02computer Network Basics

7/28/2019 02computer Network Basics

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Computer NetworkBasics

An overview of computer networkingwhich introduces many key conceptsand terminology. Sets the stage forfuture topics.


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Components of any Computer

Processor(active)

Computer

Control (brain)

Datapath (brawn)

Memory (passive)

(whereprograms,

data livewhenrunning)

DevicesInput

Output

Keyboard,Mouse

Display ,Printer

Disk,Network


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Communication Devices

Synchronous communication uses a clocksignal separate from the data signal-communication can only happen during the

tick of the timing cycle Asynchronous communication does not usea clock signal- rather, it employs a startand stop bit to begin and end the irregulartransmission of data


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Connecting to Networks (andOther I/O)

Bus - shared medium of communication that

can connect to many devicesHierarchy of Buses in a PC


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Operating systems

Developer or manufacturer Operating system

Apple Computers Inc. Mac OS 8/9/X

AT&T Bell Laboratories Unix

Be Inc. beOS

Berkeley University BSD, FreeBSD

Carnegie-Mellon University Mach 3.0

Cisco Systems Inc. IOS

HP HP-UX

IBM AIX and OS/2

Linus Thorvald Linux

Microsoft Windows XP, Vista

Novell NetWareSanta Cruz Operation Inc. (SCO) SCO XENIX, SCO UNIX, SCO MPX

Siemens SINIX

Silicon Graphics IRIX

Sun Microsystems Solaris, SunOS, JavaOS


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Operating Systems Developed forPortable Devices

Developer or manufacturer Operating system

Microsoft Windows CE

Microsoft Windows Mobile 6.0

Palm PalmOS

Symbian Symbian OS

RIM (Research In Motion Limited) RIM


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A closer look at network structure:

network edge: applications andhosts

network core: routersnetwork of

networks


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General Architecture of ComputerNetworks

Cloud

Externalnodes

Internal nodes

(or stations)

(swithing devices)


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The Network Core

mesh of interconnectedroutersthe fundamentalquestion: how is datatransferred through net?

circuit switching : dedicated circuit percall: telephone netpacket-switching : data sent thru net indiscrete chunks


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Connection of Networks

networks or subnetworks

router or gateway

node(host,

station)


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Network Topology

a) bus, b) star, c) ring, d) tree structure

a) b) c) d)


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Classification of the networks according

to the connection establishing

Line switched networkPacket switched networkRadiating/data disseminating systemsPoint-to-point connected networks


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Wired media

Telephone lineThin CoaxThick CoaxUnshielded Twisted Pair (UTP)Shielded Twisted Pair (STP)Fibre


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(Data) Reliability

A network service is (data) reliableif the sender application can rely onthe error-free and ordered deliveryof the data to the destinationIn the Internet the reliability canobtained mainly byacknowledgements and

retransmissionIn such a way the losses in theunderlying layers can be retrieved


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Flow-control and CongestionPrevention

Flow-control : to protect thereceiver against the overload

I.e.: the sender (source) sends moredata than the receiver can processit is mainly necessary in link andtransport level

Congestion prevention : to preventthe intermediate nodes against theoverload

it is mainly necessary in network level


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Overload and Congestion

Overload : Too many packets occur in asubnetwork in the same time, whichprevent each other and in such a waythe throughput decreasesCongestion : the queues in the routersare too long, the buffers are full.

As a consequence some packages aredropped if the buffers of the routers areoverloaded

In extreme case: grid-lock , lock-up


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Deadlock

Deadlock : the most serious situation of thecongestion, the routers wait for each other Direct store and forward deadlock : the

buffers of two neighbouring routers arefull with the packets to be sent to theother routerIndirect store and forward deadlock : thedeadlock occurred not between twoneighbouring routers but in a subnetwork,where any of the routers has not freebuffer space for accepting packets


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Network: physical connection that allows two computers tocommunicate

Packet: unit of transfer, bits carried over the networkNetwork carries packets from on CPU to anotherDestination gets interrupt when packet arrives

Protocol: agreement between two parties as to howinformation is to be transmittedBroadcast Network: Shared Communication MediumDelivery: How does a receiver know who packet is for?

Put header on front of packet: [ Destination | Packet ]Everyone gets packet, discards if not the target

Arbitration: Act of negotiating use of shared mediumPoint-to-point network: a network in which every physicalwire is connected to only two computersSwitch: a bridge that transforms a shared-bus(broadcast) configuration into a point-to-point networkRouter: a device that acts as a junction between twonetworks to transfer data packets among them

Review: Networking Definitions


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The Need for a Protocol Architecture

Procedures to exchange data betweendevices can be complexHigh degree of cooperation requiredbetween communicating systems

destination addressing, pathreadiness to receivefile formats, structure of datahow commands are sent/received andacknowledgedetc.


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Layered Protocol Architecture

Modules arranged in a vertical stackEach layer in stack:

Performs related functions

Relies on lower layer for more primitivefunctionsProvides services to next higher layerCommunicates with corresponding peer layer ofneighboring system using a protocol


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Network LayeringLayering: building complex services from simpler ones

Each layer provides services needed by higher layers by utilizing services

provided by lower layersThe physical/link layer is pretty limitedPackets are of limited size (called the Maximum Transfer Unit or MTU:often 200-1500 bytes in size)Routing is limited to within a physical link (wire) or perhaps through aswitch

Our goal in the following is to show how to construct a secure, ordered,message service routed to anywhere:

Physical Reality: Packets Abstraction: MessagesLimited Size Arbitrary Size

Unordered (sometimes) OrderedUnreliable Reliable

Machine-to-machine Process-to-processOnly on local area net Routed anywhere

Asynchronous Synchronous


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Key Features of a Protocol

Set of rules or conventions to exchangeblocks of formatted dataSyntax : data formatSemantics : control information(coordination, error handling)Timing: speed matching, sequencing

Actions: what happens when an eventoccurs


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Operation of Protocols

(interlayer) protocol layerprotocol

Host Host

Physical connection

(n-1). layer protocol entity

(n-1). layer protocol entity

(n+1). layer protocol entity

n. layer protocol entity

(n+1). layer protocol entity

n. layer protocol entity

... ...


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The OSI Model

Physical Layer(Data) Link Layer

Network LayerTransport LayerSession Layer

Presentation LayerApplication Layer


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Physical Layer

Transmission of energy onto themedium

Collection of energy from the medium

This layer is concerned with the physicaltransmission of raw bitsThis bits are transmitted throughmechanical, electrical, and procedural

interfaces which include interface card standard modem standards certain portions of the ISDN and LAN MAN

standards


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(Data) Link Layer

Transmission of frames over one link or networkOften subdivided into the MAC and LLCIt receives bits from the physical layer, converting bitsto frames

frame boundariesUsing protocols (e.g. HDLC), this layer corrects errorsthat might have occurred during transmission across a linkIn addition this layer provides an error -freetransmission channel to the next layer known as thenetwork layer: error control

ARQduplicates

Flow control


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The previous two layers were concerned with getting error-free data across a link The network layer establishes connections between nodes,routes data packets through the network, and accounts for them

End-to-end transmission of packets (possibly over multiplelinks)Controls the operation of the subnetRouting

staticdynamic

Congestion controlAt this stage, there may be congestion due to many packets waiting to be routed

Some packets may be lost during congestion

Network Layer I


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Network Layer IIAccounting

packetsbytesetc.

InternetworkingThis layer is also concerned with internetworking where there is talking between technologies, such as the traditional Internet connected to ATM segmentationaddressingsequencingaccounting

Broadcast subnets: thin network layer


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Transport Layer I

This layer presumes the ability to pass through a network and provides additional services to end-users, such as and-to-and

packet reliability End-to-end delivery of a complete message(end-to-end communication path, usuallyreliable)Isolation from hardware Multiplexing/demultiplexingDivide message into packetsReassemble (possibly out of order packets)

into the original message of the distant end


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Transport Layer II

End-to-end flow controlAcknowledgmentsTypes of service

error-free, point-to-point, in sequence,flow controlledno correctness guaranteesno sequencing

Establishing/terminating connectionsnaming/addressingintra-host addressing (process, ports)


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This layer enables users to establish sessions across a network between machines In addition, it offers session management services Set up and management of end-to-end conversation

Establish and terminate sessionssuperset of connections

Assignment of logical portsDialogue controlToken management

for critical operationsSynchronization

checkpoints/restarts

Session Layer


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Presentation Layer

This layer is concerned with the syntax and semantics of messages, code conversions between machines, and other data conversion services Some of these services are data compression and data encryption Interface between lower layers and applicationFormatting

Syntax & semantics of messagesData encoding (e.g.: ASCII to EBCDIC)CompressionEncryption/Decryption

Authentication


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Application Layer

This layer provides support for the user's network applications Some application layer services have been standardized,e.g.:

File Transfer and Management (FTAM)Message Handling Services for electronic mail (X.400)Directory Services (X.500)Electronic Data Interchange (EDI)

Program youre running,applications file transfer, access & managemente-mailvirtual terminals

WWW


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The OSI Protocol Stack


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Operationof themodel

Virtualtransmission

Real datatransmission

Applicationlayer entity

Session layer entity

Transportlayer entity

Network layer entity

Datalink layer entity

Presentationlayer entity

Intermediate



Intermediate



Endsystem

Applicationlayer entity

Session layer entity

Transportlayer entity


Presentationlayer entity


Physical medium

Endsystem

Physical layer entity





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Names of the Nodes, Connections andData Units

Layer name Node Connection Data unit

Application layer application network service e.g. file (ADU)

Presentation layer host session data structure (PPDU)

Session layer host transport connection message (SPDU)

Transport layer host network path message (TPDU)

Network layer host, router line (data)packet (NPDU)

(Data)link layer station (physical) channel (data)frame (LLC PDU)

Physical layer switch physical transmissionmedium

bit


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Communication among the layers

Connection oriented network service(virtual circuits, eg. ATM )

Reliable transport service Unreliable transport serviceConnectionless network service(datagram service, eg. IP )

Reliable transport service (eg. TCP) Unreliable transport service (eg. UDP)


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Network Tools

Repeater : connects network segmentslogically to one networkHub : multiport repeaterBridge : datalink level connection of twonetworksSwitch : multiport bridgeRouter : connects networks that arecompatible in transport level

subnetworks are connected to the interfaces ofthe repeater

Gateway (proxy server) : router betweentwo individual network. The Way Out


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Physical Layer Devices

Repeater

Hubdumb level-1 hubmulti-port repeater


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Data Link Layer Devices

BridgeCascaded vs. BackboneSingle

Multiple

Switch (switched hub)


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Routers

Provide link between networksAccommodate network differences:

Addressing schemes

Maximum packet sizesHardware and software interfacesNetwork reliability

Congestion/Traffic Management


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Devices of the Network Connection

Application layer

Presentation layer

Session layer

Transport layer

Datalink layer

Network layer

Physical layer

Gateway

or

Proxy server

Router or Gateway

Bridge or Switch

Repeater or Hub

Application layer

Presentation layer

Session layer

Transport layer

Datalink layer

Network layer

Physical layer


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Architectural Implementation of theLANs

Ethernet (IEEE 802.3)FDDIGigabit EthernetToken Bus (IEEE 802.4)Token Ring (IEEE 802.5)


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Characteristics of High-Speed LANs

Fast Ethernet Gigabit Ethernet Fibre Channel Wireless LAN

Data Rate 100 Mbps 1 Gbps, 10 Gbps 100 Mbps 3.2Gbps 1 Mbps 2 Gbps

Transmission Mode UTP,STP, OpticalFiberUTP, shielded

cable, optical fiberOptical fiber,

coaxial cable, STP2.4 GHz, 5 GHz

Microwave

Access Method CSMA/CD CSMA/CD Switched CSMA/CA Polling

SupportingStandard IEEE 802.3 IEEE 802.3

Fibre ChannelAssociation IEEE 802.11


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Wide Area Network Connections

Solutions for connecting LANs to theInternet

Ethernet (ring or star topology) Managed Leased Line Network (MLLN)ATM (Asynchronous Transfer Mode)Switched line

ISDN line

f d d


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Soft and Hard States

State: the data collection, which are necessary forkeeping the connection between two protocol entitiesHard state

If the connection is established once, it is never timed out, evenif it is not in usageTo cancel the connection one of the participants of the connectionmust explicitly close itThe history of the state is stored

Soft state To keep the connection the participants must send occasionallykeep-alive messages, since without keep-alive message the stateinformation is timed out after a certain periodThe state is called as soft since in the ordinary operation thestate can change easilyThe history of the state is not stored


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Packet switching versus circuit switching

Great for bursty dataresource sharingno call setup (less start-up delay)

HoweverPackets can experience delays , so not for real -timeapplicationsexcessive congestion leads to packet delay and loss

protocols (like TCP) are needed for reliable datatransfer, and congestion control

Is packet switching best in every case?

P f C id i


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Performance ConsiderationsBefore continue, need some performance metrics

Overhead: CPU time to put packet on wire

Throughput: Maximum number of bytes per second Depends on wire speed, but also limited by slowest router (routingdelay) or by congestion at routers

Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop

Contributions to LatencyWire latency: depends on speed of light on wire

about 11.5 ns/footRouter latency: depends on internals of router

Could be < 1 ms (for a good router)

Router Router

LR1 LR2 LW1 LW2 Lw3


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Delay in packet-switched networks

packets experience delayon end-to-end pathfour sources of delayat each hop

Nodal processing : check bit errorsdetermine output link

Queueing :time waiting at output

link for transmissiondepends on congestionlevel of router

A

B

propagation transmission

nodalprocessing queueing


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Delay in packet-switched networks

Transmission delay : R=link bandwidth (bps)L=packet length (bits)time to send bits intolink = L/R

Propagation delay :d = length of physical links = propagation speed inmedium (~2x108 m/sec)

propagation delay = d/s

A

B

propagation transmission

nodal

processing queueing


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Queueing delay (revisited)

R=link bandwidth (bps)L=packet length (bits)a=average packetarrival rate

traffic intensity = La/R

La/R ~ 0: average queueing delay smallLa/R -> 1: delays become largeLa/R > 1: more work arriving than can beserviced, average delay infinite!

I l k


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Internet protocol stack

Application:supporting networkapplicationsftp, smtp, http

Transport: host-host data transfertcp, udp

Network: routing of datagrams fromsource to destinationip, routing protocols

Network access: data transfer betweenneighboring network elements

ppp, ethernetPhysical: bits on the wire


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Layering: logical communication

applicationtransportnetwork

linkphysical


linkphysical application

transportnetworklink

physical

application


physical

networklinkphysical

data

data

E.g.: transport take data from appadd addressing,reliability checkinfo to formdatagram send datagram topeerwait for peer toack receiptanalogy: postoffice

data

transport

transport

ack


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Layering: physical communication


linkphysical


linkphysical application


physical


linkphysical

networklink

physical

data

data


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Protocol layering and data

Each layer takes data from aboveadds header information to create new data unitpasses new data unit to layer below


linkphysical


linkphysical

source destinationM M M M

Ht Ht Hn Ht Hn Hl

M M M M

Ht Ht Hn Ht Hn Hl

message segment datagram frame


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IP over ATM

ATM AdaptationLayer (AAL): interface to upperlayers

end-systemsegmentation/rea

ssemblyATM Layer: cellswitchingPhysical

AAL5ATM

physical

AAL5ATM

physical

AAL5ATM

physical

AAL5ATMphysical

ATMphysical

applicationTCP/UDP

IP

applicationTCP/UDP

IP

applicationTCP/UDP

IP

applicationTCP/UDP

IP


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Physical

Data Link

Network

Transport

Session

Presentation

Application

Network Access

IP

TCP UDP

Application

Sockets

The Internet Protocol Stack

N t k P t l


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Network ProtocolsProtocol: Agreement between two parties as to howinformation is to be transmitted

Example: system calls are the protocol between the operatingsystem and applicationNetworking examples: many levels

Physical level: mechanical and electrical network (e.g. how are 0 and 1represented)

Link level: packet formats/error control (for instance, the CSMA/CDprotocol)

Network level: network routing, addressing Transport Level: reliable message delivery

Protocols on todays Internet:

Ethernet ATM Packet radio

IPUDP TCP

RPCNFS WWW e-mail ssh

Physical/Link

Network

Transport

B ilding a messaging ser ice


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Building a messaging serviceProcess to process communication

Basic routing gets packets from machine machine

What we really want is routing from process process Example: ssh, email, ftp, web browsingSeveral IP protocols include notion of a port , which isa 16-bit identifiers used in addition to IP addresses

A communication channel (connection) defined by 5 items:[source address, source port, dest address, dest port, protocol]

UDP: The User Datagram ProtocolUDP layered on top of basic IP (IP Protocol 17)

Unreliable, unordered, user-to-user communication

UDP Data

16-bit UDP length 16-bit UDP checksum16-bit source port 16-bit destination port

IP Header(20 bytes)

B ilding mess ging ser ice (cont)


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Building a messaging service (con t) UDP: The Unreliable Datagram Protocol

Datagram: an unreliable, unordered, packet sent from

source user dest user (Call it UDP/IP)Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered anti -social none of the well -

behaved aspects of (say) TCP/IP

But we need ordered messagesCreate ordered messages on top of unordered ones IP can reorder packets! P 0,P1 might arrive as P 1,P0

How to fix this? Assign sequence numbers to packets 0,1,2,3,4.. If packets arrive out of order, reorder before delivering touser application For instance, hold onto #3 until #2 arrives, etc.

Sequence numbers are specific to particular connection

TCP/IP k t Eth t f


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Message

TCP/IP packet, Ethernet frameApplication sends message

TCP data

TCP Header

IP Header

IP DataEH

Ethernet Hdr

Ethernet Hdr

TCP breaks into 64KBsegments, adds 20B header

IP adds 20B header, sendsto networkIf Ethernet, broken into1500B frames with headers,trailers (24B)All Headers, trailers havelength field, destination, ...

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02computer Network Basics