DICT 301: Computer Networks

Engr. Md. Nazim UddinB.Sc. Engg., M.Sc. Eng., Ph.D. (on going)

nazim.cse.kuet@gmail.com

Open Systems Interconnection(OSI) Reference Model

Content

TCP/IP Protocol Model

OSI Reference Model

Data Encapsulation and Decapsulation

Communication over the Network

Binary Number System and Conversion

Hands on

1-3

Protocol Layers (TCP/IP Model)

Network Protocols

Pre-determined rules that govern communications

A group of inter-related protocols that are necessary to perform a communication function is called a protocol suite

Role of Protocols

The format or structure of the message, such as how much data to put into each segment

The process by which intermediary devices share information about the path to the destination

The method to handle error and system messages between intermediary devices

The process to setup and terminate communications or data transfers between hosts

Layered Task

We use the concept of layers in our daily life.

Networks are complex!

Many “pieces/ devices/ parts”:▫ hosts▫ routers▫ links of various media▫ applications▫ protocols▫ hardware, software

1-7

Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Layering of airline functionality

Layers: each layer implements a service▫ via its own internal-layer actions▫ relying on services provided by layer below

Why a layered model?

▫ Easier to teach communication process.

▫ Speeds development, changes in one layer does not affect how the other levels works.

▫ Standardization across manufactures.

▫ Allows different hardware and software to work together.

▫ Reduces complexity

1-10

Internet protocol stack (TCP/IP model)

application: supporting network applications▫ FTP, SMTP, HTTP

transport: host-host data transfer▫ TCP, UDP

network: routing of datagrams from source to destination▫ IP, routing protocols

link: data transfer between neighboring network elements▫ PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

TCP/IP Reference Model

Application

Transport

Internet

Network Access(Host-to-network)

Layer

HTTP TELNET FTP SMTP SNMP

Protocols

TCP UDP

IP ICMP

ETHERNET PACKET RADIO

1-12

messagesegment

datagram

frame

source

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHnHl M

HtHn M

HtHnHl M HtHnHl M

router

switch

Encapsulation

Protocols at the application layer

HTTP: browser and web server communication

FTP : file transfer protocol

TELNET: remote login protocol

POP3: Retrieve email POP3 is designed to delete mail on the server as soon as the user has

downloaded it

IMAP (Internet Message Access Protocol ) Retrieve emails, retaining e-mail on the server and for organizing it in folders on the

serve

Protocols at the transport layer

Transmission control protocol (TCP), Connection oriented

Connection established before sending data Reliable

user datagram protocol (UDP) Connectionless

Sending data without establishing connection Fast but unreliable

Protocol at the network layer

IP Path selection , routing and addressing

ICMP (Internet Control Message Protocol ) sends error messages relying on IP

a requested service is not available a host or router could not be reached

Protocols at the link layer

• Ethernet ▫ Uses CSMA/CD

• Token Ring

Data Formats

Application data

dataTCPheader dataTCP

header dataTCPheader

dataTCPheader

IPheader

dataTCPheader

IPheader

Ethernetheader

Ethernettrailer

applicationlayer

transportlayer

networklayer

data linklayer

message

segment

packet

frame

Packet Encapsulation (TCP/IP)

The data is sent down the protocol stack Each layer adds to the data by pretending headers

22Bytes20Bytes20Bytes 4Bytes

64 to 1500 Bytes

Encapsulation

Data usually transferred in blocks Protocol data units (PDUs) Each PDU contains data and control information Some PDUs only control

Three categories of control Address

Of sender and/or receiver Error-detecting code

e.g. frame check sequence Protocol control

Additional information to implement protocol functions Addition of control information to data is encapsulation Data accepted or generated by entity and encapsulated

into PDU Containing data plus control information e.g. TFTP, HDLC, frame relay, ATM, AAL5, LLC, IEEE 802.3, IEEE

802.11 [Advance]

Assists in protocol design▫ protocols that operate at a specific layer have defined

information that they act upon and a defined interface to the layers above and below

Fosters competition▫ products from different vendors can work together

Prevents technology or capabilities changes in one layer from affecting other layer above and below

Provides a common language to describe networking functions and capabilities

Benefits of a Layered Model

[Advance]

OSI Reference Model

2 types of networking models

A protocol model provides a model that closely matches the structure of a particular protocol suite▫ Transmission Control Protocol/Internet Protocol (TCP/IP)

A reference model provides a common reference for maintaining consistency within all types of network protocols and services ▫ it is not intended to be an implementation specification or to

provide a sufficient level of detail to define precisely the services of the network architecture

▫ the primary purpose is to aid in clearer understanding of the functions and processes involved

▫ Open Systems Interconnection (OSI) model

Protocol and Reference Models

The OSI Reference Model

The OSI reference model is the primary model for network communications.

Allows you to view the network functions that occur at each layer.

It is a framework that you can use to understand how information travels throughout a network

7 layers -- each of which illustrates a particular network function.

AP

S T

N D

P

Summary of layers

The TCP/IP model describes the functions that occur at layer of protocols within the TCP/IP suite (Protocol Model)

The OSI model is used for network design, operation specifications and troubleshooting (Reference Model)

TCP/IP and OSI Models

The functions of the Application, Presentation and Session layers of the OSI model are combined into one Application layer in the TCP/IP model

The Data Link and Physical layers of the OSI model combine to make the Network Access layer of the TCP/IP model

The key parallel between the two models occur at layers 3 and 4 of the OSI model– TCP operates at the transport

layer– IP operates at the Internet

layer

Comparing the OSI and TCP/IP Models

Communicating over the Network

Creation of data at the application layer of the originating source end device

Segmentation and encapsulation of data as it passes down the protocol stack in the source end device

Generation of the data onto the media at the network access layer of the stack

Transportation of the data through the internetwork, which consists of media and any intermediary devices

Reception of the data at the network access layer of the destination end device

Decapsulation and reassembly of the data as it passes up the stack in the destination device

Passing this data to the destination application at the application layer of the destination end device

Communication Process

1

2

3

4

5

6

7

Source Destination

Communication Process cont…

Communication with OSI Model

Data

(Port) Segment

(IP) Packet

(MAC) Frame

Device Operation Layer

Devices Operate at Layer

Computer Layer-7 (6,5,4)

Router Layer-3

Switch Layer-2

Hub Layer-1

The virtual interaction between layers

[Advance]

Protocol data unit (PDU) is the generic term for “data” at each level

Encapsulation is the process of adding control information as it passes through the layered model

Protocol Data Unit and Encapsulation

The Application layer begins the process by delivering the data to the Transport layer

The Application data is broken into TCP segments and each segment is given a label, called a header▫ the header contains information about which process running

on the destination computer should receive the message▫ it also contains the information to enable the destination

process to reassemble the segments

The TCP segment is sent to the Internet layer and encapsulated within an IP packet, which adds an IP header▫ the IP header contains the source and destination IP

addresses

Sending Process

The IP packet is sent to the Network Access layer where it is encapsulated with a frame header and trailer▫ each frame header contains the source and destination physical

addresses▫ the trailer contains error checking information

Finally the bits are encoded onto the Ethernet media by the end-device network interface card (NIC)

Sending Process cont…

Sending Process cont…

Receiving Process

Data Encapsulation and Decapsulation

An exchange using the OSI model

[Advance]

Layer 7 Application Browsers Application

Layer 6 PresentationCommon Data Format

Application

Layer 5 SessionNFS, SQL, RPC, X-Win

Dialogues and Conversations

Application

Layer 4 Transport Computer Segment TCP and UDPQuality of Service, and Reliability

Transport

Layer 3 Network Router

Segment Network into Smaller Broadcast Domains

Packet

Routable Protocols. (IP, IPX, AppleTalk)

Path Selection, Routing, and Addressing

Internet

Data ProtocolsWords to Remember

TCP/IP Model

OSI Model Layer

EquipmentEquipment Purpose

OSI Model Name

Layer 2

Bridge (2 Ports) or Switch and NIC

Segment Network into Smaller Collision Domains

FrameNDIS, ODI, MAC Address, Ether Talk

Physical

Data Link -MAC -LLC

Layer 1 Physical BitSignals and Media

Network Access

Computer Data

Redirector, FTP, Telnet, SMTP, SNMP, Netware Core

Regular Computer or A Special

Gateway. Used to combine networks

using different communication

protocols

One Collision AND One Broadcast

Domain

Frames and Media Access Control (MAC)

Network Access

Repeater, Hub (Multi-port), Cabling

How the OSI and TCP/IP Models Relate in a Networking Environment

Addressing

Various types of addresses are needed to successfully deliver the data from a source application running on one host to the correct destination application running on another▫ Data Link physical addresses (MAC)▫ Network logical addresses (IP)▫ Transport service port numbers (Port)

Addressing

MAC Address

IP Address Port Number

MAC Addresses (6 Byte unique)

A MAC address is a unique 6-byte address that is burned into each network interface or more specifically, directly into the PROM chip on the NIC.

IPv4 Address

Each device on a network must be uniquely defined.

The packets of the communication need to be identified with the source and destination addresses of the two end systems

IPv4 is 32 bits in length and difficult for human to remember. Therefore, represent IPv4 addresses using dotted decimal format.

10101100.00010000.00000100.00010100 (172.16.4.20)

IP has two portion: network and host

network host

This is the physical address of the host (or end device)▫ in a LAN using Ethernet, this address is called the Media Access

Control (MAC) address Layer 2 addresses are used to communicate between

devices on a single local network

(packet)

Layer 2 Address (MAC address)

Ch 2 - 45

This is the logical address Layer 3 addresses are primarily used to move data from

one local network to another local network▫ enable intermediary network devices to locate hosts on

different networks

(segment)Network

209.165.200Device230

Layer 3 Address (IP address)

Why MAC and IP both Address?

1. Source and destination MAC change each time to transport packet from one device to another device.

2. Source and destination IP address is fixed.

[Advance]

Ch 2 - 47

At layer 4, the information contained in the PDU header identifies the specific process or service running on the destination host device▫ the separately running programs are examples of individual

processes

Layer 4 Service Port Number

Bank

Service Port Example

Cash Pay Counter

Cash Receive Counter

Electric Bill

Counter

Remittance Service Counter

Information Desk

………

Networking (PC)

Email/ SMTP

File Transfer/

FTPWeb/ HTTP

Telnet ……….

Different Service, different Counter

Different Service, different Port

Service Port List

Service Protocol Port

Mail SMTP 25

POP3 POP, POP3 110

Web HTTP 80

File Transfer FTP 20, 21

Telnet Telnet 23

Domain Name Resolution DNS 53

Dynamic Host Configuration DHCP 67, 68

Port numbers

Well known ports (Numbers 0 to 1023)

Registered Ports (Numbers 1024 to 49151)

Dynamic or Private Ports (Numbers 49152 to 65535)

The Internet Assigned Numbers Authority (IANA) assigns port numbers.

netstat

Relationship of layers and addresses in TCP/IP

[Advance]

Getting Data to the End Device

The host physical address, is contained in the header of the Layer 2 PDU, called a frame.

Layer 2 is concerned with the delivery of messages on a single local network.

The Layer 2 address is unique on the local network and represents the address of the end device on the physical media.

In a LAN using Ethernet, this address is called the Media Access Control (MAC) address.

When two end devices communicate on the local Ethernet network, the frames that are exchanged between them contain the destination and source MAC addresses.

Once a frame is successfully received by the destination host, the Layer 2 address information is removed as the data is decapsulated and moved up the protocol stack to Layer 3.

[Advance]

Getting the Data Through the Internetwork Layer 3 protocols are primarily designed to move data

from one local network to another local network within an internetwork.

Layer 3 addresses must include identifiers that enable intermediary network devices to locate hosts on different networks

At the boundary of each local network, an intermediary network device, usually a router, decapsulates the frame to read the destination host address contained in the header of the packet, the Layer 3 PDU

Routers use the network identifier portion of this address to determine which path to use to reach the destination host.

[Advance]

Getting Data to the Right Application

Think about a computer that has only one network interface on it. How to differentiate various type of data?

Each application or service is represented at Layer 4 by a port number

When the data is received at the host, the port number is examined to determine which application or process is the correct destination for the data

Example of popular port numbers?

[Advance]

OSI Reference Model at a Glance

Division of Layers

Upper Layers

Lower Layers

Middle Layer

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

OSI – The Application Layer

Provides network services to the user's applications.

It does not provide services to any other OSI layer

***Think of any network application you use daily

OSI – The Presentation Layer

It ensures that the information that the application layer of one system sends out is readable by the application layer of another system.

*** Think of any common file formats (JPEG, txt etc)

OSI – The Session Layer

*** After you prepare your data, you need to establish the communication channels to send data

This layer establishes, manages, and terminates sessions between two communicating hosts.

It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange.

Application Layer Protocols

Domain Name Service (DNS)▫ used to resolve Internet names to IP addresses

Hypertext Transfer Protocol (HTTP)▫ used to transfer files that make up the web pages of the

world wide web Simple Mail Transfer Protocol (SMTP)

▫ used for the transfer of mail messages and attachments Telnet (terminal emulation protocol)

▫ used to provide remote access to servers and networking devices

File Transfer Protocol (FTP)▫ used for interactive file transfers between systems

Application Layer Software

Network-aware applications▫ implement the application layer protocols and are able

to communicate directly with the lower layers of the protocol stack

▫ e-mail clients and web browsers

Application layer services▫ are programs that interface with the network and

prepare the data for transfer▫ different types of data (text, graphics or video) require

different network services to ensure that it is properly prepared for processing by the functions occurring at the lower layers of the OSI model

DNS Service and Protocol

DNS uses TCP/UDP port 53 Domain names were created to convert the numeric IP

address into a simple, recognizable name

– domain names are easier to remember than actual numeric addresses

– any change to the address is transparent to the user since the domain name is the same

DNS Directory Structure

Uses a hierarchical structure to create a name database hierarchy is an inverted tree with the root at the top and branches

below The different top-level domains (TLD) represent either the country origin

or the type of organization

• After TLD are second-level domain names, and below them are other lower level domains

[Advance]

WWW Service and Protocol

• The web browser establishes a connection to the web service running on the server using the Hypertext Transfer Protocol (HTTP)▫ request a page using an URL (uniform resource locator) or web

address▫ a web browser is a client application running on a client device

• HTTP uses TCP port 80• An URL consists of three parts

▫ the protocol – hypertext transfer protocol (http)▫ the server name▫ the specific file name requested

• Browsers can interpret and present many data types▫ plain text and Hypertext Markup Language (HTML)▫ other data types require another service or program such as

plug-ins or add-ons

WWW Service and HTTP cont…

• The web client makes a connection to the HTTP server and requests a page

• In response to the request, the HTTP server returns the code for a web page

• The browser interprets the HTML code and displays a web page

HTTP Protocol

• HTTP specifies a request/response protocol▫ HTTP protocol uses three common messages – GET, POST and

PUT

• HTTP is not a secure protocol– POST messages in

plain text can be intercepted and read

– HTML pages are not encrypted

• HTTP Secure (HTTPS) protocol can use authentication and encryption to secure the data

E-mail Services and SMTP/POP3

User composes an e-mail using an application called a mail user agent (MUA) or e-mail client

Client sends e-mails to a server using Simple Mail Transfer Protocol (SMTP) and receives e-mails using Post Office Protocol version 3 (POP3)

SMTP uses TCP port 25 POP uses UDP port 110

Mail Transfer Agent

• The MTA process is used to forward e-mail▫ the MTA receives messages from a MUA or another MTA▫ based on the message header, it determines how a message has

to be forwarded to reach its destination

– if the recipient resides on the local server, the mail is passed to the mail delivery agent (MDA)

– if the recipient is not on the local server, the MTA routes the e-mail to the MTA on the appropriate server

Mail Delivery Agent

The MDA receives the inbound mail from the MTA and delivers the mail to the appropriate users’ mailboxes

It can also resolve final delivery issues, such as virus scanning, spam filtering and return-receipt handling

SMTP and POP

• POP and POP3 are inbound mail delivery protocols ▫ MDA listens for a client connection to the server

• SMTP governs the transfer of outbound e-mail from the sending client to the MDA, as well as the transport of e-mail between MTA– enables e-mail to

be transported across data networks between different types of server and client software

[Advance]

File Transfer Protocol

FTP uses TCP ports 20 (data) and 21 (commands and replies)

An FTP client is used to push and pull files from a server running the FTP daemon (FTPd)

TCP port 21

TCP port 20

• Client establishes the first connection to the server on TCP port 21– for control traffic

such as client commands and server replies

• Client establishes the second connection to the server over TCP port 20– for file transfer in

both direction

Telnet

• Telnet uses TCP port 23• Provides a method of emulating text-based terminals over

the network▫ allows a local device to access a remote device as if the

keyboard and monitor are connected to the remote device directly

• A connection using Telnet is called a virtual terminal (VTY) session

• The Telnet server runs a service called the Telnet daemon

OSI – The Transport Layer

Data will be segmented and send to destination device. Transport layer of destination device will reassemble them.

This layer handles details of reliable transfer. (ensures that the data arrive completely )

Basic functions of the Transport layer

Segmentation and Reassembly Conversation Multiplexing Plus :

▫ Connection-oriented conversations▫ Reliable delivery▫ Ordered data reconstruction▫ Flow control

Protocols

The two most common Transport layer protocols of TCP/IP protocol suite are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

User Datagram Protocol (UDP)

UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery. The pieces of communication in UDP are called datagrams. These datagrams are sent as "best effort" by this Transport layer

protocol.

8 bytes overhead

User Datagram Protocol

Applications that use UDP include:▫ Domain Name System (DNS)▫ Video Streaming▫ Voice over IP (VoIP)

Transmission Control Protocol (TCP)

TCP is a connection-oriented protocol, described in RFC 793.

Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data.

Same order delivery

Reliable delivery

Flow control.

Transmission Control Protocol

Applications that use TCP are:▫ Web Browsers▫ E-mail ▫ File Transfers

OSI – The Network Layer

Many paths to the same destination. So, which path to follow?

Segmented data needs address to reach the destination (network address)

This layer handle 2 above stated issues.

Ch 5 - 80

Network Layer Processes

• Addressing▫ Network layer must provide a method for addressing the end

devices▫ each device must have a unique address

• Encapsulation▫ Network layer receives the Layer 4 PDU and adds a Layer 3

header▫ the Layer 3 header contains a source address, destination

address and other control information▫ this Layer 3 PDU is known as a packet▫ the packet is sent down to the Data Link

layer• Routing

▫ the Network layer must provide services to direct these packets to the destination host

▫ intermediary devices, called routers, are used to direct packets toward the destination

[Advance]

Ch 5 - 81

Network Layer Processes cont…

• Decapsulation▫ the destination host examines the destination address to verify

that the packet was addressed to this device▫ the packet is decapsulated by the Network layer and the Layer

4 PDU contained in the packet is passed up to the appropriate service at the Transport layer

encapsulation

decapsulation

[Advance]

OSI – The Data Link Layer

It provides means for exchanging data frames over a common media

To detect and possibly correct errors that may occur in the Physical layer

Physical Addressing, topologies and flow control

OSI – The Physical Layer

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

Voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes defined by physical layer specifications.

Hands on

Binary Number System

Decimal: 10 digit 0 1 2 3 4 5 6 7 8 9 Octal: 8 digit 0 1 2 3 4 5 6 7 Hexadecimal: 16 digit 0 1 2 3 4 5 6 7 8 9 A B C D E F Binary: 2 digit 0 1

Computer only understand binary

Decimal: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Binary: 0 1 10 11 100 101 110 111 1000 1001 1010 1011 1100 1101 1110 1111

{0,1} each called a bit , Binary Digit = Bit8 bit = 1 Byte1000 Byte (1024) = 1 Kilobyte1000 Kilobyte (1024) = 1 Megabyte1000 Megabyte (1024) = 1 Gigabyte1000 Gigabyte (1024) = 1 Terabyte …

MAC- Hexadecimal IP- Decimal

Decimal to Binary Conversion and vise versa

Thank you…