Transmission Control Protocol and Cisco Public Information

Topics: Transmission Control Protocol, File Transfer Protocol, Internet Protocol Pages: 10 (2626 words) Published: March 27, 2013
Learning Objectives
Be able to explain the purpose of a protocol analyzer (Wireshark). Be able to perform basic PDU capture using Wireshark.
Be able to perform basic PDU analysis on straightforward network data traffic. Experiment with Wireshark features and options such as PDU capture and display filtering.

Wireshark is a software protocol analyzer, or "packet sniffer" application, used for network troubleshooting, analysis, software and protocol development, and education. Before June 2006, Wireshark was known as Ethereal. A packet sniffer (also known as a network analyzer or protocol analyzer) is comput er software that can intercept and log data traffic passing over a data network. As data streams travel back and forth over the network, the sniffer "captures" each protocol dat a unit (PDU) and can decode and analyze its content according to the appropriate RFC or other specifications. Wireshark is programmed to recognize the structure of different network protocols. This enables it to display the encapsulati on and individual fields of a PDU and interpret their meaning.

It is a useful tool for anyone working with networks and can be used with most labs in the CCNA courses for data analysis and troubleshooting.
For information and to download the program go to -

To capture PDUs the computer on which W ireshark is installed must have a working connection to the network and Wireshark must be running before any data can be captured.
W hen Wireshark is launched, the screen below is displayed.

To start data capture it is first necessary to go to the Capture menu and select the Options choice. The Options dialog provides a range of settings and filters which determines which and how much data traffic is captured.

All contents are Copyright © 1992–2007 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information. Page 2 of 12

First, it is necessary to ensure that Wireshark is set to monitor the correct interface. From the Interface drop down list, select the network adapter in use. Typically, for a computer this will be the connected Ethernet Adapter. Then other Options can be set. Among those available in Capture Options, the two highlighted below are worth examination.

Setting Wireshark to capture packets in promiscuous mode
If this feature is NOT checked, only PDUs destined for this computer will be captured. If this feature is checked, all PDUs d estined for this computer AND all those detected by the computer NIC on the same network segment (i.e., those that "pass by" the NI C but are not destined for the computer) are captured. Note: The capturing of these other PDUs depends on the intermediary device connecting the end device computers on this network. As you use different intermediary devices (hubs, switches, routers) thro ughout these courses, you will experience the different Wireshark results. Setting Wireshark for network name resolution

This option allows you to control whether or not Wireshark translates network addresses found in PDUs into names. Although th is is a useful feature, the name resolution process may add extra PDUs to your captured data perhaps distorting the analysis. There are also a number of other capture filtering and process settings available. Clicking on the Start button starts the data capture process and a message box displays the progress of this process.

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As data PDUs are captured, the types and number are indicated in the message box

The examples above show the capture of a ping process and then accessing a web page . When the Stop button is clicked, the capture process is terminated and the main screen is displayed . This main display window of Wireshark has three panes.

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