北交计网第二章作业

Chapter 2注：括弧之中红色标注为与课本习题对应的编号2.1，2.2节后作业：1.(R2)For a communication session between a pair of processes, which process is the client andwhich is the server?发起通信的进程为客户机，等待联系的进程是服务器。

2.(R3)What is the difference between network architecture and application architecture?网络体系结构是指以分层的方式来描述通信过程的组织体系。

应用体系结构是由应用程序的研发者设计，并规定应用程序的主要结构。

3.(R4)What information is used by a process running on one host to identify a process runningon another host?目的主机的IP 地址和目的套接字的端口号。

4.(R7)Suppose you wanted to do a transaction from a remote client to a server as fast aspossible. Would you use UDP or TCP? Why?UDP，因为UDP不需要建立连接，速度更快。

5.(R8)List the four broad classes of services that a transport protocol can provide. For each ofthe service classes, indicate if either UDP or TCP (or both) provides such a service.Reliable Data Transfer,Throughput,Timing,Security.TCP: Reliable Data Transfer, SecurityUDP: Security6.(R9)Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?因为与这些协议相联系的应用都要求应用数据能够被无差错的有序的接收。

TCP 提供可靠的数据传输服务，而UDP 提供的是不可靠数据传输。

7.(R11)What is meant by a handshaking protocol?TCP协议提供可靠的连接服务,采用三次握手建立一个连接。

2.3，2.4节后作业8.(R15)Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends amessage to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.信息从Alice 的主机发送到她的邮件服务器，使用HTTP 协议。

然后邮件从Alice 的邮件服务器发送到Bob的邮件服务器，使用SMTP 协议。

最后Bob 将邮件从他的邮件服务器接收到他的主机，使用POP3 协议。

9.(R17)From a user's perspective, what is the difference between the download-and-deletemode and the download-and-keep mode in POP3?在下载并删除方式下，在用户从POP 服务器取回他的邮件后，信息就被删除。

在下载并保留方式下，在用户取回邮件后，邮件不会被删除。

10. (R18)Is it possible for an organization's Web server and mail server to have exactly the samealias for a hostname (for example, foo. com )? What would be the type for the RR that contains the hostname of the mail server?可以。

MX 。

11. (R19)Why is it said that FTP sends control information "out-of-band"?FTP 使用两个并行的TCP 连接，一个连接用来传送控制信息（例如一个传送文件的请 求），另一个连接用于准确地传输文件。

因为控制信息不是在文件传输地连接上传送，所以FTP 的控制信息是带外传送的。

2.5，2.6节后作业12. (P1)True or false?a. A user requests a Web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages.b. Two distinct Web pages (for example, /research.Html and /students. html) can be sent over the same persistent connection.c. With nonpersistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.d. The Date: header in the HTTP response message indicates when the object in the response was last modified.a.Fb.Tc.Fd.F13. (P7)Suppose within your Web browser you click on a link to obtain a Web page. The IPaddress for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT of n RTT RTT ,...,1. Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let 0RTT denote the RTT between the local host and the server containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?2RTT0 + RTT1+ RTT2 + Λ + RTTn .14. (P8)Referring to Problem P7, suppose the HTML file references three very small objects onthe same server. Neglecting transmission times, how much time elapses witha. Non-persistent HTTP with no parallel TCP connections?b. Non-persistent HTTP with parallel connections?c. Persistent HTTP (with pipelining)?a. RTT1 +Λ+ RTTn+2RTT0＋3×2RTT0＝8RTT0＋RTT1 +Λ+ RTTnb. RTT1 +Λ+ RTTn+2RTT0＋2RTT0＝4RTT0＋RTT1 +Λ+ RTTnc. RTT1 +Λ+ RTTn+2RTT0＋RTT0＝3RTT0＋RTT1 +Λ+ RTTn2.7，2.8节后作业15.(P9)Consider Figure2.12, for which there is an institutional network connected to the Internet.Suppose that the average object size is 900,000 bits and that the average request rate from the institution’s browsers to the origin servers is 10 requests per second. Also suppose that the amount of time it takes from when the routers on the Internet side of the access link forwards an HTTP request until it receives the response is two seconds on average (see Section 2.2.5).Model the total average response time as the sum of the average access delay (that is, the delay from Internet router to institution router) and the average Internet delay. For the average access delay, use△/(1-△β), where △is the average time required to send an object over the access link andβis the arrival rate of objects to the access link.a.Find the total average response time.b.Now suppose a cache is installed in institution LAN. Suppose the hit rate is 0.6. Find thetotal response time.a.通过一个传输速率为R 的链路传输长度为L 的对象需要的时间是L/R。