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首页Computer Networking: A Top-Down Approach 第二版的习题答案(英文)
Computer Networking: A Top-Down Approach<br/>Featuring the Internet<br/>Solutions to Review Questions and Problems<br/>Version Date: August 12, 2002<br/><br/>This document contains the solutions to review questions and problems for the 2nd<br/>edition of Computer Networking: A Top-Down Approach Featuring the Internet by Jim<br/>Kurose and Keith Ross. These solutions are being made available to instructors ONLY.
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Computer Networking: A Top-Down Approach
Featuring the Internet
Solutions to Review Questions and Problems
Version Date: August 12, 2002
This document contains the solutions to review questions and problems for the 2
nd
edition of Computer Networking: A Top-Down Approach Featuring the Internet by Jim
Kurose and Keith Ross. These solutions are being made available to instructors ONLY.
Please do NOT copy or distribute this document to others (even other instructors). We’ll
be happy to provide a copy (up-to-date) of this solution manual ourselves to anyone who
asks.
All material © copyright 1996-2002 by J.F. Kurose and K.W. Ross. All rights reserved
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, Internet-connected PDAs, WebTVs, etc.
2. Suppose Alice, an ambassador of country A wants to invite Bob, an ambassador of
country B, over for dinner. Alice doesn’t simply just call Bob on the phone and say,
“come to our dinner table now”. Instead, she calls Bob and suggests a date and time.
Bob may respond by saying he’s not available that particular date, but he is available
another date. Alice and Bob continue to send “messages” back and forth until they
agree on a date and time. Bob then shows up at the embassy on the agreed date,
hopefully not more than 15 minutes before or after the agreed time. Diplomatic
protocols also allow for either Alice or Bob to politely cancel the engagement if they
have reasonable excuses.
3. A networking program usually has two programs, each running on a different host,
communicating with each other. The program that initiates the communication is the
client. Typically, the client program requests and receives services from the server
program.
4. The Internet provides its applications a connection-oriented service (TCP) and a
connectionless service (UDP). Each Internet application makes use of one these two
services. The two services will be discussed in detail in Chapter 3. Some of the
principle characteristics of the connection-oriented service are:
• Two end-systems first “handshake” before either starts to send application data to
the other.
• Provides reliable data transfer, i.e., all application data sent by one side of the
connection arrives at the other side of the connection in order and without any
gaps.
• Provides flow control, i.e., it makes sure that neither end of a connection
overwhelms the buffers in the other end of the connection by sending to many
packets to fast.
• Provides congestion control, i.e., regulates the amount of data that an application
can send into the network, helping to prevent the Internet from entering a state of
grid lock.
The principle characteristics of connectionless service are:
• No handshaking
• No guarantees of reliable data transfer
• No flow control or congestion control
5. Flow control and congestion control are two distinct control mechanisms with distinct
objectives. Flow control makes sure that neither end of a connection overwhelms the
buffers in the other end of the connection by sending to many packets to fast.
Congestion control regulates the amount of data that an application can send into the
network, helping to prevent congestion in the network core (i.e., in the buffers in the
network routers).
6. The Internet’s connection-oriented service provides reliable data transfer by using
acknowledgements and retransmissions. When one side of the connection doesn’t
receive an acknowledgement (from the other side of the connection) for a packet it
transmitted, it retransmits the packet.
7. 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.
8. In a packet switched network, the packets from different sources flowing on a link do
not follow any fixed, pre-defined pattern. In TDM circuit switching, each host gets
the same slot in a revolving TDM frame.
9. 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 t1, it can begin to transmit the
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
.
10. In a VC network, each packet switch in the network core maintains connection state
information for each VC passing through it. Some of this connection state
information is maintained to a VC-number translation table. (See page 25)
11. The cons of VCs include (i) the need to have a signaling protocol to set-up and tear-
down the VCs; (ii) the need to maintain connection state in the packet switches. For
the pros, some researchers and engineers argue that it is easier to provide QoS
services - such as services that guarantee a minimum transmission rate or services
that guarantee maximum end-to-end packet delay – when VCs are used.
12. One advantage of message segmentation is that it allows for pipelined transmission
over a series of links, as discussed in the text. (See also online Java applet.) Imagine
the store-and-forward delays in sending an MP3 or a DVD video as a single message!
Another advantage is that without it, small messages can get stuck behind large
messages in router buffers.
13. 1. Dial-up modem over telephone line: residential; 2. DSL over telephone line:
residential or small office; 3. Cable to HFC: residential; 4. 100 Mbps switched
Etherent: company; 5. Wireless LAN: mobile; 6. Cellular mobile access (for example,
WAP): mobile
14. A tier-1 ISP connects to all other tier-1 ISPs; a tier-2 ISP connects to only a few of
the tier-1 ISPs. Also, a tier-2 ISP is a customer of one or more tier-1
15. A POP is a group of one or more routers in an ISPs network at which routers in other
ISPs can connect. NAPs are localized networks at which many ISPs (tier-1, tier-2 and
lower-tier ISPs) can interconnect.
16. 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.
17. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.
For an X Mbps Ethernet (where X = 10, 100, 1,000 or 10,000), a user can
continuously transmit at the rate X Mbps if that user is the only person sending data.
If there are more than one active user, then each user cannot continuously transmit at
X Mbps.
18. Ethernet most commonly runs over twisted-pair copper wire and “thin” coaxial cable.
It also can run over fibers optic links and thick coaxial cable.
19. Dial up modems: up to 56 Kbps, bandwidth is dedicated; ISDN: up to 128 kbps,
bandwidth is dedicated; ADSL: downstream channel is .5-8 Mbps, upstream channel
is up to 1 Mbps, bandwidth is dedicated; HFC, downstream channel is 10-30 Mbps
and upstream channel is usually less than a few Mbps, bandwidth is shared.
20. 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.
21. 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.
22. 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 1.7.2.
23. Routers process 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 layer four as
well.)
Chapter 1 Problems
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