Distributed and Concurrent Systems                      4.4.2008

Exercise 4

No X-exercise this week.

19) Let us continue the dinner time agreement of ex 5 (week 1) by
    calculating the usage of time in a synchronous world. Let us assume
    that an SMS message is delivered by the network in 0..1 minutes,
    each person reacts in 0..4 min, makes the decision in 0..5 min, and
    writes a new message in 1..3 minutes. Calculate the time (minimum
    and maximum) of the whole agreement when we assume that message
    delivery and persons are reliable. Are to time limits realistic?

20) Recompute the total time (min&max) again if any single message (but
    no more messages) may be lost. Calculate the time it takes to detect
    a lost message.

For the following tasks, use the linear protocol (instead of 1->all),
and assume that all messages are acknowledged to the sender.

21) Draw the message deliveries and logical (Lamport's) clocks for four
    person agreement. See, e.g., Fig 11.6 in CDK4; 10.6 in CDK3)

22) Draw the vector clock for three person agreement (e.g., Fig 11.7 in
    CDK4; 10.7 in CDK3)

23) One goal of the peer-to-peer applications (Gnutella, BitTorrent,
    Freenet, etc) has been to hide all centralized actions to disturb
    possible copyright actions. Consider and seek from web how the
    responsibilities of possible illegalities are distributed in
    different peer-to-peer networks.

24) Consider different (technical, juridical, physical) actions to
    enforce the copyright issues without hindering legal usage of the
    applications. Are such actions sensible/efficient?