习题课(4)

17. Consider sending a 4,000-byte datagram into a link that has an MTU of 400 bytes. Suppose

the original datagram is stamped with the identification number 422-How many fragments are generated? What are their characteristics?

Consider the following network. With the indicated link costs, use Dijkstra's shortest-path algorithm to compute the shortest path from r to all network nodes. Show how the algorithm works by computing a table similar to Table 4.3.

18. Consider the network shown below, and assume that each node initially knows the costs to

each of its neighbors. Consider the distance-vector algorithm and show the distance table entries at node z.

19. Consider the network fragment shown below. -r has only two attached neighbors, w and y. w

has a minimum-cost path to destination u (not shown) of 5,and y has a minimum-cost path to u of 6. The complete paths from w and y to z (and between w and y) are not shown. All link costs in the network have strictly positive integer values.

a. Give x's distance vector for destinations w, y, and u.

b. Give a link-cost change for either c(x,w) or c(x,y) such that -r will inform its neighbors of a new minimum-cost path to u as a result of executing the distance-vector algorithm.

c. Give a link-cost change for either c(x,w) or c(x,y) such that x will not inform its neighbors of a new minimum-cost path to u as a result of executing the distance-vector algorithm. （17、18、19可选讲）

Chapter 5

1. What are some of the possible services that a link-layer protocol can offer to the network

layer? Which of these link-layer services have corresponding services in IP? In TCP? 2. If all the links in the Internet were to provide reliable delivery service, would the TCP reliable

delivery service be redundant? Why or why not? 3. Suppose nodes A, B, and C each attach to the same broadcast LAN (through their adapters).

If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the

MAC address of B, will C's adapter process these frames? If so, will C's adapter pass the IP datagrams in these frames to the network layer C? How would your answers change if A sends frames with the MAC broadcast address?

4. How big is the MAC address space? The IPv4 address space? The IPv6 address space?

5. Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within a frame with a specific destination MAC address?

6. In CSMA/CD, after the fifth collision, what is the probability that a node chooses K = 4? The result K = 4 corresponds to a delay of how many seconds on a 10 Mbps Ethernet?

7. Suppose the information content of a packet is the bit pattern 1110110010001010 and an

even parity scheme is being used. What would the value of the field containing the parity bits

be for the case of a two-dimensional parity scheme? Your answer should be such that a minimum-length checksum field is used.

8. Consider the previous problem, but suppose that D has the value a. 10010001. b. 10100011. c. 01010101

9. Consider three LANs interconnected by two routers, as shown in Figure 5.38. a. Redraw the diagram to include adapters.

b. Assign IP addresses to all of the interfaces. For Subnet I use addresses of the form 111.111.111.xxx; for Subnet 2 uses addresses of the form 122.122.122.xxx; and for Subnet 3

use addresses of the form 133.133.133.xxx.

c. Assign MAC addresses to all of the adapters.

d. Consider sending an IP datagram from Host A to Host F. Suppose all of the ARP tables are up to date. Enumerate all the steps, as done for the single-router example in Section 5.4.2. e. Repeat (d), now assuming that the ARP table in the sending host is empty(and the other tables are up to date).

10. Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and the propagation delay

between the two nodes is 225 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide? For our purposes, it suffices to work out the following example. Suppose A and B begin transmission at t = 0 bit times. They both detect collisions at t = 225 bit times. They finish transmitting a jam signal at t = 225 + 48 = 273 bit times. Suppose KA?0 and KB?1, At what time does B schedule its retransmission? At what time does A begin transmission? (Note: The nodes must wait for an idle channel after returning to Step 2-see protocol.) At what time does A's signal reach B? Does B refrain from transmitting at its scheduled time?

11. Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and the propagation delay

between the two nodes is 225 bit times. Suppose node A begins transmitting a frame and,

before it finishes, node B begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why or why not? If the answer is yes, then A incorrectly believes that its frame was successfully transmitted without a collision. Hint: Suppose at time t = 0 bit times, A begins transmitting a frame. In the worst case, A transmits a minimum-sized frame of 512 + 64 bit times. So A would finish transmitting the frame at t = 512 + 64 bit times. Thus, the answer is no, if B's signal reaches A before bit time t = 512 + 64 bits. In the worst case, when does B's signal reach A?

12. Suppose two nodes, A and B, are attached to opposite ends of a 900 m cable, and that they

each have one frame of 1,000 bits (including all headers and preambles) to send to each other. Both nodes attempt to transmit at time t = 0.Suppose there are four repeaters between A and B, each inserting a20-bit delay. Assume the transmission rate is 10Mbps, and CSMA/CD with backoff intervals of multiples of 512 bits is used. After the first collision, A draws K = 0 and B draws K = 1 in the exponential backoff protocol. Ignore the jam signal and the 96-bit time delay.

a. What is the one-way propagation delay (including repeater delays) between A and B in seconds? Assume that the signal propagation speed is 2?10 m/sec. b. At what time (in seconds) is A s packet completely delivered at B?

c. Now suppose that only A has a packet to send and that the repeaters are replaced with switches. Suppose that each switch has a 20-bit processing delay in addition to a store-and-forward delay. At what time, in seconds, is A’s packet delivered at B? 13. 数据链路层必须执行：链路管理、传输帧、流量控制与（ ）等功能。（中山大学2006年试题）

A、流量控制 B、面向连接确认服务 C、差错控制 D、面向字符型 解析:数据链路层的主要功能有:

8(1)链路管理

(2)帧定界 (3)流量控制 (4)差错控制

(5)将数据和控制信息区分开 (6)透明传输

(7)寻址。所以选项C为正确答案。

答案:C

14. 在通信过程中产生的传输差锖是由随机差错与（ ）共同:组成的`(中山大学2006年试

题)

A、字节差错 B、连接差错 C、突发差错 D、字符差错 解析:传输过程中,差错主要是由通信过程中的噪声引起的。通信信道的噪声分为两类:热噪声和冲击噪声。其中,热噪声引起的差错是随机差错,或随机错:冲击噪声引起的差错是突发差错,或突发错,引起突发差错的位长称为突发长度。在通信过程中产生的传输差错,是由随机差错与突发差错共同构成的c所以选项C为正确答案。 答案:C

15. 下属协议中，（ ）不是链路层的标准。（北京邮电大学2005年试题） A、ICMP B、BSC C、PPP D、SLIP

解析：ICMP协议是TCP/IP协议子集中的一个子协议,属于网络层协议。因此,ICMP不是链路层的标准。所以选项A为正确答案。

BSC规程(也可称为BISYNC,即Binary Synchronous Cor11munication的缩写)是IBM的二进制同步通信规程,属于面向字符的数据链路层协议。所以排除选项B。

串行线路互联网协议SLIP(Serial Line Internet Protocol)与点对点通信协议PPP是互联网中使用最为广泛的数据链路层协议。所以排除选项C、D。 答案：A

16. 3比特连续ARQ协议，发送窗口的最大值为（ ）。（北京邮电大学2005年试题） A、2 B、3 C、7 D、8

解析:当用n个比特进行编号时,若接收窗口的大小为1,则只有在发送窗口的大小WT?2?1时,连续ARQ协议才能正确运行。因此,当采用3bit编码时,发送窗口的最大值为7。所以选项C为正确答案。 答案:C

17. 计算机接入Internet时，可以通过公共电话网进行连接。以这种方式连接并连接时分配

到个临时性IP地址的用户，通常使用的是（ ）。（华中科技大学2003年试题） A、拨号连接仿真终端方式 B、经过局域网连接的方式 C、SLIP/PPP协议连接方式 D、经分组网连接的方式

解析：仿真终端方式：用这种连接方式用户计算机和因特网的连接是没有IP协议的间接连接,在建立连接期间,通信软件的仿真功能使用户计算机成为服务系统的仿真终端。这种连接方式很简单,也很容易实现,适合于信息传输量小的个人和单位。但是,服务范围往往受到一定的限制。

局域网接入方式:将一个局域网连接到Intemet主机有两种方法。第一种是通过局域网的服务器局域网中所有计算机共享服务器的一个IP地址:第二种是通过路由器,局域网上的所有主机都可以有自己的IP地址。采用这种接入方式的用户,软硬件的初始投资较高,通信线路费用也较高。这种方式是唯一可以满足大信息量因特网通信的方式,最适合希望多台主机都押

n

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