Networking 201

Copyright Priscilla Oppenheimer

Lab 2 IP Fragmentation

Objectives

Step 1. Calculate the maximum message size

  1. What is the maximum size (including header, data, and trailer) of an Ethernet frame?
  2. What are the fields in an Ethernet header and trailer and how long is each field (in bytes)?
  3. How many total bytes are in an Ethernet header and trailer?
  4. How many bytes are in an IP header? (Don't include any options.)
  5. How many bytes are in an ICMP header?
  6. If you send an ICMP Echo Request (also known as a ping), how much of the packet will be overhead (headers)? Add up your answers to Questions 3, 4, and 5 to answer this question.
  7. So, what is the maximum size of the message in a single ping packet when the ping packet is sent on an Ethernet network? Subtract your answer for Question 6 from your answer to Question 1 to answer this question. This answer is your maximum message size.

Step 2. Experiment with IP fragmentation

  1. On Windows open a Command Prompt window.
    1. Select Run from the Start menu and type cmd and Enter.
  2. Type ping -f -l <size> <ip_address>, where size is the result of Question 7 above and ip_address is the address of another host on your network. The -f parameter indicates that the packet should not be fragmented. The -l size parameter defines the size of the Message field in the ping packet.
  3. Was your ping successful? It should be if you calculated your maximum message size correctly.
  4. Now, increase the size to see the reaction. Add 1 to the size you obtained in Question 7 above and repeat your ping, entering the ping -f -l <size> <ip address> command again.
  5. Was this ping successful? What message do you see?

Step 3: Analyze IP fragments

  1. Start a packet capture with Wireshark.
  2. Ping once more but this time do not use the -f parameter. On the other hand, do use the -l parameter and use a vary large MTU, such as 5000 bytes.
  3. Stop capturing traffic.
  4. Find a packet that is an IP fragment and briefly describe the fields in the IP header that are relevant to IP fragmentation.

Step 4: Hand in your work

Submit the answers to the questions and your packet description.