computer networks unit-8

8/7/2019 Computer Networks UNIT-8

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C o n f i d e n t i a l

Course : BCA

Semester : IV

Subject Code : BC 0048

Subject Name : Computer Networks

Unit number : 8

Unit Title : Network Layer

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Unit-8 Network Layer

Network Layer

Objectives

After going through the presentation, you should be able to:

Describe Design issues of Network layer .

Discuss Principles of Routing

Discuss Routing algorithms

Explain congestion control

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Lecture outline

Introduction

Design issues of Network layer

Principles of Routing

Routing algorithms

Congestion

IP protocol (IPV4)

Summary

Network Layer

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Introduction

The network layer is one of the most important layers in OSI seven

layered architectural model and in TCP/IP reference model.

It is responsible for end to end (source to destination) packet delivery,

whereas the data link layer is responsible for node to node (hop to hop)

packet delivery.

The Network layer performs network routing, flow control, network

segmentation/de-segmentation, and error control functions.

The network layer deals with transmitting information all the way from its

source to its destination - transmitting from anywhere, to anywhere.

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The important design issues pertaining to the network layer are as

follows :

1 Nature of the service provided :

The network layer provides two well-defined classes of services, namely

connectionless and connection oriented services to the numerous nodes

or hosts communicating through subnet.

2. Internal organization :

Another important design issue in the network layer is the internal

organization of the subnet. The organization may be either a datagram

based that employs connectionless services or virtual circuit based that

employs a connection-oriented service.

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The important design issues : ( CONTINUED..)

3 Routing :

Routing of packets across the subnet is considered the most important

function of the network layer. Two basic types of routing static and

dynamic .

Static routing is also referred to as non-adaptive and dynamic is also

referred to as Adaptive routing methods.

4. Congestion control :

An important problem that any subnet faces is the congestion because of

heavy traffic in the subnet than the load that it can handle. The subnet

gets increasingly loaded with packets causing increase in delay in the

delivery of packets, which can lead to more retransmission, and

ultimately increasing more and more traffic.

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The important design issues : ( CONTINUED..)

5. Internetworking :

A subnet can be considered as an interconnection of multiple subnets.

The network layer now has to perform a job of internetworking among

the subnets.

Different subnets may use

Different routing and congestion control techniques,

Different packet length

Different frame format

Different data rates etc.

Hence internetworking is a very crucial job. For this purpose a separate

sub-layer has been created as the top part of the Network layer within an

OSI model in order to take care of internetworking task.

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The main function of the network layer is routing packets from the source

machine to the destination machine. In computer networking the term

routing refers to selecting paths in a computer network along which to

send data.

There are two types of routing algorithm depending up on the

communication subnet.

Session routing: if subnet uses virtual circuits

Forwarding: if subnet uses datagram

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Classes of routing algorithms :

Routing algorithms can be grouped into two major classes.

1. Static or Non-adaptive Routing algorithms

2. Dynamic or Adaptive Routing algorithms

Static or Non-adaptive Routing algorithms do not base their routingdecisions on measurements or estimates of the current traffic and

topology. The choice of the route to use to get from source to destination

or from one node to other is computed in advance

Dynamic or adaptive Routing algorithms in contrast change their routing

decisions to reflect changes in topology and usually the traffic as well.

Dynamic routing attempts to solve this problem by constructing routing

tables automatically.

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Properties of routing algorithms :

Correctness

Simplicity

Robustness since usually the entire network cant be re-booted

Stability routing algorithm reaches equilibrium in a reasonable time

Fairness, and

Optimality

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Routing algorithms

Different routing Algorithms :

1. Shortest path algorithm :

It is the simplest and widely used algorithm.

It is a static algorithm.

2. Flooding :This is also a static algorithm in which every incoming packet is sent out

on every outgoing line except the one it arrives on. This method usually

generates vast number of duplicate packets.

3. Distance vector routing :

It is a dynamic routing algorithm. Distance vector routing algorithm

consists of a data structure called a routing table. Distance vector

algorithms use the Bellman-Ford algorithm.

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Routing algorithms

Different routing Algorithms : (continued..)

4. Hierarchical routing :

When hierarchical routing is used, the routers are divided into regions.

Each router knows all details about how to route packets to destinations

within its own region. But it does not have any idea about internalstructure of other regions.

5. Link state routing :

When applying link-state algorithms, each node uses as its fundamental

data a map of the network in the form of a graph.Using this map, each router then independently determines the least-cost

path from itself to every other node using a standard shortest paths

algorithm such as Dijkstra's algorithm.

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Congestion

When does congestion occurs ?

Network congestion occurs when a link or node is carrying so much data

that its quality of service deteriorates. Typical effects include queuing

delay, packet loss or the blocking of new connections.

Modern networks use congestion control and network congestion

avoidance techniques to try to avoid congestion collapse. These include:

exponential back off in protocols such as 802.11's CSMA/CA and the

original Ethernet, window reduction in TCP, and fair queuing in devices

such as routers.

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Congestion

Factors of congestion :

Congestion can be brought by many factors which are listed below:

If all of a sudden, streams of packets begin to arrive from most of

the input lines and all need the same output line.

If there is insufficient memory to hold all the packets that arrive,

then packets may be lost and increase traffic because of

retransmission of those dropped packets.

Adding more memory also increases congestion.

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Congestion

Factors of congestion : (continued..)

Slow processors also cause congestion. As queues can be build up,

even though there is excess line capacity.

Low bandwidth lines also cause congestion.

Finally congestion tends to feed upon itself and become worse.

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IP protocol (IPV4)

IP protocol :

An IP datagram consists of two parts

Header: 20 byte fixed

Optional: variable to a maximum of 40 bytes.

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IP protocol (IPV4)

Version field: It is 4 bits long. It keeps track of which version of the

protocol the datagram belongs to. Here for IPV4 the value it takes is

4.

IHL: It is 4 bits long. It is provided to tell how long the header is in

units of 32-bit words. The minimum value is 5 and maximum is 15,which implies header to be maximum of 60 bytes..

Type of service: It is 6 bits long. It allows the host to tell the subnet

what kind of service it wants.

Total length: specifies length of the datagram including header and

data. The maximum value is 65535 bytes.

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IP protocol (IPV4)

Identification field: It specifies the fragment number.

DF: the data gram should not be further broken down into fragments.

MF: all fragments except the last one have this bit set.

Time to live field: is a counter used like the hop counter discussed

in flooding. It is used to limit the packet life time allowing a maximum

of 255 sec.

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IP protocol (IPV4)

Protocol: tells which of the protocols are used at Transport layer

(TCP or UDP)

Header checksum: this checksum verifies the header only.

Address field: it uses 32 bit addressing called an IP address.

Option field: is variable length. It was provided to permit

experiments to try out new ideas.

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Summary

The network layer provides service to transport layer.

It can be based on the virtual circuits or datagrams.

Apart from this there are pother major task that are carried out by the

network layer such as routing.

Routing is a very complex task and are of two types. One of which is

static routing, which does not take any changes in topology or traffic

variations into account for decisions.

And the other is dynamic or adaptive, that considers the topology

changes and traffic variations into account for decisions.

Finally another important task taken care is congestion control.

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computer networks unit-8

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