Exploring TCP and UDP based on Kurose and Ross (Computer Networking: A Top-Down Approach) May 15, 2018

Transcription

1 Exploring TCP and UDP based on Kurose and Ross (Computer Networking: A Top-Down Approach) May 15, 2018

2 Exploring TCP Description Capturing a bulk TCP transfer from your computer to a remote server. In this lab, we ll investigate the behavior of 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 also briefly consider TCP connection setup and we ll investigate the performance of the TCP connection between your computer and the server.

3 Exploring TCP Preparing for upload STEPS (Preparing for upload): 1. Start up your web browser. Go to and retrieve an ASCII copy of Alice in Wonderland. Store this file somewhere on your computer. 2. Next go to 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). Do not press the Upload alice.txt file button yet.

4 Exploring TCP Capturing the upload STEPS (Capturing the upload): 4. 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.

5 Exploring TCP Exercise 1 a) What is the IP address and TCP port number used by your client computer (source) to transfer the file to gaia.cs.umass.edu? Client computer (source) IP address: TCP port number: b) What is the IP address and port number used by gaia.cs.umass.edu to receive the file? Fill the gaps. Destination computer: gaia.cs.umass.edu IP address: TCP port number:

6 Exercise 1 - Hint It should be clear which two IPs and port are involved. Consider which machine is sending what types of messages to determine which belongs to the server and which to the client. If unsure: compare with your neighbor; what address would be the same and which one different. Can you now explain it with the point above?

7 Exploring TCP Exercise 2 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? Sequence number of TCP SYN = What is it in the segment that identifies the segment as a SYN segment? (Look at flags)

8 Exercise 2 Example solution Solution in sample Packet Trace

9 Exploring TCP Exercise 3 Examine the trace to answer the following questions: a) What is the sequence number of the SYNACK segment sent by gaia.cs.umass.edu to the client computer in reply to the SYN? Seq number of SYNACK segment = b) What is the value of the ACKnowledgement field in the SYNACK segment? How did gaia.cs.umass.edu determine that value? ACKnowledgement field = c) What is it in the segment that identifies the segment as a SYNACK segment? (Check flags)

10 Exploring TCP Exercise 4 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. Sequence number of TCP segment =

11 Exercise 4 Sample solution Solution in sample Packet Trace

12 Exploring TCP Exercise 5 Consider the TCP segment containing the HTTP POST as the first segment in the TCP connection. We aim to calculate Estamated RTT after 3 rd segment: 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 three segments? What is the EstimatedRTT value after 3 rd segment in the TCP connection? Assume that the value of the EstimatedRTT is equal to the measured RTT for the first segment, and then is computed using the EstimatedRTT formula.

13 Exploring TCP Exercise 5 step by step gathering the information Fill the table based on your packet capturing file Sequence Number Sent Time ACK Received Time RTT (seconds) Segment #1 Segment #2 Segment #3

14 Exploring TCP Exercise 5 step by step calculating RTT Calculate estimatedrtt after 3 rd segment reception in your captured packet file EstimatedRTT =

15 Exploring TCP Exercise 6 What are the lengths of TCP segments? Segment #1 Segment #2 Segment #3 Length of TCP segment

16 Exploring UDP Capture some UDP traffic There are many ways to cause your computer to send and receive UDP messages since UDP is widely used as a transport protocol. The easiest options are to: 1. Do nothing but wait for a while. UDP is used for many system protocols that typically run in the background and produce small amounts of traffic, e.g., DHCP for IP address assignment and NTP for time synchronization. 2. Use your browser to visit sites. UDP is used by DNS for resolving domain names to IP addresses, so visiting fresh sites will cause DNS traffic to be sent. Be careful not to visit unsafe sites; pick recommended sites or sites you know about but have not visited recently. Simply browsing the web is likely to cause a steady stream of DNS traffic. 3. Start up a voice-over-ip call with your favorite client. UDP is used by RTP, which is the protocol commonly used to carry media samples in a voice or video call over the Internet.

17 Exploring UDP EXERCISE 1 Select one packet. Answer the following questions directly from what you observe in the packet trace. (Do not look in the textbook!) 1. From this packet, determine how many fields there are in the UDP header. Name these fields.

18 EXERCISE 1 Sample solution Solution in sample packet trace

19 Exploring UDP EXERCISE From the packet content field, determine the length (in bytes) of each of the UDP header fields. 3. The value in the Length field is the length of what? Verify your claim with your captured UDP packet. 4. What is the protocol number for UDP? Give your answer in both hexadecimal and decimal notation. To answer this question, you ll need to look into the IP header.

20 Exercise 4 solution

21 Lessons Learned Practiced Wireshark skills and the ability to extract specific pieces of information. Seen the TCP and UDP protocols in practice. Try to link this to the theory about them, e.g. can you recognize the TCP congestion avoidance in action? what types of applications use UDP why is UDP chosen over TCP for these?