Ethernet or WiFi). Recall from the discussion from section1.5in the text (Figure

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Ethernet or WiFi). Recall from the discussion from section1.5in the text (Figure 1.241)Read your first lab’s intro and instructions here. Download here.Please be sure you complete all lab steps.
This will require you to download and install Wireshark, a packet-sniffing utility we will use. READ BELOW WHAT YOU WILL NEED TO SUBMIT HERE FOR THIS LAB:
You will complete all lab steps, and when you get to Step 9 of the lab, you will complete Step 9 and then upload a screenshot of your own screen (which will look similar to Figure 5). This interesting and fun exercise will help you understand how to view network packet details.
that messages exchanged by higher layer protocolssuch as HTTP, FTP, TCP, UDP,
DNS, or IP all are eventually encapsulated in link-layer frames that are transmitted over
physical media such as an Ethernet cable or an 802.11 WiFi radio. Capturing all link-
layer frames thus gives you all messages sent/received across the monitored link from/by
all protocols and applications executing in your computer.
Figure 1: packet sniffer structure
The second component of a packet sniffer is the packet analyzer, which displays the
contents of all fields within a protocol message. In order to do so, the packet analyzer
must “understand” the structure of all messages exchanged by protocols. For example,
suppose we are interested in displaying the various fields in messages exchanged by the
HTTP protocol in Figure 1. The packet analyzer understands the format of Ethernet
frames, and so can identify the IP datagram within an Ethernet frame. It also understands
the IP datagram format, so that it can extract the TCP segment within the IP datagram.
Finally, it understands the TCP segment structure, so it can extract the HTTP message
contained in the TCP segment. Finally, it understands the HTTP protocol and so, for
example, knows that the first bytes of an HTTP message will contain the string “GET,”
“POST,” or “HEAD,” as shown in Figure 2.8 in the text.
We will be using the Wireshark packet sniffer [http://www.wireshark.org/] for these labs,
allowing us to display the contents of messages being sent/received from/by protocols at
different levels of the protocol stack. (Technically speaking, Wireshark is a packet
analyzer that uses a packet capture library in your computer. Also, technically speaking,
Wireshark captures link-layer frames as shown in Figure 1, but uses the generic term
“packet” to refer to link-layer frames, network-layer datagrams, transport-layer segments,
and application-layer messages, so we’ll use the less-precise “packet” term here to go
along with Wireshark convention). Wireshark is a free network protocol analyzer that
runs on Windows, Mac, and Linux/Unix computers. It’s an ideal packet analyzer for our
labs – it is stable, has a large user base and well-documented support that includes a user-
guide (http://www.wireshark.org/docs/wsug_html_chunked/), man pages
1 References to figures and sections are for the 8th edition of our text, Computer Networks, A Top-down
Approach, 8th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2020.
(http://www.wireshark.org/docs/man-pages/), and a detailed FAQ
(http://www.wireshark.org/faq.html), rich functionality that includes the capability to
analyze hundreds of protocols, and a well-designed user interface. It operates in
computers using Ethernet, serial (PPP), 802.11 (WiFi) wireless LANs, and many other
link-layer technologies.
Getting Wireshark
In order to run Wireshark, you’ll need to have access to a computer that supports both
Wireshark and the libpcap or WinPCap packet capture library. The libpcap software will
be installed for you, if it is not installed within your operating system, when you install
Wireshark. See http://www.wireshark.org/download.html for a list of supported
operating systems and download sites.
Download and install the Wireshark software:
• Go to http://www.wireshark.org/download.html and download and install the
Wireshark binary for your computer.
The Wireshark FAQ has a number of helpful hints and interesting tidbits of information,
particularly if you have trouble installing or running Wireshark.
Running Wireshark
When you run the Wireshark program, you’ll get a startup screen that looks something
like the screen below. Different versions of Wireshark will have different startup screens
– so don’t panic if yours doesn’t look exactly like the screen below! The Wireshark
documentation states “As Wireshark runs on many different platforms with many
different window managers, different styles applied and there are different versions of the
underlying GUI toolkit used, your screen might look different from the provided
screenshots. But as there are no real differences in functionality these screenshots should
still be well understandable.” Well said.
Figure 2: Initial Wireshark Screen
There’s not much that’s very interesting on this screen. But note that under the Capture
section, there is a list of so-called interfaces. The Mac computer we’re taking these
screenshots from has just one interface – “Wi-Fi en0,” (shaded in blue in Figure 2) which
is the interface for Wi-Fi access. All packets to/from this computer will pass through the
Wi-Fi interface, so it’s here where we’ll want to capture packets. On a Mac, double click
on this interface (or on another computer locate the interface on startup page through
which you are getting Internet connectivity, e.g., mostly likely a WiFi or Ethernet
interface, and select that interface).
Let’s take Wireshark out for a spin! If you click on one of these interfaces to start packet
capture (i.e., for Wireshark to begin capturing all packets being sent to/from that
interface), a screen like the one below will be displayed, showing information about the
packets being captured. Once you start packet capture, you can stop it by using the
Capture pull down menu and selecting Stop (or by clicking on the red square button next
to the Wireshark fin in Figure 2). 2
2 If you are unable to run Wireshark, you can still look at packet traces that were captured on one
of the author’s (Jim’s) computer. Download the zip file http://gaia.cs.umass.edu/wireshark-
labs/wireshark-traces-8E.zip and extract the file wireshark-intro-trace. The traces in this zip file
were collected by Wireshark running on one of the author’s (Jim’s) computers, while performing
the steps indicated above. Once you have downloaded the trace, you can load it into Wireshark
and view the trace using the File pull down menu, choosing Open, and then selecting the
wireshark-intro-trace trace file. The resulting display should look similar to Figures 3 and 5.
(The Wireshark user interface displays just a bit differently on different operating systems, and in
different versions of Wireshark).
Figure 3: Wireshark window, during and after capture
This looks more interesting! The Wireshark interface has five major components:
• The command menus are standard pulldown menus located at the top of the
Wireshark window (and on a Mac at the top of the screen as well; the screenshot
in Figure 3 is from a Mac). Of interest to us now are the File and Capture menus.
The File menu allows you to save captured packet data or open a file containing
previously captured packet data and exit the Wireshark application. The Capture
menu allows you to begin packet capture.
• The packet-listing window displays a one-line summary for each packet
captured, including the packet number (assigned by Wireshark; note that this is
not a packet number contained in any protocol’s header), the time at which the
packet was captured, the packet’s source and destination addresses, the protocol
type, and protocol-specific information contained in the packet. The packet listing
can be sorted according to any of these categories by clicking on a column name.
The protocol type field lists the highest-level protocol that sent or received this
packet, i.e., the protocol that is the source or ultimate sink for this packet.
• The packet-header details window provides details about the packet selected
(highlighted) in the packet-listing window. (To select a packet in the packet-
listing window, place the cursor over the packet’s one-line summary in the
packet-listing window and click with the left mouse button.). These details
include information about the Ethernet frame (assuming the packet was
sent/received over an Ethernet interface) and IP datagram that contains this
packet. The amount of Ethernet and IP-layer detail displayed can be expanded or
minimized by clicking on the plus/minus boxes or right/downward-pointing
triangles to the left of the Ethernet frame or IP datagram line in the packet details
window. If the packet has been carried over TCP or UDP, TCP or UDP details
will also be displayed, which can similarly be expanded or minimized. Finally,
details about the highest-level protocol that sent or received this packet are also
provided.
• The packet-contents window displays the entire contents of the captured frame,
in both ASCII and hexadecimal format.
• Towards the top of the Wireshark graphical user interface, is the packet display
filter field, into which a protocol name or other information can be entered in
order to filter the information displayed in the packet-listing window (and hence
the packet-header and packet-contents windows). In the example below, we’ll
use the packet-display filter field to have Wireshark hide (not display) packets
except those that correspond to HTTP messages.
Taking Wireshark for a Test Run
The best way to learn about any new piece of software is to try it out! We’ll assume that
your computer is connected to the Internet via a wired Ethernet interface or a wireless
802.11 WiFi interface. Do the following:
1. Start up your favorite web browser, which will display your selected homepage.
2. Start up the Wireshark software. You will initially see a window similar to that
shown in Figure 2. Wireshark has not yet begun capturing packets.
3. To begin packet capture, select the Capture pull down menu and select Interfaces.
This will cause the “Wireshark: Capture Interfaces” window to be displayed (on a
PC) or you can choose Options on a Mac. You should see a list of interfaces, as
shown in Figures 4a (Windows) and 4b (Mac).
Figure 4a: Wireshark Capture interface window, on a Windows computer
Figure 4b: Wireshark Capture interface window, on a Mac computer
4. You’ll see a list of the interfaces on your computer as well as a count of the
packets that have been observed on that interface so far. On a Windows machine,
click on Start for the interface on which you want to begin packet capture (in the
case in Figure 4a, the Gigabit network Connection). On a Windows machine,
select the interface and click Start on the bottom of the window). Packet capture
will now begin – Wireshark is now capturing all packets being sent/received
from/by your computer!
5. Once you begin packet capture, a window similar to that shown in Figure 3 will
appear. This window shows the packets being captured. By selecting Capture
pulldown menu and selecting Stop, or by click on the red Stop square, you can
stop packet capture. But don’t stop packet capture yet. Let’s capture some
interesting packets first. To do so, we’ll need to generate some network traffic.
Let’s do so using a web browser, which will use the HTTP protocol that we will
study in detail in class to download content from a website.
6. While Wireshark is running, enter the URL:
http://gaia.cs.umass.edu/wireshark-labs/INTRO-wireshark-file1.html
and have that page displayed in your browser. In order to display this page, your
browser will contact the HTTP server at gaia.cs.umass.edu and exchange HTTP
messages with the server in order to download this page, as discussed in section
2.2 of the text. The Ethernet or WiFi frames containing these HTTP messages (as
well as all other frames passing through your Ethernet or WiFi adapter) will be
captured by Wireshark.
7. After your browser has displayed the INTRO-wireshark-file1.html page (it is a
simple one line of congratulations), stop Wireshark packet capture by selecting
stop in the Wireshark capture window. The main Wireshark window should now
look similar to Figure 3. You now have live packet data that contains all protocol
messages exchanged between your computer and other network entities! The
HTTP message exchanges with the gaia.cs.umass.edu web server should appear
somewhere in the listing of packets captured. But there will be many other types
of packets displayed as well (see, e.g., the many different protocol types shown in
the Protocol column in Figure 3). Even though the only action you took was to
download a web page, there were evidently many other protocols running on your
computer that are unseen by the user. We’ll learn much more about these
protocols as we progress through the text! For now, you should just be aware that
there is often much more going on than “meet’s the eye”!
8. Type in “http” (without the quotes, and in lower case – all protocol names are in
lower case in Wireshark) into the display filter specification window at the top of
the main Wireshark window. Then select Apply (to the right of where you entered
“http”) or just hit return. This will cause only HTTP message to be displayed in
the packet-listing window. Figure 5 below shows a screenshot after the http filter
has been applied to the packet capture window shown earlier in Figure 3. Note
also that in the Selected packet details window, we’ve chosen to show detailed
content for the Hypertext Transfer Protocol application message that was found
within the TCP segment, that was inside the IPv4 datagram that was inside the
Ethernet II (WiFi) frame. Focusing on content at a specific message, segment,
datagram and frame level lets us focus on just what we want to look at (in this
case HTTP messages).
Figure 5: looking at the details of the HTTP message that contained a GET of
http://gaia.cs.umass.edu/wireshark-labs/INTRO-wireshark-file1.html
9. Find the HTTP GET message that was sent from your computer to the
gaia.cs.umass.edu HTTP server. (Look for an HTTP GET message in the “listing
of captured packets” portion of the Wireshark window (see Figures 3 and 5) that
shows “GET” followed by the gaia.cs.umass.edu URL that you entered. When
you select the HTTP GET message, the Ethernet frame, IP datagram, TCP
segment, and HTTP message header information will be displayed in the packet-
header window 3 . By clicking on ‘+’ and ‘-‘ and right-pointing and down-pointing
arrowheads to the left side of the packet details window, minimize the amount of
Frame, Ethernet, Internet Protocol, and Transmission Control Protocol
information displayed. Maximize the amount information displayed about the
HTTP protocol. Your Wireshark display should now look roughly as shown in
Figure 5. (Note, in particular, the minimized amount of protocol information for
all protocols except HTTP, and the maximized amount of protocol information for
HTTP in the packet-header window).
10. Exit Wireshark
Congratulations! You’ve now completed the first lab!
3 Recall that the HTTP GET message that is sent to the gaia.cs.umass.edu web server is contained within a
TCP segment, which is contained (encapsulated) in an IP datagram, which is encapsulated in an Ethernet
frame. If this process of encapsulation isn’t quite clear yet, review section 1.5 in the text
What to hand in
The goal of this first lab was primarily to introduce you to Wireshark. The following
questions will demonstrate that you’ve been able to get Wireshark up and running, and
have explored some of its capabilities. Answer the following questions, based on your
Wireshark experimentation:
1. List 3 different protocols that appear in the protocol column in the unfiltered
packet-listing window in step 7 above.
2. How long did it take from when the HTTP GET message was sent until the HTTP
OK reply was received? (By default, the value of the Time column in the packet-
listing window is the amount of time, in seconds, since Wireshark tracing began.
To display the Time field in time-of-day format, select the Wireshark View pull
down menu, then select Time Display Format, then select Time-of-day.)
3. What is the Internet address of the gaia.cs.umass.edu (also known as www-
net.cs.umass.edu)? What is the Internet address of your computer?
4. Print the two HTTP messages (GET and OK) referred to in question 2 above. To
do so, select Print from the Wireshark File command menu, and select the
“Selected Packet Only” and “Print as displayed” radial buttons, and then click

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