Communication Networks [Netw501] Spring 2018 Tutorial 3 Packet Tracer Activity 3 Use of the TCP/IP Protocols and the OSI Model in Packet Tracer Introduction: In Packet Tracer simulation mode, detailed information about packets and how they are processed by networking devices may be viewed. Common TCP/IP Protocols are modeled in Packet Tracer, including DNS, HTTP, TFTP, DHCP, Telnet, TCP, UDP, ICMP, and IP. How these protocols are used by networking devices in creating and processing packets is displayed, in Packet Tracer, using a representation of the OSI Model. The term protocol data unit, or PDU, is a generic description of what are known as segments at the transport layer, packets at the network layer, and frames at the data link layer. Objectives: 1. Explore how PT uses the OSI Model and TCP/IP Protocols. Creating a Simple PDU (test packet) Switching from Real-time to Simulation Mode 2. Examine Packet Processing and Contents Accessing the PDU Information Window, OSI Model View Investigating Device algorithms in the OSI Model View Inbound and Outbound PDUs Animations of packet Flow Task 1: Create Network Topology [web server connected to a web client] and explore the PT interface Step 1 Using the correct Ethernet cable, connect a workstation with a server. Which cable was used for the connection between the two end devices? Use the following chart to apply a Layer 3 address to the end devices and give them the perspective names End Device Name IP Address Subnet Mask Web Server 192.168.1.254 255.255.255.0 Web Client 192.168.1.1 255.255.255.0
Step 2 Switching from Real-time to Simulation Mode In the far lower right of the PT interface is the toggle between Real-time and Simulation mode. PT always starts in real-time mode, in which networking protocols operate with realistic timings. However, a powerful feature of Packet Tracer is it lets the user "stop time" by switching to simulation mode. In simulation mode, packets are displayed as animated envelopes, time is event driven, and the user can step through networking events. Click on Simulation mode. Task 2: Examine Packet Contents and Processing Step 1 Creating a Packet and Accessing the PDU Information Window Click on the Web Client PC. Choose the Desktop tab. Open the Web Browser. Enter the IP address of the Web Server into the browser, 192.168.1.254. Clicking on Go will initiate a web server request. Minimize the Web Client configuration window. Since time in simulation is event driven, you must use the Capture/Forward button to display network events. Two packets appear in the event list, one of which has an eye next to it. An eye next to a packet means it is displayed as an envelope on the logical topology. Find the first HTTP packet in the Event List, and click on the colored square in the Info column. Step 2 Investigating device algorithms in the OSI Model view When you click on the Info square for a packet in the event list (or equivalently, you click on a packet envelope displayed on the logical topology, the PDU Information window opens). The OSI model organizes this window. In the case of the first packet we are viewing, notice the HTTP request (at Layer 7) is then encapsulated, successively, at Layers 4, 3, 2, and 1. If you click on these layers, the algorithm used by the device (in this case, the PC) is displayed. View what is going on at each layer. Step 3 Inbound and Outbound PDUs When opening the PDU Information window, the default is the OSI Model view. Now click on the Outbound PDU Details tab. Scroll down to the bottom of this window. There you will see that HTTP (the web page request that started this series of events) is encapsulated as data in a TCP segment, which in turn is encapsulated in an IP packet, which in turn is encapsulated in an Ethernet frame, which in turn is transmitted as bits on the medium. If a device is the first device involved in a series of events, packets at that device will only have an Outbound PDU Details tab; if a device is the last device in a series of events, packets at that device it has only an Inbound PDU Details tab. In general, you will see both Outbound and Inbound PDU details, which give details about how Packet Tracer is modeling that device.
Step 4 Packet tracing: animations of packet flow The first time through a packet animation, you are actually capturing the packets, as in a protocol sniffer. Hence, the Capture/Forward button means "Capture" one set of events at a time. Step through the web page request. Note that you are only displaying HTTP-related packets; but other protocols like TCP and ARP also have packets that are not being displayed. At any time in your packet capture, you can open the PDU Information window. Run through the entire animation until the "No More Events" message is reached. Experiment with this packet tracing process -- running the animation again, examining packets, predicting what will happen next, and investigating your predictions.
Tutorial 3 Packet Tracer Activity 4 Using Packet Tracer to View Protocol Data Units Introduction: Simulation mode in Packet Tracer captures all network traffic flowing through the entire network but only supports a limited number of protocols. To come as close as possible to approximating Lab 2.6.2 we will use a network consisting of a PC that is directly connected to a web server and capture a ping to the server from the PC's command prompt and a request for a web page using a URL. Objectives: 1. Capture a ping from a PC command prompt. Run the simulation and capture the traffic. Examine the captured traffic. 2. Capture a web request using a URL from a PC. Run the simulation and capture the traffic. Examine the captured traffic. Task 1: Capture a ping from a PC command prompt. Step 1 Run the simulation and capture the traffic. You will find the network setup in the attached file (pka4.pkt). Open it and follow the following steps. In the far lower right of the PT interface is the toggle between Real-time and Simulation mode. Click on Simulation mode. Click on the PC. Choose the Desktop tab. Open the Command Prompt. Enter the command ping 192.168.1.2, the IP address of the server. Pressing the Enter key will initiate four ICMP echo requests. Minimize the PC configuration window. Two packets appear in the Event List, the first ICMP echo request and an ARP request needed to resolve the IP address of the server to its hardware MAC address. Click the Auto Capture / Play button to run the simulation and capture events. Click OK when the "No More Events" message is reached. Step 2 Examine the captured traffic. Find the first packet in the Event List, and click on the colored square in the Info column. When you click on the Info square for a packet in the event list the PDU Information window opens. The OSI model organizes this window. In the case of the first packet we are viewing, notice the ICMP echo request (at Layer 3) is encapsulated at Layer 2. If you click on these layers, the algorithm used by the device (in this case, the PC) is displayed. View what is going on at each layer. When opening the PDU Information window, the default is the OSI Model view. Now click on the Outbound PDU Details tab. Scroll down to the bottom of this window, and you will see that the
ICMP echo request is encapsulated as data in an IP packet. Examine the PDU information for the remaining events in the exchange. Close the PDU information window and reset the simulation by clicking the Reset Simulation button. Task 2: Capture a web request using a URL from a PC. Step 1 Run the simulation and capture the traffic. Click on the PC. Close the Command Prompt window. Open the Web Browser. Enter www.example.com into the browser. Clicking on Go will initiate a web server request. Minimize the Web Client configuration window. A packet appears in the Event List, a DNS request needed to resolve the URL to the IP address of the server. Click the Auto Capture / Play button to run the simulation and capture events. Click OK when the "No More Events" message is reached. Step 2 Examine the captured traffic. Examine the PDU information for the events in this exchange. Locate the DNS Query, TCP connection setup and HTTP request & response in the event list Check the encapsulation by investigating one of the HTTP messages and clicking on the info colored square next to it. o Identify the protocol type used in each layer. Switch to the Inbound/Outbound detail tab o Investigate the Ethernet frame. Check the MAC source and destination addresses. Are they as expected? o Inspect the IP Packet. Check the IP source and destination addresses. Are they as expected? o Look into the TCP segment. Check the port numbers used for source and destination. o Check the HTTP message content.