Solutions to Chapter 1 - MEST Center



Solutions to Chapter 1

1a. Describe the step-by-step procedure that is involved from the time you deposit a letter in a mailbox to the time the letter is delivered to its destination. What role do names, addresses and mail codes (such as ZIP codes or postal codes) play? How might the letter be routed to its destination? To what extent can the process be automated?

Solution:

The steps involved in mailing a letter are:

1. The letter is deposited in mailbox.

2. The letter is picked up by postal employee and placed in sack.

3. The letter is taken to a sorting station, where it is sorted according to destination, as

determined by the mail code and grouped with other letters with the same destination mail

code. (If there is no mail code, then it is determined by the largest geographical unit, for

example, country (if specified), otherwise state (if specified), otherwise city (if specified).)

4. The letter is shipped to the post office that handles the mail for the specific mail code (or

country or city).

5. The letter is then sorted by street address.

6. The letter is picked up at the post office by the postal worker responsible for delivering to the

specified address.

7. The letter is delivered according to the number and street.

The name is not really used, unless the street address is missing or incorrect. Then the name

might be used to determine where the letter belongs. (Unless of course the letter is being sent to

a small town, where most inhabitants are known to the postal worker.)

The mail delivery process can be automated by using optical recognition on the mail code. The

letter can then be sorted and routed to the destination postal station, and even to the destination

neighborhood, depending on the amount of geographical detail built into the mail code.

1b. Repeat part (a) for an e-mail message. At this point, you may have to conjecture different approaches about what goes on inside the computer network.

Solution:

The steps involved in e-mailing a message are:

1. The message is sent electronically by clicking 'Send'. (In Chapter 2 we see that the Simple

Mail Transfer Protocol (SMTP) is used to do this.)

2. The mail provider of the sender sends a request to a name server for the network address of

the mail provider or the recipient. The mail provider is determined by the information following

the @ symbol.

3. If the mail provider finds a network address of the recipient's mail provider, then it sends the

message to that address in the e-mail address.

4. Upon receipt of the message the mail provider either informs the recipient automatically that

there is mail or waits until mail is requested by the recipient, depending on how the mail

service is set up. The recipient is determined by the information before the @ symbol.

1c. Are the procedures in parts (a) and (b) connection-oriented or connectionless?

Solution:

The transfer service of letters in a postal system is connectionless. Users do not set up

connections prior to the transfer of each letter. (Note however that the postal system may have

regularly scheduled shipments of bundles of mail from city to city, because the volume of mail

between cities is predictable. These regular shipments can be viewed as pre-arranged

"connections.")

E-mail transfer is also a connectionless service in that the user does not set up end-to-end

connections to send a message. We will see in Chapter 2, however, that e-mail protocols use

connection-oriented procedures to exchange messages.

NOTE: For parts (a) and (b) other step-by-step procedures are possible.

4a. Suppose that the letter in problem 1 is sent by fax. Is this mode of communications connectionless or

connection-oriented? Real-time or non-real time?

Solution:

In order to send the letter by fax, a telephone connection must first be established. Therefore the

mode of communications is connection-oriented. The transfer of information across the network

occurs in real-time.

4b. Repeat part (a) if a voice mail message is left at a given telephone.

Solution:

Again, the process of leaving a voice mail message is connection-oriented, since there must be an end-to-end connection between the caller and the receiver with the answering service. The leaving of voice mail is also done in real-time. However, the mode of communication between the person that leaves the voice mail and the person that listens to the voice mail is connectionless and non-real-time. The people involved do not need to establish a simultaneous connection with each other in order to communicate; also, they record and listen to the messages at different times.

19. Use your Web browser to access a search engine and retrieve the article "A Brief History of the Internet," by

Leiner, Cerf, Clark, Kahn, Kleinrock, Lynch, Postel, Roberts, and Wolff. Answer the following questions:

Solutions follow questions:

a. Who was J. Licklider, and what was his "Galactic Network" concept?

J. Licklider, of MIT, was the first head of DARPA. In 1962 he envisioned a global network of

computers where users could access data or programs from any site.

b. Who coined the term packet?

Donald Davies and Roger Scantlebury of NPL (National Physics Laboratory) in the United

Kingdom first coined the term.

c. What (who?) is an IMP?

IMP stands for Interface Message Processor. An IMP is a packet switch and was a key

component in the design of the ARPANET.

d. Did the ARPANET use NCP or TCP/IP?

The ARPANET used a host-to-host protocol called Network Control Protocol or NCP.

e. Was packet voice proposed as an early application for Internet?

In the early development of TCP, work on packet voice in particular prompted the separation of

TCP into two components: TCP and IP. IP was to provide addressing and forwarding; TCP

provided special services such as recovery from packet losses (important for packet voice).

f. How many networks did the initial IP address provide for?

The initial IP addressing scheme provided for 256 networks.

Solutions to Chapter 2

2a. What universal set of communication services is provided by TCP/IP?

Solution:

The TCP/IP protocol stack provides two basic types of communications services through its two

transport layer protocols: TCP provides reliable connection-oriented transfer of a byte stream; UDP provides for best-effort connectionless transfer of individual messages.

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2b. How is independence from underlying network technologies achieved?

Solution:

The two basic communications services provided by TCP and UDP are built on the connectionless packet transfer service provided by the Internet Protocol (IP). Many network interfaces are defined to support IP. The salient part of the above figure is that all of the higher layer protocols access the network interfaces through IP. This is what provides the ability to operate over multiple networks.

2c. What economies of scale result from (a) and (b)?

Solution:

Once a network interface for IP is defined for a given network technology, then hosts connected using the given network technology can connect to the Internet. This allows the reach of the Internet to grow rapidly, leveraging multiple coexisting networks technologies. Thus investment in new network technologies extend the reach of the Internet.

6. Which OSI layer is responsible for the following?

Solutions follow questions:

a. Determining the best path to route packets.

Layer 3 (network layer) determines the best path to route packets.

The network layer is concerned with the selection of paths across the network.

b. Providing end-to-end communications with reliable service.

Layer 4 (transportation layer) provides end-to-end communications with reliable services.

The transport layer is concerned with providing reliable service on an end-to-end basis across the

network.

c. Providing node-to-node communications with reliable service.

Layer 2 (data link layer) provides node-to-node communications with reliable services.

The data link layer provides for the reliable transfer of information between adjacent nodes in a

network.

10. Give two features that the data link layer and transport layer have in common. Give two features in which they differ. Hint: Compare what can go wrong to the PDUs that are handled by these layers.

Solution:

Features they have in common:

• Both layers insert a header to enable recovery from transmission errors.

• Both layers can provide flow control.

• Both layers provide a service that may be connection-oriented or connectionless.

Features in which they differ:

• The transport layer is end to end and involves the interaction of peer processes across the

network.

• The data link layer involves the interaction of peer-to-peer processes across a single hop.

• In general, the time that elapses in traversing a data link is much smaller than the time traversing a network, where packets can become trapped in temporary routing loops. Consequently, transport layer protocols must be able to deal with much larger backlog of PDUs than data link layers.

27. What is the difference between a physical address, a network address, and a domain name?

Solution:

The physical address is the unique hardware address that identifies an interface of a machine on a physical network such as a LAN. Physical addresses are used in the data link layer.

A network address is a machine's logical address on a network. The network address is used in the network layer. The network address used on the Internet is the IP address.

Domain names are used as an aid to identify hosts and networks in the Internet, since names are

easier to remember than numbers. The DNS system is used to translate between domain names and IP addresses. The domain name for the network address 128.100.132.30 is toronto.edu.

30. Suppose a machine is attached to several physical networks. Why does it need a different IP address for each attachment?

Solution:

The IP address dictates through which network the packets are sent to and from the machine.

Therefore each network connection must have a different address.

31. Suppose a computer is moved from one department to another. Does the physical address need to change? Does the IP address need to change? Does it make a difference if the computer is a laptop?

Solution:

The physical address does not change. It is globally unique to the computer's NIC card.

The IP address needs to be changed to reflect the new subnetwork id and host id.

The situation is the same for laptops.

41. Which of the TCP/IP transport protocol (UDP or TCP) would you select for the following applications: packet voice, file transfer, remote login, multicast communication (i.e., multiple destinations).

Solution:

Packet Voice - This example involves the transfer of a stream of information in real time across the network. At first, it may appear that TCP is suitable because of its connection orientation. However the acknowledgment and retransmission mechanisms in TCP introduce too much delay in the transfer of packets, and so UDP is the preferred approach to transferring a real-time voice stream across the network.

File Transfer - In general, file transfer requires reliable transfer and so TCP is preferred.

Remote Login - TCP is preferable because it provides for the reliable transfer of the stream of

keystrokes that forms the basis for a remote login application.

Multicast Communication - In multicast services, a source sends information to a subset of

destinations attached to the network. It is easy to imagine multicast applications that require reliable transfer of a stream of information to a set of destinations, and multicast applications that require only best effort transfer of individual messages. Therefore neither TCP nor UDP is preferred. A more pertinent point is that providing reliable multicast stream transfer service is quite difficult to implement, and TCP is not designed for this.

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