坏了计算机网络英文版习题答案

2021年1月27日发(作者：impaction)
P63
#5
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.
e
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?
t
s
+[(F+h)/R]Q


e 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?
[(F+2h)/R]Q

y,
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 ts setup time and h bits of
header
appended
to
the
packet,
how
long
does
it
take
to
send
the
packet?
t
s
+(F+h)/R

P64 #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 meters and suppose
that
the
propagation
speed
along
the
link
is
s
meters/sec.
Host
A
sends 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 queueing delays, obtain an expression
for the end-to- end delay.
[d]
Suppose Host A begins to transmit the packets 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 x 10
8
, L=100 bits and R=28kbps. Find the distance
m so that d
prop
= d
trans
.

[a] d
prop
= m/s
[b] d
trans
= L/R
[c] end-to-end delay = d
prop
+ d
trans
=m/s+L/R
[d] The beginning position of the link.
[e] On the channel between A and B.
[f] On the host B.
[g] m/s = L/R = > m = sL/R = > m = 892.86 km


P65 #10 Consider the queueing delay in a router buffer. Suppose that
all
packets
are
L
bits,
the
transmission
rate
is
R
bps,
and
that
N
packets simultaneously arrive at the buffer every LN/R seconds. Find
the average queueing delay of a packet (in terms of L, R and N). (Hint:
The queueing delay for the first packet is zero; for the second packet
L/R;
for
the
third
packet
2L/R.
The
Nth
packet
has
already
been
transmitted when the second batch of packets arrives.)

As the Nth packet has already been transmitted when the next batch
of
packets
arrive,
we
only
need
to
consider
the
delay
for
a
single
batch of packets.
Average delay = Total delay / Number of packets
Delay for 1st packet = 0
Delay for 2nd packet = L/R
Delay for 3rd packet = 2L/R
...
...
Delay for Nth packet = (N-1)L/R
Total delay for N packets = (0 + 1 + 2 ... +(N-1) ) * (L/R)
Using the formulas for sum of integer series, this can be written as:
Total delay for N packets = (N-1) * (N/2) * (L/R)
Therefore, average delay for N packets = ((N-1) * L) / 2R

P170
#12
What
is
the
difference
between
persistent
HTTP
with
pipelining and persistent HTTP without pipelinning? Which of the two
is used by HTTP/1.1?

For the persistent connection without pipelining, the client issues a new request only
when the previous has been received. In this case, the client experiences one RTT in
order to request and receive each of the referenced objects.
For the persistent connection with pipelining, the client issues a request as soon as it
encounters
a
reference.
It
is
possible
for
only
RTT
to
be
expended
for
all
the
referenced objects.

P170
#14
Telnet
into
a
Web
server
and
send
a
multiline
request
message.
Include
in
the
request
message
theIf-modified-since:
header line to force a response message with the 304 Not Modified
status code.
GET/somedir/ HTTP/1.1
Host:


Connection: close
User-agent: Mozilla/4.0
If-Modified-Since: Thu, 30 May 2007 12:00:00 GMT
Accept-language: fr
P172
#6
Suppose
within
your
web
browser
you
click
on
a
link
to
obtain
a
web
page.
The
IP
address
for
the
associated
URL
is
not
cached in your local host, so a DNS look-up is necessary to obtain the
IP address. Suppose that n DNS servers are visited before your host
receives the IP address from DNS; the successive visits incur an RTT
(Round
Trip
Time)
of
RTT1, ...
RTTn.
Further
suppose
that
the
web
page associated with the link contains exactly one object, consisting
of a small amount of HTTP text. Let RTT0 denote the RTT between the
local host and the remote server containing the object. Assuming zero
transmission
time
of
the
object,
how
much
time
elapses
from
when
the client clicks on the link until the client receives the object? (Hint:
read pages 90 .. 93)

Time
to
visit
DNS
servers
and
get
IP
address
=
RTT1
+
RTT2
+ ...
+
RTTn
Time to establish TCP connection (SYN and SYNACK) = RTT0
Time to send HTTP request and receive reply = RTT0
Total time = 2 * RTT0 + (RTT1 + RTT2 + ... + RTTn)

P171
#16
Suppose
Alice
with
a
Web-based
e-mail
account
(such
as
Yahoo!
Mail
or
Hotmail)
sends
a
message
to
Bob,
who
accesses
his
mail
from
his
mail
server
using
POP3.
Discuss
how
the
message gets from Alice
’
s host to Bob
’
s host. Be sure to list the
series
of
application-layer
protocols
that
are
used
to
move
the
message between the two hosts.
The series of application-layer protocols: HTTP
、
SMTP
、
POP3


Suppose
that
you
send
an
e-mail
message
whose
only
data
is
a
Microsoft
Excel
attachment.
What
might
the
header
lines
(including
MIME lines) look like?
From: 123@
To: 456@
Subject: hello
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content- Type: Application/MS-Excel

P286
#5
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.

much data is in the first segment?
a.20 bytes

e
that
the
first
segment
is
lost
but
the
second
segment
arrives
at
B.
In
the
acknowledgement
that
host
B
sends
to
host
A,
what will be the acknowledgement number?
90

P291 #27 Consider the following plot of TCP window size as a function
of
time.
(reproduced
below
for
you)
Assuming
TCP
Reno
is
the
protocol
experiencing
the
behavior
shown
above,
answer
the
following
questions.
In
all
cases,
you
should
provide
a
short
discussion justifying your answer.

a. Identify the intervals of time when TCP slow start is operating.
b.
Identify
the
intervals
of
time
when
TCP
congestion
avoidance
is
operating.c.
After
the
16th
transmission
round,
is
segment
loss
detected by a tripleduplicate ACK or by a timeout?
d. After the 22nd transmission round, is segment loss detected by a
triple duplicate ACK or by a timeout?
e.
What
is
the
initial
value
of
Threshold
at
the
first
transmission
round?
f. What is the value of Threshold at the 18th transmission round?
g. What is the value of Threshold at the 24th transmission round?
h. During what transmission round is the 70th segment sent?
i.
Assuming
a
packet
loss
is
detected
after
the
26th
round
by
the
receipt
of
a
triple
duplicate
ACK,
what
will
be
the
values
of
the
congestion-window size and of Threshold?