vago计算机网络作业题(2011年12月版)

2021年1月27日发(作者：乱码英文)

计算机网络作业题
(2011
年
12
月版
)
第一章

1. 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 delays. Propagation delay is negligible.
a. Suppose the network is a packet-switched virtual-circuit network. Denote the VC setup time by
t
s

seconds. Suppose the sending layers add a total of
h
bits of header to the packet. How long
does it take to send the file from source to destination?
b.
Suppose
the
network
is
a
packet-switched
datagram
network
and
a
connectionless
service
is
used.
Now
suppose
each
packet
has
2h

bits
of
header.
How
long
does
it
take
to
send
the
packet?
c.
Finally,
suppose
that
the
network
is
a
circuit-switched
network.
Further
suppose
that
the
transmission rate of the circuit between source and destination is
R
bps. Assuming
t
s
setup time
and
h
bits of header appended to the packet, how long does it take to send the packet?
1
、

a.

t
s
+(F+h)/R*Q
b.


(F+2h)/R*Q
c.


C.t
s
+(F+h)/R
6.
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

meter,
and
suppose
the
propagation
speed along the link is
s
meters/sec. Host A is to send a packet of size
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 queuing delays, obtain an expression for the end-to-end dealy.
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?
8
g. Suppose
s=
2.
5
·
10
,
L
=
100bits, and
R=
28kbps. Find the distance
m
so that
d
prop
equals
d
trans
.
2
、

a)

m/s
b)

L/R
c)

m/s+ L/R
d)

Interface of host A
e)

s*d
trans
=s*L/R
f)

Host B
g)

m/s= L/R=> m=8.928*10
5


第二章

1. In the following figure, Alice sends an e-mail to Bob, and Bob gives a reply to Alice. If Alice
and Bob both use mail user agent, please write what protocol is used in procedur
e
①～⑥
? If both
Alice and Bob use Web browser, what protocol is used in procedure
①～⑥
?

（
1
）①
SMTP;
②
①
POP3 or IMAP;
③
SMTP;
⑥
SMTP

③

④
SMTP;
⑤
POP3 or IMAP;
⑤


②

Alice
Alice
’
s mail
server
④

1
⑥

Bob
’
s mail
server
Bob

（
2
）①
HTTP;
②
HTTP;
③
SMTP;
④
SMTP;
⑤
HTTP;
⑥
HTTP

2.
A
client
requests
a
Web
page
/jwzx/

from
a
server
through
IE
browser. The version of HTTP is 1.1 and default connection is persistent connection. The page is
supported in Chinese. Please write the request message.

If the server gives a reply to the client, the response message is created at 8:00:00 on 27-09-2011,
and the object is modified at 00:00:00 on 18-09-2011. Please write the response message.
The known- conditions are: server is apache/2.0.40; the object is a text file and consists 1796 bytes.
If there is a proxy server between the client and the Web server, how does the proxy server check
the page it caches is newer or older?
（
1
）

GET

/jwzx/

HTTP/1.1


Host:




Connection:

Keep-Alive


User-agent:

Mozilla/4.0


Accept-language:

zh-cn
（
2
）

HTTP/1.1

200

OK


Date:

Tue, 27 Sep 2011 08:00:00 GMT


Server:

Apache/2.0.40


Content-Length:

1796


Content-Type:

text/html


Last-Modified:

Sun, 18 Sep 2011 00:00:00 GMT
（
3
）

the proxy server uses conditional GET


GET

/jwzx/

HTTP/1.1


Host:




If-modified-since:

Sun, 18 Sep 2011 00:00:00 GMT
第三章

1.
Consider
a
TCP
connection
between
Host
A
and
Host
B.
Suppose
that
the
TCP
segments
traveling from Host A to Host B have source port number
x
and destination port number
y
.
What are the source and destination port numbers for the segments traveling from Host B to
Host A?

Source port number is
y
, destination port number is
x
.
2. True of False?
(1) Host A is sending Host B a large file over a TCP connection. Assume Host B has no data to
send
Host
A.
Host
B
will
not
send
acknowledgments
to
Host
A
because
Host
B
can
not
piggyback the acknowledgments on data.
×

(2) The size of the TCP
RcvWindow
never changes throughout the duration of the connection.

×

(3)
Suppose
Host
A
is
sending
Host
B
a
large
file
over
a
TCP
connection.
The
number
of
unacknowledged bytes that A sends cannot exceed the size of the receive buffer.
×

(4)
Suppose
Host
A
is
sending
a
large
file
to
Host
B
over
a
TCP
connection.
If
the
sequence
number for a segment of this connection is
m
, then the sequence number for the subsequent
segment will necessarily be
m+
1.
×

(5) The TCP segment has a field in its header for
RcvWindow
.

√

(6)
Suppose
that
the
last
SampleRTT

in
a TCP
connection
is
equal
to
1
sec.
Then
the
current
value of
TimeoutInterval
for the connection will necessarily be

≥
1 sec.
×


2

(7) Suppose Host A sends over a TCP connection to Host B one segment with sequence number 38
and 4 bytes of data. In this same segment the acknowledgment number is 42.
×

(8) Consider congestion control in TCP. When the timer expires at the sender, the threshold is set
to one half of its previous value.
×

3. Suppose Host A sends two TCP segments back to back to Host B over a TCP connection. The
first segment has sequence number 90; the second has sequence number 110.
(1) How much data is in the first segment? 20 bytes
(2)
Suppose
that
the
first
segment
is
lost
but
the
second
segment
arrives
at
B.
In
the
acknowledgment that Host B sends to Host A, what will be acknowledgment number? 90
4.
Suppose
you
have
the
following
six
16-bit
bytes:
01010000
11001101,
00110011
01100110,
1000, please compute the 16-bit checksum.


0100
习题课

1. congestion window
计算

2. Latency.
计算（静态）

3. checksum
计算

参考

第三版教材
P291
：
27
题，
P 293
：
34
—
37
题

第四章

1.
There
is
a
host
A
in
the
Internet.
Its
IP
address
is
210.100.89.204/20.
Please
answer
the
following questions:
(1) Network address. (2)Broadcast address.
(3) What is the subnet mask?
(4) The fist host IP address.
(5) The last host IP address.
(6) How many hosts are there in the network?
Solution:
(1) 210.100.80.0
(2)210.100.95.255
(3) 255.255.240.0
(4) 210.100.80.1
(5) 210.100.95.254
(6) 2
32-20
=2
12
=4094

2.
For the given topology of the network, use Dijstra’s shortest path algorithm to compute the
shortest path from A to all network nodes. Give a shortest path tree and nod
e A’s routing table.


F
F

2
1

1
3

C
C
5

2
E
D
E
D

1

1
1
1
1
2
2

2
2

5
B
A
A


3
B