REFERENCE

MODELS

• Over the past couple of decades many of the networks

that were built used different hardware and software

implementations, as a result they were incompatible and it

became difficult for networks using different specifications

to communicate with each other.

• To address the problem of networks being incompatible

and unable to communicate with each other, the

International Organisation for Standardisation (ISO)

researched various network schemes.

• The ISO recognised there was a need to create a

NETWORK MODEL that would help vendors create

interoperable network implementations.

• A reference model is a conceptual layout that

describes how communication between devices

should occur. A reference model has many

advantages such as it defines standards for

building network components thereby permitting

multiple-vendor development and also defines

which functions should be performed at each

layer of the model thereby promoting the

standardization of network.

• In 1984 in order to aid network interconnection without

necessarily requiring complete redesign, the Open Systems

Interconnection (OSI) reference model was approved as an

international standard for communications architecture.

• The Open Systems Interconnection (OSI) reference model is a

descriptive network scheme. It ensures greater compatibility

and interoperability between various types of network

technologies.

• The OSI model describes how information or data makes its

way from application programmes (such as spreadsheets)

through a network medium (such as wire) to another

application programme located on another network.

• The OSI reference model divides the problem of moving

information between computers over a network medium into

SEVEN smaller and more manageable problems.

In 1983, Day and Zimmerman laid down certain principles that were

applied to arrive at the seven layers can be briefly summarized as

follows:

A layer should be created where a different abstraction is needed.

Each layer should perform a well-defined function.

The function of each layer should be chosen with an eye toward

defining internationally standardized protocols.

The layer boundaries should be chosen to minimize the information

flow across the interfaces.

The number of layers should be large enough that distinct functions

need not be thrown together in the same layer out of necessity and

small enough that the architecture does not become unwieldy

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Layer 5

Layer 6

Layer 7 Application

Presentation

Session

Application

DATADATA

Format

When to start

Signaling9Mukesh Chinta, Asst Prof, CSE

Transport

Network

Data Link

PhysicalLayer 1

Layer 2

Layer 3

Layer 4 Reliability

0101

0101

Data transmission

at the node interval

Routing

Physical connection

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Provides physical interface for transmission ofinformation.

Defines rules by which bits are passed from onesystem to another on a physical communicationmedium.

Covers all - mechanical, electrical, functional andprocedural - aspects for physical communication.

Such characteristics as voltage levels, timing ofvoltage changes, physical data rates, maximumtransmission distances, physical connectors, andother similar attributes are defined by physical layerspecifications.

The physical layer is responsible for movements of

individual bits from one hop (node) to the next.

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Data link layer attempts to provide reliablecommunication over the physical layer interface.

Breaks the outgoing data into frames and reassemblethe received frames.

Create and detect frame boundaries.

Handle errors by implementing an acknowledgementand retransmission scheme.

Implement flow control.

Supports point-to-point as well as broadcastcommunication.

A special sublayer Medium access control sublayerdeals with the problem of access control of sharedchannel.

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The data link layer is responsible for moving

frames from one hop (node) to the next.

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15

16

Implements routing of frames (packets) through the

network.

Defines the most optimum path the packet should take

from the source to the destination

Defines logical addressing so that any endpoint can be

identified.

Handles congestion and Quality of service issues in the

network.

Facilitates interconnection between heterogeneous

networks (Internetworking).

The network layer also defines how to fragment a

packet into smaller packets to accommodate different

media.

In broadcast networks, the network layer is

thin(nonexistent)

The network layer is responsible for the

delivery of individual packets from

the source host to the destination host.

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Purpose of this layer is to provide a reliablemechanism for the exchange of data between twoprocesses in different computers.

Accepts data from above, split into smaller units inneeded

Ensures that the data units are delivered error free.

Ensures that data units are delivered in sequence.

Ensures that there is no loss or duplication of dataunits.

Provides connectionless or connection orientedservice.

Provides for the connection management.

Multiplex multiple connection over a single channel.

The transport layer is responsible for the delivery

of a message from one process to another.

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Session layer provides mechanism for controlling the dialogue

between the two end systems. It defines how to start, control

and end conversations (called sessions) between applications.

This layer requests for a logical connection to be established

on an end-user’s request. Responsible for maintenance and

termination of sessions.

Token Management: Prevents two parties from attempting the

same critical operation simulatenously.

This layer provides services like dialogue discipline which can

be full duplex or half duplex: dialog control

Session layer can also provide check-pointing mechanism

such that if a failure of some sort occurs between checkpoints,

all data can be retransmitted from the last checkpoint:

Synchronization.

The session layer is responsible for dialog

control and synchronization.

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Presentation layer defines the format in which the data is to beexchanged between the two communicating entities. It dealswith syntax and semantics of the information transmitted.

Also handles data compression and data encryption(cryptography).

Responsible for protocol conversion, character conversions,data encryption / decryption, expanding graphics commandsand data compression

• Used for applications specially written to run over the network

• Allows access to network services that support applications

• Directly represents the services that directly support user

applications (e.g., file transfer and email)

• What the user sees or does

• Contains a variety of protocols that are commonly needed by

users

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The application layer is responsible for

providing services to the user.

End

System

Intermediate

Systems

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30

31

32

Meet the Protocol Family in the Internet

DNS

IP QoS

Winsock SMTP

NTCIP POP3

SLIP

testerdeveloper

administrator

OSPF

BGPRTP

WWW

TCP/IP originated out of the investigative research into networking

protocols that the US Department of Defense (DoD) initiated in 1969.

In 1968, the DoD Advanced Research Projects Agency (ARPA) began

researching the network technology that is called packet switching.

The network that was initially constructed as a result of this research

to provide a communication that could function in wartime., then

called ARPANET, gradually became known as the Internet. The TCP/IP

protocols played an important role in the development of the

Internet. In the early 1980s, the TCP/IP protocols were developed. In

1983, they became standard protocols for ARPANET.

Because of the history of the TCP/IP protocol suite, it's often referred

to as the DoD protocol suite or the Internet protocol suite 34

35

The Internet Protocol Suite (commonly known

as TCP/IP) is the set of communications

protocols used for the Internet and other similar

networks.

It is named from two of the most important protocols

in it: the Transmission Control Protocol (TCP)

The Internet Protocol (IP), which were the first two

networking protocols defined in this standard.

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TCP/IP Network Architecture

Application Layer

Transport Layer

Network Layer

Link Layer

operating-system/computer-architecture independent

LAN/MAN/WAN applicable

physical-medium independent

host host

network network

media media

process process

client-server model

TCP/IP Protocol Suite

Ethernet

ARP RARP

IPv4 IPv6

TCP UDP

ICMP

TelnetFTPping SNMP TFTP

Serial line

PPP

SMTP POP3

21 23 11025 69161port

number

IP

address

TCP\IP Protocol Suite

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The internet layer is the linchpin that holds the whole architecture

together. Its job is to permit hosts to inject packets into any network

and have them travel independently to the destination (potentially on

a different network).

Internet Protocol (IP) is the most important protocol in this layer. It

is a connectionless protocol and does not provide reliability, flow

control, or error recovery. IP provides a routing function that

attempts to deliver transmitted messages to their destination.

A message unit in an IP network is called an IP datagram. This is

the basic unit of information transmitted across TCP/IP networks.

Other internetwork-layer protocols are ICMP {Internet Control

Message Protocol}, IGMP {Internet Group Management Protocol},

ARP {Address Resolution Protocol}, and RARP {Reverse ARP}.

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Transport layer (host-to-host) is designed to allow peer entities on the

source and destination hosts to carry on a conversation, just as in the OSI

transport layer. Two end-to-end transport protocols namely TCP

{Transmission Control Protocol} and UDP {User Datagram Protocol}

have been defined.

TCP is a reliable connection-oriented protocol that permits a byte stream

originating on one machine to be transported without error on any

machine in the internet. It divides the incoming byte stream into discrete

message and passes each one onto the internet layer. It reassembles

these messages at the receiver. It also handles flow control.

UDP is an unreliable, connectionless protocol for applications that do not

want TCP's sequencing or flow control and wish to provide their own. It is

also widely used for one-shot, client-server-type request-reply queries

and applications in which prompt delivery is more important than

accurate delivery, such as transmitting speech or video.

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The network interface layer, also called the link layer or the

data-link layer, is the interface to the actual network hardware. This

interface may or may not provide reliable delivery, and may be

packet or stream oriented. In fact, TCP/IP does not specify any

protocol here, but can use almost any network interface available,

which illustrates the flexibility of the IP layer.

Examples are IEEE 802.2, X.25, ATM, FDDI, and even SNA.

It varies from implementation to implementation with vendors

supplying their own version

• The Application layer is equivalent to the combined OSI session,

presentation and Application Layers.

• All the functions handled by these 3 layers in the OSI model are handled by

the Application layer in TCP/IP model.

• It provides a way for applications to have access to networked services.

• The Application Layer has the responsibility for authentication, data

compression, and end-user services such as terminal emulation, file transfer,

e-mail, web browsing/serving, and other network control and management

services. An application header and following data are packaged as a

message

• Application layer is present on the top of the Transport layer. It includes all

the higher-level protocols which are virtual terminal (TELNET), file

transfer (FTP), electronic mail (SMTP) & HTTP (Hyper Text Transfer

Protocol) 43

• IMAP4 - Internet Mail Access Protocol version 4 lets clients access an IMAP4 mail

server to download their e-mail to a local computer program. It works using TCP as

its transport protocol.

• FTP - File Transfer Protocol uses TCP as transport and allows the transfer of files

between two computer systems with login required by the requester.

• Telnet – Sometimes incorrectly called Terminal Emulation across a network, it is

used to remotely open a session on another computer acting as a server. It relies on

TCP for transport.

• SMTP - Simple Mail Transfer Protocol is a TCP-transported application layer

protocol used to send electronic mail.

• HTTP - Hypertext Transfer Protocol uses the TCP transport protocol to carry web

browsing requests to a web server, and web pages from web servers to web

browsers.

• POP3 - Post Office Protocol version 3 uses TCP as a way to offer clients access to a

POP3 mail server to transfer their e-mail to a local program on their computer.

• BGP4 - Border Gateway Protocol version 4 is a routing protocol most often used

between organizations. Two routers using BGP will establish a TCP connection to

send each other their BGP routing tables. In that exchange is information about

reachable networks including the full path to all BGP-known networks. 44

• DNS3 - Domain Names System provides the ability to refer to IP devices

using names instead of numerical IP addresses. It lets Domain Name Servers

resolve these names to their corresponding IP addresses.

• DHCP - Dynamic Host Configuration Protocol uses UDP as its transport

protocol to dynamically and automatically assign IP addresses and other

networking configuration information to computers starting up on a given

network.

• TFTP - Trivial File Transfer Protocol is a UDP-transported protocol that

allows file transfer between two computers with no login or user required for

its limited use.

• SNMP - Simple Network Management Protocol is used to manage all types of

network elements based on various data sent and received using UDP as its

transport protocol.

• RIP2 - Routing Information Protocol is an internal routing protocol used to

dynamically update router tables on internal organization networks. It uses

UDP as its transport protocol. 45

• ARP - Address Resolution Protocol supports the packaging of IP data into

Ethernet frames. It finds the local Ethernet (MAC) address that matches a

specific local IP address.

• ICMP4 - Internet Control Message Protocol provides diagnostics and logical

error reporting to help manage the sending of data between computers. Its

best-known function is ping.

• IGMP - Internet Group Management Protocol supports multicasting by

letting multicast routers track group memberships on each of its connected

networks.

• IPsec - Internet Protocol Security is an end-to-end security scheme for

securing Internet Protocol (IP) communications by authenticating and

encrypting each IP packet of a communication session.

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• IP4 - Internet Protocol provides connectionless communication support

for all protocols’ data, except ARP, by packaging that data into an IP

datagram.

• OSPF - Open Shortest Path First is an internal routing protocol for use

inside an organization. It checks the function of its link to each of its

neighbor OSPF routers. Then, it sends the acquired routing information to

those neighbor routers.

• EIGRP - Enhanced Interior Gateway Routing Protocol is a local routing

protocol that is proprietary to Cisco. It is an advanced distance-vector

routing protocol that shares internal organizational routing information

found in three tables.

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• The application layer in TCP/IP handles the responsibilities of multiple

layers in the OSI model.

• The OSI model numbers and names its layers, whereas the TCP/IP stack

only names the layers.

• Unlike the transport layer in OSI, TCP/IP only guarantees reliable delivery

of packets when TCP is the chosen protocol.

• OSI has much more complexity in its 7 layers than TCP/IP has in its 4

layers.

• In TCP/IP, protocols are deliberately designed to have more layer

flexibility than the strict layers of the OSI model.

• TCP/IP functions are implemented, then standardized. OSI is standardized

in concept only, though some functions work.

• OSI has more limited Network Management and Network Security.48