Tài liệu Chapter 5 - OSI Network Layer CCNA Exploration version 4.0 ppt

Overview ° Identify the role of the Network Layer, as it describes communication from one end device to another end device e Examine the most common Network Layer protocol, Internet

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Chapter 5 - OSI Network Layer

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CCNA Exploration version 4.0

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Overview

° Identify the role of the Network Layer, as it describes

communication from one end device to another end device

e Examine the most common Network Layer protocol, Internet

Protocol (IP), and its features for providing connectionless and

best-effort service

e Understand the principles used to guide the division or grouping

of devices into networks

e Understand the hierarchical addressing of devices and how this

allows communication between networks

e Understand the fundamentals of routes, next hop addresses and

packet forwarding to a destination network

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Network Layer Protocols and Internet Protocol (IP)

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BE The Network Layer

_Application } As we communicate our data

—

Network “=———— And the Network layer

Data Link enables devices to reach

each other!

Physical |

Routers connected to each other

° Define the basic role of the Network Layer in data networks

e The protocols of the OSI model Network layer specity,addressing and

processes that enable Transport layer data to be packaged and

transported The Network layer encapsulation allows its contents to be

oassed to the destination within a network or on another network with

minimum overhead

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Network Layer — Communication from Host to Host

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e Layer 3 uses four basic processes:

1 Addressing

2 Encapsulation

3 Routing: Intermediary devices that connect the networks are called

routers The role of the router is to select paths for and direct packets toward their destination This process is known as routing

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Network Layer Protocols

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° Protocols implemented at the Network layer that carry user

data include:

— Internet Protocol version 4 (IPv4)*

— Internet Protocol version 6 (IPv6)*

— Novell Internetwork Packet Exchange (IPX)

— AppleTalk

— Connectionless Network Service (CLNS/DECNet)

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The roles of IPv4 protocol

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e |Pv4 is currently the most widely-used version of IP

e The Internet Protocol was designed as a protocol with low

overhead It provides only the functions that are necessary to

deliver a packet from a source to a destination over an

interconnected system of networks

e The protocol was not designed to track and manage the flow of

packets These functions are performed by other protocols in

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The IPv4 protocol — Connectionless

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° Describe the implications for the use of the IP protocol

as it is connectionless

Connectionless Communication

The receiver doesn't know:

» ifthe receiver is present * when it is coming

¢ ifthe letter arrived

¢ ifthe receiver can read the letter

a1

A packet is sent

The sender doesn"t know: The receiver doesn't know:

« ifthe receiver is present « when it is coming

* ifthe packet arrived

* ifthe receiver can read the

packet

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The IPv4 protocol — Best Effort

° Describe the implications for

the use of the IP protocol as it

capability to manage, and — |

corrupt packets

layers can manage reliability, cee epssting tet celven

IP is allowed to function very

efficiently at the Network

layer

As an unreliable Network layer protocol, IP does not guarantee

that all sent packets will be received

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The IPv4 protocol — Media independent

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° Describe the implications for the

use of the IP as it is media

independent

e One major characteristic of the Media Independence

media that the Network layer

considers: the maximum size of

PDU that each medium can

transport This characteristic is

referred to as the Maximum

Transmission Unit (MTU) Part of

the control communication

between the Data Link layer and

the Network layer is the

establishment of a maximum size

for the packet

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IPv4 packet - Packaging the Transport layer PD

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e In some cases, an intermediary device - usually a router - will

need to split up a packet when forwarding it from one media to a

media with a smaller MTU This process is called fragmenting the

IP Packet

In TCP/IP based networks, the Network layer PDU is the IP packet

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IPv4 packet header

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° Identify the major header fields in the IPv4 protocol

and describe each field's role in transporting packets

IPv4 Packet Header Fields

| Byte 1 | Byte 2 | Byte 3 | Byte 4 9

ý*

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Network Layer Fields

identification Flags Fragment Offset

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° Indicates datagram header ref ec bit words

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assigned by upper-layer protocol

oe

° Specifies the level of importance as been

14

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° Specifies the length of the entire pac! Bet in bytes,

including data and header

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° Identifies the current datagra

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- 0: Fragment if necessary 4 o>

- 1: Do not fragment ‡> uP

° Q: This is the last fragment

- 1: More fragment follow this fragment

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identification Flags C | Fragment Offset >

Time to Live Protocol Header Checksum

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VERS HLEN Service Type Total Lenth

identi Flags Fragment Offset

Cc Time to Live Protocol Header Checksum

- Specifies the number of hops a packet aval: This

number is decreased by one as the packet travels through a

router

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VERS HLEN Service Type Total Lenth

° Indicates which upper-layer protocol, su 5 6) or

UDP(17), receives incoming packet ocessing has

been completed

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VERS HLEN Service Type ' TotalLenth

Identification | Flags | Fragment Offset

°_Helps ensure IP header integri “về

° Not caculated for the encapsul on data

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° Specifies the sending node adress

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° Specifies the receiving node IPiadatess

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° Allows IP to support various ctlons Such a security

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e Extra zeros are added to this field to ensure that the

IP header is always a multiple of 32 bits

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VERS HLEN Service Type _| Total Lenth

eet Flags Fragment Offset

° Variable length up to 64 Kb Miu poe” A4

° Contains upper-layer information®

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Grouping Devices into Networks

e Reasons for grouping devices into sub-networks and

define several terms used to identify the sub-networks

— Geographic

Network designers have to ask: on

what basis should the network be

divided?

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° Reasons for grouping devices into sub-networks and

— Purpose

Network designers have to ask: on what basis should the network be

divided?

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° List several different reasons for grouping devices into

sub-networks and define several terms used to identify the sub-networks

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° Reasons for grouping devices into sub-networks and

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Broadcast domain ? - Performance

Am

° A broadcast is a message sent from one host to all

other hosts on the network

° Managing the size of broadcast domains by dividing a

network into subnets or replace switch by router

All devices in this network are connected in one broadcast domain when the switch is setto the |

factory default settings Since switches forward broadcasts by default, broadcasts are processed =! Replacing the middle switch with a router creates 2 IP subnets, hence, 2 distinct broadcast

by all devices in this network domains, All devices are connected but local broadcasts are contained

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Why separate hosts into networks ? - Security

° List several ways in which dividing a large network can

increase network security

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° Explain the communication problems that emerge

when very large numbers of devices are included in

one large network

Hosts do not know how to deliver data to devices in a remote

network - this is the role of the gateway

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Hierarchical Addressing

° Hierarchical addressing solves the problem of devices

communicating across networks of networks

TO: Jane Doe 170 West Tasman Drive, San Jose, CA 95134, USA

Which Which zip

At each step of delivery, the post office need only examine the next hierarchical level

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° Router only need to know network portion(network

address) to indicate the best path to reach the

N etwo rk Hierarchical IPv4 Address

< 8 bits —> = 8 bits — <— 8 bits —> = 8 bits —>

(Octet) (Octet) (Octet) (Octet)

Network Host

Complete IPv4 Address

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Routing Protocols

° Routing is an OSI Layer 3

7 Application function It is a hierarchical

6 Presentation scheme and allows individual

addresses to be group

3 Network <<— | Routing ° Routing is the process of

2 Data Link finding the most effiefent path

The network layer is responsible for routing packets through a network

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The network layer is responsible for routing packets through a network

e Two key functions of a router:

— Maintain routing tables and make sure other routerS*know‘ef changes in

the network topology

— Use the routing table to determine where to sendthem

° Routing metrics are values used in determining the advantage of one route

over another They use various combinations of metrics for determining the

best path for data

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Routing vs Switching

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Routing and Layer 2 Switching

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ee | LAN Layer 2 switching

based on MAC address based on MAC address

Layer 2 switching takes place within the LAN Layer 3 routing moves traffic between broadcast domains This requires the hierarchical addressing format

that a Layer 3 addressing scheme like IP provides

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Router and Switch

Features Router Switch

OSI =— Layer 3 Layer 2

The speed and security are relative comparisons, and depend on the

configurations of the device

e Each computer and router interface maintains an ARP table for hayer 2 communication The ARP table is only effective for the:broadcast

domain (or LAN) that it is connected to

e MAC addresses are not logically organized,ut IP@ddresses are

organized In a hierarchical manner

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Routed versus Routing

Routed protocol transport data from one end-station to another

° Routed protocol: used at the network layer that transfer,data from one host

to another across a router

° Routing protocols: allow routers to choose the bestath for data from

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Network Destination Exit are used between

Protocol Network Interface | oiters to determine paths

1012020 ¬ and maintaining routing tables RIP ni can at After the path is determined

IGRP -10.0 a router can route a routed protocol!

Routing protocol = RIP, IGRP

e Provides processes for sharing route information

° Allows routers to communicate with other routers to-Update and maintain the routing tables

e Examples: Routing Information Protocol (RIP),-<interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway

Protocol (BGP) and Enhanced IGRP (EIGRP)

Tiêu đề	Osi Network Layer Ccna Exploration Version 4.0
Trường học	Học viện mạng Bách khoa
Thể loại	Tài liệu

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