PowerPoint Presentation
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SIT182 – Real World Practices For Cyber Security
Trimester 2 – 2021
Deakin College
Week 4 – Part 1
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• Basic Networking Background
2
Topics,
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NOTE
• This unit is not aiming to teach you ALL about networking. You will have other units for this in your
degree.
• Concepts covered here are just the very minimum knowledge that you need to have so we can
cover some of the basics of network security.
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Protocol
• A Protocol is an agreement on how to communicate
• Specifies the syntax
• Format of messages
• Order in which they’re exchanged
• Specifies the semantics
• What messages mean
• What to do on message send or receive, or at specified times
• Defines a language for communications
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Internet Protocol (IP)
• The Internet Protocol (IP) is what lets computers communicate around the
world.
• Has a well-defined binary representation for transmission over the
networks it comprises.
• Everything is big-endian (aka Network byte order)
• Any IP-enabled host receiving an IP packet knows how to handle it,
because of the protocol.
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End-to-End Principles
A and B are End hosts
• on the periphery of the network
• not physically connected
• communicate through the network
Ri are Routers
• in the interior of the network
• Routing specifies how to get from A to B
• Forwarding moves traffic towards B
End-to-end Principles: knowledge/control of connections exclusively at
the periphery
Interior nodes only forward packets based on local delivery rules
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An Analogy
Postal service:
You write a letter to your friend
You seal that letter in an envelope
You write your friend’s address on the envelope
You drop that envelope in themail
Australia Post picks it up
Internet:
You click on a link in your webbrowser
Your browser constructs a packet with the GET request
Your browser adds the server’s address to the packet
Your computer sends that packet out its network interface
The gateway router receives that packet
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An Analogy
Postal service: Envelope → Destination with successively smaller hops
State -> City -> Street -> Address -> Apartment number
⇒ Australia Post never knows contents of envelope
Internet: Packet → Destination with successively smaller hops
ISP (Internet Service Provider) -> Region -> Organization’s network -> Office’s
subnet -> Server
⇒ Routers never look at payload of packet
Layers
• Abstract network protocol into separate layers
• Each layer controls some aspect of
communications
• A layer relies on services provided by the layer
below it
• A layer provides services to the layer above it
(Similar to how software is designed)
• Each layer has a distinct role
The Network Stack
• Open Systems Interconnection (OSI) Model
• Defines 7-Layer Model for the network stack
• We will only worry about 5 of these -☺
• As with software stack, each layer has a distinct
role
• These roles are codified/implemented with
protocols
How Protocol Stack Works
Physical Layer
How to encode bits for a single physical link
►Voltage levels
►RF modulation
►Photon wavelengths/intensities
Lots of technologies
►Coaxial cable
►Twisted-pair cable
►RF broadcast
►Fiber-optic cable
Link Layer
• Combines bits into frames
• Provides local addressing (MAC—Media Access
Control)
• Supports point-to-point and often broadcast
delivery
MAC address of a device
shown in MS Windows OS
Link Layer
• Link-layer connections grouped into subnets
• Link-layer protocol used to transmit messages within this subnet. MAC addresses must be unique.
⇒ Device can join any subnet
⇒ Address space partitioned by manufacturers (MAC is fixed and unique, which is included by the
device manufacturer)
What is a subnet?
• A subnet is a logical partition of an IP
network into multiple, smaller
network segments.
• The internet is composed of many
networks that are run by many
organizations. In turn, each
organization’s network can be
composed of many smaller networks,
or subnets. Each subnet allows its
connected devices to communicate
with each other, and routers are used
to communicate between subnets.
Network Layer
• Bridges subnets for end-to-end connectivity
• Provides global addressing (IP addresses — 32 bits)
• Delivery is best-effort
⇒ No retransmissions
⇒ No message integrity
• Data is encapsulated in packets
⇒ payload for Link-layer (Layer 2) frames
Internet: network of networks using the Internet Protocol
https://thumbs.dreamstime.com/z/tcp-ip-network-model-28867280.jpg
Network Layer
• IP addresses are locally unique
⇒ assigned within a subnet (CIDR notation: 192.168.0.0/16)
⇒ some hosts not globally addressable
• A router is a device that implements up through Network layer (Layer 3)
⇒ connects multiple subnets
⇒ different IP addresses on each interface
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IP Networks – Simply Put
• Data is divided into packets
• Internet delivers packets between IP addresses
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Routers – Simply Put
• Each router forwards packets to the next based on the destination address
• Unreliable (best effort), routers may drop packets
• Not through predefined route – different factors such as load affect the route a packet
takes
Packet Switching: example
A
C
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D
F
E
3 2 1Packets
A
C
B
D
F
E
3 2
1
Packets
Packet Switching: example
A
C
B
D
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3
21
Packets
Packet Switching: example
A
C
B
D
F
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321
Packet Switching: example
Computer Networks
24
Transport Layer
End-to-end communication between processes
UDP (User Datagram Protocol)
► UDP socket contains the port number and
IP address of the destination only
► unreliable, best effort, no packet retransmission
if it gets lost
► datagram-based (single-packet messages)
TCP (Transmission Control Protocol)
► TCP socket contains the port numbers and IP addresses of the source & destination
► reliable, keeps track of data sent/received
► retransmission of lost packets
► byte-based (messages/sessions span possibly many packets)
Computer Networks
25
Application Layer
What users/processes interact with
Choice of transport depends on what is needed
► Web browsing ⇒ TCP
► Email ⇒ TCP
► Voice calls ⇒ UDP
Defines its own data formats and protocols, within TCP or UDP
► Web browsing ⇒ HTTP (Hypertext Transfer Protocol)
► Email ⇒ SMTP (Simple Mail Transfer Protocol)
► Voice calls ⇒ RTP (Real-time Transport Protocol)
Computer Networks
26
Endpoint Names
Each protocol layer has its own way of naming endpoints:
• Ethernet (MAC) addresses in the link layer (e.g. 00-B0-D0-05-04-7E)
• IP address in the network layer (e.g. 157.58.56.101)
• TCP port number + IP address
• URL in web services (e.g. http://www.tempurl.org/myservice)
Computer Networks
27
Network Protocols in 1-slide
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References and Further Reading
What’s a Network: https://www3.nd.edu/~cpoellab/teaching/cse40814_fall14/networks.pdf
[Chapter: An Introduction to Information Networks] Introduction to Computer Networks and Cybersecurity, J.
Irwin, CRC Press.
https://www3.nd.edu/~cpoellab/teaching/cse40814_fall14/networks.pdf
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Acknowledgement
Acknowledging the kind support and contribution of:
Dr Arash Shaghaghi (Deakin University, Australia), Prof. Chang-Tsun Li (Deakin University, Australia), Prof. Sanjay
Jha (The University of New South Wales, Australia), Dr. Nicolas Courtois (University College London, UK), Dr George
Danezis (University College London, UK), and Dr Michael March (University of Maryland, USA).