第一部分：Multiple choice （共4道题）
1 、The application of ( ) is loss-tolerant.
A、 stored audio/video
B、 file transfer
C、 Web documents
D、 text messaging
参考答案：A
2、The UDP segment includes （ ）.
A、 checksum
B、 receive window
C、 sequence number
D、 acknowledgement number
参考答案：B
3 、How does an operating system identify the application to which data coming in from the network should be delivered? ( ).
A、 By the protocol with which the data was sent
B、 By the port number carried by the transport protocol
C、 By IP address of the data
D、 By the file extension of the data
参考答案：B
4 The service of TCP is（ ）.
A、 connectionless transmission
B、 unreliable data transmission
C、 datagram
D、 reliable data transmission
参考答案：D
第二部分：Blank filling （共1道题）
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. So the source port numer is ( ) and destination port number is ( )
for the segments traveling from Host B to Host A.
参考答案：
y
、 x
第三部分：True or False （共8道题）
1 、In TCP, the acknowledgement number that a host puts in a segment is the sequence number of the next byte the host is expecting from the sender.
A、 True
B、 False
参考答案：A
2 、Suppose that host A wants to send data over TCP to host B, and host B wants to send data to host A over TCP. Two separate TCP connections - one for each direction - are needed.
A、 True
B、 False
参考答案：B
3、The TCP segment has a field in its header for rwnd.
A、 True
B、 False
参考答案：A
4、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.
A、 True
B、 False
参考答案：A
5、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
A、 True
B、 False
参考答案：B
6、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
A、 True
B、 False
参考答案：B
题目解析：
In the same segment, seq. Number does nothing with Ack Number.
only A->B, B->A, different segments:
Ack No.of (B->A) = Seq. NO. of (A->B) + length of (A->B) data.
7、The size of the TCP rwnd never changes throughout the duration of the connection.
A、 True
B、 False
参考答案：B
题目解析：
The receive window, denoted rwnd is set to the amount of spare room in the buffer:
rwnd = RcvBuffer – [LastByteRcvd – LastByteRead]
Because the spare room changes with time, rwnd is dynamic.
8、Consider congestion control in TCP. When the timer expires at the sender, the value of ssthresh is set to one half of its previous value.
A、 true
B、 false
参考答案：B
题目解析：
False, it is set to half of the current value of the congestion window:
1/2 Cwnd, not 1/2 of ssthresh previous value.
第四部分：comprehensive problems （共3道题）
1 [ 组合题 ]
UDP and TCP use 1s complement for their checksums.
1 [ 填空题 ]
Suppose you have the following three 8-bit bytes: 01010011，01100110，01110100. What is the 1s complement of the sum of these 8-bit bytes? (Note that although UDP and TCP use 16-bit words in computing the checksum, for this problem you are being asked to consider 8-bit sums.)
参考答案：
11010001
题目解析：
．求和----1分
0101 0011
+0110 0110
1011 1001
1011 1001
+ 0111 0100
10010 1101
2．回卷
回卷 0010 1101
+ 1
0010 1110
3求反：1101 0001
2 [ 单选题 ]
Is it possible that a 1-bit error will go undetected?
A、 Yes
B、 No
参考答案：B
题目解析：
All one-bit errors will be detected, but two-bit errors can be undetected (e.g., if the last digit of the first word is converted from 1 to 0 and the last digit of the second word is converted from 0 to 1).
3\Is it possible that a 2-bit error will go undetected?
A、 Yes
B、 No
参考答案：A
题目解析：
All one-bit errors will be detected, but two-bit errors can be undetected (e.g., if the last digit of the first word is converted from 1 to 0 and the last digit of the second word is converted from 0 to 1).
2 [ 组合题 ]
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?
Anser: ( )Bytes.
参考答案：
20
题目解析：
110-90
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? （ ）
参考答案：
90
题目解析：
see to Figure 3.37 of Page 285: see the 2nd & 3rd segment.
3 [ 组合题 ]
Consider Figure below. 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.
A、 [1,6]
B、 [23,26]
C、 [1,6] and [23,26]
D、 [1,14]
参考答案：C
b. Identify the intervals of time when TCP congestion avoidance is
operating.
A、 [6,16]
B、 [6,16] and [17,22]
C、 [17,22]
D、 [1,14]
参考答案：B
c. After the 16th transmission round, is segment loss detected by a triple
duplicate ACK or by a timeout?
A、 a triple duplicate ACK
B、 a timeout
参考答案：A
题目解析：After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1.
d. After the 22nd transmission round, is segment loss detected by a triple
duplicate ACK or by a timeout?
A、 a triple duplicate ACK
B、 a timeout
参考答案：B
题目解析：After the 22nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1.
e. What is the initial value of ssthresh at the first transmission round? （ ）
参考答案：
32
题目解析：The threshold is initially 32, since it is at this window size that slowtart stops and congestion avoidance begins.
f. What is the value of ssthresh at the 18th transmission round? （ ）
参考答案：
21
题目解析：The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18th transmission round
g. What is the value of ssthresh at the 24th transmission round? （ ）
参考答案：
14.5 或 14 或 15
题目解析：
The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 22, the congestion windows size is 29. Hence the threshold is 14.5（or 14 or 15） during the 24th transmission round.
h. During what transmission round is the 70th segment sent?
A、 6
B、 7
C、 8
D、 16
参考答案：B
题目解析：
1+2+4+8+16+32=63(the first 6 segments), and the 7th segment sent 33 segments.
==>
During the 1st transmission round, packet 1 is sent; packet 2-3 are sent in the 2nd transmission round; packets 4-7 are sent in the 3rd transmission round; packets 8-15 are sent in the 4th transmission round; packets15-31 are sent in the 5th transmission round; packets 32-63 are sent in the 6th transmission round; packets 64 – 96 are sent in the 7th transmission round. Thus packet 70 is sent in the 7th transmission round.
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 ssthresh?
Answer：cwnd value is （ ） and the ssthresh value is （ ）。
参考答案：
7
、 4
题目解析：
The threshold will be set to half the current value of the congestion window (8) when the loss occurred. Thus the new values of the threshold will be 4,and the cwnd = ssthread +３