Wireshark Lab 6: Internet Protocol

2. If you are using a Windows platform, start up pingplotter and enter the name of a target destination in the “Address to Trace Window.” Enter 3 in the “# of times to Trace” field, so you don’t gather too much data. Select the menu item Edit->Advanced Options->Packet Options and enter a value of 56 in the Packet Size field and then press OK. Then press the Trace button

3. Send a set of datagrams with a longer length, by selecting Edit->Advanced Options->Packet Options and enter a value of 2000 in the Packet Size field and then press OK. Then press the Resume button.

4.Finally, send a set of datagrams with a longer length, by selecting Edit->Advanced Options->Packet Options and enter a value of 3500 in the Packet Size field and then press OK. Then press the Resume button.

5.Stop Wireshark tracing.

Questions:

1. What is the IP address of your computer?

• The IP address of my computer is 192.168.1.110

2. Within the IP packet header, what is the value in the upper layer protocol field?

• The value of the upper layer protocol field is ICMP (0X01)

3. How many bytes are in the IP header? How many bytes are in the payload of the IP datagram? Explain how you determined the number of payload bytes.

• There are 20 bytes in the IP header which leaves 36 bytes for the payload of the IP datagram because we were sending a packet of length 56 bytes.

4. Has this IP datagram been fragmented? Explain how you determined whether or not the datagram has been fragmented.

• The fragment offset is set to 0, therefore, the packet has not been fragmented.

5. Which fields in the IP datagram always change from one datagram to the next within this series of ICMP messages sent by your computer?

• The header checksum and the Identification changes from each datagram to the next.

6. Which fields stay constant? Which of the fields must stay constant? Which fields must change? Why?

Fields that stay constant:

• Version(IPv4)
• Length of header
• Source IP(sending from same place)
• Destination IP(contacting same site)
• Upper layer protocol(always using ICMP)

Fields that must stay constant:

The fields that must change are:

• The header checksum (header changes)
• Identification(to verify packets)

7. Describe the pattern you see in the values in the Identification field of the IP datagram

• The pattern in the identification field is that the field increases by one in each strand of echo requests. (see video for proof)

8. What is the value in the Identification field and the TTL field?

• Identification: 28451
• TTL: 10

9. Do these values remain unchanged for all of the ICMP TTL-exceeded replies sent to your computer by the nearest (first hop) router? Why?

• The Identification field changes from all of the replies because this field has to have a unique value. If they(2 or more replies) have the same value then the replies must be fragments of a bigger packet.
• The TLL field does not change because the time to live to the first hop router is always the same.

10. Find the first ICMP Echo Request message that was sent by your computer after you changed the Packet Size in pingplotterto be 2000. Has that message been fragmented across more than one IP datagram?

• Yes, that message has been fragmented across more than one IP datagram.

11. Print out the first fragment of the fragmented IP datagram. What information in the IP header indicates that the datagram been fragmented? What information in the IP header indicates whether this is the first fragment versus a latter fragment? How long is this IP datagram?

• The fact that the flag is set for more segments shows that the the datagram has been fragmented (see above).The fragment offset is set to 0 indicating that this is the first fragment rather than a latter fragment where that value is is set to (1480). The datagram has a total length of 1500.

12. Print out the second fragment of the fragmented IP datagram. What information in the IP header indicates that this is not the first datagram fragment? Are the more fragments? How can you tell?

• The second fragment is obvious because it now has a a fragment offset of 1480. There are no more fragments because it no longer has a flag set for more fragments

.

13. What fields change in the IP header between the first and second fragment?

• The fields that change are

1. Length
2. Flags Set
3. Fragment offset
4. header checksum

14. How many fragments were created from the original datagram?

• After switching to 3500 bytes, 3 fragements are created.

15. What fields change in the IP header among the fragments?

• The fields that change are the fragment offset (0, 1480, 2960) and checksum. The first 2 packets also have lengths of 1500 and more fragments flags set, while the last fragment is shorter (540) and does not have a flag set.