Distributed and Concurrent Systems                      7.3.2008

Exercise 2

Draw a figure on each task. For tasks 7-9 you can seek information from
the Internet. These tasks still are based mostly on common sense,
previous CS knowledge, and on seeking information from the Internet.

7) In the Internet, the delivery time of any single packet can vary
   considerably. Also, any packet can be lost. What kind of quality
   requirements for data delivery set the following applications: a)
   Internet phone calls (VoIP), b) Internet video calls, c) Internet
   radios (constant stream), d) watching news clips later (non-real
   time), e) watching TV on real time (IPTV), f) buying and downloading
   music over the net? Make an educated guess what techniques those
   applications use to fulfill the quality requirements over the
   unreliable Internet?.

8) What kind of security risks there are in different services listen in
   the previous question? How we could protect against them?


9) Compare the Internet implementation of each of the services of task 7
   against their old technology counterpart (e.g., land-line phone,
   FM-radio, digital TV (DVB), compact audio disc (CD)). Estimate
   usability, usage of resources, and scalability for large amount of
   users.

10) List some simple (not distributed, but one used in PCs) applications
    that benefit from a) concurrency (several threads of execution), b)
    parallel computing (computing power of several processors).

The following X1 exercise is obligatory for all students. The answers to
X-exercises have to be unique for every student. No copies of the same
answer are allowed. The answer has to be sent by Thursday 2:00 pm (the
previous day). You will receive an acknowledgment upon successful
processing. Answers will be graded. The answer must also contain a short
self-evaluation in which you describe whether the algorithm works,
nearly works, or does not probably work; how efficient it is, etc. A
correct and proper self-evaluation is worth one point (in case of a
proper answer).

Send your answer to using cs to user sjuva with a subject HS_X1_username
and the answer (with self-evaluation) as the body of the message (no
attachments). At simplest using program mail at cs:

        /usr/ucb/mail -s HS_X1_username sjuva < answer.txt

Where username is your cs username and answer.txt is a text file
containing your answer. This first X-exercise is also a rehearsal and a
test of the automatic processing system.

X1) Let us continue the dinner time agreement of task 6. Now any single
    message may be lost (in addition of any single person). Further, now
    we have to account for the possibility that two (or more) persons
    start the agreement procedure concurrently (without knowing the
    other's intentions). Even in this case, only one time must be
    agreed. You can assume the participants can find at least one
    applicable time.

    Describe your algorithm and messaging protocol accurately enough so
    that all participants will work correctly. If your algorithm has
    weaknesses/uncertainness, tell about them in your self-evaluation.
    How many messages your algorithm requires (as a function of the
    number of participants).