Click the ⫹ sign next to Hypertext Transfer Protocol. Here, you will see the host

44 | Lesson 2 7. Click the ⴙ sign next to Transmission Control Protocol. Here, you will see the out- bound port used by your computer to connect to the web server known as a source port, as well as the inbound port 80 that the web server uses known as a Dst or destination port.

8. Spend some time analyzing the information listed, and match it to the appropriate

layer of the OSI model. Devices known as gateways reside on the application layer. These are not to be confused with gateway devices like routers on the network layer. An application layer gateway is a computer that translates from one protocol suite to another, such as from TCPIP to IPXSPX. An example, albeit an out of date one, would be Client Services for NetWare when loaded on a Windows client computer. Reviewing the OSI Layers CERTIFICATION READY What do you need to know to review all of the OSI layers? 3.1 Application Presentation Transmitting Computer Session Transport Network Data Link Physical Application Presentation Receiving Computer Session Transport Network Data Link Physical Figure 2-8 OSI layers revisited In general, data transactions start at the sending computer, travel down the OSI layers start- ing with the application layer and ending with the physical layer, are transmitted across the physical medium be it wired or wireless, and travel back up the layers of the OSI model at the receiving computer. For example, if you wanted to connect to a Web site, you would type the name of the site in your web browser’s address field. Then, when you press Enter, the HTTP protocol would take effect at the application layer. The packets of data would be compressed with gzip and possibly encrypted HTTPS by way of SSL or TLS at the presentation layer. The web server would acknowledge the session with the client web browser at the session layer. The information would then be transmitted as TCP information on the transport layer, where ports are also selected. The TCP information would be broken up into easy-to-send The OSI model contains seven layers, each of which work collectively to define the trans- mission of data from one computer to another. The mnemonic device All People Seem To Need Data Processing can help you memorize the layer order. Although earlier in the lesson, we defined each of the OSI layers starting at the bottom, the physical layer, and moving upward from there, quite often, you will see the layers listed from the top down, with the application layer at the top and the physical layer at the bottom, as shown in Figure 2-8. However, in Wireshark and other protocol analyzers, the physical layer will be displayed at the top. It all depends on what application or technical document you are looking at, so be ready to encounter both orientations. Defining Networks with the OSI Model | 45 packets on the network layer, and IP addressing information would be added. The packets would then be sent to the data link layer, where the network adapter would encapsulate them into frames of data. Then, at the physical layer, the network adapter would break the frames up into a serial bit stream to be sent over the cable media. When the serial bit stream arrived at the destination client computer via the web browser, it would be reconfigured by the client’s network adapter as frames of information. The header information of the frames would be checked for authenticity and stripped, leaving the pack- ets to be sent to the operating system. The operating system would then put these packets together to form the web page that is displayed on your computer screen. Of course, this all happens 10,000 times faster than explained here, and it happens many times each second. For instance, if your computer has a 100 Mbps connection, it can take in about 12 MB maxi- mum of data per second. Packets of information are variable in size, and they can be between approximately 60 and 1,500 bytes. Say that you are downloading a large file. This file will be broken down into the largest packet size possible, around 1,500 bytes. We can therefore calculate that an average computer can take in 8,000 packets of data per second. By the way, most client computers probably do not take advantage of this maximum data throughput, but servers and power workstations do. Table 2-3 reviews the OSI layers and shows the corresponding devices, protocols, and network standards that apply to each layer. Table 2-3 The OSI model layers and corresponding components L AYER P ROTOCOL D EVICE 7. Application FTP, HTTP, POP3, SMTP Gateway 6. Presentation Compression, Encryption NA 5. Session LogonLogoff NA 4. Transport TCP, UDP NA 3. Network IP, ICMP, ARP, RIP Routers 2. Data Link 802.3, 802.5 NICs, switches, bridges, WAPs 1. Physical 100BASE-T, 1000BASE-X Hubs, patch panels, RJ45 jacks Although the OSI model is a reference model, the TCPIP model also known as the DoD model or Internet model is more descriptive, defining principles such as “end-to-end” and “robustness,” which describe strong endpoint connections and conservative transmission of data. This model is maintained by the Internet Engineering Task Force IETF. The four layers in the TCPIP model are as follows: • Layer 1: Data link layer also simply known as the link layer • Layer 2: Network layer also known as the Internet layer • Layer 3: Transport layer • Layer 4: Application layer The OSI physical layer is skipped altogether, and the application layer comprises the OSI application, presentation, and session layers. CERTIFICATION READY How can you define the TCPIP model? 3.1 Defining the TCPIP Model The TCPIP or TCP model is similar to the OSI model. It is often used by software manufacturers who are not as concerned with how information is sent over physical media, or how the data link is actually made. This model is composed of only four layers.