Exercise 9 (12. -16.4.) (Tanenbaum pp. 374 -424)
1. Answer shortly the following questions ("review type" problem: answers
are usually found in
lecture notes or in Tanenbaum's book).
a) Why is flow control much easier
than congestion control?
b) Buffers of a router are overflown. Which
packets will be destroyed, the arriving ones or
some others?
c) What is tunneling?
d) Why a firewall has both routers and
a gateway?
e) Both IP and ATM protocols count
checksums only for the header? Why for the header and
why only for it?
f) Is fragmentation needed in
concatenated virtual circuit internets, or only in datagram systems?
g) Why are ARP and RARP protocols needed?
What is common to both these protocols?
How do they differ
from each other?
2. a) What kind of errors the network layer has to prepare
for? Is it possible to have the data link
layer or
transport layer to take care of these errors?
b) What all things have to be negotiated when
establishing a virtual circuit across many networks?
Explain in each case
what happens when wishes/ requirements are not compatible?
3. A PC is connected to a 6 Mbps network, where
token bucket method is used for congestion
control. Tokens are generated at
a rate of 1 Mbps. The token bucket is initially filled with to
maximum capasitity of 8 megabits. How long
can the computer transmit at the full 6 Mbps?
(The answer is not 1.33 s!)
.
4. a) Tunneling through a concatenated virtual circuit subnet
is straightforward: the multiprotocol
router at one end just sets
up a virtual circuit to the other end and passes packets through it.
Can tunneling also be used in
datagram subnets? If so, how?
b) A person with a portable computer travels to
Berkeley and to his surprise finds out that
there is a wireless
LAN. So he doesn't have to plug his computer in, but can he use it?
If he can, how is it done?
c) In Berkeley there is also a GSM network. Supposing
the local operator has a roaming contract
with some finnish
operator (calls from one operator are transferred to another).
Explain how
a travelling person can
read his email from Berkeley, or is it not possible?
5. a) Convert the IP address whose hexadecimal representation
is C22F1582 to dotted decimal
notation.
b) A class B network on the Internet
has a subnet mask 255.255.240.0. What is the maximum
number of hosts
per subnet?
6. a) Describe, in suitable detail, how IP fragments
are reassembled at the destination.
b) Some fragments can get lost in the transmission.
For this reason, the reassembly of fragments
uses a timer.
In case of timeout the unfinished packet is destroyed. Suppose
a datagram is
fragmented into four fragments.
The first three fragments arrive, but the last one is delayed.
Eventually the timer goes
off and the three fragments in the receiver's memory are discarded.
A little later, the last
fragments arrives. What should be done with it?
c) An IP datagram using the Strict
source routing option. has to be fragmented. Do you think
the option has to be copied
into each fragment, or is it sufficient to just put it in the first
fragment? Explain your answer.