【正文】 responsibilities of each of these layers?10. Which layers in the Internet protocol stack does a router process? Which layers does a linklayer switch process? Which layers does a host process?11. What is an applicationlayer message? A transportlayer segment? A networklayer datagram? A linklayer frame? 12. 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 the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B.a. Express the propagation delay, , in terms of m and s.b. Determine the transmission time of the packet, in terms of L and R.c. Ignoring processing and queuing delays, obtain an expression for the endtoend delay.d. Suppose Host A begins to transmit the packet at time t = 0. At time ,where is the last bit of the packet?e. Suppose is greater than . At time t = ,where is the first bit of the packet? f. Suppose is less than . At time t = , where is the first bit of the packet? g. Suppose , L = 100bits, and R = 28 kbps. Find the distance m so that equals .13. In modern packetswitched networks, the source host segments long, applicationlayer messages (for example, an image or a music file) into smaller packets and sends the packets into the network. The receiver then reassembles the packets back into the original message. We refer to this process as message segmentation. Figure illustrates the endtoend transport of a message with and without message segmentation. Consider a message that is bits long that is to be sent from source to destination in Figure . Suppose each link in the figure is 2 Mbps. Ignore propagation, queuing, and processing delays.a. Consider sending the message from source to destination without message segmentation. How long does it take to move the message from the source host to the first packet switch? Keeping in mind that each switch uses storeandforward packet switching, what is the total time to move the message from source host to destination host?b. Now suppose that the message is segmented into 4,000 packets, with each packet being 2,000 bits long. How long does it take to move the first packet from source host to the first switch? When the first packet is being sent from the first switch to the second switch, the second packet is being sent from the source host to the first switch. At what time will the second packet be fully received at the first switch?c. How long does it take to move the file from source host to destination host when message segmentation is used? Compare this result with your answer in part (a) and ment.d. Discuss the drawbacks of message segmentation.14. 下列說法中,正確的是( )。,幣在較大范圍內(nèi)布置的一定是廣域網(wǎng),廣域網(wǎng)是基于交換技術(shù)發(fā)展起來的向絡(luò)解答:D。有時候在一個不大的范圍內(nèi)采用廣域網(wǎng),這取決于應(yīng)用的需要和是否采用單一網(wǎng)絡(luò)等多種因素。城域網(wǎng)是利用廣域網(wǎng)技術(shù)、滿足一定區(qū)域需求的一種網(wǎng)絡(luò),事實上,城域網(wǎng)的范圍彈性非常大。 ?15. 相對于o⒏的7層參考模型的低4層,TCP/IP協(xié)議集內(nèi)對應(yīng)的層次有( )。根據(jù)TCP/P分層模型可知,其對應(yīng)OSI低4層的分別是傳輸層、互聯(lián)網(wǎng)層、網(wǎng)絡(luò)接口層。,服務(wù)器響應(yīng)請求、進(jìn)行處理并返回結(jié)果,以避免向服務(wù)器請求服務(wù)解答:A。服務(wù)器必須總能而不是有時可以同時為多個客戶機(jī)服務(wù),否則網(wǎng)絡(luò)就沒有了存在的價值。從表面上看,服務(wù)器可能是網(wǎng)絡(luò)的瓶頸, 但事實上:在多數(shù)情況下網(wǎng)絡(luò)的主要瓶頸不在服務(wù)器，而在通信線路。解答:報文交換網(wǎng)絡(luò)與分組交換的原理都是:將用戶數(shù)據(jù)加上源地址、自的地址、長度、校驗碼等輔助信息封裝成PDU,發(fā)送給下一個節(jié)點。每個PDU可單獨選擇到達(dá)目的節(jié)點的路徑。兩者的不同之處是:分組交換所生成的PDU的長度較短,而且是固定的。正是這一差別,使得分組交換具有獨特的優(yōu)點:緩沖區(qū)易于管理。更易標(biāo)準(zhǔn)化。所以現(xiàn)在的主流網(wǎng)絡(luò)基本上都可以看成是分組交換網(wǎng)絡(luò)。 【2】在大多數(shù)網(wǎng)絡(luò)中,數(shù)據(jù)鏈路層都是用請求重發(fā)已損壞了幀的辦法來解決發(fā)送出錯問題。 【5】通信子網(wǎng)不包括( d )。 【7】RFC是（）。RFC(reguest for ments)的意思就是“請求評論”。但應(yīng)注意,并非所有的RFC文檔都是因特網(wǎng)標(biāo)準(zhǔn),只有一小部分RFC文檔最后才能變成因特網(wǎng)標(biāo)準(zhǔn)。一個RFC文檔更新后就使用一個新的編號,并在文檔中指出原來老編號的RFC文檔已成為陳舊的。所以選項A為正確答案。(華中科技大學(xué)2003年試題) 解析:集線器屬于LAN與大型機(jī)以及LAN與WAN的互連。集線器的設(shè)計目標(biāo)主要是優(yōu)化網(wǎng)絡(luò)布線結(jié)構(gòu)，簡化網(wǎng)絡(luò)管理，主要功能是對接收到的信號進(jìn)行再生整形放大，以擴(kuò)大網(wǎng)絡(luò)的傳輸距離，同時把所有節(jié)點集中在以它為中心的結(jié)點上。網(wǎng)關(guān)不僅具有路由器的全部功能,同時還可以完成因操作系統(tǒng)差異引起的通信協(xié)議之間的轉(zhuǎn)換。它要求兩個互連網(wǎng)絡(luò)在數(shù)據(jù)鏈路層以上采用相同或兼容的網(wǎng)絡(luò)協(xié)議。它位于0SI參考模型中物理層。所以選項B為正確答案。因此,網(wǎng)絡(luò)層處于數(shù)據(jù)鏈路層與傳輸層之間。答案:B【10】完成路徑選擇功能是在OSI參考模型的（ ）。數(shù)據(jù)鏈路層:分為MAC和LLC,傳送以幀為單位的數(shù)據(jù),采用差錯控制,流量控制方法。傳輸層:是向用戶提供可靠的端到端服務(wù),透明地傳送報文,使用TCP協(xié)議。所以選項C為正確答案。（華中科技大學(xué)2003年試題） 解析:TCP/IP協(xié)議族把整個協(xié)議分成四個層次:(1)網(wǎng)絡(luò)接口層:負(fù)責(zé)接收P數(shù)據(jù)報,并把該數(shù)據(jù)報發(fā)送到相應(yīng)的網(wǎng)絡(luò)上。 .(2)網(wǎng)絡(luò)層(也叫網(wǎng)際層):網(wǎng)絡(luò)層解決了計算機(jī)到計算機(jī)通信的問題。(3)傳輸層:傳輸層提供一個應(yīng)用程序到另一個應(yīng)用程序之間端到端的通信。(4)應(yīng)用層:是TCP/IP協(xié)議的最高層,與0sI參考模型的上三層的功能類似。由此可知,網(wǎng)際層解決了計算機(jī)到計算機(jī)通信的問題。答案:B【12】TCP/IP參考模型的網(wǎng)際層用于實現(xiàn)地址轉(zhuǎn)換的協(xié)議有( )。ARP協(xié)議的基本功能就是通過目標(biāo)設(shè)備的IP地址,查詢目標(biāo)設(shè)備的MAC地址,以保證通信的順利進(jìn)行。ICMP協(xié)議是TCP/IP協(xié)議集中的一個子協(xié)議,屬于網(wǎng)絡(luò)層協(xié)議,主要用于在主機(jī)與路由器之間傳遞控制信息,包括報告錯誤、交換受限控制和狀態(tài)信息等。用戶數(shù)據(jù)報協(xié)議(UDP)是ISO參考模型中一種無連接的傳輸層協(xié)議,提供面向事務(wù)的簡單不可靠信息傳送服務(wù)。由此可知,UDP不具備地址轉(zhuǎn)換功能,排除選項C。因此，排除選項D。解析:面向連接服務(wù)是電話系統(tǒng)服務(wù)模式的抽象。無連接服務(wù)是郵政系統(tǒng)服務(wù)模式的抽象。這些資源將在數(shù)據(jù)傳輸時動態(tài)地進(jìn)行分配。當(dāng)數(shù)據(jù)交換結(jié)束后,則應(yīng)終止這個連接。接收到的數(shù)據(jù)與發(fā)送方的數(shù)據(jù)在內(nèi)容和順序上是一致的。無連接服務(wù)不能保證報文到達(dá)的先后順序,先發(fā)送煦報文不一定先到。Chapter 21. For a munication session between a pair of processes, which process is the client and which is the server?2. What is the difference between network architecture and application architecture?3. What information is used by a process running on one host to identify a process running on another host?4. Suppose you wanted to do a transaction from a remote client to a server as fast as possible. Would you use UDP or TCP? Why?5. List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service.6. Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?7. What is meant by a handshaking protocol?8. Suppose Alice, with a Webbased account (such as Hotmail or gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice39。s host. Be sure to list the series of applicationlayer protocols that are used to move the message between the two hosts.9. From a user39。s Web server and mail server to have exactly the same alias for a hostname (for example, foo. )? What would be the type for the RR that contains the hostname