Wireshark TCP Packet Analysis

Task and Annotation Instructions

Annotate the printout[ 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.] 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 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? 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?

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?  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 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.

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

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.

Answer the following questions for the TCP segments the packet trace tcp-ethereal-trace-1 in 

13.Use the Time-Sequence-Graph(Stevens) plotting tool to view the sequence number versus time plot of segments being sent from the client.  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.