Chapter 2. Chapter 2. Network ModelsNetwork Models

1

LAYERED TASKSLAYERED TASKSWe use the concept of layers in

our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

2.2

2.3

Layered Tasks, ExampleLayered Tasks, Example

THE OSI MODELTHE OSI MODELEstablished in 1947, the International

Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

Note: ◦ ISO is the organization.◦OSI is the model.

2.4

Seven layers of the OSI Seven layers of the OSI modelmodel

2.5

Interfaces b/w LayersInterfaces b/w Layers

2.6

2.7

Exchange using the OSI Exchange using the OSI ModelModel

LAYERS IN THE OSI MODELLAYERS IN THE OSI MODELPhysical LayerData Link LayerNetwork LayerTransport LayerSession LayerPresentation LayerApplication Layer

2.8

Physical LayerPhysical LayerThe physical layer is responsible for

movements of individual bits from one hop (node) to the next.◦ Physical characteristics of interface and

medium: pin assignment, connector, cables◦ Representation of bits: encoding ◦ Data rate◦ Synchronization of bits◦ Line configuration: point-to-point, multipoint◦ Physical topology◦ Transmission mode: simplex, half-duplex, full-

duplex

2.9

Data Link LayerData Link LayerThe data link layer is responsible

for moving frames from one hop (node) to the next.◦Framing◦Physical addressing◦Flow control◦Error control◦Access control

2.10

Hop-to-hop DeliveryHop-to-hop Delivery

2.11

Network LayerNetwork LayerThe network layer is responsible

for the delivery of individual packets from the source host to the destination host.◦Logical addressing◦Routing

2.12

Source-to-destination Source-to-destination deliverydelivery

2.13

Transport layerTransport layerThe transport layer is responsible

for the delivery of a message from one process to another.◦Service-point addressing◦Segmentation and reassembly◦Connection control◦Flow control◦Error control

2.14

Segmentation and Segmentation and ReassemblyReassembly

2.15

Reliable process-to-process Reliable process-to-process delivery of a messagedelivery of a message

2.16

Session layerSession layerThe session layer is responsible

for dialog control and synchronization.

2.17

Presentation layerPresentation layerThe presentation layer is

responsible for translation, compression, and encryption.

2.18

Application layerApplication layerThe application layer is

responsible for providing services to the user.

2.19

Summary of layersSummary of layers

2.20

TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITEThe layers in the TCP/IP protocol suite

do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.

2.21

TCP/IP and OSI modelTCP/IP and OSI model

2.22

ADDRESSINGADDRESSINGFour levels of addresses are used

in an internet employing the TCP/IP protocols: ◦Physical address

Ex. Ehternet address, machine address

◦Logical address IP address

◦Port number◦Specific

URL, Email address, domain name

2.23

Network layer in TCP/IPNetwork layer in TCP/IPAddress resolutionReverse address resolutionInternet control messageInternet group message

2.24

Transport layer in TCP/IPTransport layer in TCP/IPUser Datagram Protocol (UDP)

◦Connectionless, unreliableTransmission Control Protocol

(TCP)◦Connection-oriented, reliable

Stream Control Transmission Protocol (SCTP)◦For supporting multimedia and VOIP

2.25

Relationship of layers and Relationship of layers and addresses in TCP/IPaddresses in TCP/IP

2.26

Example 2.1Example 2.1In Figure 2.19 a node with

physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

2.27

Physical addressesPhysical addressesThe physical addresses will

change from hop to hop,but the logical addresses usually

remain the same.

2.28

TCP/IP in a NetworkTCP/IP in a Network

ECE/IUPUIIntro to Computer Communication Networks 2-29

IP (1,2)

NI3487

data

data1,2

data3487

IP(1,3)

NI6537

data

1,3

6537

data1,2

1,3

Example 2.2Example 2.2As we will see in Chapter 13,

most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:◦07:01:02:01:2C:4B

2.30

Example 2.3Example 2.3Figure 2.20 shows a part of an internet

with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

2.31

IP addressesIP addresses

2.32

TCP/IP thru a RouterTCP/IP thru a Router

ECE/IUPUIIntro to Computer Communication Networks 2-33

IP (1,2)

NI3487

data

data1,2

data3487

IP(2,5)

NI3903

data

NI6543

NI1002

IP(1,1) (2,3)

2,5 data1,2

2,5

data1,2

2,5

6543data

1002

3903

Host A Router Host B

Example 2.4Example 2.4Figure 2.21 shows two computers

communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination.

2.34

Port addressesPort addresses

2.35

Example 2.5Example 2.5As we will see in Chapter 23, a

port address is a 16-bit address represented by one decimal number.

2.36

Server in TCP/IPServer in TCP/IP A process waiting a packet on a specific port number Duplicate a connection after establishing Connection?

◦ 5 tuple – (PT, SA, SP, DA, DP) protocol type, source address, source port, destin address, destin port.

◦ A server waits on (pt, sa, sp, any-DA, any-DP)

◦ Upon a request from the client, any-DA and any-DP are filled with specific value.

Server Port Number – 0 to 1023 are reserved to well-known services.

ECE/IUPUIIntro to Computer Communication Networks 2-37

Protocol Reserved Port #

Comments

FTP 21 File Transfer

telnet 23 remote login

SMTP 25 E-mail

DNS 53 Domain name system

HTTP 80 World Wide Web

Berkeley SocketBerkeley SocketAPI – interface available to programmer

◦ socket(), bind(), listen(), accept(), connect(), sendto(), recvfrom(), htonl(), htons(), ntohl(), ntohs()

◦ Utility function - gethostbyname(), gethostbyaddr()Support multiple communication protocols

◦ Internet Domain◦ Unix Domain◦ Xerox NS Domain (XNS)◦ ATM Domain (Recent progress in several

implementation)References

◦ Richard Stevens, “TCP/IP Programming”, or◦ http://en.wikipedia.org/wiki/Internet_socket

ECE/IUPUIIntro to Computer Communication Networks 2-38

Example of ProtocolExample of Protocol Interaction between two peer entities Server/Client

◦ Server – a process waiting a request Listening a specific port in TCP/IP Httpd (Apache™) , telnetd, ftpd, …

◦ Client – a process making a request A request to server address and the port number Netscape™, telnet, gftp, ws_ftp, …

A connection in a server/client model is a 5-tuple◦ Protocol type, ◦ Source address, ◦ Destination address◦ Source port number, ◦ destination port number

ECE/IUPUIIntro to Computer Communication Networks 2-39

Example of Protocol, http Example of Protocol, http (1)(1)

ECE/IUPUIIntro to Computer Communication Networks 2-40

Server Client

Listening on port 80 Request to 134.68.80.4:80

Connection established

Send a requestGET/index.html HTTP 1.0

Send a result codeHTTP/1.1 200

Read and send the file<html> <head> …

Disconnect the connection

Interpret and display the html

Connection established

time

TCP and Socket in TCP and Socket in Client/ServerClient/Server

8-41

connect (blocks)

Host A (Client)Host B (Server)socke

tbind

listen

socket

connect returns

accept (returns)

write

read returns

accept (blocks)

write

read (blocks)

read returns

read (blocks)