【优质】四川大学林峰计算机网络第三次作业

不同的是，当传输层传输一个

TCP

段至应用层的时候，它

并不需要明确指定

IP

地址，因为套接字标识符会协助它指定。

R14.

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.

a.

How much data is in the first segment?

答

:110-90=20

b. 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

the

acknowledgment number?

答

:90

R15.

True or false?

(F)

a.

The

size

of

the

TCP

RcvWindow

never

changes

throughout

the

duration

of the connection.

(T)

b.

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.

(F)

c.

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 cannot piggyback the acknowledgments on data.

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

(F)

e.

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.

(F) f. Suppose that the last Sample RTT in a TCP connection is equal to

1 sec. The current value of Timeout Interval for the connection will

necessarily be

≥

1 sec.

(T)

g.

Suppose

Host

A

sends

one

segment

with

sequence

number

38

and

4

bytes

of

data

over

a

TCP

connection

to

Host

B.

In

this

same

segment

the

acknowledgment number is necessarily 42.

R17.

True

or

false?Consider

congestion

control

in

the

timer

expires at the sender,the threshold is set to one half of its previous

value.

Problems

P1.

Suppose Client A initiates a Telnet session with Server S. At about

the same time, Client B also initiates a Telnet session with Server S.

Provide possible source and destination port numbers for

a. The segments sent from A to S.

The source port number is 467, destination number is 23.

b. The segments sent from B to S.

The source port is 513, the destination number is 23.

c. The segments sent from S to A.

The source port is 23, the destination number is 467.

d. The segments sent from S to B.

The source port is 23, the destination number is 513.

e. If A and B are different hosts, is it possible that the source port

number in the segments from A to S is the same as that from B to S?

Yes.

f. How about if they are the same host?

No.

P2.

Consider

Figure

3.5.

What

are

the

sources

and

destination

port

values

in

the

segments

flowing

from

the

server

back

to

the

clients

’

processes?

What are the IP addresses in the network-layer datagrams carrying the

transport-layer segments?

< /p>

答

:(1)

返回值包含了客户端和服务器 的

IP

地址以及

port

码，

还有用户索求的内

容。

(2)

网络层数据报中携带的

IP

地址包含目的端口的

IP

地址、

码、传输层添

加的头（

< br>message

）

、网络层添加的段（

< br>segment

）

。

P5

.

a. Suppose you have the following 2 bytes: 01011100 and 01010110. What

is the 1s complement of the sum of these 2 bytes?

01011100 + 01010110 = 10110010

取反

= 0110010

b. Suppose you have the following 2 bytes: 11011010 and 00110110. What

is the 1s complement of the sum of these 2 bytes?

11011010 + 00110110 = 100010000

去首位补至末位

= 00010001

取反

= 11101110

c. For the bytes in part (a), give an example where one bit is flipped

in each of the 2 bytes and yet the 1s complement doesn

’

t change.

P18.

Consider

the

GBN

protocol

with

a

sender

window

size

of

3

and

a

sequence

number range of 1,024. Suppose that at time t, the next in-order packet

that the receiver is expecting has a sequence number of k. Assume that

the medium does not reorder messages. Answer the following questions:

a.

What are the possible sets of sequence numbers inside the sender

’

s

window at time t? Justify your answer.

答

:

因为窗口大小为

3

，

接收方期待的下一列序号为

k

所以

t

时刻发送窗口中 序

列号应为

k,k+1,k+2

。

b.

What

are

all

possible

values

of

the

ACK

field

in

all

possible

messages

currently

propagating

back

to

the

sender

at

time

t?

Justify

your

answer.

答

:

由于序列号为

0-1024

，所以在

t

时刻，所有可能的

ACK

序号应为

0

≤

ACK

number

≤

1024.

P19.

Answer true or false to the following questions and briefly justify

your answer:

(T) a. With the SR protocol, it is possible for the sender to receive

an ACK for a packet that falls outside of its current window.

(T) b. With GBN, it is possible for the sender to receive an ACK for a

packet that falls outside of its current window.

(T) c. The alternating-bit protocol is the same as the SR protocol with

a sender and receiver window size of 1.

(T)

d.

The

alternating-bit

protocol is

the

same

as

the

GBN

protocol

with

a sender and receiver window size of 1.

P21

.

Consider

the

GBN

and

SR

protocols.

Suppose

the

sequence

number

space

is

of

size

k.

What

is

the

largest

allowable

sender

window

that

will

avoid

the occurrence of problems such as that in Figure 3.27 for each of these

protocols?

答

:

首先我们必须保证发送窗口和接受窗口中序列号都不重复。假设窗口的序列

号为

x

，那么序列号

n

必须保持

n>= 2x

，才能使窗口中的序列号不重复。