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TitleWireshark_TCP_SOLUTION_v6.0b.pdf
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Wireshark Lab: TCP
SOLUTION


Supplement to Computer Networking: A Top-Down
Approach, 6th ed., J.F. Kurose and K.W. Ross

© 2005-21012, J.F Kurose and K.W. Ross, All Rights Reserved





The answers below are based on the trace file tcp-ethereal-trace-1 in in
http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip

TCP Basics

Answer the following questions for the TCP segments:


1. What is the IP address and TCP port number used by your client computer
(source) to transfer the file to gaia.cs.umass.edu?

2. What is the IP address and port number used by gaia.cs.umass.edu to receive the
file.


Solution: Client computer (source)
IP address: 192.168.1.102
TCP port number: 1161

Destination computer: gaia.cs.umass.edu
IP address: 128.119.245.12
TCP port number: 80


3. If you did this problem on your own computer, you’ll have your own solution

©2013 Pearson Education, Inc. Upper Saddle River, NJ. All Rights Reserved.

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Figure 1: IP addresses and TCP port numbers of the client computer (source) and gaia.cs.umass.edu


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?


Solution: Sequence number of the TCP SYN segment is used to initiate the TCP
connection between the client computer and gaia.cs.umass.edu. The value is 0 in this
trace.

The SYN flag is set to 1 and it indicates that this segment is a SYN segment.


©2013 Pearson Education, Inc. Upper Saddle River, NJ. All Rights Reserved.

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Figure 4: Sequence number of the TCP segment containing the HTTP POST command


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 page 237 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 237 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.


Solution: The HTTP POST segment is considered as the first segment. Segments 1 – 6
are No. 4, 5, 7, 8, 10, and 11 in this trace respectively. The ACKs of segments 1 – 6 are
No. 6, 9, 12, 14, 15, and 16 in this trace.

Segment 1 sequence number: 1
Segment 2 sequence number: 566
Segment 3 sequence number: 2026
Segment 4 sequence number: 3486

©2013 Pearson Education, Inc. Upper Saddle River, NJ. All Rights Reserved.

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Segment 5 sequence number: 4946
Segment 6 sequence number: 6406

The sending time and the received time of ACKs are tabulated in the following table.

Sent time ACK received time RTT (seconds)
Segment 1 0.026477 0.053937 0.02746
Segment 2 0.041737 0.077294 0.035557
Segment 3 0.054026 0.124085 0.070059
Segment 4 0.054690 0.169118 0.11443
Segment 5 0.077405 0.217299 0.13989
Segment 6 0.078157 0.267802 0.18964

EstimatedRTT = 0.875 * EstimatedRTT + 0.125 * SampleRTT

EstimatedRTT after the receipt of the ACK of segment 1:
EstimatedRTT = RTT for Segment 1 = 0.02746 second

EstimatedRTT after the receipt of the ACK of segment 2:
EstimatedRTT = 0.875 * 0.02746 + 0.125 * 0.035557 = 0.0285

EstimatedRTT after the receipt of the ACK of segment 3:
EstimatedRTT = 0.875 * 0.0285 + 0.125 * 0.070059 = 0.0337

EstimatedRTT after the receipt of the ACK of segment 4:
EstimatedRTT = 0.875 * 0.0337+ 0.125 * 0.11443 = 0.0438

EstimatedRTT after the receipt of the ACK of segment 5:
EstimatedRTT = 0.875 * 0.0438 + 0.125 * 0.13989 = 0.0558

EstimatedRTT after the receipt of the ACK of segment 6:
EstimatedRTT = 0.875 * 0.0558 + 0.125 * 0.18964 = 0.0725
second


©2013 Pearson Education, Inc. Upper Saddle River, NJ. All Rights Reserved.

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Figure 10: Sequence numbers of the segments from the source (192.168.1.102) to the destination
(128.119.245.12)


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 247 in the text).


Solution: The acknowledged sequence numbers of the ACKs are listed as follows.
acknowledged sequence number acknowledged data
ACK 1 566 566
ACK 2 2026 1460
ACK 3 3486 1460
ACK 4 4946 1460
ACK 5 6406 1460
ACK 6 7866 1460
ACK 7 9013 1147
ACK 8 10473 1460
ACK 9 11933 1460
ACK 10 13393 1460
ACK 11 14853 1460
ACK 12 16313 1460

The difference between the acknowledged sequence numbers of two consecutive ACKs
indicates the data received by the server between these two ACKs. By inspecting the
amount of acknowledged data by each ACK, there are cases where the receiver is

©2013 Pearson Education, Inc. Upper Saddle River, NJ. All Rights Reserved.

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ACKing every other segment. For example, segment of No. 80 acknowledged data with
2920 bytes = 1460*2 bytes.


Figure 8: Cumulative ACKs (No. 80, 87, 88, etc) where the receiver is ACKing every other received
segment.


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

Explain how you calculated this value.
Solution: The computation of TCP throughput largely depends on the selection of
averaging time period. As a common throughput computation, in this question, we select
the average time period as the whole connection time. Then, the average throughput for
this TCP connection is computed as the ratio between the total amount data and the total
transmission time. The total amount data transmitted can be computed by the difference
between the sequence number of the first TCP segment (i.e. 1 byte for No. 4 segment)
and the acknowledged sequence number of the last ACK (164091 bytes for No. 202
segment). Therefore, the total data are 164091 - 1 = 164090 bytes. The whole
transmission time is the difference of the time instant of the first TCP segment (i.e.,
0.026477 second for No.4 segment) and the time instant of the last ACK (i.e., 5.455830
second for No. 202 segment). Therefore, the total transmission time is 5.455830 -
0.026477 = 5.4294 seconds. Hence, the throughput for the TCP connection is computed
as 164090/5.4294 = 30.222 KByte/sec.


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? Note that in this “real-
world” trace, not everything is quite as neat and clean as in Figure 3.54 (also

©2013 Pearson Education, Inc. Upper Saddle River, NJ. All Rights Reserved.

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