Transcript Lec 1 - The University of Sydney

Network Architectures
Week 3 – OSI and The Internet
What do we want from a
communications network?
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We want to
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transfer messages from a process in one
computer
to a process in another computer
reliably
quickly
and in an understandable form.
We also know that:
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Messages have to pass through a number of
switching nodes from one host to another, and thus
the network has to route messages correctly
The various links may be of different media at
different bandwidths
The signals carrying data are subject to interference
and degradation, and thus messages may be
corrupted or lost
The byte coding structure in one computer may be
different from the other
We need a series of protocols
to address these issues
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Remember the definition of a protocol:
“A protocol defines the format and order of
messages exchanged between two
communicating entities, and the actions taken
on receipt or transmission of a message.”
Those “communicating entities” may be as
limited as the two devices on either end of a
physical link.
Network Architectures
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Data communication developed in the 60’s
By early 70s suppliers were developing own
architectures – IBM/SNA DEC/DECnet
But these did not help inter-organisation data
communications
Two approaches:
 OSI model
 Internet’s TCP/IP
OSI – Open Systems Interconnect
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Developed by the International
Standards Organisation in 1974
It is a reference model
Describes a network and a framework
for developing network protocols
Incorporates work done by a number of
organisations, particularly DEC
Not ever fully implemented
OSI Continued
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Its real benefit is that is defines a
layered architecture and thus the model
is still used
The objectives of the model were:
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Must provide a high degree of connectivity
Must be reliable
Must be easy to implement, to use and to
modify
OSI Layers
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7
6
5
4
3
2
1
– Application
– Presentation
– Session
– Transport
– Network
– Data link
- Physical
As a layered architecture, each layer is isolated from the others,
And thus its internals can be modified or replaced.
Key points about a layered
approach
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The source process only wants to know that the
message it sent gets to the destination process
– it does not need to know how
The Network stack is implemented in the
operating system of both hosts and the various
switches
As such the software in each operating system
will be different code – but implementing the
same protocol
Application – Layer 7
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Provides an application or a service to
the user application
Examples are
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FTAM – File transfer Access Mode
X.400 – message handling
X.500 – network directory services
Presentation – Layer 6
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Structures data in agreed format
Carries out code conversion (ASCII to
EBCDIC
Carries out data compression
Carries out data encryption
Session – Layer 5
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Co-ordinates connection &
disconnection of dialogs between
processes
Synchronises the flow of data checkpoints
Re-establishes the connection if it fails
Transport – Layer 4
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Provides end-to-end, error free delivery
of messages, based on level of service
required
These include:
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Error control
Flow control
Partitioning and reassembling messages
Network – Layer 3
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Responsible for end-to-end routing of
data packets across the network
Logical addressing
Routing
Performs network management
Formats packages
Data Link – layer 2
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Responsible for reliable transfer of data
across a link in frames
Provides for error detection and control
Organises data into frames
Provides flow control – if receiver
slower than transmitter
Negotiates access control between the
two devices
Physical – Layer 1
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Transmits bits across the physical
medium
Accepts data in frames and translates
into signals on the medium
Concerned about the medium being
used , the signalling scheme and the
connectors
Why is OSI not the prevailing
standard?
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A case of De facto prevailing over De jure
While OSI was defined, manufacturers were slow to
implement it
Europeans were keen, but the US had TCP/IP
US government proclaimed GOSIP (Government OSI
Profile) in 1992
But TCP/IP took off with the Internet in the early 90s
It worked, was common, and most organisations just
accepted it
TCP/IP, Internet & The WWW
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TCP/IP are two protocols at the
Transport and Network levels
The Internet is a “Network of Networks”
that use TCP/IP as key layers in its
protocol stack
The World Wide Web is an application
that runs on the Internet
TCP/IP
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TCP (Transmission Control Protocol
IP (Internet Protocol)
Developed at the same time as OSI, but as a
product not an international model
Developed for the ARPANET – Dept of
Defence, defence contractors, Universities
and the Military
To enable communication across analogue
lines, packet radios and Ethernet networks
To be a Network of networks
TCP/IP (cont.)
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Developed by Vint Cerf & Robert Kahn
Uni of California included it is BSD UNIX
National Science Foundation mandated it and
ran the backbone 1985
While the requirements were much the same
as for OSI, it was not built as a layered
product.
Many of its attributes reflect the environment
it was developed in
Commercial services started in the 1980’s and
NSF stopped providing the backbone in 1995.
New Data link level protocol
added in 1990
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RFC 1149: A Standard for the Transmission of IP
Datagrams on Avian Carriers.
www.faqs.org/rfcs/rfc1149.html
Later followed up with
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RFC 2549: IP over Avian Carriers with Quality of
Service
The Principles set out for
TCP/IP
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Autonomy – a network should be able to
work on its own without change
Best effort service – Lost messages would be
retransmitted
Stateless servers – Routers should not need
to maintain the state of a connection
Decentralised control – No global control over
the Internet
Comparing The Internet & OSI
Internet “layers”
PDU
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Application – layer 5
Transport – layer 4
Network – layer 3
Data Link – layer 2
Physical – layer 1
PDU (Protocol Data Unit)
Layers as per Kurose & Rose
Message
Segment
Datagram
Frame
Bit
Generic functions that may be
at each layer
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Error control
Flow or congestion control
Segmentation & re-assembly
Multiplexing – higher level sessions
sharing a single lower level connection
Connection set-up