Description
Wireshark Lab
Q. 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. Before beginning this lab, you’ll probably want to review sections 3.5 and 3.7 in
the textbook.
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.
1 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.
• 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.
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 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-trace1 in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip (download the trace and
open that trace in Wireshark; see footnote 1 in red font). Whenever possible, when answering a
question, attach screenshots.
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”.
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?
Use the trace that you created (your trace file) to 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 will see a series of TCP segments sent between your computer and gaia.cs.umass.edu. We
will use the packet trace that you have captured (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:
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.
Additional work for Grad Students (Answer earlier Questions 1 through 6 also)
Consider the TCP segment containing the HTTP POST as the first segment in the TCP
connection.
1. 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 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 in textbook for all subsequent segments.
2. What is the length of each of the first six TCP segments?
3. What is the minimum amount of available buffer space advertised at the receiver for the
entire trace? Does the lack of receiver buffer space ever throttle the sender?
4. Are there any retransmitted segments in the trace file? What did you check for (in the
trace) in order to answer this question?
5. 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 textbook).
6. What is the throughput (bytes transferred per unit time) for the TCP connection? Explain
how you calculated this value.
