Exercises 3 (29.3.2007)

1. How is it possible that a message in a data communication network can
   1. totally disappear
   2. multiply
   3. pop up to a receiver much later than many of its followers (in principal even many days later)?

2. Cookies
   What are benefits from using cookies? Who benefit from them? What drawbacks they have? Can cookies be security risks? Is it possible to disable cookies? What results can follow from disabling? Do you know what cookies you have in your files?

3. E-mail
   Student T. Terävä from the University of Helsinki sends email to her friend M. Smart to the University of Berkeley in California. She starts a mail program in her PC, writes a short message "Hello! How are you?" addressed to M.Smart@cs.berkeley.edu and sends it.

   1. What protocols are needed in different phases when the message is transferred from the mail program to the Helsinki university mail server and from there to the mail server of the receiver and then to the mail program of the receiver M. Smart.
   2. T.Terävä would like to include into the message a picture of herself. Why was this a problem in the original SMTP mail system? How does a multimedia message including a picture look like when it is on transfer between the mail servers?

4. Name service
   1. Assume that a student at the University of Helsinki clicks an URL but the IP address of its hostname is not found in the local name server. Explain how the IP address of this hostname is resolved after the click when using i) iterative queries, ii) recursive queries.
   2. Become acquainted with nslookup program and use it to find the IP addresses for www.cs.helsinki.fi, www.helsinki.fi, www.funet.fi and www.princeton.edu. What does it mean that the answer is " non-authoritative"? What is meant by a canonical name?

5. Well-known port numbers
   1. Visit http://www.iana.org . What are the well-known port numbers for following protocols: SSH Remote Login Protocol, Kerberos, IRC (Internet Relay Chat Protocol), SFTP (simple file transfer protocol) ja NNTP (network news transfer protocol)?
   2. What other things IANA takes care of? What are it's tasks?

6. Checksums
   UDP and TCP use one's complement in their checksums.
   1. Calculate an 8 bit UDP checksum for the following three 8 bit words: 0101 0101, 0111 0000, 1100 1100.
      In the checksum calculation the sum is calculated using one's complement sum: bits are added normally, carry bits including, except the last carry bit(s) that is/are added to the calculated sum.

                              1011
            			1101
            			====
                              1000
            			   1  (This is the last carry bit!)
                             =====
            			1001  (That is the final sum.)
      
            			0110  (Here is the one's complement of the sum
            			        = the checksum to be added.)
      

   2. How does the receiver detect that an error has occured in the transfer?
   3. Is it possible that the receiver does not detect one bit error (ie, the change of one bit from one to zero or vice verse)? What about two or more bit errors?