Assignment I
Computer Networks
Bu-Ali Sina University, Hamedan, Iran
By: Mohammad Nassiri
Due: Tuesday, 1388/8/24
1. In chapter 1 of Kurose’s book solve problems P5, P7, P17, P18, P19, and P22.
2. In chapter 2 of Kurose’s book solve problems P1, P4, P5, P7, P8, P17, and P18.
3. Network Performance : In the following figure, two hosts are connected through 3
types of network : Wifi, Satellite & Ethernet. The throughput is respectively 100 M bps,
10 M bps, and 1 M bps for Ethernet, Wifi, and Satellite. Routers interconnect the Wifi
and Ethernet LAN to the satellite network. The satellite behaves like a store and forward
switch (the packet must be completely received before being forwarded).
Geostationary Satellite
1 Mbps
1 Mbps
Web Server
Web Client
Wifi 10Mbps
Ethernet 100Mbps
The user working on the web client downloads a page of 10000 bytes from the web server.
We assume that data are directly transmitted on the wire without any additional encapsulation (no overhead). We also ignore processing and queuing delays. In addition, the
time needed for client to establish a connection to the web server is one second.
Let’s consider two switching modes : packet-switching
(the web page is divided into ten
ISP
1000-byte packets that are sent one after the other) and message-switching (the web page
is sent through a unique 10000-byte message).
(a) Assuming packet-switching mode, how long does it take to download the page (between the click and the instant when the page is downloaded) ?
X
Router
(b) Consider the previous question for message-switching
mode.
Note : Geostationary orbits are useful because they cause a satellite to appear stationary
with respect to a fixed point on the rotating Earth. As a result, an antenna can point in a
60 hosts
20 hosts
fixed direction and maintain
a link with the satellite. The satellite orbits in the direction
of the Earth’s rotation, at an altitude of 35,786 km (22,236 mi) above ground (Assume
Dpt C
8 m ).
propagation speed of signal
2 ∗ 10
Dpt A in the air to 500
hosts
s
1
Dpt B
1-30
0
4. Packet/Circuit Switching : Consider sending a packet of F bits over a path of Q links.
Each link transmits at R bps. The network is lightly loaded so that there are no queuing
delay. Propagation delay is also negligible.
6/15/06
(a) Suppose the network is a packet-switched datagram network and a connectionPage 20 service is used. Suppose each packet has h×F bits of header where 0 < h < 1.
oriented
Assuming ts setup time, how long does it take to send the packet ?
4:34 PM
(b) Suppose that the network is a circuit-switched network. Furthermore, suppose that
the transmission rate of the circuit between source and destination is R/24 bps.
Assuming ts setup time and no bits of header appended to the packet, how long does
it take to send the packet ?
STUDY COMPANION FOR COMPUTER NETWORKING, THIRD EDITION
(c) When is the delay longer for packet switching than for circuit switching assuming
h = 0.5 ? Interpret your result.
5. Caching and delays : Consider the networks shown in the figure below. There are
d. Now
suppose there is noand
HTTP
cache ininnetwork
a.com. a.com.
What is Suppose
the
two user
machines–m1.a.com
m2.a.com
the network
the user at
maximum
rate
at
which
machines
in
a.com
can
make
requests
for
file
m1.a.com types in the URL www.b.com/bigfile.htm into a browser the
to retrieve
a 1Gbit
www.b.com/bigfile.htm
while keeping the time from when a request is
(1000 Mbit)
file from www.b.com.
made to when it is satisfied non-infinite in the long run?
10.(a) Persistent
versus non-persistent TCP connections. Suppose within your
List the sequence of DNS and HTTP messages sent/received from/by m1.a.com, as
Web browser you click on a link to obtain a Web page. Suppose the IP adwell as any other messages that leave/enter the a.com network that are not directly
dress for the associated URL is cached in your local host, so that a DNS
sent/received by m1.a.com from the point that the URL is entered into the browser
lookup is not necessary. Denote RTT as the roundtrip time between the local
until the file is completely received. Indicate the source and destination of each
host and the server containing the Web page. Assume the Web page consists
message. You can assume that every HTTP request by m1.a.com is first directed
of a base HTML file and three small images. Assume the transmission times
to the HTTP cache in a.com, that the cache is initially empty, and that all DNS
for all of the objects are negligible in comparison with the RTT. How much
requests are iterated queries.
time elapses (in terms of RTTs) from when the user clicks on the link until
(b) the
How
longreceives
does itthe
take
to accomplish
theeach
steps
youfollowing?
outlined in your answer to the
client
entire
Web page with
of the
previous
question
regarding
the
m1.a.com
HTTP
and
DNS messages. Explain how
a. Non-persistent HTTP with no parallel connections
you arrived at your answer. In answering this question, you can assume the following :
b. Non-persistent HTTP with up to five parallel connections
c. Persistent HTTP with pipelining 2
11. In-band versus out-of band control. What does it mean when we say that
control messages are “in-band”? What does it mean when we say that control messages are “out-of-band”? Give an example of a protocol that has inband control messages and an example of a protocol that has out-of-band
control messages.
– The packets containing DNS commands and HTTP commands such as GET are
very small compared to the size of the file. Therefore, their transmission times (but
not their propagation times) can be neglected.
– Propagation delays within the local area networks (LANs) are small enough to be
ignored. The propagation from router R1 to router R2 is 100 msec.
– The one-way propagation delay from anywhere in a.com to any other site in the
Internet (except b.com) is 500 msec.
(c) Now assume that machine m2.a.com makes a request to the same URL that m1.a.com
requested. List the sequence of DNS and HTTP messages sent/received from/by
m2.a.com as well as other messages that leave/enter the a.com network that are not
directly sent/received by m2.a.com from the point that the URL is entered into the
browser until the file is completely received. Indicate the source and destination of
each message. (Hint : be sure to consider caching.)
(d) Now suppose there is no HTTP cache in network a.com. What is the maximum rate
at which machines in a.com can make requests for the file www.b.com/bigfile.htm
while keeping the time from when a request is made to when it is satisfied non-infinite
in the long run ?
3