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Department of Computer Science
Southern Illinois University Carbondale
CS591 – Wireless & Network Security
Lecture 2: Overview of Computer
Networking
Dr. Kemal Akkaya
E-mail: [email protected]
Kemal Akkaya
Wireless & Network Security
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What’s the Internet: “nuts and
bolts” view
 Millions of connected computing
devices: hosts = end systems
 Running network apps
 Communication links
 fiber, copper, radio, satellite
 transmission rate = bandwidth
router
server
workstation
mobile
local ISP
 Routers: forward packets (chunks
of data)
 Protocols control sending,
receiving of msgs
regional ISP
 e.g., TCP, IP, HTTP, FTP, PPP
 Internet: “network of networks”
 Loosely hierarchical
 Public Internet vs Private intranet
 Internet standards
 RFC: Request for Comments
 IETF: Internet Engineering Task Force
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company
network
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The network edge
 End systems (hosts)
 Run application programs
 e.g., Web, email
 At “edge of network”
 Client/Server model
 Client host requests, receives
service from always-on server
 e.g. Web browser/server; email
client/server
 Peer-Peer model
 Minimal (or no) use of dedicated
servers
 e.g., Napster, Gnutella, KaZaA
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What’s the Internet: a service view
 Communication infrastructure
enables distributed applications
 Network edge: applications and hosts
 Network core:
 routers
 network of networks
 Access networks, physical media:
communication links
 Applications: Web, email, games, ecommerce, file sharing
 Communication services
provided to apps:
 Connectionless unreliable
 Connection-oriented reliable
 Various protocols are used for
communication services
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What’s a protocol?
Human protocols:
 “What’s the time?”
 “I have a question”
 introductions
… specific msgs sent
… specific actions taken
when msgs received,
or other events
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Network protocols:
 Machines rather than
humans
 All communication
activity in Internet
governed by protocols
Protocols define format,
order of messages sent
and received among
network entities, and
actions taken on
message transmission,
receipt
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What’s a protocol?
 A human protocol and a computer network protocol:
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
Q: Other human protocols?
<file>
time
 Key Elements of a Protocol:
 Timing
 Syntax
Data formats
Signal levels
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Speed matching
Sequencing
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 Semantics
Control information
Error handling
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Protocol “Layers”
Networks are complex!
 Many “pieces”:






 Consider the following analogy:
hosts
routers
links of various media
applications
protocols
hardware, software
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
Question:
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
Is there any hope of
organizing structure
of network?
Or at least our
discussion of
networks?
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Organization of air travel
airplane routing
 A series of steps
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Layering of airline functionality
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
intermediate air-traffic
control centers
ticket
arrival
airport
 Layers: Each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
 Why Layering?
 Dealing with complex systems
 Explicit structure allows identification, relationship of complex system’s pieces
 Layered reference model for discussion
 Modularization eases maintenance, updating of system
 Change of implementation of layer’s service transparent to rest of system
 e.g., change in gate procedure doesn’t affect rest of system
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Standardized Protocol Architectures
 Required for devices to communicate
 Vendors have more marketable products
 Customers can insist on standards based equipment
 Two standards:
 OSI Reference model
 Open Systems Interconnection
 Developed by the International Organization for Standardization (ISO)
 Seven layers
 Never lived up to early promises
 TCP/IP protocol suite
 Most widely used
 De facto standard
 Also: IBM Systems Network Architecture (SNA)
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OSI - The Model
 A layer model
 Each layer performs a subset of the
required communication functions
 Each layer relies on the next lower
layer to perform more primitive
functions
 Each layer provides services to the
next higher layer
 Changes in one layer should not
require changes in other layers
 Why OSI did not take over the world




Bad timing
Bad technology
Bad implementations
Bad politics
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The OSI Environment
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OSI as Framework for Standardization
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OSI Layers
 Transport
 Physical
 Physical interface between
devices




Mechanical
Electrical
Functional
Procedural
 Data Link
 Means of activating,
maintaining and
deactivating a reliable link
 Error detection and control
 Higher layers may assume
error free transmission
 Network
 Transport of information
 Higher layers do not need to
know about underlying
technology
 Not needed on direct links
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





Exchange of data between end systems
Error free
In sequence
No losses
No duplicates
Quality of service
 Session




Control of dialogues between applications
Dialogue discipline
Grouping
Recovery
 Presentation
 Data formats and coding
 Data compression
 Encryption
 Application
 Means for applications to access OSI
environment
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Use of a Relay
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TCP/IP Protocol Architecture
 Developed by the US Defense Advanced Research
Project Agency (DARPA) for its packet switched network
(ARPANET)
 Used by the global Internet
 No official model but a working one.





Application layer
Host to host or transport layer
Internet layer
Data link layer
Physical layer
 Problems:
 Service, interface, and protocol not distinguished
 Not a general model
 No exact separate mention of physical and data link layers
 Sometimes called host-to-network layer
 Minor protocols deeply entrenched, hard to replace
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Layer Descriptions
 Physical Layer
 Physical interface between data transmission device (e.g. computer) and
transmission medium or network
 Characteristics of transmission medium
 Signal levels
 Data rates
 Data Link Layer
 Exchange of data between neighboring network nodes
 Invoking services like priority
 Internet (IP) Layer
 Systems may be attached to different networks
 Routing functions across multiple networks
 Implemented in end systems and routers
 Transport Layer (TCP)
 Reliable delivery of data
 Ordering of delivery
 Application Layer
 Support for user applications: HTTP, SMTP
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OSI vs TCP/IP
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Protocols and networks in the TCP/IP model
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PDUs in TCP/IP
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source
message
segment Ht
datagram Hn Ht
frame
Hl Hn Ht
M
M
M
M
Encapsulation
application
transport
network
link
physical
Hl Hn Ht
M
link
physical
Hl Hn Ht
M
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
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Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
Hl Hn Ht
M
M
router
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Some Protocols in TCP/IP Suite
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