6. Modelul OSI-IsO -Reprezentare Detaliata

8/13/2019 6. Modelul OSI-IsO -Reprezentare Detaliata

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Table of Contents

Review the OSI Model

LAN Devices & Technologies

IP Addressing

CIDR Notation

Routing

Transport Layer

Go There!

Go There!

Go There!

Go There!

Go There!

Go There!


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Arhitectura generala a Internetului


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Arhitectura Internet - 2


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Componente Internet


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Protocol de retea - definitie

un protocol definete formatul i ordinea mesajelor schimbatentre dou sau mai multe entiti ce comunic ntre ele, precumi aciunile ce sunt ntreprinse odat cu transmiterea sau recepiaunui mesaj sau a unui alt eveniment.


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Application

Presentation

Session

Transport

Network

Data-Link

Physical

THE OSI MODEL

Review The Model

Open SystemsInterconnected Reference

Model

Table of Contents


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Why A Layered Model?

Reduces complexity Standardizes interfaces

Facilitates modular

engineering Ensures interoperable

technology

Accelerates evolution

Simplifies teaching &learning

Application

Presentation

Session

Transport

Network

Data-Link

Physical


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Application Layer

Provides network services(processes) to applications.

For example, a computer ona LAN can save files to a

server using a networkredirector supplied by NOSslike Novell.

Network redirectors allowapplications like Word andExcel to see the network.

Application

Presentation

Session

Transport

Network

Data-Link

Physical


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Presentation Layer

Provides data representationand code formatting.

Code formatting includescompression and encryption

Basically, the presentationlayer is responsible forrepresenting data so that

the source and destinationcan communicate at theapplication layer.

Application

Presentation

Session

Transport

Network

Data-Link

Physical


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Session Layer

Provides inter-hostcommunication by establishing,maintaining, and terminatingsessions.

Session uses dialog control and

dialog separation to manage thesession

Some Session protocols: NFS (Network File System)

SQL (Structured Query Language)

RCP (Remote Call Procedure)

ASP (AppleTalk Session Protocol)

SCP (Session Control Protocol)

X-window

Application

Presentation

Session

Transport

Network

Data-Link

Physical


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Transport Layer

Provides reliability, flow control,and error correctionthrough theuse of TCP.

TCP segments the data, adding aheader with control information

for sequencing andacknowledging packets received.

The segment header alsoincludes source and destinationports for upper-layer applications

TCP is connection-oriented anduses windowing.

UDP is connectionless. UDP doesnot acknowledge the receipt of

packets.

Application

Presentation

Session

Transport

Network

Data-Link

Physical


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Network Layer

Responsible for logicallyaddressingthe packet andpath determination.

Addressing is done through

routed protocols such as IP,IPX, AppleTalk, and DECnet.

Path Selection is done byusing routing protocols such

as RIP, IGRP, EIGRP, OSPF,and BGP.

Routers operate at theNetwork Layer

Application

Presentation

Session

Transport

Network

Data-Link

Physical


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Data-Link Layer

Provides access to the media Handles error notification,

network topology issues, andphysically addressing the

frame.

Media Access Control througheither...

Deterministictoken passing

Non-deterministicbroadcasttopology (collision domains)

Important concept: CSMA/CD

Application

Presentation

Session

Transport

Network

Data-LinkPhysical


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Peer-to-Peer Communications

Peers communicate using the PDU of theirlayer. For example, the network layers of thesource and destination are peers and usepackets to communicate with each other.

Application Application

Presentation Presentation

Session Session

Transport TransportNetwork Network

Data-Link Data-Link

Physical Physical

Data

Segments

Packets

Frames

Bits

Data

Data


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Encapsulation Example

Transport segmentsthedata using TCP and hands itto the Network Layer foraddressing

Network addresses thepacketusing IP.

Data-Link then encaps. thepacket in a frameand

addresses it for localdelivery (MACs)

The Physical layer sends thebitsdown the wire.

Application

Presentation

Session

Transport

Network

Data-LinkPhysical


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Application

Presentation

Session

Transport

Network

Data-LinkPhysical

THE OSI MODEL

LAN Devices &

TechnologiesThe Data-Link &Physical Layers

Table of Contents

Data-LinkPhysical


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Devices

What does it do? Connects LAN

segments;

Filters traffic based

on MAC addresses;and

Separates collisiondomains based upon

MAC addresses.

What layer device?


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Devices

What does it do? Concentrates LAN

connections frommultiple devices into

one location Repeats the signal (a

hub is a multi-portrepeater)

What layer device?


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Devices

What does it do? Interconnects networks

and provides broadcastcontrol

Determines the path

using a routing protocolor static route

Re-encapsulates thepacket in the appropriateframe format and

switches it out theinterface

Uses logical addressing(i.e. IP addresses) todetermine the path

What layer device?


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Media Types


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LAN Technologies

Three Most

Common UsedToday in

Networking


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Ethernet/802.3

Cable Specifications: 10Base2

Called Thinnet; uses coax

Max. distance = 185 meters (almost 200)

10Base5 Called Thicknet; uses coax

Max. distance = 500 meters

10BaseT Uses Twisted-pair

Max. distance = 100 meters

10 means 10 Mbps


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Ethernet/802.3

Ethernet is broadcast topology. What does that mean?

Every devices on the Ethernet segment seesevery frame.

Frames are addressed with source anddestination ______ addresses.

When a source does not know the destinationor wants to communicate with every device, it

encapsulates the frame with a broadcastMACaddress: FFFF.FFFF.FFFF

What is the main network traffic problemcaused by Ethernet broadcast topologies?


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Ethernet/802.3

Ethernet topologies are also sharedmedia.

That means media access is controlled

on a first come, first serve basis. This results in collisions between the

data of two simultaneously transmitting

devices. Collisions are resolved using what

method?


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Ethernet/802.3

CSMA/CD (Carrier Sense Multiple Access withCollision Detection)

Describe how CSMA/CD works: A node needing to transmit listens for activity on

the media. If there is none, it transmits. The node continues to listen. A collision is

detected by a spike in voltage (a bit can only be a0 or a 1--it cannot be a 2)

The node generates a jam signal to tell all devices

to stop transmitting for a random amount of time(back-off algorithm).

When media is clear of any transmissions, thenode can attempt to retransmit.


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Address Resolution Protocol

In broadcast topologies, we need a way toresolve unknown destination MAC addresses.

ARP is protocol where the sending devicesends out a broadcast ARP request which

says, Whats you MAC address? If the destination exists on the same LAN

segment as the source, then the destinationreplies with its MAC address.

However, if the destination and source areseparated by a router, the router will notforward the broadcast (an important functionof routers). Instead the router replies with its

own MAC address.


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Application

Presentation

Session

Transport

Network

Data-LinkPhysical

THE OSI MODEL

IP Addressing

Subnetting Review

Table of Contents

Network


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Network vs. Host

N H H H

Class A: 27

= 126 networks; 224

> 16 million hosts

N N H H

Class B : 214= 16,384 networks; 216> 65,534 hosts

N N N H

Class C : 221> 2 million networks; 28= 254 hosts


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Why Subnet?

Remember: we are usually dealing witha broadcast topology.

Can you imagine what the network

traffic overhead would be like on anetwork with 254 hosts trying todiscover each others MAC addresses?

Subnetting allows us to segment LANsinto logical broadcast domains calledsubnets, thereby improving networkperformance.


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Four Subnetting Steps

To correctly subnet a given networkaddress into subnet addresses, askyourself the following questions:

1. How many bits do I need to borrow?2. Whats the subnet mask?

3. Whats the magic number or multiplier?

4. What are the first three subnetworkaddresses?

Lets look at each of these questions indetail


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1. How many bits to borrow?

First, you need to know how many bitsyou have to work with.

Second, you must know either how

many subnets you need or how manyhosts per subnet you need.

Finally, you need to figure out the

number of bits to borrow.


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How many bits do I have to work with? Depends on the class of your network

address.

Class C: 8 host bits

Class B: 16 host bits

Class A: 24 host bits

Remember: you must borrow at least 2 bits

for subnets and leave at least 2 bits forhost addresses.

2 bits borrowed allows 22- 2 = 2 subnets


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Class C Example: 210.93.45.0 Design goals specify at least 5 subnets

so how many bits do we borrow?

How many bits in the host portion dowe have to work with (TB)?

Whats the BB in our TB = BB + BLformula? (8 = BB + BL)

2 to the what power will give us at least5 subnets?

23- 2 = 6 subnets


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How many bits are left for hosts?TB = BB + BL

8 = 3 + BL

BL = 5 So how many hosts can we assign to

each subnet?

25- 2 = 30 hosts


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Class B Example: 185.75.0.0 Design goals specify no more than 126 hostsper subnet, so how many bits do we need toleave (BL)?

How many bits in the host portion do we haveto work with (TB)?

Whats the BL in our TB = BB + BL formula?(16 = BB + BL)

2 to the what power will insure no more than126 hosts per subnet and give us the mostsubnets?

27- 2 = 126 hosts


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How many bits are left for subnets?TB = BB + BL

16 = BB + 7

BL = 9 So how many subnets can we have?

29- 2 = 510 subnets


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2. Whats the subnet mask?

We determine the subnet mask by adding upthe decimal value of the bits we borrowed.

In the previous Class C example, we borrowed3 bits. Below is the host octet showing the

bits we borrowed and their decimal values.

128 64 32 16 8 4 2 1

1 1 1

We add up the decimal value of these bits and get 224.Thats the last non-zero octet of our subnet mask.

So our subnet mask is 255.255.255.224


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3. Whats the magic number?

To find the magic number or themultiplier we will use to determine thesubnetwork addresses, we subtract thelast non-zero octet from 256.

In our Class C example, our subnetmask was 255.255.255.224. 224 is ourlast non-zero octet.

Our magic number is 256 - 224 = 32


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Last Non-Zero Octet

Memorize this table. You should be able to: Quickly calculate the last non-zero octet whengiven the number of bits borrowed.

Determine the number of bits borrowed given thelast non-zero octet.

Determine the amount of bits left over for hostsand the number of host addresses available.

Bits

Borrowed

Non-Zero

Octet Hosts

2 192 623 224 30

4 240 14

5 248 6

6 252 2


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4. What are the subnets?

We now take our magic number anduse it as a multiplier.

Our Class C address was 210.93.45.0.

We borrowed bits in the fourth octet, sothats where our multiplier occurs 1st subnet: 210.93.45.32

2nd subnet: 210.93.45.64

3rd subnet: 210.93.45.96 We keep adding 32 in the fourth octet

to get all six available subnet addresses.


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Host & Broadcast Addresses

Now you can see why we subtract 2 whendetermining the number of host address.

Lets look at our 1st subnet: 210.93.45.32

What is the total range of addresses up to ournext subnet, 210.93.45.64?

210.93.45.32 to 210.93.45.63 or 32 addresses

.32 cannot be assigned to a host. Why?

.63 cannot be assigned to a host. Why? So our host addresses are .33 - .62 or 30 host

addresses--just like we figured out earlier.


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Application

Presentation

Session

Transport

Network

Data-LinkPhysical

THE OSI MODEL

CIDR Notation

A Different Way toRepresent a Subnet Mask

Table of Contents

Network


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CIDR Notation

Classless Interdomain Routing is a method ofrepresenting an IP address and its subnetmask with a prefix.

For example: 192.168.50.0/27

What do you think the 27 tells you? 27 is the number of 1 bits in the subnet mask.

Therefore, 255.255.255.224

Also, you know 192 is a Class C, so we borrowed 3bits!!

Finally, you know the magic number is 256 - 224 =32, so the first useable subnet address is197.168.50.32!!

Lets see the power of CIDR notation.


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202.151.37.0/26

Subnet mask? 255.255.255.192

Bits borrowed? Class C so 2 bits borrowed

Magic Number? 256 - 192= 64

First useable subnet address? 202.151.37.64

Third useable subnet address? 64 + 64 + 64 = 192, so 202.151.37.192


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198.53.67.0/30

Subnet mask? 255.255.255.252

Bits borrowed? Class C so 6 bits borrowed

Magic Number? 256 - 252= 4

Third useable subnet address? 4 + 4 + 4 = 12, so 198.53.67.12

Second subnets broadcast address? 4 + 4 + 4 - 1 = 11, so 198.53.67.11


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200.39.89.0/28

What kind of address is 200.39.89.0? Class C, so 4 bits borrowed

Last non-zero octet is 240

Magic number is 256 - 240 = 16

32 is a multiple of 16 so 200.39.89.32 is asubnet address--the second subnetaddress!!

Whats the broadcast address of200.39.89.32?

32 + 16 -1 = 47, so 200.39.89.47


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194.53.45.0/29

What kind of address is 194.53.45.26? Class C, so 5 bits borrowed Last non-zero octet is 248

Magic number is 256 - 248 = 8

Subnets are .8, .16, .24, .32, ect.

So 194.53.45.26 belongs to the third subnetaddress (194.53.45.24) and is a host address.

What broadcast address would this host use

to communicate with other devices on thesame subnet? It belongs to .24 and the next is .32, so 1 less is

.31 (194.53.45.31)


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No Worksheet Needed!

After some practice, you should never need asubnetting worksheet again.

The only information you need is the IPaddress and the CIDR notation.

For example, the address 221.39.50/26

You can quickly determine that the firstsubnet address is 221.39.50.64. How?

Class C, 2 bits borrowed 256 - 192 = 64, so 221.39.50.64

For the rest of the addresses, just domultiples of 64 (.64, .128, .192).


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The Key!!

MEMORIZE THIS TABLE!!!

Bits

Borrowed

Non-Zero

Octet Hosts

2 192 62

3 224 30

4 240 14

5 248 6

6 252 2


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Practice On Your Own

Below are some practice problems. Take outa sheet of paper and calculate... Bits borrowed

Last non-zero octet

Second subnet address and broadcast address

1. 192.168.15.0/26

2. 220.75.32.0/30

3. 200.39.79.0/29

4. 195.50.120.0/275. 202.139.67.0/28

6. Challenge:132.59.0.0/19

7. Challenge:64.0.0.0/16

Answers


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Application

Presentation

Session

Transport

Network

Data-LinkPhysical

THE OSI MODEL

Routing Basics

Path Determination &Packet Switching

Table of Contents

Network


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A Routers Functions

A router is responsible for determiningthe packets path and switching thepacket out the correct port.

A router does this in five steps:1. De-encapsulates the packet

2. Performs the ANDing operation

3. Looks for entry in routing table

4. Re-encapsulates packet into a frame

5. Switches the packet out the correctinterface


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Routed v. Routing Protocols

What is a routed protocol? Routed protocols are protocols that enable data to

be transmitted across a collection of networks orinternetworks using a hierarchical addressingscheme.

Examples include IP, IPX and AppleTalk.

A routable protocol provides both a network andnode number to each device on the network.Routers AND the address to discover the networkportion of the address.

An example of a protocol that is not routable isNetBEUI because it does not have a network/nodestructure.


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Routed v. Routing Protocols

What is a routing protocol?A routing protocol is a protocol that

determines the path a routed protocol willfollow to its destination.

Routers use routing protocols to create amap of the network. These maps allowpath determination and packet switching.

Maps become part of the routers routingtable.

Examples of routing protocols include: RIP,IGRP, EIGRP, & OSPF


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S


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Dynamic v. Static Routing

Dynamic routing refers to the process ofallowing the router to determine the path tothe destination.

Routing protocols enable dynamic routing

where multiple paths to the same destinationexist.

D i S i R i


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Dynamic v. Static Routing

Static routing means that the networkadministrator directly assigns the path routerare to take to the destination.

Static routing is most often used with stub

networks where only one path exists to thedestination.

D f l R


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Default Routes

A default route is usually to a border orgateway router that all routers on a networkcan send packets to if they do not know theroute for a particular network.

R ti P t l Cl


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Routing Protocol Classes

Routing protocols can be divided intothree classes:

Distancevector: determines the routebased on the direction (vector) anddistance to the destination

Link-state: opens the shortest path first tothe destination by recreating an exact

topology of the network in its routing table Hybrid: combines aspects of both


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Di t t R ti


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Distance-vector Routing

Each router receives a routing table periodicallyfrom its directly connected neighboring routers.

For example, in the graphic, Router B receivesinformation from Router A. Router B adds a

distance-vector number (such as a number ofhops), and then passes this new routing table toits other neighbor, Router C.

Li k t t R ti


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Link-state Routing

Link-state protocols maintain complex databasesthat summarize routes to the entire network.

Each time a new route is added or a route goesdown, each router receives a message and thenrecalculates a spanning tree algorithm and

updates its topology database.

C i th T


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Comparing the Two

DISTANCE-VECTOR LINK-STATE

Views network topology from

neighbors perspective

Gets common view of entire

network topology

Adds distance vectors from

router to router

Calculates the shortest path to

other routers

Frequent, periodic updates:

slow convergence Event triggered updates: fastconvergencePasses copies of routing tables

to neighbors

Passes link-state routing updates

to all routers in the system.

H b id R ti


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Hybrid Routing

Ciscos proprietary routing protocol,EIGRP, is considered a hybrid.

EIGRP uses distance-vector metrics.

However, it uses event-triggeredtopology changes instead of periodicpassing of routing tables.


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Application

Presentation

Session

Transport

Network

Data-LinkPhysical

THE OSI MODEL

Transport Layer

A Quick Review

Table of Contents

Transport

T t L F ti


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Transport Layer Functions

Synchronization of the connection Three-way handshake

Flow Control

Slow down, youre overloading mymemory buffer!!

Reliability & Error Recovery

Windowing: How much data can I sendbefore getting an acknowledgement?

Retransmission of lost or unacknowledgedsegments

T t T P t l


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Transports Two Protocols

TCP Transmission Control

Protocol

Connection-oriented

Acknowledgment &Retransmission ofsegments

Windowing

Applications: Email

File Transfer

E-Commerce

UDP User Datagram

Protocol

Connectionless

NoAcknowledgements

Applications:

Routing Protocols

Streaming Audio

Gaming

Video Conferencing

6. Modelul OSI-IsO -Reprezentare Detaliata

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