第二章
1. (Q2) For a communication session between a pair of processes, which process is the client and which is the server?
Answer: The process which initiates the communication is the client; the process that waits to be contacted is the server. .
2. (Q3) What is the difference between network architecture and application architecture?
Answer: Network architecture refers to the organization of the communication process into layers (e.g., the five-layer Internet architecture). Application architecture, on the other hand, is designed by an application developer and dictates the broad structure of the application (e.g., client-server or P2P)
3. (Q4) What information is used by a process running on one host to identify a process running on another host?
Answer: The IP address of the destination host and the port number of the destination socket.
4. (Q6) Referring to Figure 2.4, we see that none of the application listed in Figure 2.4 requires both no data loss and timing. Can you conceive of an application that requires no data loss and that is also highly time-sensitive?
Answer: There are no good example of an application that requires no data loss and timing. If you know of one, send an e-mail to the authors
5. (Q9) Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?
Answer: The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not.
6. (Q11) What is meant by a handshaking protocol?
Answer: A protocol uses handshaking if the two communicating entities first exchange control packets before sending data to each other. SMTP uses handshaking at the application layer whereas HTTP does not.
7. (Q13) Telnet into a Web server and send a multiline request message. Include in the request message the If-modified-since: header line to force a response message with the 304 Not Modified status code.
Answer: Issued the following command (in Windows command prompt) followed by the HTTP GET message to the “utopia.poly.edu” web server: > telnet utopia.poly.edu 80
Since the index.html page in this web server was not modified since Fri, 18 May 2007 09:23:34 GMT, the following output was displayed when the above commands were issued on Sat, 19 May 2007. Note that the first 4 lines are the GET message and header lines input by the user and the next 4 lines (starting from HTTP/1.1 304 Not Modified) is the response from the web server.
8. (Q14) Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies.
Answer: When the user first visits the site, the site returns a cookie number. This cookie number is stored on the user’s host and is managed by the browser. During each subsequent visit (and purchase), the browser sends the cookie number back to the site. Thus the site knows when this user (more precisely, this browser) is visiting the site.
9. (Q15) Suppose Alice, with a Web-based e-mail 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 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.
Answer: Message is sent from Alice’s host to her mail server over HTTP. Alice’s mail server then sends the message to Bob’s mail server over SMTP. Bob then transfers the message from his mail server to his host over POP3.
10. (Q10) Recall that TCP can be enhanced with SSL to provide process-to-process security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do?
Answer: SSL operates at the application layer. The SSL socket takes unencrypted data from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, she has to include the SSL code in the application.
11. (Q16) Print out the header of an e-mail message you have recently received. How many Received: header lines are there? Analyze each of the header lines in the message.
Answer: from 65.54.246.203 (EHLO bay0-omc3-s3.bay0.hotmail.com)
Received： (65.54.246.203) by mta419.mail.mud.yahoo.com with SMTP; Sat, 19 May 2007
16:53:51 -0700
from hotmail.com ([65.55.135.106]) by bay0-omc3-s3.bay0.hotmail.com
Received: with Microsoft SMTPSVC(6.0.3790.2668); Sat, 19 May 2007 16:52:42 -0700
Received: from mail pickup service by hotmail.com with Microsoft SMTPSVC; Sat,19 May
2007 16:52:41 -0700
Message-ID:
Received: from 65.55.135.123 by by130fd.bay130.hotmail.msn.com with HTTP; Sat, 19 May
2007 23:52:36 GMT
From: \ To: Bcc:
Subject: Test mail
Date: Sat, 19 May 2007 23:52:36 +0000 Mime-Version:1.0
Content-Type: Text/html; format=flowed Return-Path:
Figure: A sample mail message header
Received: This header field indicates the sequence in which the SMTP servers send and receive the mail message including the respective timestamps.
In this example there are 4 “Received:” header lines. This means the mail message passed through 5 different SMTP servers before being delivered to the receiver’s mail box. The last (forth) “Received:” header indicates the mail message flow from the SMTP server of the sender to the second SMTP server in the chain of servers. The sender’s SMTP server is at address 65.55.135.123 and the second SMTP server in the chain is by130fd.bay130.hotmail.msn.com. The third “Received:” header indicates the mail message flow from the second SMTP server in the chain to the third server, and so on.
Finally, the first “Received:” header indicates the flow of the mail message from the forth SMTP server to the last SMTP server (i.e. the receiver’s mail server) in the chain.
Message-id: The message has been given this number BAY130-(by
bay0-omc3-s3.bay0.hotmail.com. Message-id is a unique string assigned by the mail system when the message is first created.
From: This indicates the email address of the sender of the mail. In the given example, the sender is
2.-第二章课后习题及标准答案
