CN unit 2_2

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Computer networks

Name: K.SUDHA

Designation: Lecturer

Department: Electrical and Electronics Engineering

Subject code: CS2361

Year: III

Unit: II

Title: Introduction to network layers

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Internetworking

An internetworkis a collection of individual networks, connected by

intermediate networking devices, that functions as a single large

network.

different kinds of network technologies that can be interconnectedby routers and other networking devices to create an internetwork

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Types

Local-area networks (LANs)enabled multiple users in a relatively small

geographical area to exchange files and messages, as well as access

shared resources such as file servers and printers.

Wide-area networks (WANs) interconnect LANs with geographicallydispersed users to create connectivity.

technologies used for connecting LANs include T1, T3, ATM, ISDN,

ADSL, Frame Relay, radio links, and others.

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ETH

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IPV4 Packet Header

Version HLen TOS Length

Ident FlagsOffset

TTL Protocol Checksum

SourceAddr

Destination Addr

Options(variable) Pad(variable)

Data

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Datagram Delivery

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Packet Format

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IPV4 Packet header

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Fragmentation and Reassembly

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(RARP)Reverse Address Resolution

Protocol (RARP) is a Link layer networking protocol

RARP is described in internet EngineeringTask ForceETF) publication

RFC 903

It has been rendered obsolete by the Bootstrap Protocol (BOOTP) and

the modern Dynamic Host Configuration Protocol(DHCP) BOOTP configuration server assigns an IP address to each client from a

pool of addresses.

BOOTP uses the User Datagram Protocol (UDP)

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RouterA router is a device that determines the next network pointto which a packet should be forwarded toward itsdestination

Allow different networks to communicate with each other

A router creates and maintain a table of the availableroutes and their conditions and uses this information todetermine the best route for a given packet.

A packet will travel through a number of network pointswith routers before arriving at its destination.

There can be multiple routes defined. The route with alower weight/metric will be tried first.

Routing

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Routing

Routing

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Routing ProtocolsStatic Routing

Dynamic Routing

IGP (Interior Gateway Protocol): Route data within an Autonomous System

RIP (Routing Information Protocol)

RIP-2 (RIP Version 2)OSPF (Open Shortest Path First)

IGRP (Interior Gateway Routing Protocol)

EIGRP (Enhanced Interior Gateway Routing Protocol)

IS-IS

EGP (Exterior Gateway Protocol): Route data between AutonomousSystems

BGP (Border Gateway Protocol)

Routing

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Iskra Djonova-Popova

The Routing Algorithm

u the shortest path tree is contained

in the routing table

u Calculations are based on the

Bellman-Ford algorithm

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The Centralized Version of the

Algorithm

Cycle Node B C D E

Initial (., ) (., ) (., ) (., )

1 (1, 1) (2, 2) (3, 1) (4, 2)

A B C

D E

2

3 45

6

1A B C

D E

2

3 4

1

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The Distributed Version

A B C

D E

1 2

3 4

5

6

Example of simple network with 5 nodes (routers)

and 6 links (interfaces)

The cost of all links is assumed to be 1

From A to Link Cost

B 1 1C 1 2

D 3 1

E 1 2

Routing table for A

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Advantages

simple to implement

low requirement in processing and memoryat the nodes

suitable for small networks

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Disadvantages

Slow convergence

Bouncing effect

Counting to infinity problem

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Slow Convergence

A B C

D E

2

3 4

5

6

XXX

When a link breaks the routers are supposed to reestablish the routing tables

link 1 breaks

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The Bouncing Effect

link 2 breaks and A sends its routing

table to B before B sends it to A

A B C

D E

3 4

5

6

XXX

1

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Counting to Infinity Problems

D E

3

A B C2

4

5

XXX

XXX

Links 1 and 6

break.

A sends its old routing table

before D sends the new routing table

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Subnets

Each organization assigns IP addresses tospecific computers on its networks

IP addresses are assigned so that all

computers on the same LAN have similaraddresses

Each of these lans is known as a TCP/IP subnet

Any portion of the IP address can bedesignated as a subnet using a subnet mask*

* Subnet masks tell computers what part of an IP address is to be used to

determine whether a destination is on the same or a different subnet

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Subnet Addressing

Figure 5-6

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Subnet Addressing

Example 1

Suppose that the first two bytes are the subnet indicator with addressesof the form 131.156.x.x

Then, 131.156.29.156 and 131.156.34.215 would be on the samesubnet.

The subnet mask would be 255.255.0.0, which corresponds to

11111111.11111111.00000000.00000000, where 1 indicates thatthe position is part of the subnet address and a 0 indicates that itis not.

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Subnet Addressing

Example 2

Partial bytes can also be used as subnets.

For example, consider the subnet mask 255.255.255.128, which is

11111111.11111111.11111111.10000000.

Here, all computers with the same first three bytes and last byte from

128 to 254 would be on the same subnet.

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Providing Addresses

Providing addresses to networked

computers

Static addressing

Dynamic addressing

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Static Addressing

Each computer is given an address through aconfiguration file

Stored on individual computers

Problems Moves, changes, adds and deletes

Individuals could change their own IP address

Network renumbered

Companies do not have a good way of trackingthe addresses

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Dynamic Addressing

Server supplies a network layer addressautomatically Each time user logs in

For a specific lease period

Two standards for dynamic addressing Bootstrap protocol (bootp) - developed in 1985

Dynamic host control* protocol (DHCP), developedin 1993

* some say C = configuration

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Dynamic Addressing

Bootp and DHCP

Software installed on the client instructs the

client to contact the server using data link layer

addresses

Message asks server to assign the client a

unique network layer address

Server runs corresponding software that sendsthe client its network address and subnet mask

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Leasing

Bootp or DHCP server can either:

Assign the same network layer address to the

client each time the client requests it (bootp)

Lease the network address from the next

available on a list of authorized addresses for

as long as the client is connected or for a

specified amount of time -- common with ispsand dial-up users (DHCP)

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Address Resolution

The process of:

Translating an application layer address to a

network address (server name resolution)

Translating the server name address to a data

link layer address (data link layer address

resolution)

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Address Resolution

Server name resolution

Accomplished by the use of domain name

service (DNS)

Computers called name servers provide these

DNS services

Address data base includes: server names and their

corresponding IP address

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Client

computer

DNS Servernetmgr.cso.niu.edu

131.156.1.11

DNS Request

LAN

LAN

Internet

DNS Request

Root DNS Server

for .EDU

domain

Weber State University

Northern Illinois University

DNS Request

DNS Response

DNS Response

DNS ResponseDNS Server

sol.acs.unt.edu

137.90.2.122

Figure 5-7

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Address Resolution

Data link layer address resolution

Broadcast message is sent to all computers in

its subnet

if your IP address is xxx.Yyy.Zzz.Ttt, please send

your data link layer address

Uses address resolution protocol (ARP)

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Network Routing

The process of determining the route amessage will take through the network

Centralized

Decentralized Static routing

Dynamic routing

Broadcast or multicast routing

Connectionless

Connection-oriented routing

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Route and Route Table

Computer B

Destination Route

A A

C C

D AE E

F E

G C

A

BC

G

E

D F

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Internet Routes

UEN

WSU CanadaOther destinations

West Coast

Europe Asia

WSU

Destination RouteUEN Utah

Oxford Europe

U of Toronto Canada

U of Singapore Asia

UC Stanford West Coast

Other Other

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Types of Routing

Centralized routing

Static routing (decentralized)

Dynamic routing (adaptive and

decentralized) Distance vector

Link state

Other types Broadcast routing

Multicast routing

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Centralized Routing

All routing decisions are made by one computer

Main routing for star and mesh topologies

Routing tables located on each computer

Central computer sends updated tables as needed

Routing table tells the device where to send

messages

Simplicity - no wasted resources Hardware failures or changing conditions cause

table to be out of sync

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Decentralized Routing

Each of the following types of routing fall under the

heading of decentralized routing

Each device makes its own routing decisions with the use

of a formal routing protocol Routing protocols are self-adjusting

Can automatically adapt to changes in the network configuration

Drawbacks

Slows down the network with status messages Requires more processing by each computer

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Static Routing

Routing table developed by the networkmanager or some type of committee

Initial table sent to each computer which then

updates the routing table as needed Reroutes as needed with down or removed

circuits

Updated when new devices announce their

presence

Used in relatively static networks that have fewrouting options

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Dynamic Routing (Adaptive)

Routing messages over the fastest route Used when there are multiple routes in the

network Improves network performance by selecting the

fastest route to avoid bottlenecks or busy circuits

Initial table developed by network manager Dynamically updated with changing conditions by

the devices themselves Monitors message transmission time or each

device reports how busy it is to avoid bottlenecks

Disadvantages Requires more processing by each computer Wastes network capacity

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Dynamic Routing (Adaptive)

Distance vector dynamic routing The number of hops along a route

Exchange information with the neighboring computers every few

minutes

Link state dynamic routing The number of hops along a route

The speed of the circuits on the route

How busy the route is

Exchanges information with other routing devices every 15-30

minutes

Tries to determine the fastest route

Converges reliable routing information more quickly

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Routing Protocols

RIP, IGP, OSPF, EGP, BGP

Distance vector routing protocols (RIP,Appletalk,IPX, IGRP)

Routers inform neighboring routers of table Closest router is used to route packets

Link State routing protocols (OSPF)

Routers have at least a partial map of the network Changes are flooded throughout network

Routes are recomputed

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Interior and Exterior Routing

Interior routing is within an autonomoussystem (collection of routers under a singleadministrative control)RIP, OSPF

Exterior routing occurs between autonomoussystems

Network access protocols operate at Layer 2.

Transport of IP datagrams IP over point-to-point connections is used by ISP

when you dial in

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Routing Protocols

Border Gateway Protocol

Internet Control Message Protocol

Routing Information Protocol

Open Shortest Path First

Enhanced Interior Gateway Routing

Protocol

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Routing Protocols

Internet protocols BGP (border gateway protocol)

Exchanges information between autonomous systemsabout the condition of the internet

Complex, hard to administer, exterior routing protocol

ICMP (internet control message protocol) Simple, interior routing protocol used with the internet

Reports routing errors but is limited in the ability toupdate

RIP (routing information protocol) Dynamic distance vector interior routing protocol

Counts the number of devices on each route

Selects the route with the least number of devices

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Routing Protocols

OSPF (open shortest path first) Link state interior routing protocol used on the internet

Counts number of computers, network traffic, network errorrates to select the best route

Doesnt broadcast to all devices just to routing devices

Preferred TCP/IP, but also used by IPX/SPX

EIGRP (enhanced interior gateway routing protocol) Link state interior routing protocol developed by CISCO

Uses route transmission capacity, delay, reliability and load toselect best route

Stores multiple routing tables SAP (service advertisement protocol)

Netware servers send SAP advertisements

Novells broadcast protocol

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Broadcast Routing

Sends the message to all computers on the

network

Only computer with correct addressprocesses the message

Used only in bus networks

Wastes network bandwidth

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Multicasting

Similar to broadcasting Only works within one LAN or subnet

Messages sent from one computer to another on the networkis called a unicastmessage

Messages sent to a group of computers is called a multicast

message Targeting a specific work group

IGMP (internet group management protocol) Sends an IGMP multicast request to the routing computer

Assigned a special class D IP address to identify the group

The routing computer sets the data link layer address All participating machines will process messages sent to this

address

Sends a IGMP message notifying of end of session

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CN unit 2_2

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