OSI and TCP Reference Models

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To understand Basic definitions

Protocol Application

Understand communication process using Reference models ◦ OSI Reference Model◦ TCP/IP - Internet Model

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A network protocol defines the◦ format and the order of messages ◦exchanged between two or more

communicating entities, as well as the◦ actions taken on the transmission

and/or receipt of a message or other events.

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mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

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Messages format and order From a client to a mail server

◦ HELO◦ MAIL FROM: <address>◦ RCPT TO: <address>◦ DATA<This is the text end with a line with a single .>

◦ QUIT From a mail server to a client

◦ status code 1xx - Informative message 2xx - Command ok 3xx - Command ok so far, send the rest of

it. 4xx - Command was correct, but couldn't

be performed for some reason. 5xx - Command unimplemented, or

incorrect, or a serious program error occurred.

◦ content

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragentuser

agent

mailserver

useragent

SMTP

SMTP

SMTP

POP3,IMAPSMTP

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Network Applications are Programs that run on different end systems and communicate over a network.

application

transport

networkdata link

physical

application

transport

networkdata link

physical

application

transport

networkdata link

physical

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Number of Hosts on the Internet:

Aug. 1981 213

Oct. 1984 1,024

Dec. 1987 28,174

Oct. 1990 313,000

Jul. 1993 1,776,000

Jul. 1996 19,540,000

Jul. 1999 56,218,000

Jul. 2004 285,139,000

Jul. 2005 353,284,000

Jul. 2007 489,774,000

Jul. 2008 570,937,000

Jan. 2009 625,226,000

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All standards of the Internet are published as RFC (Request for Comments) e.g., the SMTP protocol is specified in RFC821

A typical (but not the only) way of standardization: Internet draft RFC proposed standard draft standard (requires 2 working implementations) Internet standard (declared by Internet Architecture Board)

We reject: kings, presidents, and voting. We believe in: rough consensus and running code.

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Why need standards? The use of standards in protocols ensures that

products from different manufacturers can work together

Do you know of name of a standards body for :◦ food industry

FSA◦ Computer Science and Engineering

IEEE And many others

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The problem of moving information between computers over a network medium is divided into smaller and more manageable problems

This separation into smaller more manageable functions is known as layering.

Two Reference Models:◦ OSI -7 Layer Model◦ TCP/IP Model

Also known as the Internet model

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여보세요

She is saying hello

Ambassador in Korea

Translator

Secretary

The Korean ambassador said

“Hello”

Bonjour!

Ambassador in France

Translator

Secretary

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A standard reference model called OSI (Open Systems Interconnect ) is now considered the primary Architectural model for inter-computer communications

The OSI model divides the networking process into seven layers.

Each layer in the model has a unique function in the communication process and communicates with the layer directly above and below it.

OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Information passed from source to destination has to pass through each layer on each side.

Which layer does the user interact with: Application or Physical ?

Which layers are connected by a cable?

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Phrase to remember 7 layers is:All People Seem To Need Data Processing What is a protocol? Examples of some network applications? Need for standardisation? Example of a standard? How many layers in OSI model?

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Physical Layer◦ Media◦ Topology◦ Making Cables

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