Section 4 : The OSI Network Layer

CSIS 479R Fall 1999

“Network +”

George D. Hickman, CNI, CNE

Objectives

Identify the basic purpose of the OSI Network layerIdentify the uses of the addresses defined at the Network LayerIdentify the characteristics of the three switching methodsIdentify the characteristics of the two route discovery methods

Objectives (con’t)

Identify the characteristics of the two route selection methodsIdentify the characteristics of Network layer connection servicesIdentify the basic purpose of gateway servicesDescribe why connectivity devices are needed on networks

Objectives (con’t)

Describe the function of Ethernet and token ring repeaters

Describe the function of bridges

Explain how a transparent bridge works

Explain how a source-routing bridge works

Explain how switching-hubs work

Objectives (con’t)

Describe how token ring switches work

Describe routing protocols

Identify nonroutable protocols and how to deal with them

Describe combination devices

Decide when to use bridges,switches, and routers

Basic Purpose of theOSI Network Layer

RoutingMoving information between multiple

independent (often dissimilar) networks called internetworks

Routes Data using:SwitchingNetwork Layer Addressing (Logical, not physical or MAC)

Routing Algorithms

Processes and Methods

Logically separate networks must have unique network addressesSwitching determines how connections are madeImplement routing to have “best” data path determinedDifferent levels of connection services can be implemented, based on errors expected

Processes and Methods (con’t)

AddressingLogical NetworkService

SwitchingPacketMessageCircuit

Processes and Methods (con’t)

Route DiscoveryDistance VectorLink-state

Route SelectionStaticDynamic

Processes and Methods (con’t)

Connection servicesNetwork-layer flow controlError controlPacket sequence control

Gateway servicesNetwork layer translation

Network Layer AddressingLogical network address Logically distinguish two networks in an

internetwork Routers connect two networks with different logical

network addresses

Service address Every entity (hardware or software) needs its own

address (port or socket) to send and receive data Ports identity an upper layer software process or

protocol. (Computers can run several concurrently) See figure 4-3 on page 4-10

Switching Method Characteristics

Circuit Switching A Single path is set up for the duration of a single

conversation.

Message Switching Conversations are divided into messages, each of

which MAY take a different path.

Packet Switching Combination of both Messages broken into packets small enough to fit

in RAM. A sequence number is added to each packet Most LANs use Packet Switching

Packet Switching

Virtual Circuit Packet SwitchingLogical connections between sender and

receiverAppears as a point-to-point link, but is actually

a logical switched pathDatagram Packet Switching

Used when no logical connection is required In common use on the internetRelies on the network layer to navigate paths

and correct errors

Route Discovery TermsHop Count Number of routers data must pass through to

reach the destination

Tick count Amount of time required to reach the destination

network

Relative expense An assignable number based on user-defined

costs

Route discovery uses two methods Distance Vector Link-state

Distance VectorRoute Discovery Method

Routers keep tables of routes to various destinations

These tables are communicated to neighboring routers

If router A tells router B that it can reach network 100 in 6 hops, router B would show it can reach network 100 in 7 hops

Updating routing tables is called convergence

Link StateRoute Discovery Method

Considered more intelligent and faster than distance-vectorAfter an initial table is received, routers only broadcast information that has changed (not all routes)Link State Route Discovery requires less network overhead, but can require considerable planning

Route Selection Methods

DynamicRouters can change in response to

changing network conditionsRoute selections are made at each router

StaticData packets always follow a

predetermined pathPath defined by network administrator or

by an assigned network device

Network LayerConnection Services

Data Link-LLC sublayer connection services control the amount of data transmitted and notification of bad frames

Data Link layer connection services manage communication between source and destination devices

Network Layer connection services build on these services, add reliability

Connection Services

Network Layer Flow ControlSimilar to Data Link layer flow control

Can be guaranteed rate or static/dynamic windows

Can involve intelligent path selection (congestion control)

Connection Services (Con’t)

Network Layer Error ControlPrimarily concerned with packet loss,

duplicate packets, altered packetsLost packets handled by acknowledgmentsDuplicate packets are handled by dropping all

but one of the packetsAltered packets are detected (sometimes

corrected) by use of a CRC check CRC is calculated at each router, since the address

field is changed at each hop

Connection Services (Con’t)

Network Layer Packet Sequence ControlUpper layer messages are rebuilt by

putting arrived packets back into the original order

Packet sequence control can take place at Network layer, but at the Transport layer is more common.

Gateway ServicesA Gateway interprets and translates the rules on two separate networksAddress, Route Discovery, Route Selection, and Connection Service rules may vary from one network to anotherBook example: Two networks use data units of different sizes. A Network Layer gateway fragments and reassembles data into acceptable sizes for both networksThe Default Gateway or Default Router is where all packets are sent that are destined for devices not on the local network segment

Connectivity Devices

LAN Media limitations Media may limit number of devices, length of cable

run, or available bandwidth Repeaters and Bridges address these issues

Data destination across multiple networks Routers forward data from network to network

Incompatible Systems Two systems without a common protocol Gateway acts as a translator

Bridges, Repeaters, Switching Hubs, Routing Protocols

Are covered in the book; we will not have class time to cover them

Connection DevicesBroutersBridge and Router in a single deviceCheaper than both devices, more

expensive than either one

Switching Hub and Router CombinationLeading-edge technology, which means

implementation problems

Nonroutable Protocols

Protocols that do not use Network Layer for routingProtocols that use only static routes that cannot be updatedDeal with them by encapsulation (tunneling) or bridging. Routing SNA NetBIOS NetBEUI LAT

Network Layer

This is a major layer in the OSI model, and has many important features we were unable to discuss in class

It is critical that you read the material in the book. Email me if you have any questions about the content.