M.Sc. Computer Science
Computer Systems
Additional Questions on Week 8 Materials – Part II
Question #1: What is the difference between a host and an end system? List several different types of
end systems. Is a Web Server an end system?
Basically the devices or the computing systems connected to the computer network are sometimes
referred to as end systems. They are so called because they sit at the edge of the computer network and
are operated by an end user. These are usually connected together by a network of communication links
and packet switches.
Coming to the point, in the Internet, host and end systems are interchangeable terms which tells that
there is no specific difference between them.
The types of end systems are email servers, Workstations, Web servers, TVs, Cell Phones, Tablets etc.
Yes, a web server is considered as an end system.
Question #2: Define the role of a firewall and draw a diagram that shows where a firewall should be
positioned with relation to protecting a local network. In defining the role of a firewall, you should
discuss the techniques that a firewall uses at different levels to prevent external attacks on the network
and control traffic flow through the firewall.
The following figure shows a typical setting, where are firewall is placed between a private
network and the public Internet. It shows a local area network structure and the firewall is a single
interface point or gateway to the outside world.
The firewall mainly deals with the following activities / techniques:
Denying connection requests to known ports, such as ftp, sendmail on hosts within the local
network.
Packet filtering based on keywords or encryption status.
Suspicious packet traffic patterns that represent denial of service or packet spoofing attacks on
local hosts from external IP addresses.
Question #3: Suppose there is exactly one packet switch between a sending host and a receiving host.
The transmission rates between the sending host and the switch and between the switch and the
receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward packet
switching, what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagation
delay, and processing delay).
At time t0 the sending host begins to transmit. At time t1 = L/R1 , the sending host completes
transmission and the entire packet is received at the router (no propagation delay). Because the router
has the entire packet at time t1 , it can begin to transmit the packet to the receiving host at time t1 . At
time t2 = t1 + L/R2 , the router completes transmission and the entire packet is received at the receiving
host (again, no propagation delay). Thus, the end-to-end delay is L/R1 + L/R2.
Question #4: What advantages does a circuit-switched network have over a packet-switched network?
A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of
a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end
guarantees for bandwidth.
Question #5: Suppose users share a 2Mbps link. Also suppose each user transmits continuously at 1
Mbps when transmitting, but each user transmits only 20 percent of the time.
a) When circuit-switching is used, how many users can be supported?
2 users can be supported because each user requires half of the link bandwidth.
b) When packet-switching is used, why will there be essentially no queuing delay before the link if two
of fewer users transmit at the same time? Why will there be a queuing delay if three users transmit at
the same time?
Since each user requires 1Mbps when transmitting, if two or fewer users transmit simultaneously, a
maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there
will be no queuing delay before the link. Whereas, if three users transmit simultaneously, the
bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link. In
this case, there will be queuing delay before the link.
Question #6: Consider an application that transmits data at a steady rate (for example, the sender
generates an N-bit unit of data every k time units, where k is small and fixed). Also, when such an
application starts, it will continue running for a relatively long period of time. Answer the following
questions, briefly justifying your answers:
a) Would a packet-switched network or a circuit-switched network be more appropriate for this
application? Why?
A circuit-switched network would be well suited to the application, because the application involves
long sessions with predictable smooth bandwidth requirements. Since the transmission rate is known
and not bursty, bandwidth can be reserved for each application session without significant waste. In
addition, the overhead costs of setting up and tearing down connections are amortized over the lengthy
duration of a typical application session.
b) Suppose that a packet-switched network is used and the only traffic in this network comes from such
applications as described above. Furthermore, assume that the sum of the application data rates is less
than the capacities of each and every link. Is some form of congestion control needed? Why?
In the worst case, all the applications simultaneously transmit over one or more network links.
However, since each link has sufficient bandwidth to handle the sum of all of the applications’ data
rates, no congestion (very little queuing) will occur. Given such generous link capacities, the network
does not need congestion control mechanisms.
Question #7: Why will two ISPs at the same level of the hierarchy often peer with each other? How
does an IXP earn money?
If the two ISPs do not peer with each other, then when they send traffic to each other they have to send
the traffic through a provider ISP (intermediary), to which they have to pay for carrying the traffic. By
peering with each other directly, the two ISPs can reduce their payments to their provider ISPs. An
Internet Exchange Points (IXP) (typically in a standalone building with its own switches) is a meeting
point where multiple ISPs can connect and/or peer together. An ISP earns its money by charging each
of the the ISPs that connect to the IXP a relatively small fee, which may depend on the amount of
traffic sent to or received from the IXP.
Question #8: Why is a content provider considered a different Internet entity today? How does a
content provider connect to other ISPs? Why?
Google’s private network connects together all its data centers, big and small. Traffic between the
Google data centers passes over its private network rather than over the public Internet. Many of these
data centers are located in, or close to, lower tier ISPs. Therefore, when Google delivers content to a
user, it often can bypass higher tier ISPs. What motivates content providers to create these networks?
First, the content provider has more control over the user experience, since it has to use few
intermediary ISPs. Second, it can save money by sending less traffic into provider networks. Third, if
ISPs decide to charge more money to highly profitable content providers (in countries where net
neutrality doesn’t apply), the content providers can avoid these extra payments.