Transcript Computer Networks 0000

Computer Networks
0010-ISO-OSI
Gergely Windisch
[email protected]
How can we do effective communication?
• Take a look at the client-server image from the
previous lecture
• Where should all the communication take place?
Protocol hierarchy
Layers, Layers everywhere
Why OSI?
• Networks were being developed at different places
– universities, companies, research institutions
– different technology, different naming conventions –
how are they going to talk to each other?
• ISO – common standard model should be created
– Open System Interconnect
• open means that the details of implementation are openly
avaliable – not necessarily free and can also be tied to a
vendor
– To have a common base – same nomenclature so that
people can collaborate
OSI – Open System Interconnect
• 7 layers have been defined
– number of layers is arbitrary, implementations do not
have to follow
• Having layers is important
– inside is a black box
– layer separation is important – layers should be as
independent from each other as possible - make sure
that they are separated well
• Layers are service providers
source: http://www.infocellar.com/networks/osi-model.htm
Learn tehese by heart. You’ll thank me later.
OSI model
Sidenote: mnemonics to help you remember
• All People Seem To Need Data Processing
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Princess Diana Never Tried Screwing Prince Andrew
Please Do Not Take Sales People's Advice
Philys Did Networking Till She Passed Away
American Presidents Should Try New Dating Practices
All Penguins Stand Too Near Deep Pools
Angus Prefers Sausages To Nibbling Dried Pork
A Pathetic Silly Trick Never Does Please
People Don't Need This Stuff Presented Anyway
Programmers Do Not Throw Sausage Pizza Away
Arse Pounders Seem To Neglect Delicious Pussy
All Parents Should Teach New Dads Parenting
Apply Proper Sense To Network Data Path
People Don't Need To Study Protocol Analysis
Please Do Not Touch Steve's Pet Alligator
Phil Donahue Never Televises Sick People Anymore
Please Dear, Not Tonight, Soft Penis Already
All People Studying This Need Drastic Psychotherapy
People Design Networks To Send Packets Accurately
Australian Post Sucks They Never Deliver Parcels
Philosophers Deign Not To Solipstically Ponder Anachronisms
Please Don't Network These Stupid People Again
Any Person Standing Totally Nude Doesn't Perspire
All Pussy Seems To Need Douching Periodically
Layer 1 - Physical
• Transfers bits to the other side
• Access medium is handled on the Physical layer
– light
– radio signals
– electrical impulse
• Physical layer is not reliable, bits get lost,
duplicated, corrupted
Layer 2 – Data link
• Uses the services of the dumb phyisical layer to
send bits
• Provides the service of sending large chunks of bits
to the other side reliably
– creates frames, adds synchronizing bits to the beginning
of the frame and crc to the end
– CRC is used for error detection
– Error correction has been used earlier, but physical
layers are too reliable now – error correction became
extinct
• 1 wrong bit in 1 billion
Layer 3 – Network
• Receives packets and sends them to any host in the
world*
• Interesting questions that might arise
– how do we know who we want to send it to?
– how the hell do we get there?
*: For a given value of any
Layer 3 – Network
• Receives packets and sends them to any host in the
world*
• Interesting questions that might arise
– how do we know who we want to send it to?
– how the hell do we get there?
• Interesting answers to the interesting questions
– logical addressing (IP)
– routing protocols
• Routing protocols help find the destination – one
hop at a time
– how many hops does it take to get from this room to a
computer lab in china?
*: For a given value of any
Layer 4 – Transport
• Layer 3 can transport individual packets, but we are
not in the business of transferring packets – we
want to transfer data – documents, music files (only
legal of course), cute pictures of cats, video files of
our favorite Britney Spears concert and so on.
• The transport layer takes the data apart, creates
datagrams and gives them to Layer 3 for
transportation.
• Transport layer helps for the data to get to the given
application
– ports
• TCP and UDP are the most important protocols
Layer 5 – Session
• Session layer handles the connection properties
between two computers
– user names, passwords, whatnot
Layer 6 – Presentation
• Data conversion
– byte order for example, code pages
– encoding if necessary (session layer handles key
excange, presentation layer uses them)
Layer 7 – Application
• The actual program that the user uses
• There are an infinite amount of application
protocols – ftp, http, smtp and many others
source: http://www.infocellar.com/networks/osi-model.htm
Encapsulation