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Introduction to Networks & OSI Layers
Dr Lachlan Andrew
• 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
How to be successful
University of Melbourne
• 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
Outline of this half
University of Melbourne
• 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
University of Melbourne 4
Academic Integrity
http://academicintegrity.unimelb.edu.au/
• History of the Internet
• Structure of the Internet
• Layered network protocol models / services
• Other network models: OSI
• Protocol stack
This lecture
University of Melbourne
• 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 – early 1970’s)
– NSFNET (1970’s – early 1980’s)
– Internet (1980’s – present)
• . . . the rise and rise of social media, Web 2.0+ (present)
The Internet
University of Melbourne
The Internet – High Level Overview
University of Melbourne
TN 6th 1-16
• 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
The Internet – a brief history
University of Melbourne
• ARPANET (1969-1990)
– Advanced Research Projects Agency Network funded by the
Department of Defence (ARPA was part of DoD)
– Initially started with just 4 sites: UCLA, SRI International, University
of California Santa Barbara, University of Utah
– TCP/IP was developed at ARPANET
– Misconception that it was designed to survive a nuclear attack –
robustness was a necessity in a world of unreliable communication
ory-internet/
• , co-inventor of packet switching, did have that goal
The Internet – a brief history
University of Melbourne
• Inter national Network Working Group (1972)
– Chaired by (one of the father’s of the internet)
– Proposed a packed switched datagram based network standard
– Submitted to International Telegraph and Telephone Consultative
Committee (CCITT, now ITU-T) for standardisation, but rejected
– Cerf resigned as chairman in 1975; went to work with at
– Cerf & Kahn had already published the foundations of TCP/IP in
1974 in their paper “A Protocol for Packet Network
Intercommunication”
– Cerf & Kahn developed internet protocols in a restricted
environment (ARPANET) that they could control
The Internet – a brief history
University of Melbourne
• OSI Model
– Remaining members of INWG regrouped under the International
Standards Organisation (ISO) working group to design the OSI
– This left a bitter rivalry between those at ARPANET and the OSI
working group
– First plenary meeting in 1978, published as an international
standard in 1984
– OSI looked liked the dominant player – even the US Department of
Defence recommended moving away from TCP/IP to OSI
– By the late 80’s the slow development of the OSI model was leading
to increased frustration
The Internet – a brief history
University of Melbourne
• NSFNet (National Science Foundation) created in 1986 to
provide researchers access to supercomputer sites in the
• 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
The Internet – a brief history
University of Melbourne
• Many of the underlying protocols were designed without
consideration for an adversary on the network
– Security has been retrofitted, with many insecure protocols still widely
used (DNS)
• The rivalry between the two groups led to TCP/IP working
groups rejecting OSI concepts out of principle
– The so called “palace revolt” of 1992 in which the limitations of IPv4
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, one of the important TCP/IP protocols (IS-IS routing) was
developed by the OSI and adopted by the TCP/IP community
The Internet
University of Melbourne
• Two protocol stacks:
– TCP/IP effectively standardised post implementation
– OSI standardised pre-implementation, but not widely implemented
• Why do we need a model?
– Interoperability – Open, ideally not proprietary
– A reference model to develop and validate against independently
– Since networks are multi-dimensional, a reference model can serve
to simplify the design process.
– It’s engineering best practice to have an abstract reference model,
and a reference model and corresponding implementations are
always required for validation purposes
The outcome
University of Melbourne
• 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.
Network Models
Layer N protocol
Layer N-1 protocol
Layer N/N-1
Host 1 Host 2 Host 3 Host 4
University of Melbourne
• 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
Services to protocols relationship
University of Melbourne
TN 6th 1-31
• Connection Oriented (TCP):
– connect, use, disconnect
– negotiation inherent in connection setup similar to telephone
• Connectionless (UDP):
– Use message routed through intermediate nodes
– similar to postal service or text message
• The choice of service type affects the reliability, quality and
cost of the service itself.
Connection-oriented and
Connectionless services
University of Melbourne
• 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
– When we map protocols surrounding TCP/IP to the OSI model, don’t
be surprised to see protocols straddle layers or for there to be
ambiguity as to which layer a protocol belongs to
TCP/IP vs OSI
University of Melbourne
• A layer should be created where a different abstraction is
• 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.
Open Systems Interconnection
(OSI) reference model
University of Melbourne
University of Melbourne
TN 6th 1-32
• Use data
• Tidy up e2e
• Get data e2e
• Tidy up p2p
• Get data p2p
University of Melbourne 23
Point-to-point, end-to-end
• TCP/IP – Transmission Control Protocol/Internet Protocol –
was designed to be independent of data link and physical
layers (Cerf & Kahn, 1974)
TCP/IP model
University of Melbourne
TN 6th 1-33
Using protocols
University of Melbourne
encapsulation
K&R 7th 1.24
We will be concentrating on the top three layers – the
application level and a high-level overview of TCP, UDP and IP.
The protocol stack
University of Melbourne
• Why did the OSI initiative fail?
– Politics? Poor design? Vested interests? Bad timing?
– Design by committee – too open
• “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’s of engineers would attend meetings and would argue of even trivial
details (e.g. the difference between “you will comply” and “you should
• Internet standardization has become much like this
• In software engineering terms: Waterfall vs agile
And finally…
University of Melbourne
• The slides were based on slides prepared by ,
based on material developed previously by: ,
, , and .
• 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.
– (And also) Computer Networks, 6th Edition, Tanenbaum A., Wetherall. D.
https://ebookcentral.proquest.com/lib/unimelb/detail.action?docID=6481879
• Textbook Reference: Chapter 1, especially section 1.5, 1.7
Acknowledgement
University of Melbourne
https://ebookcentral.proquest.com/lib/unimelb/detail.action?docID=6481879
Introduction to Networks & OSI Layers
How to be successful
Outline of this half
Academic Integrity
This lecture
The Internet
The Internet – High Level Overview (2)
The Internet – a brief history
The Internet – a brief history (2)
The Internet – a brief history (3)
The Internet – a brief history (4)
The Internet – a brief history (5)
The Internet (2)
The outcome
Network Models (2)
Services to protocols relationship
Connection-oriented and Connectionless services
TCP/IP vs OSI
Open Systems Interconnection (OSI) reference model
Point-to-point, end-to-end
TCP/IP model (2)
Using protocols
The protocol stack
And finally…
Acknowledgement
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