Wire shark

All I want the answers of the questions in each attachment. The questions are based on data you gather using Wireshark.

You dont have to answer any question for extra credit…

WiresharkLab: Ethernet

and ARP v6.01

Supplement to Computer Networking: A Top-Down
Approach, 6th ed., J.F. Kurose and K.W. Ross

“Tell me and I forget. Show me and I remember. Involve me and I
understand.” Chinese proverb

In this lab, we’ll investigate the Ethernet protocol and the ARP protocol. Before
beginning this lab, you’ll probably want to review sections 5.4.1 (link-layer addressing
and ARP) and 5.4.2 (Ethernet) in the text1. RFC 826 (ftp://ftp.rfc-editor.org/in-
notes/std/std37.txt) contains the gory details of the ARP protocol, which is used by an IP
device to determine the IP address of a remote interface whose Ethernet address is
known.

1. Capturing and analyzing Ethernet frames

Let’s begin by capturing a set of Ethernet frames to study. Do the following2:

• First, make sure your browser’s cache is empty. To do this under Mozilla Firefox
V3, select Tools->Clear Recent History and check the box for Cache. For Internet
Explorer, select Tools->Internet Options->Delete Files. Start up the Wireshark
packet sniffer

• Enter the following URL into your browser
http://gaia.cs.umass.edu/wireshark-labs/HTTP-ethereal-lab-file3.html
Your browser should display the rather lengthy US Bill of Rights.

1 References to figures and sections are for the 6th edition of our text, Computer Networks, A Top-down
Approach, 6th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2012.
2 If you are unable to run Wireshark live on a computer, you can download the zip file
http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file ethernet–ethereal-trace-1.
The traces in this zip file were collected by Wireshark running on one of the author’s computers, while
performing the steps indicated in the Wireshark lab. 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
ethernet-ethereal-trace-1 trace file. You can then use this trace file to answer the questions below.

• Stop Wireshark packet capture. First, find the packet numbers (the leftmost
column in the upper Wireshark window) of the HTTP GET message that was sent
from your computer to gaia.cs.umass.edu, as well as the beginning of the HTTP
response message sent to your computer by gaia.cs.umass.edu. You should see a
screen that looks something like this (where packet 4 in the screen shot below
contains the HTTP GET message)

• Since this lab is about Ethernet and ARP, we’re not interested in IP or higher-
layer protocols. So let’s change Wireshark’s “listing of captured packets” window
so that it shows information only about protocols below IP. To have Wireshark do
this, select Analyze->Enabled Protocols. Then uncheck the IP box and select OK.
You should now see an Wireshark window that looks like:

In order to answer the following questions, you’ll need to look into the packet details and
packet contents windows (the middle and lower display windows in Wireshark).

Select the Ethernet frame containing the HTTP GET message. (Recall that the HTTP
GET message is carried inside of a TCP segment, which is carried inside of an IP
datagram, which is carried inside of an Ethernet frame; reread section 1.5.2 in the text if
you find this encapsulation a bit confusing). Expand the Ethernet II information in the
packet details window. Note that the contents of the Ethernet frame (header as well as
payload) are displayed in the packet contents window.

Answer the following questions, based on the contents of the Ethernet frame containing
the HTTP GET message. Whenever possible, when answering a question you should
hand in a printout of the packet(s) within the trace that you used to answer the question
asked. Annotate the printout3 to explain your answer. To print a packet, use File->Print,
choose Selected packet only, choose Packet summary line, and select the minimum
amount of packet detail that you need to answer the question.

1. What is the 48-bit Ethernet address of your computer?
2. What is the 48-bit destination address in the Ethernet frame? Is this the Ethernet

address of gaia.cs.umass.edu? (Hint: the answer is no). What device has this as its
Ethernet address? [Note: this is an important question, and one that students
sometimes get wrong. Re-read pages 468-469 in the text and make sure you
understand the answer here.]

3. Give the hexadecimal value for the two-byte Frame type field. What upper layer
protocol does this correspond to?

4. How many bytes from the very start of the Ethernet frame does the ASCII “G” in
“GET” appear in the Ethernet frame?

Next, answer the following questions, based on the contents of the Ethernet frame
containing the first byte of the HTTP response message.

5. What is the value of the Ethernet source address? Is this the address of your
computer, or of gaia.cs.umass.edu (Hint: the answer is no). What device has this
as its Ethernet address?

6. What is the destination address in the Ethernet frame? Is this the Ethernet address
of your computer?

7. Give the hexadecimal value for the two-byte Frame type field. What upper layer
protocol does this correspond to?

8. How many bytes from the very start of the Ethernet frame does the ASCII “O” in
“OK” (i.e., the HTTP response code) appear in the Ethernet frame?

3 What do we mean by “annotate”? If you hand in a paper copy, please highlight where in the printout
you’ve found the answer and add some text (preferably with a colored pen) noting what you found in what
you ‘ve highlight. If you hand in an electronic copy, it would be great if you could also highlight and
annotate.

2. The Address Resolution Protocol

In this section, we’ll observe the ARP protocol in action. We strongly recommend that
you re-read section 5.4.1 in the text before proceeding.

ARP Caching

Recall that the ARP protocol typically maintains a cache of IP-to-Ethernet address
translation pairs on your comnputer The arp command (in both MSDOS and
Linux/Unix) is used to view and manipulate the contents of this cache. Since the arp
command and the ARP protocol have the same name, it’s understandably easy to confuse
them. But keep in mind that they are different – the arp command is used to view and
manipulate the ARP cache contents, while the ARP protocol defines the format and
meaning of the messages sent and received, and defines the actions taken on message
transmission and receipt.

Let’s take a look at the contents of the ARP cache on your computer:

• MS-DOS. The arp command is in c:\windows\system32, so type either “arp” or
“c:\windows\system32\arp” in the MS-DOS command line (without quotation
marks).

• Linux/Unix/MacOS. The executable for the arp command can be in various
places. Popular locations are /sbin/arp (for linux) and /usr/etc/arp (for some Unix
variants).

The Windows arp command with no arguments will display the contents of the ARP
cache on your computer. Run the arp command.

9. Write down the contents of your computer’s ARP cache. What is the meaning of
each column value?

In order to observe your computer sending and receiving ARP messages, we’ll need to
clear the ARP cache, since otherwise your computer is likely to find a needed IP-Ethernet
address translation pair in its cache and consequently not need to send out an ARP
message.

• MS-DOS. The MS-DOS arp –d * command will clear your ARP cache. The –d
flag indicates a deletion operation, and the * is the wildcard that says to delete all
table entries.

• Linux/Unix/MacOS. The arp –d * will clear your ARP cache. In order to run
this command you’ll need root privileges. If you don’t have root privileges and
can’t run Wireshark on a Windows machine, you can skip the trace collection part
of this lab and just use the trace discussed in the earlier footnote.

Observing ARP in action

Do the following4:

• Clear your ARP cache, as described above.
• Next, make sure your browser’s cache is empty. To do this under Mozilla Firefox

V3, select Tools->Clear Recent History and check the box for Cache. For Internet
Explorer, select Tools->Internet Options->Delete Files.

• Start up the Wireshark packet sniffer
• Enter the following URL into your browser

http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-lab-file3.html
Your browser should again display the rather lengthy US Bill of Rights.

• Stop Wireshark packet capture. Again, we’re not interested in IP or higher-layer
protocols, so change Wireshark’s “listing of captured packets” window so that it
shows information only about protocols below IP. To have Wireshark do this,
select Analyze->Enabled Protocols. Then uncheck the IP box and select OK.
You should now see an Wireshark window that looks like:

4 The ethernet-ethereal-trace-1 trace file in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip
was created using the steps below (in particular after the ARP cache had been flushed).

In the example above, the first two frames in the trace contain ARP messages (as does the
6th message). The screen shot above corresponds to the trace referenced in footnote 1.

Answer the following questions:

10. What are the hexadecimal values for the source and destination addresses in the
Ethernet frame containing the ARP request message?

11. Give the hexadecimal value for the two-byte Ethernet Frame type field. What
upper layer protocol does this correspond to?

12. Download the ARP specification from
ftp://ftp.rfc-editor.org/in-notes/std/std37.txt. A readable, detailed discussion of
ARP is also at http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/arp.html.

a) How many bytes from the very beginning of the Ethernet frame does the
ARP opcode field begin?

b) What is the value of the opcode field within the ARP-payload part of the
Ethernet frame in which an ARP request is made?

c) Does the ARP message contain the IP address of the sender?

d) Where in the ARP request does the “question” appear – the Ethernet
address of the machine whose corresponding IP address is being queried?

13. Now find the ARP reply that was sent in response to the ARP request.
a) How many bytes from the very beginning of the Ethernet frame does the

ARP opcode field begin?
b) What is the value of the opcode field within the ARP-payload part of the

Ethernet frame in which an ARP response is made?
c) Where in the ARP message does the “answer” to the earlier ARP request

appear – the IP address of the machine having the Ethernet address whose
corresponding IP address is being queried?

14. What are the hexadecimal values for the source and destination addresses in the
Ethernet frame containing the ARP reply message?

15. Open the ethernet-ethereal-trace-1 trace file in
http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip. The first and second
ARP packets in this trace correspond to an ARP request sent by the computer
running Wireshark, and the ARP reply sent to the computer running Wireshark by
the computer with the ARP-requested Ethernet address. But there is yet another
computer on this network, as indicated by packet 6 – another ARP request. Why
is there no ARP reply (sent in response to the ARP request in packet 6) in the
packet trace?

Extra Credit

EX-1. The arp command:

arp -s InetAddr EtherAddr

allows you to manually add an entry to the ARP cache that resolves the IP address
InetAddr to the physical address EtherAddr. What would happen if, when you
manually added an entry, you entered the correct IP address, but the wrong
Ethernet address for that remote interface?

EX-2. What is the default amount of time that an entry remains in your ARP cache
before being removed. You can determine this empirically (by monitoring the
cache contents) or by looking this up in your operation system documentation.
Indicate how/where you determined this value.

Wireshark Lab: ICMP v6.0

Supplement to Computer Networking: A Top-Down
Approach, 6th ed., J.F. Kurose and K.W. Ross

“Tell me and I forget. Show me and I remember. Involve me and I
understand.” Chinese proverb

In this lab, we’ll explore several aspects of the ICMP protocol:

• ICMP messages generating by the Ping program;
• ICMP messages generated by the Traceroute program;
• the format and contents of an ICMP message.

Before attacking this lab, you’re encouraged to review the ICMP material in section 4.4.3
of the text1. We present this lab in the context of the Microsoft Windows operating
system. However, it is straightforward to translate the lab to a Unix or Linux
environment.

1. ICMP and Ping

Let’s begin our ICMP adventure by capturing the packets generated by the Ping program.
You may recall that the Ping program is simple tool that allows anyone (for example, a
network administrator) to verify if a host is live or not. The Ping program in the source
host sends a packet to the target IP address; if the target is live, the Ping program in the
target host responds by sending a packet back to the source host. As you might have
guessed (given that this lab is about ICMP), both of these Ping packets are ICMP packets.

Do the following2:

1 References to figures and sections are for the 6th edition of our text, Computer Networks, A Top-down
Approach, 6th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2012.
2 If you are unable to run Wireshark live on a computer, you can download the zip file
http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file ICMP-ethereal-trace-1.
The traces in this zip file were collected by Wireshark running on one of the author’s computers, while
performing the steps indicated in the Wireshark lab. 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
ICMP-ethereal-trace-1 trace file. You can then use this trace file to answer the questions below.

• Let’s begin this adventure by opening the Windows Command Prompt application

(which can be found in your Accessories folder).
• Start up the Wireshark packet sniffer, and begin Wireshark packet capture.
• The ping command is in c:\windows\system32, so type either “ping –n 10

hostname” or “c:\windows\system32\ping –n 10 hostname” in the MS-DOS
command line (without quotation marks), where hostname is a host on another
continent. If you’re outside of Asia, you may want to enter www.ust.hk for the
Web server at Hong Kong University of Science and Technology. The argument
“-n 10” indicates that 10 ping messages should be sent. Then run the Ping
program by typing return.

• When the Ping program terminates, stop the packet capture in Wireshark.

At the end of the experiment, your Command Prompt Window should look something
like Figure 1. In this example, the source ping program is in Massachusetts and the
destination Ping program is in Hong Kong. From this window we see that the source ping
program sent 10 query packets and received 10 responses. Note also that for each
response, the source calculates the round-trip time (RTT), which for the 10 packets is on
average 375 msec.

Figure 1 Command Prompt window after entering Ping command.

Figure 2 provides a screenshot of the Wireshark output, after “icmp” has been entered
into the filter display window. Note that the packet listing shows 20 packets: the 10 Ping
queries sent by the source and the 10 Ping responses received by the source. Also note
that the source’s IP address is a private address (behind a NAT) of the form 192.168/12;
the destination’s IP address is that of the Web server at HKUST. Now let’s zoom in on

the first packet (sent by the client); in the figure below, the packet contents area provides
information about this packet. We see that the IP datagram within this packet has
protocol number 01, which is the protocol number for ICMP. This means that the payload
of the IP datagram is an ICMP packet.

Figure 2 Wireshark output for Ping program with Internet Protocol expanded.

Figure 3 focuses on the same ICMP but has expanded the ICMP protocol information in
the packet contents window. Observe that this ICMP packet is of Type 8 and Code 0 – a
so-called ICMP “echo request” packet. (See Figure 4.23 of text.) Also note that this
ICMP packet contains a checksum, an identifier, and a sequence number.

Figure 3 Wireshark capture of ping packet with ICMP packet expanded.

What to Hand In:

You should hand in a screen shot of the Command Prompt window similar to Figure 1
above. Whenever possible, when answering a question below, you should hand in a
printout of the packet(s) within the trace that you used to answer the question asked.
Annotate the printout3 to explain your answer. To print a packet, use File->Print, choose
Selected packet only, choose Packet summary line, and select the minimum amount of
packet detail that you need to answer the question.

You should answer the following questions:

1. What is the IP address of your host? What is the IP address of the destination
host?

2. Why is it that an ICMP packet does not have source and destination port
numbers?

3. Examine one of the ping request packets sent by your host. What are the ICMP
type and code numbers? What other fields does this ICMP packet have? How
many bytes are the checksum, sequence number and identifier fields?

4. Examine the corresponding ping reply packet. What are the ICMP type and code
numbers? What other fields does this ICMP packet have? How many bytes are the
checksum, sequence number and identifier fields?

2. ICMP and Traceroute

Let’s now continue our ICMP adventure by capturing the packets generated by the
Traceroute program. You may recall that the Traceroute program can be used to figure
out the path a packet takes from source to destination. Traceroute is discussed in Section
1.4 and in Section 4.4 of the text.

Traceroute is implemented in different ways in Unix/Linux/MacOS and in Windows. In
Unix/Linux, the source sends a series of UDP packets to the target destination using an
unlikely destination port number; in Windows, the source sends a series of ICMP packets
to the target destination. For both operating systems, the program sends the first packet
with TTL=1, the second packet with TTL=2, and so on. Recall that a router will
decrement a packet’s TTL value as the packet passes through the router. When a packet
arrives at a router with TTL=1, the router sends an ICMP error packet back to the source.
In the following, we’ll use the native Windows tracert program. A shareware version of a
much nice Windows Traceroute program is pingplotter (www.pingplotter.com). We’ll
use pingplotter in our Wireshark IP lab since it provides additional functionality that
we’ll need there.

Do the following4:

• Let’s begin by opening the Windows Command Prompt application (which can be
found in your Accessories folder).

• Start up the Wireshark packet sniffer, and begin Wireshark packet capture.
• The tracert command is in c:\windows\system32, so type either “tracert

hostname” or “c:\windows\system32\tracert hostname” in the MS-DOS command
line (without quotation marks), where hostname is a host on another continent.

4 If you are unable to run Wireshark live on a computer, you can download the zip file
http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file ICMP-ethereal-trace-2.
The traces in this zip file were collected by Wireshark running on one of the author’s computers, while
performing the steps indicated in the Wireshark lab. 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
ICMP-ethereal-trace-2 trace file. You can then use this trace file to answer the questions below.

(Note that on a Windows machine, the command is “tracert” and not
“traceroute”.) If you’re outside of Europe, you may want to enter www.inria.fr
for the Web server at INRIA, a computer science research institute in France.
Then run the Traceroute program by typing return.

• When the Traceroute program terminates, stop packet capture in Wireshark.

At the end of the experiment, your Command Prompt Window should look something
like Figure 4. In this figure, the client Traceroute program is in Massachusetts and the
target destination is in France. From this figure we see that for each TTL value, the
source program sends three probe packets. Traceroute displays the RTTs for each of the
probe packets, as well as the IP address (and possibly the name) of the router that
returned the ICMP TTL-exceeded message.

Figure 4 Command Prompt window displays the results of the Traceroute program.

Figure 5 displays the Wireshark window for an ICMP packet returned by a router. Note
that this ICMP error packet contains many more fields than the Ping ICMP messages.

Figure 5 Wireshark window of ICMP fields expanded for one ICMP error packet.

What to Hand In:

For this part of the lab, you should hand in a screen shot of the Command Prompt
window. Whenever possible, when answering a question below, you should hand in a
printout of the packet(s) within the trace that you used to answer the question asked.
Annotate the printout to explain your answer. To print a packet, use File->Print, choose
Selected packet only, choose Packet summary line, and select the minimum amount of
packet detail that you need to answer the question.

Answer the following questions:

5. What is the IP address of your host? What is the IP address of the target
destination host?

6. If ICMP sent UDP packets instead (as in Unix/Linux), would the IP protocol
number still be 01 for the probe packets? If not, what would it be?

7. Examine the ICMP echo packet in your screenshot. Is this different from the
ICMP ping query packets in the first half of this lab? If yes, how so?

8. Examine the ICMP error packet in your screenshot. It has more fields than the
ICMP echo packet. What is included in those fields?

9. Examine the last three ICMP packets received by the source host. How are these
packets different from the ICMP error packets? Why are they different?

10. Within the tracert measurements, is there a link whose delay is significantly
longer than others? Refer to the screenshot in Figure 4, is there a link whose
delay is significantly longer than others? On the basis of the router names, can
you guess the location of the two routers on the end of this link?

3. Extra Credit

For one of the programming assignments you created a UDP client ping program. This
ping program, unlike the standard ping program, sends UDP probe packets rather than
ICMP probe packets. Use the client program to send a UDP packet with an unusual
destination port number to some live host. At the same time, use Wireshark to capture
any response from the target host. Provide aWireshark screenshot for the response as well
as an analysis of the response.

WiresharkLab: IP v6.0

Supplement to Computer Networking: A Top-Down
Approach, 6th ed., J.F. Kurose and K.W. Ross

“Tell me and I forget. Show me and I remember. Involve me and I
understand.” Chinese proverb

In this lab, we’ll investigate the IP protocol, focusing on the IP datagram. We’ll do so by
analyzing a trace of IP datagrams sent and received by an execution of the traceroute
program (the traceroute program itself is explored in more detail in the Wireshark
ICMP lab). We’ll investigate the various fields in the IP datagram, and study IP
fragmentation in detail.

Before beginning this lab, you’ll probably want to review sections 1.4.3 in the text1 and
section 3.4 of RFC 2151 [ftp://ftp.rfc-editor.org/in-notes/rfc2151.txt] to update yourself
on the operation of the traceroute program. You’ll also want to read Section 4.4 in
the text, and probably also have RFC 791 [ftp://ftp.rfc-editor.org/in-notes/rfc791.txt] on
hand as well, for a discussion of the IP protocol.

1. Capturing packets from an execution of traceroute

In order to generate a trace of IP datagrams for this lab, we’ll use the traceroute
program to send datagrams of different sizes towards some destination, X. Recall that
traceroute operates by first sending one or more datagrams with the time-to-live
(TTL) field in the IP header set to 1; it then sends a series of one or more datagrams
towards the same destination with a TTL value of 2; it then sends a series of datagrams
towards the same destination with a TTL value of 3; and so on. Recall that a router must
decrement the TTL in each received datagram by 1 (actually, RFC 791 says that the
router must decrement the TTL by at least one). If the TTL reaches 0, the router returns
an ICMP message (type 11 – TTL-exceeded) to the sending host. As a result of this
behavior, a datagram with a TTL of 1 (sent by the host executing traceroute) will
cause the router one hop away from the sender to send an ICMP TTL-exceeded message
back to the sender; the datagram sent with a TTL of 2 will cause the router two hops

1 References to figures and sections are for the 6th edition of our text, Computer Networks, A Top-down
Approach, 6th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2012.

away to send an ICMP message back to the sender; the datagram sent with a TTL of 3
will cause the router three hops away to send an ICMP message back to the sender; and
so on. In this manner, the host executing traceroute can learn the identities of the
routers between itself and destination X by looking at the source IP addresses in the
datagrams containing the ICMP TTL-exceeded messages.

We’ll want to run traceroute and have it send datagrams of various lengths.

• Windows. The tracert program (used for our ICMP Wireshark lab) provided
with Windows does not allow one to change the size of the ICMP echo request
(ping) message sent by the tracert program. A nicer Windows traceroute
program is pingplotter, available both in free version and shareware versions at
http://www.pingplotter.com. Download and install pingplotter, and test it out by
performing a few traceroutes to your favorite sites. The size of the ICMP echo
request message can be explicitly set in pingplotter by selecting the menu item
Edit-> Options->Packet Options and then filling in the Packet Size field. The
default packet size is 56 bytes. Once pingplotter has sent a series of packets with
the increasing TTL values, it restarts the sending process again with a TTL of 1,
after waiting Trace Interval amount of time. The value of Trace Interval and the
number of intervals can be explicitly set in pingplotter.

• Linux/Unix/MacOS. With the Unix/MacOS traceroute command, the size
of the UDP datagram sent towards the destination can be explicitly set by
indicating the number of bytes in the datagram; this value is entered in the
traceroute command line immediately after the name or address of the
destination. For example, to send traceroute datagrams of 2000 bytes
towards gaia.cs.umass.edu, the command would be:

%traceroute gaia.cs.umass.edu 2000

Do the following:

• Start up Wireshark and begin packet capture (Capture->Start) and then press OK
on the Wireshark Packet Capture Options screen (we’ll not need to select any
options here).

• If you are using a Windows platform, start up pingplotter and enter the name of a
target destination in the “Address to Trace Window.” Enter 3 in the “# of times to
Trace” field, so you don’t gather too much data. Select the menu item Edit-
>Advanced Options->Packet Options and enter a value of 56 in the Packet Size
field and then press OK. Then press the Trace button. You should see a
pingplotter window that looks something like this:

Next, send a set of datagrams with a longer length, by selecting Edit->Advanced
Options->Packet Options and enter a value of 2000 in the Packet Size field and
then press OK. Then press the Resume button.

Finally, send a set of datagrams with a longer length, by selecting Edit-
>Advanced Options->Packet Options and enter a value of 3500 in the Packet Size
field and then press OK. Then press the Resume button.

Stop Wireshark tracing.

• If you are using a Unix or Mac platform, enter three traceroute commands,

one with a length of 56 bytes, one with a length of 2000 bytes, and one with a
length of 3500 bytes.

Stop Wireshark tracing.

If you are unable to run Wireshark on a live network connection, you can download a
packet trace file that was captured while following the steps above on one of the author’s
Windows computers2. You may well find it valuable to download this trace even if
you’ve captured your own trace and use it, as well as your own trace, when you explore
the questions below.

2 Download the zip file http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file ip-
ethereal-trace-1. The traces in this zip file were collected by Wireshark running on one of the author’s
computers, while performing the steps indicated in the Wireshark lab. 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 ip-ethereal-trace-1 trace file.

2. A look at the captured trace

In your trace, you should be able to see the series of ICMP Echo Request (in the case of
Windows machine) or the UDP segment (in the case of Unix) sent by your computer and
the ICMP TTL-exceeded messages returned to your computer by the intermediate
routers. In the questions below, we’ll assume you are using a Windows machine; the
corresponding questions for the case of a Unix machine should be clear. Whenever
possible, when answering a question below you should hand in a printout of the packet(s)
within the trace that you used to answer the question asked. When you hand in your
assignment, annotate the output so that it’s clear where in the output you’re getting the
information for your answer (e.g., for our classes, we ask that students markup paper
copies with a pen, or annotate electronic copies with text in a colored font).To print a
packet, use File->Print, choose Selected packet only, choose Packet summary line, and
select the minimum amount of packet detail that you need to answer the question.

1. Select the first ICMP Echo Request message sent by your computer, and expand
the Internet Protocol part of the packet in the packet details window.

What is the IP address of your computer?

2. Within the IP packet header, what is the value in the upper layer protocol field?
3. How many bytes are in the IP header? How many bytes are in the payload of the

IP datagram? Explain how you determined the number of payload bytes.
4. Has this IP datagram been fragmented? Explain how you determined whether or

not the datagram has been fragmented.

Next, sort the traced packets according to IP source address by clicking on the Source
column header; a small downward pointing arrow should appear next to the word Source.
If the arrow points up, click on the Source column header again. Select the first ICMP
Echo Request message sent by your computer, and expand the Internet Protocol portion
in the “details of selected packet header” window. In the “listing of captured packets”
window, you should see all of the subsequent ICMP messages (perhaps with additional
interspersed packets sent by other protocols running on your computer) below this first
ICMP. Use the down arrow to move through the ICMP messages sent by your computer.

5. Which fields in the IP datagram always change from one datagram to the next
within this series of ICMP messages sent by your computer?

6. Which fields stay constant? Which of the fields must stay constant? Which fields
must change? Why?

7. Describe the pattern you see in the values in the Identification field of the IP
datagram

Next (with the packets still sorted by source address) find the series of ICMP TTL-
exceeded replies sent to your computer by the nearest (first hop) router.

8. What is the value in the Identification field and the TTL field?
9. Do these values remain unchanged for all of the ICMP TTL-exceeded replies sent

to your computer by the nearest (first hop) router? Why?

Fragmentation

Sort the packet listing according to time again by clicking on the Time column.

10. Find the first ICMP Echo Request message that was sent by your computer after
you changed the Packet Size in pingplotter to be 2000. Has that message been
fragmented across more than one IP datagram? [Note: if you find your packet has
not been fragmented, you should download the zip file
http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the ip-
ethereal-trace-1packet trace. If your computer has an Ethernet interface, a packet
size of 2000 should cause fragmentation.3]

11. Print out the first fragment of the fragmented IP datagram. What information in
the IP header indicates that the datagram been fragmented? What information in
the IP header indicates whether this is the first fragment versus a latter fragment?
How long is this IP datagram?

3 The packets in the ip-ethereal-trace-1 trace file in http://gaia.cs.umass.edu/wireshark-labs/wireshark-
traces.zip are all less that 1500 bytes. This is because the computer on which the trace was gathered has an
Ethernet card that limits the length of the maximum IP packet to 1500 bytes (40 bytes of TCP/IP header
data and 1460 bytes of upper-layer protocol payload). This 1500 byte value is the standard maximum
length allowed by Ethernet. If your trace indicates a datagram longer 1500 bytes, and your computer is
using an Ethernet connection, then Wireshark is reporting the wrong IP datagram length; it will likely also
show only one large IP datagram rather than multiple smaller datagrams.. This inconsistency in reported
lengths is due to the interaction between the Ethernet driver and the Wireshark software. We recommend
that if you have this inconsistency, that you perform this lab using the ip-ethereal-trace-1 trace file.

12. Print out the second fragment of the fragmented IP datagram. What information in
the IP header indicates that this is not the first datagram fragment? Are the more
fragments? How can you tell?

13. What fields change in the IP header between the first and second fragment?

Now find the first ICMP Echo Request message that was sent by your computer after you
changed the Packet Size in pingplotter to be 3500.

14. How many fragments were created from the original datagram?
15. What fields change in the IP header among the fragments?

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