1995 Problem Set
Problem 1 (15 Points)
In some asymmetric networks the available bandwidth in one
direction is much less than the available bandwidth in the other
direction. Consider a network in which communication from a
server to clients is handled via broadcast at 1
Gigabit/second. Unfortunately, the back-channel from each client
to the server is only 1 Kilobit/second and has a high latency of
10 seconds.
You've been asked to design a distributed file system
using this network. Assume the server is always available and is
able to store all the files in the system. Assume the clients
each have a small local disk and some reasonable amount of main
memory for a file cache (10 Megabytes each).
In your design, how would you use the available network
resources? How would you arrange for clients to receive copies
that they need? Would you allow files on the server to be
writable as well as readable? If so, what sort of cache
consistency might you provide, if any?
Problem 2 (15 Points)
You work for a company that seems happy to purchase workstations
with large main memories and local disks for every employee, but
they are queasy about purchasing machines to be designated as
shared file servers. Nonetheless, your company needs some sort
of distributed file service. You decide to implement a
distributed file service that uses all the clients
cooperatively. In particular, to eliminate central server file
caching, you decide to allow all the clients to perform file
caching duties for each other.
How might this solve your problem? What advantages does
it have? What disadvantages does it have? (Consider such issues
as latency of file accesses, throughput, availability, cache
consistency, complexity, and trust.) What sort of cache
consistency protocol might you propose?
Problem 3 (15 Points)
A long-time goal in distributed systems has been location
transparency -- one should not need to know the physical
location of resources or users in order to communicate with
them. Some systems have taken this point to an extreme and
attempt to hide the location of entities in the system.
In networks of mobile hosts, this issue is being
reconsidered. One proposal includes making location information
about users, hosts and resources available to applications and
users. Describe the advantages and disadvantages of this
proposl. (Consider issues such as performance, network routing
choices, and privacy.) What application(s) might this proposal
enable?
Problem 4 (15 Points)
Recently it has been suggested by some researchers that DSM
(distributed shared memory (*)) is an appropriate application
for a mobile, wireless computing environment. In this
environment the hosts have about 20 Megabytes of memory each,
moderately-sized local disks, and reasonable processor speed.
Communication takes place over a 9.6 Kilobit/second network that
suffers from a high bit error rate and intermittent lapses in
connectivity. Do you think this idea is splendid, outrageously
stupid, or just so-so? Why? If you had to implement DSM in this
environment, what sorts of problems would you expect to see? How
would you address these problems? What sort of memory
consistency model(s) might you choose? What applications, if
any, would you expect to run well on top of DSM in this
environment?