Week 7 project

Lab 4: Wireshark -Exploring TCP

PART 1: Capturing a bulk TCP transfer from your computer to a remote server

STEPS:

1. 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. (From your browsers

menu, go to file , Save As and save as a .txt file)

2. Next go to http://gaia.cs.umass.edu/wireshark-labs/TCP-wireshark-file1.html.

3. 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.

4. 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).

5. 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.

6. Stop Wireshark packet capture. Your Wireshark window should look similar to the window shown

below.

——————————————————————————————————————————————

—————————-

PART 2: A first Look At the Captured Trace

Use the online capture (shown below) to answer the following:

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 your own Capture to answer the following:

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?

——————————————————————————————————————————————

—————————-

PART 3: TCP Basics

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.

To prevent Wireshark from reassembling the packets and displaying them as one response, rather than as multiple continuation packets go to Edit -> Preferences->Protocols->HTTP and unchecking the \Reassemble HTTP bodies spanning multiple TCP segments" box".

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?

8. What is the length of each of the first six TCP segments?

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?

12. What is the throughput (bytes transferred per unit time) for the TCP connection? Explain how you

calculated this value.

——————————————————————————————————————————————

PART 4: TCP Congestion Control In Action

STEPS:

1. Select a TCP segment in the Wireshark’s “listing of captured-packets” window. Then select the menu

: Statistics->TCP Stream Graph-> Time-SequenceGraph(Stevens).

QUESTIONS:

Answer Question 13 Using the provided Capture

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.

Powered by TCPDF (www.tcpdf.org)