Cpsc 1050 Networking Data Packets
  1. Start the “TCP/IP” (reliable connection simulator) applet.
  2. In the text field labeled “Your message:”, type the following: "Computer networking is essential in our world today.". Then press the button “Send a message.”
  3. Watch the entire sequence of packets that are sent for the sample message. How many "DAT" packets were sent? How many "ACK" packets were sent?
  4. If each character or blank in a packet header counts for one character, and all "DAT" packets except the last carry 10 characters (including the blanks), and "ACK" packets have no data characters (though they do have a header), count up how many characters were sent in total.
  5. There are 52 characters in the above message, including blanks and punctuation. Subtract 52 from the total number of characters sent in both directions in all packets. Divide this number by the total number of characters to get the overhead (those characters which are not really data but yet you have to transmit them), represented as a percentage.
  6. Imagine that you have a million-character message to send, perhaps a large file. How many characters in total (and to your best estimate) would be sent in all packets necessary to move it from node 0 to node 1?
  7. What would be an obvious way to decrease the overhead?
  8. Repeat Steps 1 and 2.
  9. Select "Delete packets that are touched".
  10. Delete some data packets by clicking on them as they move along the wire and watch the retransmission after timeout.
  11. What happens if you delete the re-transmitted packet?
  12. Now try deleting some ACK or NAK packets. What happens?
  13. See if you can compute the checksum as TCP/IP does. Run the applet, using any message. Select a data packet, but don’t select the last packet because it might be too short, less than 10 characters. Count up the characters, including blanks. If they do not equal 10, assume there are blanks at the end so that the character count is 10.
  14. Using the “Text Encoding in ASCII” applet, type in the characters from the data packet, and click on "translate text".
  15. Add up the values for each of the characters. Then calculate the modulus of this number using 256.
  16. Compare your manually computed checksum for the first packet of the message "Introduction to Computer Science" against what the applet shows for the packet. Do they match?
  17. Now damage your packet by altering one character. Re-compute the checksum. You should realize how the applet can spot errors.
  18. Think of a way that a packet can be damaged and still have the same checksum as the undamaged version. Give one example for this scenario.

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