware or software from vendor X, you needed to use the networking protocols created by vendor X. Imagine sitting at your desk in the late 1980s and needing to work with an IBM mainframe using SNA, a DEC minicomputer using DECnet, and a Novell server using NetWare, and having to transfer files with an Apple computer using AppleTalk. Believe it or not, it actually worked, and networks using all these different protocols were not at all uncommon.

A better solution was to create a standardized networking model that all vendors would support. The International Organization for Standardization (ISO) took on this task starting as early as the late 1970s, beginning work on what would become known as the Open Systems Interconnection (OSI) networking model. The ISO had a noble goal for the OSI: to standardize data networking protocols to allow communication between all computers across the entire planet. The OSI worked toward this ambitious and noble goal, with participants from most of the technologically developed nations on Earth participating in the process.

A second, less formal effort to create a standardized, public networking model sprouted forth from a U.S. Defense Department contract. Researchers at various universities volunteered to help further develop the protocols surrounding the original department’s work. These efforts resulting in a competing networking model called TCP/IP.
The world now had many competing vendor networking models and two competing standardized networking models. So what happened? TCP/IP won the war. Proprietary protocols are still in use today in many networks, but much less so than in the 1980s and 1990s. OSI, whose development suffered in part because of the slow formal standardization processes of the ISO, never succeeded in the marketplace. And TCP/IP, the networking model created almost entirely by a bunch of volunteers, has become the most prolific set of data networking protocols ever.

In this chapter, you will read about some of the basics of TCP/IP. Although you will learn some interesting facts about TCP/IP, the true goal of this chapter is to help you understand what a networking model or networking architecture really is and how one works.
Also in this chapter, you will learn about some of the jargon used with OSI. Will any of you ever work on a computer that is using the full OSI protocols instead of TCP/IP? Probably not. However, you will often use terms relating to OSI. Also, the INTRO exam covers the basics of OSI, so this chapter also covers OSI to prepare you for questions about it on the exam.

The TCP/IP Protocol Architecture

TCP/IP defines a large collection of protocols that allow computers to communicate. TCP/IP defines the details of each of these protocols inside document called Requests For Comments (RFCs). By implementing the required protocols defined in TCP/IP RFCs, a computer can be relatively confident that it can communicate with other computers that also implement TCP/IP.

An easy comparison can be made between telephones and computers that use TCP/IP. I can go to the store and buy a phone from one of a dozen different vendors. When I get home, I plug the phone in to the wall socket, and it works. The phone vendors know the standards for phones in their country and build their phones to match those standards. Similarly, a computer that implements the standard networking protocols defined by TCP/IP can communicate with other computers that also use the TCP/IP standards.

Like other networking architectures, TCP/IP classifies the various protocols into different categories. Table 2-2 outlines the main categories in the TCP/IP architectural model.

The TCP/IP model represented in column 1 of the table lists the four layers of TCP/IP, and column 2 of the table lists several of the most popular TCP/IP protocols. If someone makes up a new application, the protocols used directly by the application would be considered to be application layer protocols. When the World Wide Web (WWW) was first created, a new application layer protocol was created for the purpose of asking for web pages and receiving the contents of the web pages. Similarly, the network interface layer includes protocols and standards such as Ethernet. If someone makes up a new type of LAN, those protocols would be considered to be a part of the networking interface layer. In the next several sections, you will learn the basics about each of these four layers in the TCP/IP architecture and how they work together.

TCP/IP application layer protocols provide services to the application software running on a computer. The application layer does not define the application itself, but rather it defines services that applications need - like the ability to transfer a file in the case of HTTP. In short, the application layer provides an interface between software running on a computer and the network itself.

The TCP/IP Application Layer

Arguably, the most popular TCP/IP application today is the web browser. Many major software vendors either have already changed or are changing their software to support access from a web browser. And thankfully, using a web browser is easy—you start a web browser on your computer and select a web site by typing in the name of the web site, and the web page appears.
What really happens to allow that web page to appear on your web browser? These next few sections take a high-level look at what happens behind the scene.

Imagine that Bob opens his browser. His browser has been configured to automatically ask for web server Larry’s default web page, or home page. The general logic looks like that in Figure 2-1.

So what really happened? Bob’s initial request actually asks Larry to send his home page back to Bob. Larry’s web server software has been configured to know that Larry’s default web
page is contained in a file called home.htm. Bob receives the file from Larry and displays the contents of the file in the web browser window.

Taking a closer look, this example uses two TCP/IP application layer protocols. First, the request for the file and the actual transfer of the file are performed according to the Hypertext Transfer Protocol (HTTP). Many of you have probably noticed that most web sites’ URLs (Universal Resource Locators, the text that identifies a web server and a particular web page) begin with the letters “http,” to imply that HTTP will be used to transfer the web pages.

The other protocol used is the Hypertext Markup Language (HTML). HTML defines how Bob’s web browser should interpret the text inside the file he just received. For instance, the file might contain directions about making certain text be a certain size, color, and so on. In most cases, it also includes directions about other files that Bob’s web browser should get— things such as graphics images and animation. HTTP would then be used to get those additional files from Larry, the web server.

A closer look at how Bob and Larry cooperate in this example reveals some details about how networking protocols work. Consider Figure 2-2, which simply revises Figure 2-1, showing the locations of HTTP headers and data.

To get the web page from Larry, Bob sends something called an HTTP header to Larry. This header includes the command to “get” a file. The request typically contains the name of the file (home.htm in this case), or, if no filename is mentioned, the web server assumes that Bob wants the default web page. The response from Larry includes an HTTP header as well, with something as simple as “OK” returned in the header. In reality, it includes an HTTP return code. For instance, if you have ever used the web, and a web page that you looked for was not found, then you received an HTTP 404 “not found” error, which means that you received an HTTP return code of 404. When the requested file is found, the return code is 0, meaning that the request is being processed This simple example between Bob and Larry introduces one of the most important general concepts behind networking models: When a particular layer wants to communicate with the same layer on another computer, the two computers use headers to hold the information that they want to communicate. The headers are part of what is transmitted between the two computers. This process is called same-layer interaction.

The application layer protocol (HTTP, in this case) on Bob is communicating with Larry’s application layer. They each do so by creating and sending application layer headers to each other—sometimes with application data following the header and sometimes not, as seen in Figure 2-2. Regardless of what the application layer protocol happens to be, they all use the same general concept of communicating with the same layer on the other computer using application layer headers.
TCP/IP application layer protocols provide services to the application software running on a computer. The application layer does not define the application itself, but rather it defines services that applications need—like the ability to transfer a file in the case of HTTP. In short, the application layer provides an interface between software running on a computer and the network itself.

The TCP/IP Transport Layer

The TCP/IP application layer includes a relatively large number of protocols, with HTTP being only one of those. The TCP/IP transport layer consists of two main protocol options— the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). To get a true appreciation for what TCP/IP transport layer protocols do, read Chapter 6, “Fundamentals of TCP and UDP.” However, in this section, you will learn about one of the key features of TCP, which enables us to cover some more general concepts about how networking models behave.

To appreciate what the transport layer protocols do, you must think about the layer above the transport layer, the application layer. Why? Well, each layer provides a service to the layer above it. For example, in Figure 2-2, Bob and Larry used HTTP to transfer the home page from Larry to Bob. But what would have happened if Bob’s HTTP get request was lost in transit through the TCP/IP network? Or, what would have happened if Larry’s response, which includes the contents of the home page, was lost? Well, the page would not show up in Bob’s browser, as you might expect. So, TCP/IP needs a mechanism to guarantee delivery of data across a network. TCP provides that feature by using acknowledgments. Figure 2-3 outlines the basic acknowledgment logic.

As Figure 2-3 shows, the HTTP software asks for TCP to reliably deliver the HTTP get request. TCP sends the HTTP data from Bob to Larry, and the data arrives successfully. Larry’s TCP software acknowledges receipt of the data and also gives the HTTP get request to the web server software. The reverse happens with Larry’s response, which also arrives at Bob successfully.

Of course, the benefits of TCP error recovery cannot be seen unless the data is lost. Chapter 6 covers TCP, including error recovery, in detail. For now, assume that if either transmission had been lost, that HTTP would not be concerned, and that TCP would resend the data and ensure that it was received successfully. This example outlines the concepts of how adjacent layers in a networking model work together on the same computer. The higher-layer protocol (HTTP) needs to do something it cannot do (error recovery). So, the higher layer asks for the next lower-layer protocol (TCP) to perform the service, and the next lower layer performs the service. The lower layer provides a service to the layer above it.

Table 2-3 summarizes the key points about how adjacent layers work together on a single computer and how one layer on one computer works with the same networking layer on another computer.

The TCP/IP transport layer provides services to the various application layer protocols. Error recovery, as performed by TCP, is one feature. This layer also provides other functions, as detailed in Chapter 6. All the examples describing the application and transport layers ignored many details relating to the physical network. The application and transport layers purposefully were defined to work the same, way whether the endpoint host computers were on the same LAN or were separated by the Internet. The lower two layers of TCP/IP, the internetwork layer and the network interface layer, must understand the underlying physical network because they define the protocols used to deliver the data from one host to another.

The TCP/IP Internetwork Layer

Imagine that you just wrote a letter to your favorite person on the other side of the country and that you also wrote a letter to someone on the other side of town. It’s time to send the letters. Is there much difference in how you treat each letter? Not really. You put different addresses on the envelope for each letter because the letters need to go to two different places. You put stamps on both letters and put them in the same mailbox. The postal service takes care of all the details of figuring out how to get each letter to the right place—whether it is across town or across the country.

Inside the postal service, both letters are processed. One letter gets sent to another post office, then another, and so on, until the letter gets delivered across the country. The local letter might go to the post office in your town and then simply be delivered to your friend across town, without going to another post office.
So what does this all matter to networking? Well, the internetwork layer of the TCP/IP networking model, the Internet Protocol (IP), works much like the postal service. IP defines addresses so that each host computer can have a different IP address, just like the postal service defines addressing that allows unique addresses for each house, apartment, and business. Similarly, IP defines the process of routing so that devices called routers (ingenious name, huh?) can choose where to send packets of data so that they are delivered to the correct destination. Just like the postal service created the necessary post offices, sorting machines, trucks, and personnel to deliver the mail, the internetwork layer defines much of the details needed to implement the necessary networking infrastructure.

Chapter 5, “Fundamentals of IP,” describes the TCP/IP Internetwork layer further, with other details scattered throughout the book. But to help you understand the basics of the internetwork layer, take a look at Bob’s request for Larry’s home page, now with some information about IP, in Figure 2-4.

First, some basic information about the figure will help. The LAN cabling details are not important for this example, so both LANs simply are represented by the lines shown near Bob and Larry, respectively. When Bob sends the data, he is sending an IP packet, which includes the IP header, the transport layer header (TCP, in this example), the application header (HTTP, in this case), and any application data (none, in this case). The IP header includes both a source and a destination IP address field, with Larry’s IP address as the destination address and Bob’s as the source.

Bob sends the packet to R2, which makes a routing decision. R2 chooses to send the packet to R1 because the destination address of the packet is, and R1 knows enough about the network topology to know that (Larry) is on the other side of R1. Similarly, when R1 gets the packet, it forwards the packet over the Ethernet to Larry. And if the link between R2 and R1 fails, IP allows R2 to learn of the alternate route through R3 to reach

IP defines logical addresses, called IP addresses, that allow each TCP/IP speaking device (called IP hosts) to communicate. It also defines routing—the process of how a router should forward, or route, packets of data. Other protocol specifications, like OSI, have different protocols that also define addressing and routing.
Both CCNA exams cover IP fairly deeply. For the INTRO exam, this book’s Chapter 5 covers more of the basics, and Chapters 12, “IP Addressing and Subnetting,” through 14, “Introduction to Dynamic Routing Protocols,” cover many of the details.

The TCP/IP Network Interface Layer

The network interface layer defines the protocols and hardware required to deliver data across some physical network. The term network interface refers to the fact that this layer defines how to connect the host computer, which is not part of the network, to the network; it is the interface between the computer and the network. For instance, Ethernet is one example protocol at the TCP/IP network interface layer. Ethernet defines the required cabling, addressing, and protocols used to create an Ethernet LAN. Likewise, the connectors, cables, voltage levels, and protocols used to deliver data across WAN links are defined in a variety of other protocols that also fall into the network interface layer.

Chapter 3, “Data Link Layer Fundamentals: Ethernet LANs,” and Chapter 4, “Fundamentals of WANs,” cover more details about the TCP/IP network interface layer. Just like every layer in any networking model, the TCP/IP network interface layer provides services to the layer above it in the model. The best way to understand the basics of the TCP/ IP network interface layer is to examine the services that it provides to IP. IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology, things such as which routers are connected to each other, which host computers are connected to which networks, and what the IP addressing scheme looks like. However, the IP protocol purposefully does not include the details about each of the underlying physical networks. Therefore, the Internet layer, as implemented by IP, uses the services of the network interface layer to deliver the packets over each physical network, respectively.

The network interface layer includes a large number of protocols. For instance, the network interface layer includes all the variations of Ethernet protocols and other LAN standards. This layer also includes the popular WAN standards, such as the Point-to-Point Protocol (PPP) and Frame Relay. The same familiar network is shown in Figure 2-5, with Ethernet and PPP used as the two network interface layer protocols.

To fully appreciate Figure 2-5, first think a little more deeply about how IP accomplishes its goal of delivering the packet from Bob to Larry. Bob wants to send the IP packet to Larry, but it must first do so by sending the packet to R2. Bob uses Ethernet to get the packet to R2. At R2, R2 strips the Ethernet header and trailer from the IP packet. To get the IP packet from R2 to R1, R2 does not need to use Ethernet—it instead needs to use the PPP serial link. To send the IP packet from R2 to R1, R2 needs to place a PPP header in front of the IP packet and a PPP trailer at the end. Similarly, after the packet is received by R1, R1 removes the PPP header and trailer because PPP’s job is to get the IP packet across the serial link. R1 then decides that it should forward the packet over the Ethernet to Larry. To do so, R1 adds a brand-new Ethernet header and trailer to the packet and forwards it to Larry.

In effect, IP uses the network interface layer protocols to deliver the IP packet to the next router or host, with each router repeating the process until the packet arrives at the destination. Each network interface protocol uses headers to encode the information needed to successfully deliver the data across the physical network, much like other layers use headers to achieve their goals.
Many people describe the network interface layer of the TCP/IP model as two layers, the data link layer and the physical layer. The reasons for the popularity of these alternate terms are explained in the section covering OSI because the terms originated with the OSI model. In short, the TCP/IP Network Interface layer includes the protocols, cabling standards,
headers and trailers that define how to send data across a wide variety of types of physical networks.

Data Encapsulation

The term encapsulation describes the process of putting headers and trailers around some data. A computer that needs to send data encapsulates the data in headers of the correct format so that the receiving computer will know how to interpret the received data. You have seen several examples of encapsulation in this chapter already. The web server encapsulated the home page inside an HTTP header in Figure 2-2. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 2-3. IP encapsulated the TCP headers and the data inside an IP header in Figure 2-4. Finally, the network interface layer encapsulated the IP packets inside both a header and a trailer in Figure 2-5.

You can think about the complete process of data encapsulation with TCP/IP as a five-step process. In fact, previous CCNA exams referred to a specific five-step process for encapsulation. This included the typical encapsulation by the application, transport, network, and network interface (referred to as data link) layers as Steps 1 through 4 in the five-step process. The fifth step was the physical layer’s transmission of the bit stream. In case any questions remain in the CCNA question database referring to a five-step encapsulation process, the following list provides the details and explanation. Regardless, the ideas behind the process apply to any networking model and how it encapsulates data:

This five-step process happens to match the TCP/IP network model very well. Figure 2-6 depicts the concept; the numbers shown represent each of the five steps.

When each layer encapsulates data given to it from the next higher layer, that layer does not really care about the details of the data. Figure 2-7 shows the encapsulated data from the perspective of the transport, internetwork, and data link (network interface) layers.

Each layer treats the data given to it by the next higher layer simply as “data.” For instance, IP just wants to transport what TCP gives it—IP does not really care what is inside the data. So, the IP packet shown in the figure shows the rest of the bits as data, meaning that IP does not care that the data field looks like the TCP segment above it in the figure.

Also notice the specific terms used for the framing as it exists at each layer, as shown in the figure. Throughout this book and on the CCNA exams, the term frame defines all the encapsulated data. The term packet includes the IP header but not any data link headers. Finally, the term segment includes the TCP or UDP header but not the IP header or data link header or trailer.
OSI Reference Model

To pass the INTRO exam, you must be conversant in a protocol specification with which you are very unlikely to ever have any hands-on experience—the OSI reference model. The
difficulty these days when discussing the OSI protocol specifications is that you have no point of reference—you simply cannot typically walk down the hall and use a computer whose main, or even optional, networking protocols conform to OSI. OSI is the Open System Interconnection reference model for communications. Some participants in OSI’s creation and development wanted OSI to become the networking protocol used by all applications on all computers in the world. The U.S. government went so far as to require OSI support on every computer that it purchased, as of a certain date in the early 1990s, which certainly gave vendors some incentive to write OSI code. In fact, in my old IBM days, they even had charts showing how the TCP/IP-installed base would start declining by 1994, how OSI installations would increase, and how OSI would be the protocol from which the 21st-century Internet was built.

What is OSI today? Well, OSI never succeeded in the marketplace. Some of the original protocols that comprised OSI are still used. The U.S. government reversed its decision to require OSI support on computers that it bought, which was probably the final blow to the possibility of pervasive OSI implementations. So, why do you even need to think about OSI for the CCNA exam? Well, the OSI model now is mainly used as a point of reference for discussing other protocol specifications. And because being a CCNA requires you to understand some of the concepts and terms behind networking architecture and models, and because other protocols are almost always compared to OSI, you need to know some things about OSI.

OSI Layers

The OSI reference model consists of seven layers. Each layer defines a set of typical networking functions. When OSI was in active development in the 1980s and 1990s, the OSI committees created new protocols and specifications to implement the functions specified by each layer. In other cases, the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined. For instance, the IEEE defines Ethernet standards, so the OSI committees did not waste time specifying a new type of Ethernet; it simply referred to the IEEE Ethernet standards.

Today the OSI model can be used as a standard of comparison to other networking models. Figure 2-8 shows OSI, as compared with TCP/IP and Novell NetWare.

Because OSI does have a very well-defined set of functions associated with each of its seven layers, you can examine any networking protocol or specification and make some determination of whether it most closely matches OSI Layer 1, 2, or 3, and so on. For instance, TCP/IP’s internetworking layer, as implemented by IP, equates most directly to the OSI network layer. So, most people say that IP is a network layer, or Layer 3, protocol, using OSI terminology and numbers for the layer. Of course, if you numbered the TCP/IP model, starting at the bottom, IP would be in Layer 2—but, by convention, everyone uses the OSI standard when describing other protocols. So, using this convention, IP is a network layer protocol.

Cisco requires that CCNAs demonstrate an understanding of the functions defined by OSI for each layer, as well as some example protocols that correspond to each OSI layer. The names of the OSI reference model layers, a few of the typical protocols at each layer, and the functions of each layer are simply good things to memorize for the INTRO exam. And frankly, if you want to pursue your Cisco certifications beyond CCNA, these names and functional areas will come up continually.

The upper layers of the OSI reference model (application, presentation, and session—Layers 7, 6, and 5) define functions focused on the application. The lower four layers (transport, network, data link, and physical—Layers 4, 3, 2, and 1) define functions focused on end-to- end delivery of the data. Both CCNA exams focus on issues in the lower layers—in particular, with Layer 2, upon which switching is based, and Layer 3, upon which routing is based. Table 2-4 defines the functions of the seven layers, and Table 2-5 lists typical protocols considered to be comparable to the OSI layers.

OSI Layering Concepts and Benefits

Many benefits can be gained from the process of breaking up the functions or tasks of networking into smaller chunks, called layers, and defining standard interfaces between these layers. The layers break a large, complex set of concepts and protocols into smaller pieces, making it easier to talk about, easier to implement with hardware and software, and easier to troubleshoot. The following list summarizes the benefits of layered protocol specifications:
• Easier to learn—Humans can more easily discuss and learn about the many details of a protocol specification.
• Easier to develop—Reduced complexity allows easier program changes and faster product evolution.
• Multivendor interoperability—Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network.
• Modular engineering—One vendor can write software that implements higher layers for example, a web browser—and another can write software that implements the lower layers for example, Microsoft’s built-in TCP/IP software in its operating systems.

The benefits of layering can be seen in the familiar postal service analogy. A person writing a letter does not have to think about how the postal service will deliver a letter across the country. The postal worker in the middle of the country does not have to worry about the contents of the letter. Likewise, layering enables one software package or hardware device to implement functions from one layer, assuming that other software/hardware will perform the functions defined by the other layers. For instance, a web browser does not need to think about what the network topology looks like, the Ethernet card in the PC does not need to think about the contents of the web page, and a router in the middle of the network does not need to worry about the contents of the web page or whether the computer that sent the packet was using an Ethernet card or some other networking card.

OSI Terminology

First, remembering the names of the OSI layers is just an exercise in memorization. You might benefit from the following list of mnemonic phrases, with the first letters in each word being the same as the first letters of the OSI layer names, in order:
• All People Seem To Need Data Processing (Layers 7 to 1)
• Please Do Not Take Sausage Pizzas Away (Layers 1 to 7)
• Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7)
You also should know how to use the names of the layers when discussing other networking models. An example definitely helps make sense of this concept. In Figure 2-9, you see the OSI model, the TCP/IP model, and a third figure with some sample TCP/IP protocols shown at their respective layers. ¦

As shown in the figure, the layers in the TCP/IP model correlate to particular layers in the OSI model. For instance, the TCP/IP internetwork layer corresponds to the OSI network layer. Why? Well, the OSI network layer defines logical addressing and routing, as does the TCP/IP internetwork layer. So, IP is called a network layer, or Layer 3, protocol. Similarly, the TCP/IP transport layer defines many functions, including error recovery, as does the OSI transport layer—so TCP is called a transport layer, or Layer 4, protocol.

Not all TCP/IP layers correspond to a single OSI layer. For instance, the TCP/IP network interface layer defines both the physical network specifications and the protocols used to control the physical network. OSI separates the physical network specifications into the physical layer and the control functions into the data link layer. Ethernet includes functions defined by OSI Layers 1 and 2. So, depending on the context, you can refer to Ethernet as a Layer 1 or Layer 2 protocol.

The final OSI terms covered here all use the base term protocol data unit, or PDU. A PDU represents the bits that include the headers and trailers for that layer, as well as the encapsulated data. For instance, an IP packet, as shown in Figure 2-7, is a protocol data unit. In fact, an IP packet is a Layer 3 PDU because IP is a Layer 3 protocol. The term L3PDU is a shorter version of the phrase Layer 3 PDU. Figure 2-10 represents the typical encapsulation process, this time for the OSI model, with the terms used for the PDUs listed at each layer.

OSI Summary

In the first part of this chapter, you learned about how TCP/IP protocols at the various layers
work with each other and how TCP/IP encapsulates data. Those same concepts are true of
OSI, as well as other networking models. The basic ideas can be summed up as follows:
• Each layer provides a service to the layer above it in the protocol specification.
• Each layer communicates with the same layer’s software or hardware on other computers.
• To accomplish these tasks, the data is encapsulated progressively with new headers when sending the data and is de-encapsulated when receiving the data.

Foundation Summary

The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your INTRO exam, a well-prepared CCNA candidate should know, at a minimum, all the details in each “Foundation Summary” section before going to take the exam.
Table 2-6 summarizes the key points about how adjacent layers work together on a single computer and how one layer on one computer works with the same networking layer on another computer. These concepts are some of the most important concepts in this chapter.

Data encapsulation is another key concept discussed throughout this chapter. You can think about the complete process generically or with the example five-step TCP/IP encapsulation process shown in the following list and in Figure 2-11:
Step 1 Create the application data and headers—This simply means that the application has data to send.
Step 2 Package the data for transport—In other words, the transport layer (TCP or UDP) creates the transport header and places the data behind it.
Step 3 Add the destination and source network layer addresses to the data— The network layer creates the network header, which includes the network layer addresses, and places the data behind it.
Step 4 Add the destination and source data link layer addresses to the data— The data link layer creates the data link header, places the data behind it, and places the data link trailer at the end.
Step 5 Transmit the bits—The physical layer encodes a signal onto the medium to transmit the frame.

You should know the names of all the OSI and TCP/IP layers, as shown in Figure 2-12.

You should memorize the names of the layers of the OSI model. Table 2-7 lists a summary of OSI functions at each layer, along with some sample protocols at each layer.

The following list summarizes the benefits of layered protocol specifications:
• Easier to learn—Humans can more easily discuss and learn about the many details of a protocol specification.
• Easier to develop—Reduced complexity allows easier program changes and faster product evolution.
• Multivendor interoperability—Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network.
• Modular engineering—One vendor can write software that implements higher layers for example, a web browser and another can write software that implements the lower layers for example, Microsoft’s built-in TCP/IP software in its operating systems.


Quiz2: The TCP/IP and OSI Networking Models

1) (1 marks)
Which of the following terms is used specifically to identify the entity that is created when encapsulating data inside data-link headers and trailers?
None—there is no encapsulation by the data link layer
Leave blank

2) (1 marks)
Which of the following protocols are examples of TCP/IP network interface layer protocols?
Leave blank

3) (1 marks)
Which OSI layer defines the standards for data formats and encryption?
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
Layer 6
Layer 7
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4) (1 marks)
Which OSI layer defines the functions of logical network-wide addressing and routing?
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
Layer 6
Layer 7
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5) (1 marks)
Which of the following protocols are examples of TCP/IP transport layer protocols?
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