networking theory

7/30/2019 Networking Theory

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Networking Theory

(Part 1)


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Introduction

Overview of the basic concepts of

networking

Also discusses essential topics ofnetworking theory


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What is a Network?

A network is a collection of devices that

share a common communication protocol

and a common communication medium. Devices - computers, printers, telephones,

televisions, coke machines, etc.


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What is a Network?

Computing-centric model - services and

devices bound to individual machines

Network-centric model - services anddevices are distributed across a network

Network and software standards (e.g. Jini)

exist to allow devices and hardware talk toeach other over networks and to allow

instant plug-and-play functionality


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What is a Network?

Besides devices providing services, thereare also devices that keep the networkgoing, for example,

Network cards - to allow a computer to talk toa network. E.g. ethernet card.

Routers - machines that direct data to the

next "hop" in the networkHubs - allow multiple computers to access a

network

Gateways - connect one network to another.

E.g. a LAN to the Internet.


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How do Networks Communicate?

Networks consist of connections between

computers and devices.

Connections:

Wires and cables - use electricity for

transmitting data

Wireless - use infrared / radioFiber-optic cables - use light


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How do Networks Communicate?

Connections carry data (bits - 0's and 1's)

between one point (node) in the network

and another. For data to be successfully delivered to

individual nodes, these nodes must be

clearly identifiable.


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Addressing

Each node in a network is typically

represented by an address.

The manufacturer of the network interfacecard (NIC) is responsible for ensuring that

no two card addresses are alike, and

chooses a suitable addressing scheme. Each card will have this address stored

permanently, so that it remains fixed.


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Addressing

There are many addressing schemesavailable. E.g. Ethernet network cards are

assigned a unique 48-bit number. This physical address is referred to by

many names, such as:

Hardware address

Ethernet address

Media Access Control (MAC) address

NIC address


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Addressing

Often, machines are known by more than

one type of address. E.g. a network

server may have a physical Ethernetaddress as well as an Internet Protocol

(IP) address, or it may have more than

one network card. For inter-network communications, the IP

address is used.


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Data Transmission Using Packets

Sending individual bits of data from node

to node is not very cost effective.

Overhead involved - e.g address ofdestination node.

Most networks group data into packets.


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A packet consists of a header and data

segment.

Header fields Data 1101000111010100001

The header contains:

Addressing information (e.g sender & recipient)Checksums to ensure packet has not been

corrupted

Other info needed for transmission across

network


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To transmit data, a direct connection is

usually not available. So packets are sent

to their destination nodes via intermediarynodes in the network.

Due to network conditions (such as

congestion or network failures), packetsmay take arbitrary routes, and sometimes

may be lost or arrive out of sequence.


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Packet transmission and transmission of

raw bits are low-level processes.

Most network programming deals withhigh-level transmission of data.


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Communication Across Layers

The concept of layers was introduced to

acknowledge and address the complexity

of networking theory. The most popular approach to network

layering is the Open Systems

Interconnection (OSI) model created bythe International Standards Organization

(ISO)


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The OSI model groups network operations into

seven layers.


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Each layer is responsible for some form of

communication task, but each task is

narrowly defined and usually relies on theservices of one or more layers beneath it.

Generally, programmers work with one

layer at a time; details of the layers beloware hidden from view.


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Layer 1 - Physical Layer

This layer is network communication at itsmost basic level.

At this level, networking hardware transmitsequence of bits between two nodes.

Java programmers do not work at this

layer - it is the domain of hardware driverdevelopers and electrical engineers.

No real attempt is made to ensure error-free data transmission


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Layer 2 - Data Link Layer

This layer is responsible for providing amore reliable transfer of data, and for

grouping data together into frames. Frames are similar to data packets but are

blocks of data specific to a single type ofhardware architecture.

Frames have checksums to detect errorsin transmission.

Corrupted frames are discarded so that

they will not be passed to higher layers.


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Layer 3 - Network Layer

The network layer deals with data packets

which are sent across the network.

Communication at this level is still verylow-level; network programmers are rarely

required to write software services for this

layer.


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Layer 4 - Transport Layer

This layer is concerned with controlling

how data is transmitted.

It deals with issues such as automaticerror detection and correction, and flow

control (limiting the amount of data sent to

prevent overload).


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Layer 5 - Session Layer

The purpose of this layer is to facilitateapplication-to-application data exchange,

and the establishment and termination ofcommunication sessions.

Connection-oriented communication canincrease network delays and bandwidth

consumption. Some applications chooseto use a connectionless form ofcommunication.


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Layer 6 - Presentation Layer

This layer deals with data representation

and data conversion.

Different machines use different types of datarepresentation (e.g. 8-bit integers on one

system and 16-bit integers on another).

Data compressionData encryption


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Layer 7 - Application Layer

This layer is where the vast majority of

programmers write code.

Protocols for this layer dictate thesemantics of how requests for services are

made (e.g requesting a file).

In Java, almost all network softwarewritten will be for this layer.


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Advantages of Layering

Helps simplify networking protocols.

Protocols can be designed for

interoperabilitySoftware that uses Layern can talk to

software running on another machine that

supports Layern, regardless details of thelower layers.

Example: a network layer protocol can work with an

Ethernet network and a token ring network.


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Networking Theory

(part 2)


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Internet Architecture

The Internet is a worldwide collection of

smaller networks that share a common

suite of communication protocols(TCP/IP).

It is an open system, built on common

network, transport and application layerprotocols, while granting the flexibility to

connect a variety of computers, devices

and operating systems to it.


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Design of the Internet

The Internet is the result of many decades

of innovation and experimentation.

The TCP/IP protocols have been carefullydesigned, tested and improved over the

years.


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Design of the Internet

Major design goals:

Resource sharing between networks

Hardware and software independence

Reliability and robustness

Fault tolerant protocols - data could be rerouted

depending on the state of the network

"Good" protocols that are efficient and simple.


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TCP/IP Protocol Suite

Major protocols:

Internet Protocol (IP)

Internet Control Message Protocol (ICMP)

Transmission Control Protocol (TCP)

User Datagram Protocol (UDP)


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IP is a Layer 3 protocol (network layer)

It is used to transmit data packets over the

Internet

It is the most widely used networking

protocol in the world.

IP acts as a bridge between networks ofdifferent types


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IP is a packet-switching network protocol.

Information is exchanged between twohosts in the form of IP packets (IP

datagrams). Each datagram is treated as a discrete

unit - there are no "connections" between

machines at the network layer. Connection services are provided by the

higher-level protocols at the transportlayer.


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The IP datagram consists of a header and

the actual data being sent.

The header contains essential informationfor controlling how it will be delivered.


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IPV4 datagram format


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Although each machine has its own physical

address, each host machine under the Internet

Protocol must be assigned a unique IP address.

The IP address is a four-byte (32-bit) address.

Example: 192.168.1.5

The IP address is not bound to a particular

physical machine. Network programming in Java does not require

the use of the physical address; only the IP

address is used.


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Humans do not find IP addresses easy to

remember.

An addressing scheme is also used whichallows the use of textual names

(hostnames) instead of numerical values.

Example: java.sun.com


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Internet Control Message Protocol

(ICMP) The Internet Protocol provides absolutely

no guarantee of datagram delivery.

The Internet Control Message Protocol(ICMP) is a mechanism for error-control. It

is used in conjunction with the Internet

Protocol to report errors when and if theyoccur.


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(ICMP) The relationship between IP and ICMP is astrong one.

E.g: IP uses ICMP if it needs to notify another

host of an error. ICMP requires IP to send theerror message.

Note that a host cannot rely solely onICMP to guarantee delivery as there is noguarantee that ICMP messages will besent or that they will reach their intendeddestination.


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(ICMP) Five error messages are defined:

Destination Unreachable

If a gateway is unable to pass a datagram on to itsdestination, this message is sent back to the

original host.

Parameter Problem

This message is sent to the sending host if agateway is unable to process the header

parameters of an IP datagram.


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(ICMP)Redirect If a shorter path, or alternate route, is available, a

gateway may send this message to the router thatpassed on a datagram

Source Quench This message may be sent in an attempt to reduce

the number of incoming datagrams when a router,gateway or host becomes overloaded.

Time Exceeded Whenever the TTL value of a datagram reaches

zero is discarded. This message may be sent ifthis event occurs.


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(ICMP) ICMP supports several informational

messages such as:

Echo Request/Echo Reply Used to determine whether a host is alive and can

be reached.

Address Mask Request/Address Mask Reply

Provides the functionality to determine the address

mask which controls which bits of an IP address

correspond to a host, and which bits determine the

network/subnet portion.


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Transmission Control Protocol

(TCP) TCP is a Layer 4 protocol (transport layer)

that provides guaranteed delivery and

ordering of bytes. TCP uses IP to send TCP segments,

which contain additional information that

allows it to order packets and resend themif they go astray.


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Transmission Control Protocol

(TCP) TCP uses communication ports to

distinguish one application or service from

another.A host machine can have many

applications connected to one or moreports.

Although TCP provides a simplerprogramming interface, it may reducenetwork performance.


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User Datagram Protocol (UDP)

UDP is a Layer 4 protocol (transport layer)that applications can use to send packetsof data across the Internet (as opposed to

TCP, which sends a sequence of bytes). UDP also supports communication ports.

UDP does not guarantee delivery packets.

It also does not guarantee that they willarrive in the right order.

Although unreliable, UDP offers fastercommunication.


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Internet Application Protocols

Network programmers are more interested

in the protocols at the application layer.

Examples:Protocols for accessing and sending email

Protocols for transferring files

Protocols for reading Web pages


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Internet Message Access Protocol (IMAP)

Less popular than POP3 as it requirescontinual connection to the mail server.

Message are stored on a server and not on

the user's system.

Uses port 143.

Simple Mail Transfer Protocol (SMTP)

Allows messages to be delivered over the

Internet.Uses port 25.


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HyperText Transfer Protocol (HTTP)

One of the most popular protocols in use onthe Internet; it made the World Wide Web

possible.

Java provides good HTTP support.

Uses TCP port 80.


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TCP/IP Protocol Suite Layers

Internet Protocol

TCP UDP ICMP

HTTP SMTP FTP POP3

TCP/IP Stack


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TCP/IP Protocol Suite Layers

POP3 command: RETR1

TCP segment:

IP datagram:

Modem frame:

Carrier wave:

Application layer

Transport layer

Network layer

Data link layer

Physical layer

TCP header RETR1

IP header TCP header RETR1

IP header TCP header RETR1Modem header


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Security Issues: Firewalls and

Proxy Servers Firewall: while being an excellent tool for n/w

admin often is a developers worst enemy

It blocks direct UDP and TCP access, making

application that uses these protocols practicallyunusable

Need to adapt software to proxy requests usingprotocol such as HTTP

Direct UDP/TCP communication is simpler andoffers better performance

Using proxy server and HTTP can add delays


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Proxy Server

It is a machine that acts as a proxy forapplication protocols

The server accepts incoming connections frommachines within a local n/w and makes requestson their behalf to machines connected to theInternet

Advantages:Direct access to internal machines is never

established


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Proxy Server

Only popular protocols such as HTTP is

permitted access while newer application

such as games or RealAudio is notpermitted

Most proxy servers also log networking

events to allow n/w admin to track unusualcommunications and their origin

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