Networking Chapter 2 One such example is remote word processing

Chapter 2 Review Questions

1. The Web: HTTP; file transfer: FTP; remote login: Telnet; e-mail: SMTP; BitTorrent

file sharing: BitTorrent protocol

2. Network architecture refers to the organization of the communication process into

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.

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

7. One such example is remote word processing, for example, with Google docs.

8. a) Reliable data transfer

TCP provides a reliable byte-stream between client and server but UDP does not.

9. SSL operates at the application layer. The SSL socket takes unencrypted data from

10. A protocol uses handshaking if the two communicating entities first exchange control

11. The applications associated with those protocols require that all application data be

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

13. ossibly to the same

objects, even objects that are not cached, since caching reduces the traffic on links.

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

16. The message is first

transfers the message from his mail server to his host over POP3.

17.

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

Received:

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 16:52:42

–

0700

from mail pickup service by hotmail.com with Microsoft SMTPSVC; Sat,

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.

passed through 5 differe

SMTP server of the sender to the second SMTP server in the chain of servers. The

From: This indicates the email address of the sender of the mail. In the given

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

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

19. Yes

20. You should be able to see the sender’s IP address for a user with an .edu email

21. It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the

22. Recall that in BitTorrent, a peer picks a random peer and optimistically unchokes the

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

–

permanent TCP connection between A and B. An overlay network does not include

routers.

24. One server placement philosophy is called Enter Deep, which enter deep into the

access networks of Internet Service Providers, by deploying server clusters in access

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

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

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

Chapter 2 Problems

Problem 1

a) F

Problem 2

SMS (Short Message Service) is a technology that allows the sending and receiving of

text messages between mobile phones over cellular networks. One SMS message can

contain data of 140 bytes and it supports languages internationally. The maximum size of

a message can be 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters. SMS

Problem 3

Application layer protocols: DNS and HTTP

Problem 4

a) The document request was http://gaia.cs.umass.edu/cs453/index.html. The Host :

field indicates the server’s name and /cs453/index.html indicates the file name.

b) The browser is running HTTP version 1.1, as indicated just before the first <cr><lf>

Problem 5

a) The status code of 200 and the phrase OK indicate that the server was able to locate

the document successfully. The reply was provided on Tuesday, 07 Mar 2008

Problem 6

a) Persistent connections are discussed in section 8 of RFC 2616 (the real goal of this

question was to get you to retrieve and read an RFC). Sections 8.1.2 and 8.1.2.1 of

Problem 7

The total amount of time to get the IP address is

Problem 8

a)

oon

RTTRTTRTTRTT

282

1

Problem 9

a) The time to transmit an object of size L over a link or rate R is L/R. The average time

is the average size of the object divided by R:

b) The traffic intensity on the access link is reduced by 60% since the 60% of the

Problem 10

Note that each downloaded object can be completely put into one data packet. Let Tp

denote the one-way propagation delay between the client and the server.

First consider parallel downloads using non-persistent connections. Parallel downloads

would allow 10 connections to share the 150 bits/sec bandwidth, giving each just 15

bits/sec. Thus, the total time needed to receive all objects is given by:

Problem 11

a) Yes, because Bob has more connections, he can get a larger share of the link

Problem 12

Server.py

from socket import *

serverPort=12000

Problem 13

Problem 14

SMTP uses a line containing only a period to mark the end of a message body.

Problem 15

MTA stands for Mail Transfer Agent. A host sends the message to an MTA. The message

Problem 16

UIDL –

the server responds with the unique message ID for all of the messages present in the

Problem 17

a) C: dele 1

b) C: retr 2

S:

c) C: list

Problem 18

a) For a given input of domain name (such as ccn.com), IP address or network

207.69.189.21, 207.69.189.22,

207.69.189.23, 207.69.189.24,

207.69.189.25, 207.69.189.26, 207.69.189.27,

207.69.189.28

www.yahoo.com

Web Servers: www.yahoo.com (216.109.112.135, 66.94.234.13)

Mail Servers: a.mx.mail.yahoo.com (209.191.118.103)

b.mx.mail.yahoo.com (66.196.97.250)

www.hotmail.com

Web Servers: www.hotmail.com (64.4.33.7, 64.4.32.7)

Mail Servers: mx1.hotmail.com (65.54.245.8, 65.54.244.8, 65.54.244.136)

mx2.hotmail.com (65.54.244.40, 65.54.244.168, 65.54.245.40)

d) The yahoo web server has multiple IP addresses

f) An attacker can use the whois database and nslookup tool to determine the IP address

Problem 19

a) The following delegation chain is used for gaia.cs.umass.edu

a.root-servers.net

E.GTLD-SERVERS.NET

ns1.umass.edu(authoritative)

;; AUTHORITY SECTION:

Among all returned edu DNS servers, we send a query to the first one.

dig +norecurse @E.GTLD-SERVERS.NET any gaia.cs.umass.edu

b) The answer for google.com could be:

a.root-servers.net

E.GTLD-SERVERS.NET

ns1.google.com(authoritative)

Problem 20

We can periodically take a snapshot of the DNS caches in the local DNS servers. The

Web server that appears most frequently in the DNS caches is the most popular server.

Problem 21

Yes, we can use dig to query that Web site in the local DNS server.

Problem 22

For calculating the minimum distribution time for client-server distribution, we use the

following formula:

Client Server

N

10 100 1000

7680

51200

512000

300 Kbps

7680

51200

512000

Peer to Peer

N

10 100 1000

300 Kbps 7680 25904 47559

Problem 23

a) Consider a distribution scheme in which the server sends the file to each client, in

parallel, at a rate of a rate of us/N

c) From Section 2.6 we know that

DCS x {NF/us, F/dmin} (Equation 1)

Problem 24

a) Define u = u1 + u2 N. By assumption

us <= (us + u)/N Equation 1

b) Again define u = u1 + u2 uN. By assumption

us >= (us + u)/N Equation 2

The aggregate send rate of the server is

peer i is

(N-1)ri = ui

In this distribution scheme, peer i receives bits at an aggregate rate of

Thus each peer receives the file in NF/(us+u).

c) The solution to this part is similar to that of 17 (c). We know from section 2.6 that

Problem 25

There are N nodes in the overlay network. There are N(N-1)/2 edges.

Problem 26

Yes. His first claim is possible, as long as there are enough peers staying in the swarm for

Problem 27

a. N files, under the assumption that we do a one-to-one matching by pairing video

Problem 28

a) If you run TCPClient first, then the client will attempt to make a TCP connection with

Problem 29

In the original program, UDPClient does not specify a port number when it creates the

socket. In this case, the code lets the underlying operating system choose a port number.

With the additional line, when UDPClient is executed, a UDP socket is created with port

Problem 30

Yes, you can configure many browsers to open multiple simultaneous connections to a

Problem 31

For an application such as remote login (telnet and ssh), a byte-stream oriented protocol

is very natural since there is no notion of message boundaries in the application. When a

Problem 32

To create a web server, we need to run web server software on a host. Many vendors sell