Wireshark_802.11 Lab

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Wireshark Lab: 802.11 v7.0
Supplement to Computer Networking: A Top-Down
Approach, 7th 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
© 2005-2016, J.F Kurose and K.W. Ross, All Rights Reserved
In this lab, we’ll investigate the 802.11 wireless network protocol. Before beginning this
lab, you might want to re-read Section 7.3 in the text1. Since we’ll be delving a bit deeper
into 802.11 than is covered in the text, you might want to check out “A Technical
Tutorial on the 802.11Protocol,” by Pablo Brenner (Breezecom Communications),
http://www.sss-mag.com/pdf/802_11tut.pdf, and “Understanding 802.11 Frame Types,”
by Jim Geier, http://www.wi-fiplanet.com/tutorials/article.php/1447501. And, of course,
there is the “bible” of 802.11 – the standard itself, “ANSI/IEEE Std 802.11, 1999 Edition
(R2003),” http://gaia.cs.umass.edu/wireshark-labs/802.11-1999.pdf. In particular, you
may find Table 1 on page 36 of the standard particularly useful when looking through the
wireless trace.
In all of the Wireshark labs thus far, we’ve captured frames on a wired Ethernet
connection. Here, since 802.11 is a wireless link-layer protocol, we’ll be capturing frames
“in the air.” Unfortunately, many device drivers for wireless 802.11 NICs don’t provide
the hooks to capture/copy received 802.11 frames for use in Wireshark (see Figure 1 in
Lab 1 for an overview of packet capture). Thus, in this lab, we’ll provide a trace of
captured 802.11 frames for you to analyze and assume in the questions below that you are
using this trace. If you’re able to capture 802.11 frames using your version of Wireshark,
you’re welcome to do so. Additionally, if you’re really into frame capture, you can buy a
small USB device, AirPcap, http://www.cacetech.com, that captures 802.11 frames and
provides integrated support for Wireshark.
1. Getting Started
Download the zip file http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and
extract the file Wireshark_802_11.pcap. This trace was collected using AirPcap and
Wireshark running on a computer in the home network of one of the authors, consisting
1
References to figures and sections are for the 7th edition of our text, Computer Networks, A Top-down
Approach, 7th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2016.
of a Linksys 802.11g combined access point/router, with two wired PCs and one wireless
host PC attached to the access point/router. The author is fortunate to have other access
points in neighboring houses available as well. In this trace file, we’ll see frames
captured on channel 6. Since the host and AP that we are interested in are not the only
devices using channel 6, we’ll see a lot of frames that we’re not interested in for this lab,
such as beacon frames advertised by a neighbor’s AP also operating on channel 6. The
wireless host activities taken in the trace file are:
• The host is already associated with the 30 Munroe St AP when the trace begins.
• At t = 24.82, the host makes an HTTP request to
http://gaia.cs.umass.edu/wireshark-labs/alice.txt. The IP address of
gaia.cs.umass.edu is 128.119.245.12.
• At t=32.82, the host makes an HTTP request to http://www.cs.umass.edu, whose
IP address is 128.119.240.19.
• At t = 49.58, the host disconnects from the 30 Munroe St AP and attempts to
connect to the linksys_ses_24086. This is not an open access point, and so the
host is eventually unable to connect to this AP.
• At t=63.0 the host gives up trying to associate with the linksys_ses_24086 AP,
and associates again with the 30 Munroe St access point.
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_802_11.pcap trace file. The resulting display should look just like Figure 1.
Figure 1: Wireshark window, after opening the Wireshark_802_11.pcap file
2. Beacon Frames
Recall that beacon frames are used by an 802.11 AP to advertise its existence. To answer
some of the questions below, you’ll want to look at the details of the “IEEE 802.11”
frame and subfields in the middle Wireshark 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 printout2 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 are the SSIDs of the two access points that are issuing most of the beacon
frames in this trace?
2. What are the intervals of time between the transmissions of the beacon frames the
linksys_ses_24086 access point? From the 30 Munroe St. access point? (Hint: this
interval of time is contained in the beacon frame itself).
3. What (in hexadecimal notation) is the source MAC address on the beacon frame
from 30 Munroe St? Recall from Figure 7.13 in the text that the source,
destination, and BSS are three addresses used in an 802.11 frame. For a detailed
discussion of the 802.11 frame structure, see section 7 in the IEEE 802.11
standards document (cited above).
4. What (in hexadecimal notation) is the destination MAC address on the beacon
frame from 30 Munroe St??
5. What (in hexadecimal notation) is the MAC BSS id on the beacon frame from 30
Munroe St?
6. The beacon frames from the 30 Munroe St access point advertise that the access
point can support four data rates and eight additional “extended supported rates.”
What are these rates?
3. Data Transfer
Since the trace starts with the host already associated with the AP, let first look at data
transfer over an 802.11 association before looking at AP association/disassociation.
Recall that in this trace, at t = 24.82, the host makes an HTTP request to
http://gaia.cs.umass.edu/wireshark-labs/alice.txt. The IP address of gaia.cs.umass.edu is
128.119.245.12. Then, at t=32.82, the host makes an HTTP request to
http://www.cs.umass.edu.
2
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.
7. Find the 802.11 frame containing the SYN TCP segment for this first TCP session
(that downloads alice.txt). What are three MAC address fields in the 802.11
frame? Which MAC address in this frame corresponds to the wireless host (give
the hexadecimal representation of the MAC address for the host)? To the access
point? To the first-hop router? What is the IP address of the wireless host
sending this TCP segment? What is the destination IP address? Does this
destination IP address correspond to the host, access point, first-hop router, or
some other network-attached device? Explain.
8. Find the 802.11 frame containing the SYNACK segment for this TCP session.
What are three MAC address fields in the 802.11 frame? Which MAC address in
this frame corresponds to the host? To the access point? To the first-hop router?
Does the sender MAC address in the frame correspond to the IP address of the
device that sent the TCP segment encapsulated within this datagram? (Hint:
review Figure 6.19 in the text if you are unsure of how to answer this question, or
the corresponding part of the previous question. It’s particularly important that
you understand this).
3. Association/Disassociation
Recall from Section 7.3.1 in the text that a host must first associate with an access point
before sending data. Association in 802.11 is performed using the ASSOCIATE
REQUEST frame (sent from host to AP, with a frame type 0 and subtype 0, see Section
7.3.3 in the text) and the ASSOCIATE RESPONSE frame (sent by the AP to a host with
a frame type 0 and subtype of 1, in response to a received ASSOCIATE REQUEST).
For a detailed explanation of each field in the 802.11 frame, see page 34 (Section 7) of
the 802.11 spec at http://gaia.cs.umass.edu/wireshark-labs/802.11-1999.pdf.
9. What two actions are taken (i.e., frames are sent) by the host in the trace just after
t=49, to end the association with the 30 Munroe St AP that was initially in place
when trace collection began? (Hint: one is an IP-layer action, and one is an
802.11-layer action). Looking at the 802.11 specification, is there another frame
that you might have expected to see, but don’t see here?
10. Examine the trace file and look for AUTHENICATION frames sent from the host
to an AP and vice versa. How many AUTHENTICATION messages are sent
from the wireless host to the linksys_ses_24086 AP (which has a MAC address of
Cisco_Li_f5:ba:bb) starting at around t=49? .
11. Does the host want the authentication to require a key or be open?
12. Do you see a reply AUTHENTICATION from the linksys_ses_24086 AP in the
trace?
13. Now let’s consider what happens as the host gives up trying to associate with the
linksys_ses_24086 AP and now tries to associate with the 30 Munroe St AP. Look
for AUTHENICATION frames sent from the host to and AP and vice versa. At
what times are there an AUTHENTICATION frame from the host to the 30
Munroe St. AP, and when is there a reply AUTHENTICATION sent from that AP
to the host in reply? (Note that you can use the filter expression “wlan.fc.subtype
== 11and wlan.fc.type == 0 and wlan.addr == IntelCor_d1:b6:4f” to display only
the AUTHENTICATION frames in this trace for this wireless host.)
14. An ASSOCIATE REQUEST from host to AP, and a corresponding ASSOCIATE
RESPONSE frame from AP to host are used for the host to associated with an AP.
At what time is there an ASSOCIATE REQUEST from host to the 30 Munroe St
AP? When is the corresponding ASSOCIATE REPLY sent? (Note that you can
use the filter expression “wlan.fc.subtype < 2 and wlan.fc.type == 0 and wlan.addr == IntelCor_d1:b6:4f” to display only the ASSOCIATE REQUEST and ASSOCIATE RESPONSE frames for this trace.) 15. What transmission rates is the host willing to use? The AP? To answer this question, you will need to look into the parameters fields of the 802.11 wireless LAN management frame. 4. Other Frame types Our trace contains a number of PROBE REQUEST and PROBE RESPONSE frames. 16. What are the sender, receiver and BSS ID MAC addresses in these frames? What is the purpose of these two types of frames? (To answer this last question, you’ll need to dig into the online references cited earlier in this lab). ... Purchase answer to see full attachment

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