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14) Print out the header of a message that you have recently received. How many Recieved: header lines are there? Analyze each of the header lines in the message.
打印出你最近收到的报文的首部，在Received中，首部行有多少行？分析该报文首部行的每一行。
15) From a user's perspective, what is the difference between the download-and-delete mode and the download-and-keep mode in POP3? 从用户的观点来看，POP3中“下载并删除”模式和“下载并保留”模式有什么区别？
With download and delete, after a user retrieves（取回） its messages from a POP server, the messages are deleted. This poses a problem for the nomadic （游动的）user, who may want to access the messages from many different machines (office PC, home PC, etc.).
In the download and keep configuration（配置）, messages are not deleted after the user retrieves the messages. This can also be inconvenient, as each time the user retrieves the stored messages from a new machine, all of
non-deleted messages will be transferred to the new machine (including very old messages).
16) Redraw Figure 2.5-4 for when all queries from the local nameserver are iterative.
重绘时，从本地域名服务器所有查询迭代图2.5-4。
17) Each Internet host will have at least one local name server and one
authoratative name server. What role does each of these servers have in DNS? 每一台英特网主机都至少有一个本地域名服务器和一个权威域名服务器。这两种服务器在域名解析中起什么作用？
A host authoritative name server provides the official translation of a hostname to an IP address (among other things). A host local name server is a proxy(代理服务器) to the DNS system: the host first queries its local name server for a mapping; the local name server then obtains the mapping on the behalf of（代表） the querying host.
18) Is it possible that an organization's Web server and mail server have exactly the same alias（别名） for a hostname (e.g., foo.com)? What would be the \
一个机构的web服务器和邮件服务器可以有有完全相同的主机别名吗？包含了邮件服务器的主机名的RR有什么样的类型？
Yes 。An organization mail server and Web server can have the same alias for a host name. The MX record is used to map the mail server host name to its IP address.
19) Use nslookup to find a Web server that has multiple IP addresses. Does the Web server of your institution (school, company, etc.) have multiple IP addresses?
使用nslookup找到一个Web服务器有多个IP地址。贵机构的Web服务器（如学校，公司，等）是否有多个IP地址？
My insititution doesn’t have multiple IP address. Sections 2.6-2.9
20) The UDP server described in Section 2.7 only needed one socket, whereas the TCP server described in Section 2.6 needed two sockets. Why? If the TCP server were to support n simultaneous（同时发生的） connections, each from a different client host, how many sockets would the TCP server need?
2.7节描述的UDP服务器仅需要一个套接字，而2.6节描述的TCP服务器则需要两个套接字。为什么？如果TCP服务器要支持n个并行连接，且每个连接都来自不同的客户机主机，那么TCP服务器需要多少个套接字？
With the UDP server, there is no welcoming socket, and all data from different clients enters the server through this one socket. With the TCP server, there is a welcoming socket, and each time a client initiates（发起） a connection to the server, a new socket is created. Thus, to support n simultaneous connections, the server would need n+1 sockets.
21) For the client-server application over TCP described in Section 2.6, why must the server program be executed before the client program? For the client-server application over UDP described in Section 2.7, why may the client program be executed before the server program?
在2.6节所描述的运行于TCP上的客户机-服务器应用程序，为什么服务器程序要先于客户机程序运行？在2.7节描述的运行于UDP上的客户机-服务器应用程序，为什么客户机程序要先于服务器程序运行？
For the TCP application, as soon as the client is executed, it attempts to initiate a
TCP connection with the server. If the TCP server is not running, then the client will fail to make a connection.
For the UDP application, the client does not initiate connections (or attempt to communicate with the UDP server) immediately upon execution
PROBLEMS
1) True or false.
a) Suppose a user requests a Web page that consists of some text and two
images. For this page the client will send one request message and recieve three response messages? 假设一个用户请求一个网页，包含一些文本和两个图像。这个页面的客户端将发送一个请求消息和接收三个响应消息？ F
b) True or false. Two distinct Web pages (e.g.,
www.mit.edu/research.html and www.mit.edu/students.html) can be sent over the same persistent connection? 真或假。两种截然不同的网页（例如，www.mit.edu / research.html和www.mit.edu / students.html）可发送相同的持久连接？ T
c) With non-persistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages? 浏览器和源服务器之间的非持久连接，它有可能为一个单一的TCP段进行两个不同的HTTP请求消息？ F
d) The Date: header in the HTTP response message indicates when the object in the response was last modified? 日期： 在HTTP响应消息头指示的最后修订在响应对象时？ F
2) Read RFC 959 for FTP. List all of the client commands that are supported by the RFC.
FTP阅读RFC 959。列出所有的客户端支持的命令是由RFC。