計(jì)網(wǎng)第五章作業(yè)

1、Chapter 5 作業(yè)題，請(qǐng)同學(xué)們注意題目描述。1. (R1)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?答：可能提供的服務(wù)：鏈路接入，成幀，可靠傳輸，流量控制，差錯(cuò)檢測(cè)，差錯(cuò)糾正，半雙工和全雙工； In IP：成幀，差錯(cuò)檢測(cè)； In TCP: 成幀，可靠傳輸，差錯(cuò)檢測(cè)，半雙工和全雙工。2.

2、 (R2)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?答：不會(huì)多余，因?yàn)門(mén)CP保證的是傳輸層的數(shù)據(jù)傳送，而link提供穩(wěn)定可靠傳輸保證鏈路層的傳輸穩(wěn)定，二者不完全重疊，所以TCP可靠傳輸也不會(huì)多余。例如，鏈路層協(xié)議不能保證端到端的順序，就需要TCP協(xié)議進(jìn)行排序；路由器緩存溢出丟包，鏈路層協(xié)議無(wú)法解決這個(gè)問(wèn)題，但是TCP可以。3. (R8)How big

3、 is the MAC address space? The IPv4 address space? The IPv6 address space?答：248 個(gè)MAC addresses; 232 個(gè)IPv4 addresses; 2128 個(gè)IPv6 addresses4. (R9)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?答：因?yàn)樾录尤刖W(wǎng)絡(luò)的主機(jī)是不知道路由器的IP

4、的，自己也沒(méi)有IP，所以只能廣播才能得到IP，特定的主機(jī)就會(huì)特定響應(yīng)。因?yàn)槊總€(gè)主機(jī)的MAC地址都是唯一的，而ARP建立轉(zhuǎn)發(fā)表的時(shí)候會(huì)帶上MAC地址。5. (P5)Consider the 4-bit generator, G, shown in Figure 5.8, and suppose that D has the value 11111010. What is the value of R? G=1001答：的余數(shù)， R=1106. (P11)Consider a broadcast channel with N nodes and a transmission rate of R b

5、ps. Suppose the broadcast channel uses polling (with an additional polling node) for multiple access. Suppose the amount of time from when a node completes transmission until the subsequent node is permitted to transmit (that is, the polling delay) is dpoll. Suppose that within a polling round, a gi

6、ven node is allowed to transmit at most Q bits. What is the maximum throughput of the broadcast channel? 答：情況1：情況2：兩種極端情況進(jìn)行比較，明顯情況2為最大值；情況1為最小值。7. (P12)Consider three LANs interconnected by two routers, as shown in Figure 5.38.a. Redraw the diagram to include adapters.Figure 5.38 Three subnets, inte

7、rconnected by routersb. 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 122.222.222.xxx; and for Subnet 3 use addresses of the form 133.333.333.xxx. （注：本題目略有修改，見(jiàn)紅色標(biāo)注）c. Assign MAC addresses to all of the ad

8、apters.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 da

9、te).答：a.b.c.d. 1. 由A的轉(zhuǎn)發(fā)表決定了數(shù)據(jù)報(bào)應(yīng)該被轉(zhuǎn)發(fā)到路由器端口111.111.111.002.2. 由A的適配器生成一個(gè)攜帶以太網(wǎng)目的地址22-22-22-22-22-22的以太網(wǎng)分組.3. 第一個(gè)路由器即收到該分組并且解析了數(shù)據(jù)報(bào)。該路由器中的轉(zhuǎn)發(fā)表提示該數(shù)據(jù)報(bào)應(yīng)該被轉(zhuǎn)發(fā)到122.222.222.003.4. 第一個(gè)路由器發(fā)送帶有目的地址為55-55-55-55-55-55并且源地址為33-33-33-33-33-33的以太網(wǎng)分組，經(jīng)過(guò)IP地址為122.222.222.002的端口。5. 過(guò)程以此類推，直到分組到達(dá)主機(jī)F .e. 主機(jī)A的ARP模塊一定知道本網(wǎng)段網(wǎng)關(guān)的IP

10、地址為 111.111.111.002。主機(jī)A發(fā)送一個(gè)ARP請(qǐng)求(包含在一個(gè)以太網(wǎng)廣播幀中)。第一個(gè)路由器接收到這個(gè)請(qǐng)求分組并且發(fā)送給主機(jī)A一個(gè)ARP響應(yīng)分組。這個(gè)ARP響應(yīng)分組裝載于一個(gè)包含以太網(wǎng)目的地址00-00-00-00-00-00的以太網(wǎng)幀上。得知了路由器端口的MAC地址后，以他作為目的地址成幀，以后的步驟與上一問(wèn)相同。8. (P15)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.

11、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

12、 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 and , At what time does B schedule its retransmission? At what time does A begin transmission? (Note: The nodes must wait for an id

13、le channel after returning to Step 2-see protocol.) At what time does As signal reach B? Does B refrain from transmitting at its scheduled time?答：9. (P16)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 begin

14、s 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

15、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 Bs signal reaches A before bit time t = 512 + 64 bits. In the worst case, wh

16、en does Bs signal reach A?答：假設(shè)在 t = 0 時(shí)刻 A開(kāi)始傳輸。考慮到以太網(wǎng)的最短幀長(zhǎng)為576bit，最早在 t = 576 時(shí)刻, A 可以完成傳輸。在最壞的情況下，B 在 t = 224 時(shí)刻開(kāi)始傳輸, 此時(shí)它還不知道A正在傳輸。在t = 224 + 225 = 449 時(shí)刻，B 的第一個(gè)比特到達(dá)A，因?yàn)?49576，A將在完成分組傳輸之前中斷。因此A不能在他察覺(jué)到B的傳輸之前完成傳輸。10. (P19)Suppose two nodes, A and B, are attached to opposite ends of a 900 m cable, and

17、 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 a 20-bit delay. Assume the transmission rate is 10Mbps, and CSMA/CD with backoff in

18、tervals 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 m/sec.b. At what time (in seconds) is As 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