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CPS 422Computer Networks

Layered Approach To NetworkDesign

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‰ Networks are complex!‰ Composed of many “pieces”:

o hostso routerso links of various mediao applicationso protocolso hardware, software

‰ Tobetter understand the issue let us firstconsider an analogy of “Air Travel Process”

‰ Canyou break down the process of air travel into“sub-processes”

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Organization of air travel

Service Counter(ticket purchase)

Check-in Counter(baggage check)

gates (Departure)

runway (takeoff)

Service Counter(Future Travel)

Check-in Counter(baggage claim)

gates (Arrival)

runway (landing)

airplane routing

‰ a series of steps

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Organization of air travel: a different view

Service Counter(ticket purchase)

Check-in Counter(baggage check)

gates (Departure)

runway (takeoff)

Service Counter(Future Travel)

Check-in Counter(baggage claim)

gates (Arrival)

runway (landing)

airplane routing

Layers: each layer implements a serviceo via its own internal-layer actionso relying on services provided by layer below

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Layered air travel: services

Counter-to-counter Ticketing services

baggage-deposit-to-baggage-claim delivery

people transfer:Departure gate to Arrival gate

runway-to-runway delivery of plane

airplane routing from source to destination

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Dep

arti

ngai

rpor

t

arri

ving

airp

ort

Distributed implementation of layer functionality

Service Counter(purchase)

baggage (check)

gates (Departure)

runway takeoff

airplane routing

Service Counter(future booking)

baggage (claim)

gates (Arrival)

runway landing

airplane routing

We can refer to this collection as a “stack”

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Similarly……‰While developing a model for networkarchitecture, designers distilled the process oftransmitting data into its most fundamentalelements.They identified which networking functions hadrelated uses and collected those functions intodiscrete groups that became “Protocol Layers”which collectively form a “Protocol Stack”.

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Why layering?Dealing with complex systems:‰ explicit structure allows identification, relationship

of complex system’s pieceso layered reference model for discussion/design

‰ modularization eases maintenance, updating ofsystemo change of implementation of layer’s service

transparent to rest of systemo e.g., change in gate procedure doesn’t affect rest

of system

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Protocols in Simplified Architecture

This is a “Protocol Stack”

applicationtransportnetwork

linkphysical

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Layering: logical communication

Each layer:� distributed� “entities”

implementlayer functionsat each node� entities

applicationtransportnetwork

linkphysical

networklink

physical

performactions,exchangemessages withpeers

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

transport

transport

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Layering: logical communication

E.g.: transport� take data fromapp� add

addressing,reliability checkinfo to form“datagram”� send

datagram to peer

dataapplicationtransport

networklink

physical

applicationtransportnetwork

link

ack

data

networklink

physical

data

� wait forpeer to

ack receipt� analogy:

post office

physical applicationtransportnetwork

linkphysical

applicationtransport

networklink

physical

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Layering: physical communicationdata

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

dataapplicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

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Protocol layering and dataMaj Faisal CPS 422

Each layer takes data from above� adds header information to create new dataunit� passes new data unit to layer below

source destinationM

Ht MHnHt M

Hl HnHt M

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

MHt M

HnHt MHl HnHt M

messagesegmentdatagramframe

Notice theseHeaders

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Protocol Revisited

How does a Protocol Work ……

A Protocol implements its Functionality throughits “HEADER”

A Header contains all the controlinformation to complete the

protocol’s tasks

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TCP

–Pr

otoc

olD

ata

Uni

t(P

DU

) An – Example Header

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‰ Key Elements of a Protocolo Syntax - Formato Semantics - Control Info for Coord and error handlingo Timings - Sequencing and Speed matching

‰ Categories of Protocol Functionso Segmentation and Re-assemblyo Encapsulationo Connection Controlo Ordered Deliveryo Flow Controlo Error Controlo Addressingo Multiplexingo Transmission Services

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The OSI Reference ModelandTCP/IP Protocol Suite

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The OSIReferenceModel

Maj Faisal CPS 422

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The OSI EnvironmentMaj Faisal CPS 422

application

transport

network

link

physical

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TCP / IP Protocol Suite

‰ application: supporting networkapplicationso ftp, smtp, http

‰ transport: host-host data transfero tcp, udp

‰ network: routing of datagrams fromsource to destinationo ip, routing protocols

‰ link: data transfer betweenneighboring network elementso ppp, ethernet

‰ physical: bits “on the wire”

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Some PDUs in TCP/IP

Maj Faisal CPS 422

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TCP / IP ConceptsMaj Faisal CPS 422

Maj Faisal CPS 422

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Maj Faisal CPS 422

Some Protocols of TCP/IP Suite

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OSI - TCP/IP Comparison

Maj Faisal CPS 422