COSC 3P01 Assignment #1

COSC 3P01 Assignment #1
Due: Monday October 7 th at 5:00pm
1. What is a client program? What is a server program? Does a server program request and receive
services from a client program?
2. What advantage does a circuit-switched network have over a packet-switched network? What
advantage does a packet-switched network have over a circuit-switched network?
3. Suppose users share a 2Mbps link. Also suppose each user transmits continuously at 1 Mbps when
transmitting, but each user transmits only 20% of the time. (See section 1.3 for statistical multiplexing)
a) When circuit switching is used, how many users can be supported?
b) For the remainder of this problem, suppose packet switching is used. Why will there be
essentially no queueing delay before the link if two or fewer users transmit at the same time?
Why will there be a queueing delay if three users transmit at the same time?
c) What is the probability that a given user is transmitting at any specific time?
d) Suppose now there are three users. Find the probability that at any given time, all three users are
transmitting simultaneously. Find the fraction of time during which the queue grows.
4. This elementary problem begins to explore propagation delay and transmission delay, two central
concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps.
Suppose that the two hosts are separated by m metres, and suppose the propagation speed along the link
is s metres/second. Host A is to send a packet size of L bits to Host B.
a) Express the propagation delay, d prop , in terms of m and s.
b) Determine the transmission time of the packet, d trans , in terms of L and R.
c) Ignoring processing and queueing delays, obtain an expression for the end-to-end delay.
d) Suppose Host A begins to transmit the packet at time t=0. At time t=d trans , where is the last bit of
the packet?
e) Suppose d prop is greater than d trans . At time t=d trans , where is the first bit of the packet?
f) Suppose d prop is less than d trans . At time t=d trans , where is the first bit of the packet?
g) Suppose s=2.5×10 8 , L=120 bits, and R=56 kbps. Find the distance m so that d prop equals d trans .
5. Consider a packet of length L which begins at end system A and travels over three links to a
destination end system. These three links are connected by two packet switches. Let d i , s i , and R i denote
the length, propagation speed, and the transmission rate of link i, for i=1,2,3. The packet switch delays
each packet by d proc . Assuming no queueing delays, in terms of d i , s i , R i , (i=1,2,3), and L, what is the
total end-to-end delay for the packet? Suppose now the packet is 1,500 bytes, the propagation speed on
all three links is 2.5·10 8 m/s, the transmission rates of all three links are 2 Mbps, the packet switch
processing delay is 3msec, the length of the first link is 5,000km, the length of the second link is
4,000km, and the length of the last link is 1,000km. For these values, what is the end-to-end delay?
6. What is message segmentation? Discuss some advantages and drawbacks of it. (In particular,
consider it in the context of sending large files)
7. Consider the 5-layer model for network communication. i) Tell me the purpose of each layer. ii) For
each layer, what is talking to what?

8. (Warning: Very open-ended question! Only worry about being detailed and providing useful
information; not psychically knowing what I'm expecting!) Though professionals in the field will need
to know of numerous security concerns when it comes to networking and IT, everyone has at least
some responsibility to keep their communications and data safe. Tell me about three (separate!)
network/internet security concerns that a regular person should know about and be responsible for
minding: i) What it is, ii) Why it matters, iii) What they should know to do about it. (Note: I said
'regular', meaning layman. The average person probably doesn't have to worry about ARP poisoning,
etc.)
9. Using telnet or nc, download a form from the web. Send a completed form back to the server, and
retrieve the result.
You can PuTTY/SSH into sandcastle to do this if you like.
You'll probably use port 80.
You can choose whatever form you like. You can even make your own if you want.
For submission, show the raw data you receive and send (including header information), and also save
the received payloads as html files so you can load them into a browser and print that out as well.
If the server sent an instruction to set a cookie in the raw data, highlight or underline that part.
Note that there is no mention of bothering with images. If the form you chose includes images, you're
more than welcome to leave them as dead-links.
10. Using telnet or nc, send me an email. Send it from sandcastle's SMTP server, to my sandcastle
account (efoxwell@sandcastle.cosc.brocku.ca). Include a (small!) attachment (an image).
Make sure the attachment is relatively small (

• It doesn't matter if you actually save the contents of FOOTER.html to a file, or simply
download to display on the console
• Save and print the results of both channels (the control channel as one printout, and the data
channels as another)
Some questions are taken from the course textbook.
Submission Guidelines:
For 'write-y' questions, please ensure that all of your answers are entirely legible. Typing it out is
strongly suggested. Showing your work (for calculations) is also strongly advisable. For those
involving the manual use of application layer protocols, make sure you err on the side of including too
much, rather than not enough. Staple your answers together, place them in an envelope if you so
choose, and then attach a proper departmental coverpage with your student number and barcode on it.
Electronic submission is not required for this assignment. However, if you do happen to do everything
electronically, it's permitted (if you feel safer having a version submitted that can't get lost).