Carleton College CS 107, Explorations in Computer Science

Assignment:

Work through Laboratory 15, part 1, and turn in Exercises 1-3, subject to the changes and clarifications below.

This applet seems to get easily confused if you stop and start again. Whenever you are ready to send a message, first click the Reload button in your web browser to reset the applet.

Here are some clarifications and rewrites for Exercise 1:

• Question 3: Each packet contains two lines. The first line is the packet header, and the second line is the packet data. Each character or blank in a packet counts for one character, and all DAT packets except the last carry 10 characters in the packet data. The last one carries less, since there may be less than 10 characters remaining in the message. ACK packets have no characters at all in the packet data, though they still have some in the packet header. For the example message, count how many characters are sent in total, including packet headers and packet data for all DAT and ACK packets.


• Question 4.5: Explain in your own words what overhead means. How does overhead affect the speed at which your message is transmitted?


• Question 5: You may make some simplifying assumptions here, if you wish (such as all packet headers contain 10 characters, or something like that). State whatever assumptions you make.


• Question 6: How would changing the size of the data portion of the packets affect the overhead? Could changing the size of the packets to improve overhead backfire? Under what conditions?

Here are some clarifications and rewrites for Exercise 2:

• Question 5: Now try deleting some ACK packets. What happens


• Question 6: Why don't any NAK packets appear? How can you make the applet produce NAK packets?


• Question 7: The applet doesn't let you do this, but what if a NAK packet gets damaged? How should the sender handle it?

Here are some clarifications and rewrites for Exercise 3:

• Question 1-3: Calculate the checksum as TCP/IP for the first packet in the sample message, the one that has the data "A very lon". Using the "Text Translator into ASCII" applet from Lab 3b, type in the characters from the data packet, and click on translate text. Write down the ASCII codes (in decimal) that you see.


• Questions 4-5: Add up all the codes above, then find the remainder when you divide by 256. Show your work. This should be the same as the checksum in the packet. Is it?


• Question 6: Now suppose that the packet was damaged so that instead the data read "A verX lon". Re-compute the checksum. What do you get?


• Question 7: Give an example of a corrupted version of the data "A very lon" that will still have the same checksum as the undamaged version. A corrupted packet could corrupt more than one character at once.