Chapter!1!Review!Questions!
1. There is no difference. Throughout this text, the words “host” and “end system” are
used interchangeably. End systems include PCs, workstations, Web servers, mail
servers, PDAs, Internet-connected game consoles, etc.
2. From Wikipedia: Diplomatic protocol is commonly described as a set of international
courtesy rules. These well-established and time-honored rules have made it easier for
nations and people to live and work together. Part of protocol has always been the
acknowledgment of the hierarchical standing of all present. Protocol rules are based on
the principles of civility.
3. Standards are important for protocols so that people can create networking systems and
products that interoperate.
4. 1. Dial-up modem over telephone line: home; 2. DSL over telephone line: home or
small office; 3. Cable to HFC: home; 4. 100 Mbps switched Ethernet: enterprise.
5. HFC bandwidth is shared among the users. On the downstream channel, all packets
emanate from a single source, namely, the head end. Thus, there are no collisions in
the downstream channel.
6. In most American cities, the current possibilities include: dial-up; DSL; cable modem;
fiber-to-the-home.
7. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.
8. Today, Ethernet most commonly runs over twisted-pair copper wire. It also can run over
fibers optic links.
9. ADSL: up to 24 Mbps downstream and 2.5 Mbps upstream, bandwidth is dedicated;
HFC, rates up to 42.8 Mbps and upstream rates of up to 30.7 Mbps, bandwidth is
shared. FTTH: 2-10Mbps upload; 10-20 Mbps download; bandwidth is not shared.
10. There are two popular wireless Internet access technologies today:
a. Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets
to/from an base station (i.e., wireless access point) within a radius of few tens
of meters. The base station is typically connected to the wired Internet and thus
serves to connect wireless users to the wired network.
b. 3G and 4G wide-area wireless access networks. In these systems, packets are
transmitted over the same wireless infrastructure used for cellular telephony,
with the base station thus being managed by a telecommunications provider.
This provides wireless access to users within a radius of tens of kilometers of
the base station.
11. At time t
0
the sending host begins to transmit. At time t
1
= L/R
1
, 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 t
1
, it can begin to transmit the
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packet to the receiving host at time t
1
. At time t
2
= t
1
+ L/R
2
, 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/R
1
+ L/R
2
.
12. 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. FDM requires sophisticated
analog hardware to shift signal into appropriate frequency bands.
13. a) 2 users can be supported because each user requires half of the link bandwidth.
b) 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.
c) Probability that a given user is transmitting = 0.2
d) Probability that all three users are transmitting simultaneously =
= (0.2)
3
= 0.008. Since the queue grows when all the users are transmitting, the
fraction of time during which the queue grows (which is equal to the probability
that all three users are transmitting simultaneously) is 0.008.
14. 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.
15. 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.
16. The delay components are processing delays, transmission delays, propagation delays,
and queuing delays. All of these delays are fixed, except for the queuing delays, which
are variable.
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17. a) 1000 km, 1 Mbps, 100 bytes
b) 100 km, 1 Mbps, 100 bytes
18. 10msec; d/s; no; no
19. a) 500 kbps
b) 64 seconds
c) 100kbps; 320 seconds
20. End system A breaks the large file into chunks. It adds header to each chunk, thereby
generating multiple packets from the file. The header in each packet includes the IP
address of the destination (end system B). The packet switch uses the destination IP
address in the packet to determine the outgoing link. Asking which road to take is
analogous to a packet asking which outgoing link it should be forwarded on, given the
packet’s destination address.
21. The maximum emission rate is 500 packets/sec and the maximum transmission rate is
350 packets/sec. The corresponding traffic intensity is 500/350 =1.43 > 1. Loss will
eventually occur for each experiment; but the time when loss first occurs will be
different from one experiment to the next due to the randomness in the emission
process.
22. Five generic tasks are error control, flow control, segmentation and reassembly,
multiplexing, and connection setup. Yes, these tasks can be duplicated at different
layers. For example, error control is often provided at more than one layer.
23. The five layers in the Internet protocol stack are – from top to bottom – the application
layer, the transport layer, the network layer, the link layer, and the physical layer. The
principal responsibilities are outlined in Section 1.5.1.
24. Application-layer message: data which an application wants to send and passed onto
the transport layer; transport-layer segment: generated by the transport layer and
encapsulates application-layer message with transport layer header; network-layer
datagram: encapsulates transport-layer segment with a network-layer header; link-layer
frame: encapsulates network-layer datagram with a link-layer header.
25. Routers process network, link and physical layers (layers 1 through 3). (This is a little
bit of a white lie, as modern routers sometimes act as firewalls or caching components,
and process Transport layer as well.) Link layer switches process link and physical
layers (layers 1 through2). Hosts process all five layers.
26. A self-replicating malware is a piece of code that cab enter and infect our devices,
and once it infects the host, from that host it seeks entry into other hosts over the Internet.
27. Creation of a botnet requires an attacker to find vulnerability in some application or
system (e.g. exploiting the buffer overflow vulnerability that might exist in an
application). After finding the vulnerability, the attacker needs to scan for hosts that
are vulnerable. The target is basically to compromise a series of systems by exploiting
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that particular vulnerability. Any system that is part of the botnet can automatically
scan its environment and propagate by exploiting the vulnerability. An important
property of such botnets is that the originator of the botnet can remotely control and
issue commands to all the nodes in the botnet. Hence, it becomes possible for the
attacker to issue a command to all the nodes, that target a single node (for example,
all nodes in the botnet might be commanded by the attacker to send a TCP SYN
message to the target, which might result in a TCP SYN flood attack at the target).
28. Trudy can pretend to be Bob to Alice (and vice-versa) and partially or completely
modify the message(s) being sent from Bob to Alice. For example, she can easily
change the phrase “Alice, I owe you $1000” to “Alice, I owe you $10,000”.
Furthermore, Trudy can even drop the packets that are being sent by Bob to Alice (and
vise-versa), even if the packets from Bob to Alice are encrypted.
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