COMP30023 – Computer Systems
Introduction to Networks & OSI Layers
Dr Lachlan Andrew
©2021 University of Melbourne
How to be successful
• Understand the material, don’t just memorize it.
• If you fall behind, try to catch up as fast as possible.
• Attempt the workshop tasks every week. You should attempt the theory/tutorial questions before you attend your workshop.
• Check the LMS for announcements and discussion board posts.
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Outline of this half
• Wk 6: Intro to the Internet; layered protocol stacks
• Wk 7: Application layer: HTTP, DNS, FTP
• Wk 8: Transport layer: services, mux/demux, UDP, TCP
• Wk 9: Socket programming; flow and congestion control
• Wk 10: Network layer: Packet switching, addressing subnets / NAT / fragmentation
• Wk 11: Network layer: Routing; multicast, control protocols
• Wk 12: Guest lecture; Review
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Academic Integrity
• Unless otherwise specified, all work is to be done on an individual basis
• Academic Integrity is of utmost importance, and we will all follow the policies of the University and of the School of Engineering
• Please refer here for further information http://academicintegrity.unimelb.edu.au/.
• For the purpose of ensuring academic integrity, all submission attempts by a student may be inspected, regardless of the number of attempts made
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This lecture
• History of the Internet
• Structure of the Internet
• Layered network protocol models / services
• Other network models: OSI
• Protocol stack
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The Internet
• The Internet is composed of the aggregation of many smaller networks – not a single network or under a single point of control.
• Historically, the Internet developed in 3 distinct phases
– ARPANET(1960’s-early1970’s) – NSFNET(1970’s-early1980’s) – Internet(1980’s-present)
• . . . the rise and rise of social media, Web 2.0+ (present)
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The Internet – High Level Overview
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The Internet – a brief history
• It is necessary to understand at least some of the history of the internet in order to understand how the current set of standards came to be
• As with most IT projects there were opposing views, designs, and teams. Ultimately, the group that focussed on implementation over standardisation won out
• As we shall discuss throughout the lectures, the design decisions taken 40+ years ago have left us with a number of security issues today
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The Internet – a brief history
• ARPANET (1969-1990)
– AdvancedResearchProjectsAgencyNetworkfundedbythe
Department of Defence (ARPA was part of DoD)
– Initiallystartedwithjust4sites:UCLA,SRIInternational,University of California Santa Barbara, University of Utah
– TCP/IPwasdevelopedatARPANET
– Misconceptionthatitwasdesignedtosurviveanuclearattack– robustness was a necessity in a world of unreliable communication links (https://www.internetsociety.org/internet/history- internet/brief-history-internet/)
• Paul Baran, co-inventor of packet switching, did have that goal
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The Internet – a brief history
• International Network Working Group (1972)
– Chaired by Vint Cerf (one of the father’s of the internet)
– Proposedapackedswitcheddatagrambasednetworkstandard
– SubmittedtoInternationalTelegraphandTelephoneConsultative Committee (CCITT, now ITU-T) for standardisation, but rejected
– Cerfresignedaschairmanin1975;wenttoworkwithBobKahnat ARPA
– Cerf&KahnhadalreadypublishedthefoundationsofTCP/IPin 1974 in their paper “A Protocol for Packet Network Intercommunication”
– Cerf & Kahn developed internet protocols in a restricted environment (ARPANET) that they could control
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The Internet – a brief history
• OSI Model
– RemainingmembersofINWGregroupedundertheInternational
Standards Organisation (ISO) working group to design the OSI
– ThisleftabitterrivalrybetweenthoseatARPANETandtheOSI working group
– Firstplenarymeetingin1978,publishedasaninternational standard in 1984
– OSIlookedlikedthedominantplayer–eventheUSDepartmentof Defence recommended moving away from TCP/IP to OSI
– Bythelate80’stheslowdevelopmentoftheOSImodelwasleading to increased frustration
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The Internet – a brief history
• NSFNet (National Science Foundation) created in 1986 to provide researchers access to supercomputer sites in the USA
• By the late 80’s commercial internet service providers started to appear
• Also in the 80’s CERN developed their TCP/IP based network, which would ultimately lead to the creation of the World Wide Web
• TCP/IP became the protocol stack of choice and eventually the OSI became little more than theoretical abstraction
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The Internet
• Many of the underlying protocols were designed without consideration for an adversary on the network
– Securityhasbeenretrofitted,withmanyinsecureprotocolsstill widely used (DNS)
• The rivalry between the two groups led to TCP/IP working groups rejecting OSI concepts out of principle
– Thesocalled“palacerevolt”of1992inwhichthelimitationsofIPv4 were raised, but the proposed OSI solution was rejected and the leaders of the working group voted out for having suggested it. It wasn’t until 1996 that IPv6 was proposed and is still not in widespread use
– However,oneoftheimportantTCP/IPprotocols(IS-ISrouting)was developed by the OSI and adopted by the TCP/IP community
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The outcome
• Two protocol stacks:
– TCP/IPeffectivelystandardisedpostimplementation
– OSIstandardisedpre-implementation,butnotwidelyimplemented
• Why do we need a model?
– Interoperability–Open,ideallynotproprietary
– Areferencemodeltodevelopandvalidateagainstindependently
– Sincenetworksaremulti-dimensional,areferencemodelcanserve to simplify the design process.
– It’sengineeringbestpracticetohaveanabstractreferencemodel, and a reference model and corresponding implementations are always required for validation purposes
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Network Models
• Model the network as a stack of layers. Just a model
• Each layer offers services to layers above it.
• Inter-layer exchanges are conducted according to a protocol.
Layer N/N-1 Layer N interface
Layer N protocol
Layer N
Layer N-1 protocol
Layer N-1
Layer N-1
Layer N-1
Layer N-2
Layer N-2
Layer N-2
Layer N-2
Layer N-3
Layer N-3
Layer N-3
Layer N-3
Host1
Host2
Layer
Host3 N-3 Host4 protocol
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Services to protocols relationship
• Service = set of primitives that a layer provides to a layer above it – interfaces between layers
• Protocol = rules which govern the format and meaning of packets that are exchanged by peers within a layer
– packets sent between peer entities
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Connection-oriented and Connectionless services
• Connection Oriented (TCP): – connect,use,disconnect
– negotiationinherentinconnectionsetupsimilartotelephone service
• Connectionless (UDP):
– Usemessageroutedthroughintermediatenodes
– similartopostalserviceortextmessage
• The choice of service type affects the reliability, quality and cost of the service itself.
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TCP/IP vs OSI
• The TCP/IP model reflects what happens on the internet
• The OSI model helps reflect the thought process that should
be followed when designing a network or diagnosing a fault
– It remains at the core of a number accreditation schemes
• View the OSI model as idealised, but with a degree of flexibility
– WhenwemapprotocolssurroundingTCP/IPtotheOSImodel,don’t be surprised to see protocols straddle layers or for there to be ambiguity as to which layer a protocol belongs to
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Open Systems Interconnection (OSI) reference model
• A layer should be created where a different abstraction is needed.
• Each layer should perform a well defined function.
• The function of each layer should be chosen with a view
toward defining internationally standardised protocols.
• The layer boundaries should be chosen to minimise the information flow across the interfaces.
• The number of layers should be large enough that distinct functions need not to be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.
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OSI Model
(really “symbol”)
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Point-to-point, end-to-end
• Use data
• Tidy up e2e
• Get data e2e
• Tidy up p2p
• Get data p2p
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TCP/IP model
• TCP/IP – Transmission Control Protocol/Internet Protocol – was designed to be independent of data link and physical layers (Cerf & Kahn, 1974)
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Using protocols
encapsulation
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The protocol stack
We will be concentrating on the top three layers – the application level and a high-level overview of TCP, UDP and IP.
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And finally…
• Why did the OSI initiative fail?
– Politics?Poordesign?Vestedinterests?Badtiming?
– Designbycommittee–tooopen
• “The organizational problem alone is incredible. The technical problem is bigger than any one previously faced in information systems. And the political problems will challenge the most astute statesmen. Can you imagine trying to get the representatives from ten major and competing computer corporations, and ten telephone companies and PTTs [state-owned telecom monopolies], and the technical experts from ten different nations to come to any agreement within the foreseeable future?” Bachman (Chairman of OSI) 1978
– 100’sofengineerswouldattendmeetingsandwouldargueofeven trivial details (e.g. the difference between “you will comply” and “you should comply”)
• Internet standardization has become much like this
• In software engineering terms: Waterfall vs agile
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Acknowledgement
• The slides were based on slides prepared by Chris Culnane, based on material developed previously by: Michael Kirley, Zoltan Somogyi, Rao Kotagiri, James Bailey and Chris Leckie.
• Some of the images included in the notes were supplied as part of the teaching resources accompanying the text books listed on the previous slides.
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