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CS 6027 Advanced Computer
Networking
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Lecture 1
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Introduction and Overview
Just 50 years ago, networks were proprietary (ie. IBM, HP, DEC,
etc)
Both the software (protocols or rules) and hardware used to make a
network functional were proprietary.
Also, the networks’ technologies (components) were designed for a
specific purpose in mind (ie. Business, manufacturing, high/lowspeed, small/large capacity, etc..)
The certain applications could run on certain type networks
Larger corporations would typically have many different disjointed
computer networks - company mergers caused this problem too
(wasn’t perceived as a problem then)
Engineers and Scientists (or whomever) could have 3-4 terminals on
his/her desk for different uses
Customer would have to go to a specific vendor for an application or
network upgrade
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Introduction and Overview
• Back in the 70’s and early 80’s, there was a big push to
make communication systems or networks “open”
• “open” means “non-proprietary” - instead of the
“specifications” being known only by the vendor, the
specs would be be publicly known
• By having publicly published specifications, all of the
various vendors could design and manufacture network
components that were compatible and interchangeable
• Why was this a good thing ???? (Even if you were a
network-component-producing company with a
significant market share)
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Introduction and Overview
• Answer: would drive more (1) computer, (2) software
application and (3) network usage - therefore, drive
more revenue for all
• Answer: more efficiency for businesses, government,
etc..
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relates to OS’s ???
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Introduction and Overview
• The idea behind having “open systems” is to have the ability to
interconnect many different networks into a single network.
• The technology that allows this is called “Internetworking”
• Internetworking provides:
– The Interconnection of heterogeneous (different) networks
– Set of communication standards/protocols that make the
interconnected heterogeneous networks interoperate
(river, language scenario)
• Internetworking “hides” the details of the underlying hardware
and allow the network nodes to communicate independent of their
physical connection (or hardware)
• Internetworking can be called “internet technology” – notice to
lower case “i” on internet
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Introduction and Overview
• Some time ago, the government realized the benefit of internet
technology and funded a research project through an agency called
ARPA – Advanced Research Projects Agency
• Through ARPA support, the “open” system specs were realized.
• These open specs were called “TCP/IP Internet Protocol Suite”,
commonly called “TCP/IP”
• TCP – Transmission Control Protocol – dealt with higher level
issues like segmentation, reassembly and error detection.
• IP – Internetworking Protocol – dealt with datagram routing
• TCP/IP was heavenly sent in that: (1) previously disjointed
networks WITHIN companies could now function as a single
network and (2) it facilitated communications amongst
geographically dispersed sites
• With TCP/IP, the Internet was born. Notice the “I” on Internet –
called the Global Internet
• Global Internet interconnects over 170 million nodes – testimonial
for TCP/IP
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Introduction and Overview
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• Bottom Line: what makes TCP/IP so great and unique from other
network protocols ??:
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– Network Technology Independence – independent of a particular
vendor’s hardware
– Universal Interconnection – any 2 computers connected to the
internet can communicate – each computer has a unique
internationally recognized address
– End-to-End Acknowledgements – acknowledgements between the
source and destination versus intermediate nodes
– Application Protocol Standards – TCP/IP provides services (or
software) to applications needing lower level communication
services
• We will cover each of these attributes in detail throughout the
course
• Internet uses TCP/IP
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Introduction and Overview
Read more details about the history in your book
(Chapter 1)
Who is responsible the Internet ?. Internet
Architecture Board (IAB)
IAB coordinates the research and development in
relation to the TCP/IP protocols. The organization
decides which protocols are required and sets policies
Each member of the IAB chaired an Internet Task
Force responsible for investigating a set of problems or
issues each (there were 10 task forces)
The chairman of the IAB was called the Internet
Architect
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Introduction and Overview
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In 1989, the IAB was re-organized
due to commercial usage increases
The original IAB researchers were
moved under the Internet
Research Task Force – their focus
is longer term research
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The Internet Engineering
Task Force Chairman and
managers of each “working
group” forms the Internet
Engineering Steering Group
– this group is responsible
for coordination
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The Internet Engineering Task Force
is more concerned about short-term
issues and is mostly comprised of
industry types
In 1992, a group called the
Internet Society (ISOC) was form
to encourage participation on the
Internet.
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Introduction and Overview
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Documentation of TCP/IP is placed in online repositories and
made available at no charge – you will be responsible for
collecting some of this documentation
• The final and official TCP/IP documents start out as an
Internet draft (working document)
6• Upon recommendation from Internet authorities, the draft
0 may be published as a Request for Comment (RFC)
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Each RFC is edited, assigned a number and made available to all
interested parties. RFC’s go through maturity levels and are organized
according to their requirement level
• The six maturity levels are: proposed standard, draft standard, Internet
standard, historic, experimental and informational
• RFC’s are classified into 5 requirements levels: required, recommended,
elective, limited use and not recommended
• You can secure RFC’s: regular mail, e-mail, ftp or Internet
(http://www.rfc-editor.org)
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Introduction and Overview
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Introduction and Overview
• Protocols – set of rules that governs data communications
– defines what is communicated, how it is communicated
and when it is communicated
• Protocol elements are:
– Syntax – structure or format of the data (order of the bits)
– Semantics – meaning of each section of bits – how to interpret the
pattern of bits
– Timing – deals with (1) when the data should be sent and (2) how
fast it should be sent (ie. a Tx can overload a Rx and therefore data
can be lost or mis-interpreted)
• More clarity: the TCP/IP protocol allows one to specify
data communications without understanding the details of
the underlying hardware.
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Why Study OSI?
• Still an excellent model for conceptualizing and understanding
protocol architectures
• More granularity in functionality - more functional delineation
• Key points:
– Modular
– Hierarchical (chain of command, pecking order)
– Boundaries between layers (called interfaces)
NOTE: the protocols or functionality with in the layer could change
however, the interface remains the same – this facilitates the flexibility
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OSI Reference Model ?
• OSI – Open Systems Interconnection
• Set of rules of how to transmit data across a network
• at the lower levels of the model protocols define the electrical and
physical standards
• at the lower levels, the bit ordering, the transmission of the bits, and
error detecting and correcting are defined
• at the higher levels of the model, the protocols define the data
formatting, message syntax, dialogue management, message sequences
and info presentation
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Advantages of Layering
• Easier application development
• Network can change without all programs being
modified
• Breaks complex tasks into subtasks
• Each layer handles a specific subset of tasks
• Communication occurs
– between different layers on the same node or
stack (INTERFACES)
– between similar layers on different nodes or
stacks (PEER-TO-PEER PROCESSES
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OSI’s Layered Approach Example
Network A
Network B
Actual commands invoked, presentation
Top Layer
Top Layer
Facilitate the actual communications
Some Intermediate
Layer
Some Intermediate
Layer
Network interfaces, raw bits
Bottom Layer
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How does peer-to-peer
communication work ?
Bottom Layer
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OSI
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• Open Systems Interconnection
• Developed by ISO
(International Organization for Standardization)
• Contains seven layers
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Application
Presentation
Session
Transport
Network
Data Link
Physical
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OSI Reference Model ?
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• Bottom 3 layers
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• Bottom 3 layers responsible for getting the info to the
destination
• (Bottom 3 layers): at the lower levels of the model
protocols define the electrical and physical standards
• (Bottom 3 layers) at the lower levels, the bit ordering, the
transmission of the bits, and error detecting and correcting
are defined
• Top 4 layers
• at the higher levels of the model, the protocols define the
data formatting, message syntax, dialogue management,
message sequences and info presentation
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Ch 2: TCP/IP and OSI
Lecture 2
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OSI Physical Layer
• Responsible for transmission of bits
• Always implemented through hardware
• Encompasses mechanical, electrical, and functional
interfaces
• Encoding issues: how 0’s and 1’s are converted to
signals
• Transport medium: Coaxial, Twisted Pair, Optical,
etc..
• Transmission Rate/Data Rate – how fast to send bits
• Transmission mode: transmission direction (simplex,
duplex)
• Physical Topology: network layout
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OSI Data Link Layer
• Responsible for error-free, reliable transmission of
data
• Framing, Flow control, Error control
(detection/correction)
• Makes use of physical address because with in the
same network
Network Layer
Actually sends the packets (groups of
frames) from node to node using a routing
algorithm
Data Link Layer
Takes raw data (bits) and transform
them into frames, error control, etc.
Physical Layer
Transmit and receive the raw data
(bits)
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OSI Data Link Layer
High Level View of Data Link Layer’s Functions:
1. Take raw bits and transform them into frames or packets
(up)
2. Setup error detection on packets prior to them being sent
(down)
3. Perform error checking on packets received (up)
4. If error is encountered, the Data Link Layer notifies the
sender
5. Make sure not too much traffic is sent from the transmitter
to the receiver (flow control)
In general, the bullet items above deal with creating a
“transmission line”
To achieve the functions above, we must have STANDARDS.
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OSI Network Layer
• Responsible for routing of messages through networks
• Concerned with type of switching used (circuit v.
packet)
• Handles routing among different networks
• NOTE: with in the same network, only the DATA
LINK layer is needed – amongst multiple networks,
the NETWORK LAYER is needed
• No need for routing with in the same network
(LAN)
• Routing across “internetworks”
• Makes use of logical address vs physical address
because not with in same network
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OSI Network Layer
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Transport
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Network Layer
Concerned with an error-free end-to-end flow of
data
Actually sends the packets (groups of
frames) from node to node using a routing
algorithm
Data Link Layer
Takes raw data (bits) and transform them into
frames
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OSI Network Layer
High Level View of Network Layer’s Functions:
1. Transmitting data packets through a network in a timely
manner
2. There are more than one route between the source and
destination, the network layer chooses the best route (next
hop) based on some criteria.
Examples
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A
Node Cost
Link Cost
Distance
Spare Cap.
Low Util.
Z
3. Makes sure the network does not become congested when
link or node failures occur.
Passes data between two networks (differing networks)
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OSI Transport Layer
• Isolates messages from lower and upper layers
• Breaks down message size (segmentation) (down) and
performs re-assembly (up)
• Monitors quality of communications channel (oversee
all hops)
• Selects most efficient communication service
necessary for a given transmission (could change over
hops)
• Flow and Error control for Source and Sink
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OSI Session Layer
• Establishes logical connections between systems
(up/down)
• Manages log-ons, password exchange, log-offs
(up/down)
• Terminates connection at end of session (up/down)
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OSI Session Layer
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The Session Layer is responsible for (1) dialogue management, (2)
synchronization and (3) activity management.
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Dialogue Management – an example is, querying a database. Let the
DB sit on a remote server and the query is invoked from the client –
the entire process of sending the query and receiving the data is
considered “dialogue management”.
Synchronization – at the session layer, “synch points” can be
inserted in the data being transmitted. If network failures occur, the
data would be re-transmitted starting at the last synch point.
Activity Management – involves sending special messages at the
beginning and end of an activity. These messages can help the
receiver determine when to start processing (after all data is
received).
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OSI Presentation Layer
• Provides format and code conversion services
• Examples
– File conversion from ASCII to EBDIC
– Invoking character sequences to generate bold, italics, etc on
a printer
• The source and sink could operate using different
encoding schemes – the presentation layer makes the
translations
• Security
• Compression
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OSI Application Layer
• Provides access to network for end-user (end-user
being a human being or software application)
• User’s capabilities are determined by what items are
available on this layer (ie. remote log-in, file transfer,
email service, directory service, etc.)
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What happens at the End and
Intermediate Nodes ?
Rx
Tx
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Intermediate Nodes
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Recap - OSI’s Layered Approach
– between different layers on the same node or stack (INTERFACE)
– between similar layers on different nodes or stacks (PEER-TO-PEER
PROCESSES)
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An exchange using the OSI model
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Explain encapsulation
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decapsulation
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COMPLEXITY TO CONSIDER
• Any particular node in an internetwork can be
functioning as follows simultaneously:
• Tx to other internetwork nodes
• Rx from other internetwork nodes
• Intermediate node to some other internetwork nodes
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The File Transfer Program issues
a command to the Application
Layer
Application passes it to
Presentation, which may
reformat, encrypt, encode,
compress, passes to Session (adds
overhead)
Session requests a connection,
passes to Transport (adds
overhead)
Transport breaks file into chunks,
adds error-checking and flowcontrol info, process-to-process,
passes to Network (adds
overhead)
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• Network selects the data’s
route (internetworking),
passes to Data Link (adds
overhead)
• Data Link adds error-control
and flow-control info, passes
to Physical (adds overhead)
• Physical translates bits to
signal and transmits the
signal, which includes
information added by each
layer
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Physical receives signal and
translates to bits, passes to Data
Link
Data Link checks for errors and
performs flow control on bits,
formulates bits into some
formation (frames), passes to
Network
Network verifies routing (if
intermediate node, determines
next hop), passes to Transport
Transport checks for errors and
performs flow control on the
chunks, reassembles the
chunks, passes to Session
Session determines if transfer
is complete, may end session,
passes to Presentation
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Lecture 2
• Presentation may reformat,
perform conversions, decode,
decrypt, decompress, pass to
Application layer
• Application presents results to
user (e.g. updates FTP
program display)
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US Postal System Analogy
• Illustrate how the US Postal System is very similar to
how networking works
• Will help students better understand (versus memorize)
networking
Upper Layers – creating
and interpreting the
signal, data or info
Lower Layers – getting
the signal from one
place to the next
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