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Wireshark Lab: TCP v7.0
“Tell me and I forget. Show me and I remember. Involve me and I
understand.” Chinese proverb
In this lab, we’ll investigate the behavior of the celebrated TCP protocol in detail. We’ll
do so by analyzing a trace of the TCP segments sent and received in transferring a 150KB
file (containing the text of Lewis Carrol’s Alice’s Adventures in Wonderland) from your
computer to a remote server. We’ll study TCP’s use of sequence and acknowledgement
numbers for providing reliable data transfer; we’ll see TCP’s congestion control
algorithm – slow start and congestion avoidance – in action; and we’ll look at TCP’s
receiver-advertised flow control mechanism. We’ll also briefly consider TCP connection
setup and we’ll investigate the performance (throughput and round-trip time) of the TCP
connection between your computer and the server.
1. Capturing a bulk TCP transfer from your computer to a remote
server
Before beginning our exploration of TCP, we’ll need to use Wireshark to obtain a packet
trace of the TCP transfer of a file from your computer to a remote server. You’ll do so by
accessing a Web page that will allow you to enter the name of a file stored on your
computer (which contains the ASCII text of Alice in Wonderland), and then transfer the
file to a Web server using the HTTP POST method (see section 2.2.3 in the text). We’re
using the POST method rather than the GET method as we’d like to transfer a large
amount of data from your computer to another computer. Of course, we’ll be running
Wireshark during this time to obtain the trace of the TCP segments sent and received
from your computer.
Do the following:
• Start up your web browser. Go the http://gaia.cs.umass.edu/wiresharklabs/alice.txt and retrieve an ASCII copy of Alice in Wonderland. Store this file
somewhere on your computer.
• Next go to http://gaia.cs.umass.edu/wireshark-labs/TCP-wireshark-file1.html.
• You should see a screen that looks like:




Use the Browse button in this form to enter the name of the file (full path name)
on your computer containing Alice in Wonderland (or do so manually). Don’t yet
press the “Upload alice.txt file” button.
Now 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).
Returning to your browser, press the “Upload alice.txt file” button to upload the
file to the gaia.cs.umass.edu server. Once the file has been uploaded, a short
congratulations message will be displayed in your browser window.
Stop Wireshark packet capture. Your Wireshark window should look similar to
the window shown below.
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
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. A first look at the captured trace
Before analyzing the behavior of the TCP connection in detail, let’s take a high level
view of the trace.
• First, filter the packets displayed in the Wireshark window by entering “tcp”
(lowercase, no quotes, and don’t forget to press return after entering!) into the
display filter specification window towards the top of the Wireshark window.
What you should see is series of TCP and HTTP messages between your computer and
gaia.cs.umass.edu. You should see the initial three-way handshake containing a SYN
message. You should see an HTTP POST message. Depending on the version of
2
Download the zip file http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file tcpethereal-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 tcp-ethereal-trace-1 trace file.
Wireshark you are using, you might see a series of “HTTP Continuation” messages being
sent from your computer to gaia.cs.umass.edu. Recall from our discussion in the earlier
HTTP Wireshark lab, that is no such thing as an HTTP Continuation message – this is
Wireshark’s way of indicating that there are multiple TCP segments being used to carry a
single HTTP message. In more recent versions of Wireshark, you’ll see “[TCP segment
of a reassembled PDU]” in the Info column of the Wireshark display to indicate that this
TCP segment contained data that belonged to an upper layer protocol message (in our
case here, HTTP). You should also see TCP ACK segments being returned from
gaia.cs.umass.edu to your computer.
Answer the following questions, by opening the Wireshark captured packet file tcpethereal-trace-1 in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip (that is
download the trace and open that trace in Wireshark; see footnote 2). Annotate your
screenshots 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 IP address and TCP port number used by the client computer (source)
that is transferring the file to gaia.cs.umass.edu? To answer this question, it’s
probably easiest to select an HTTP message and explore the details of the TCP
packet used to carry this HTTP message, using the “details of the selected packet
header window” (refer to Figure 2 in the “Getting Started with Wireshark” Lab if
you’re uncertain about the Wireshark windows.
2. What is the IP address of gaia.cs.umass.edu? On what port number is it sending
and receiving TCP segments for this connection?
If you have been able to create your own trace, answer the following question:
3. What is the IP address and TCP port number used by your client computer
(source) to transfer the file to gaia.cs.umass.edu?
Since this lab is about TCP rather than HTTP, let’s change Wireshark’s “listing of
captured packets” window so that it shows information about the TCP segments
containing the HTTP messages, rather than about the HTTP messages. To have
Wireshark do this, select Analyze->Enabled Protocols. Then uncheck the HTTP box and
select OK. You should now see a Wireshark window that looks like:
This is what we’re looking for – a series of TCP segments sent between your computer
and gaia.cs.umass.edu. We will use the packet trace that you have captured (and/or the
packet trace tcp-ethereal-trace-1 in http://gaia.cs.umass.edu/wireshark-labs/wiresharktraces.zip; see earlier footnote) to study TCP behavior in the rest of this lab.
3. TCP Basics
Answer the following questions for the TCP segments and show annotated screenshots:
4. What is the sequence number of the TCP SYN segment that is used to initiate the
TCP connection between the client computer and gaia.cs.umass.edu? What is it
in the segment that identifies the segment as a SYN segment?
5. What is the sequence number of the SYNACK segment sent by gaia.cs.umass.edu
to the client computer in reply to the SYN? What is the value of the
Acknowledgement field in the SYNACK segment? How did gaia.cs.umass.edu
determine that value? What is it in the segment that identifies the segment as a
SYNACK segment?
6. What is the sequence number of the TCP segment containing the HTTP POST
command? Note that in order to find the POST command, you’ll need to dig into
the packet content field at the bottom of the Wireshark window, looking for a
segment with a “POST” within its DATA field.
7. Consider the TCP segment containing the HTTP POST as the first segment in the
TCP connection. What are the sequence numbers of the first six segments in the
TCP connection (including the segment containing the HTTP POST)? At what
time was each segment sent? When was the ACK for each segment received?
Given the difference between when each TCP segment was sent, and when its
acknowledgement was received, what is the RTT value for each of the six
segments? What is the EstimatedRTT value (see Section 3.5.3, page 242 in
text) after the receipt of each ACK? Assume that the value of the
EstimatedRTT is equal to the measured RTT for the first segment, and then is
computed using the EstimatedRTT equation on page 242 for all subsequent
segments.
Note: Wireshark has a nice feature that allows you to plot the RTT for
each of the TCP segments sent. Select a TCP segment in the “listing of
captured packets” window that is being sent from the client to the
gaia.cs.umass.edu server. Then select: Statistics->TCP Stream Graph>Round Trip Time Graph.
8. What is the length of each of the first six TCP segments?4
9. What is the minimum amount of available buffer space advertised at the received
for the entire trace? Does the lack of receiver buffer space ever throttle the
sender?
10. Are there any retransmitted segments in the trace file? What did you check for (in
the trace) in order to answer this question?
11. How much data does the receiver typically acknowledge in an ACK? Can you
identify cases where the receiver is ACKing every other received segment (see
Table 3.2 on page 250 in the text).
12. What is the throughput (bytes transferred per unit time) for the TCP connection?
Explain how you calculated this value.
4
The TCP segments in the tcp-ethereal-trace-1 trace file are all less that 1460 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 TCP payload). This 1500 byte
value is the standard maximum length allowed by Ethernet. If your trace indicates a TCP length greater
than 1500 bytes, and your computer is using an Ethernet connection, then Wireshark is reporting the wrong
TCP segment length; it will likely also show only one large TCP segment rather than multiple smaller
segments. Your computer is indeed probably sending multiple smaller segments, as indicated by the ACKs
it receives. This inconsistency in reported segment 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 provided trace file.
4. TCP congestion control in action
Let’s now examine the amount of data sent per unit time from the client to the server.
Rather than (tediously!) calculating this from the raw data in the Wireshark window,
we’ll use one of Wireshark’s TCP graphing utilities – Time-Sequence-Graph(Stevens) – to
plot out data.
• Select a TCP segment in the Wireshark’s “listing of captured-packets” window.
Then select the menu : Statistics->TCP Stream Graph-> Time-SequenceGraph(Stevens). You should see a plot that looks similar to the following plot,
which was created from the captured packets in the packet trace tcp-etherealtrace-1 in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip (see earlier
footnote ):
Here, each dot represents a TCP segment sent, plotting the sequence number of
the segment versus the time at which it was sent. Note that a set of dots stacked
above each other represents a series of packets that were sent back-to-back by the
sender.
Answer the following questions for the TCP segments the packet trace tcp-etherealtrace-1 in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip
13. Use the Time-Sequence-Graph(Stevens) plotting tool to view the sequence
number versus time plot of segments being sent from the client to the
gaia.cs.umass.edu server. Can you identify where TCP’s slowstart phase begins
and ends, and where congestion avoidance takes over? Comment on ways in
which the measured data differs from the idealized behavior of TCP that we’ve
studied in the text.
14. Answer each of two questions above for the trace that you have gathered when
you transferred a file from your computer to gaia.cs.umass.edu. Make sure
you clearly annotate screenshots and show which two questions you’ve
answered for this question.
15. How long did it take to complete the lab? Did you feel this lab was helpful
or valuable? Please explain your answer.

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