Computer networking a top down approach 7th solution năm 2024

Computer Networking: A Top-Down Approach,

7th Edition

Solutions to Review Questions and Problems

Version Date: December 2016

This document contains the solutions to review questions and problems for the 7th

edition of Computer Networking: A Top-Down Approach by Jim Kurose and Keith Ross.

These solutions are being made available to instructors ONLY. Please do NOT copy or

distribute this document to others (even other instructors). Please do not post any

solutions on a publicly-available Web site. We’ll be happy to provide a copy (up-to-date)

of this solution manual ourselves to anyone who asks.

Acknowledgments: Over the years, several students and colleagues have helped us

prepare this solutions manual. Special thanks goes to Honggang Zhang, Rakesh Kumar,

Prithula Dhungel, and Vijay Annapureddy. Also thanks to all the readers who have made

suggestions and corrected errors.

All material © copyright 1996-2016 by J.F. Kurose and K.W. Ross. All rights reserved

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Chapter2ReviewQuestions

1. The Web: HTTP; file transfer: FTP; remote login: Telnet; e-mail: SMTP; BitTorrent file sharing: BitTorrent protocol b) A guarantee that a certain value for throughput will bemaintained Neither c) A guarantee that data will be delivered within a specified amount of time Neither d) Confidentiality (via encryption)

2. 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. The process which initiates the communication is the client; the process that waits to be contacted is the server.

4. No. In a P2P file-sharing application, the peer that is receiving a file is typically the client and the peer that is sending the file is typically the server.

5. The IP address of the destination host and the port number of the socket in the destination process.

6. You would use UDP. With UDP, the transaction can be completed in one roundtrip time (RTT) - the client sends the transaction request into a UDP socket, and the server sends the reply back to the client's UDP socket. With TCP, a minimum of two RTTs are needed - one to set-up the TCP connection, and another for the client to send the request, and for the server to send back the reply.

7. One such example is remote word processing, for example, with Google docs. However, because Google docs runs over the Internet (using TCP), timing guarantees are not provided.

8. a) Reliable data transfer TCP provides a reliable byte-stream between client and server but UDP does not.

Neither

9. 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.

10. 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.

11. 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.

12. When the user first visits the site, the server creates a unique identification number, creates an entry in its back-end database, and returns this identification number as 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.

13. Web caching can bring the desired content “closer” to the user, possibly to the same LAN to which the user’s host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic onlinks.

14. Telnet is not available in Windows 7 by default. to make it available, go to Control Panel, Programs and Features, Turn Windows Features On or Off, Check Telnet client. To start Telnet, in Windows command prompt, issue the following command > telnet webserverver 80 where "webserver" is some webserver. After issuing the command, you have established a TCP connection between your client telnet program and the web server. Then type in an HTTP GET message. An example is given below:

Since the index.html page in this web server was not modified since Fri, 18 May 2007 09:23:34 GMT, and the above commands were issued on Sat, 19 May 2007, the server returned"304NotModified".Notethatthefirst4linesaretheGETmessage andheader lines inputed by the user, and the next 4 lines (starting from HTTP/1.1 304Not Modified) is the response from the web server.

15. A list of several popular messaging apps: WhatsApp, Facebook Messenger, WeChat, and Snapchat. These apps use the different protocols than SMS.

16. The message is first sent from Alice’s host to her mail server over HTTP. Alice’s mail server thensends the message to Bob’s mail serveroverSMTP.Bobthen transfers the message from his mail server to his host over POP3.

17. 17.

Received:

Received: from 65.54.246.203 (EHLO bay0-omc3-s3.bay0.hotmail.com) (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 with Microsoft SMTPSVC(6.0.3790.2668); Sat, 19 May 2007 16:52:420700

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:

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 messages 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 s3.bay0.hotmail.com. Messageid is a unique string assigned bythe 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

To: This field indicates the email address of the receiver of the mail. In the example, the receiver is

Subject: This gives the subject of the mail (if any specified by the sender). In the example, the subject specified by the sender is

Date: The date and time when the mail was sent by the sender. In the example, the sender sent the mail on 19th May 2007, at time 23:52:36 GMT.

Mime-version: MIME version used for the mail. In the example, it is 1.0.

Content-type: The type of content in the body of the mail message. In the example, it is

Return-Path: This specifies the email address to which the mail will be sent if the receiver of this mail wants to reply to the sender. This is also used by the sender’s mail server for bouncing back undeliverable mail messages of mailer-daemon error messages. In the example, the return path is

18. With download and delete, after a user retrieves its messages from a POP server, the messages are deleted. This poses a problem for the nomadic user, who may want to access the messages from many different machines (office PC, home PC, etc.). In the download and keep configuration, 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).

19. Yes an organization’s mail server and Web server can have the same alias for a host name. The MX record is used to map the mail server’s host name to its IPaddress.

20. You should be able to see the sender's IP address for a user with an .edu email address. But you will not be able to see the sender's IP address if the user uses a gmailaccount.

21. It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur even ifAlice provides chunks to Bob throughout a 30-second interval.

22. Recall that in BitTorrent, a peer picks a random peer and optimistically unchokes the peer for a short period of time. Therefore, Alice will eventually be optimistically unchoked by one of her neighbors, during which time she will receive chunks from that neighbor.

23. The overlay network in a P2P file sharing system consists of the nodes participating in the file sharing system and the logical links between the nodes. There is a logical link (an “edge” in graph theory terms) from node A to node B if there is a semi- permanent TCP connection between A and B. An overlay network does not include routers.

24. Oneserverplacementphilosophyis called EnterDeep,which enterdeep intothe access networks of Internet Service Providers, by deploying server clusters in access ISPs all over the world. The goal is to reduce delays and increase throughput between end users and the CDN servers. Another philosophy is Bring Home, which bring the ISPs home by building large CDN server clusters at a smaller number of sites and typically placing these server clusters in IXPs (Internet Exchange Points). This Bring Home design typically results in lower maintenance and management cost,compared with the enter-deep design philosophy.

25. Other than network-related factors, there are some important factors to consider, such as load-balancing (clients should not be directed to overload clusters), diurnal effects, variations across DNS servers within a network, limited availability of rarely accessed video, and the need to alleviate hot-spots that may arise due to popular videocontent.

Reference paper: Torres, Ruben, et al. "Dissecting video server selection strategies in the YouTube CDN." The 31st IEEE International Conference on. Distributed Computing Systems (ICDCS), 2011.

Another factor to consider is ISP delivery cost – the clusters may be chosen so that specific ISPs are used to carry CDN-to-client traffic, taking into account the different cost structures in the contractual relationships between ISPs and cluster operators.

26. 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 a connection to the server, a new socket is created. Thus, to support n simultaneous connections, the server would need n+1 sockets.

27. 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

What is the answer to computer networking?

Computer networking refers to interconnected computing devices that can exchange data and share resources with each other. These networked devices use a system of rules, called communications protocols, to transmit information over physical or wireless technologies.

What is networking used for?

Networking is the process of making connections and building relationships. These connections can provide you with advice and contacts, which can help you make informed career decisions. Networking can even help you find unadvertised jobs/internships.