PowerPoint Presentation
OSI Model
Dr John C. Murray
Principal Lecturer
Workshop Recap
Wireshark
Allows us to view packets sent to and from other nodes
On the same ‘viewable’ network
What does this mean?
Allows filters to be applied such as:
TCP, UDP, DNS
Anything else?
Follow TCP/UDP Stream
What else can we do?
What about User IDs and Passwords?
FTP?
ASCII vs Binary
Questions
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Questions
Who is responsible for distributing IP addresses?
IANA is responsible for global coordination of the Internet Protocol addressing systems, as well as the Autonomous System Numbers used for routing Internet traffic.
https://www.iana.org/numbers
Recap
Last week we looked at:
Packets
What is a Packet?
What is a Packet Header?
What information does it contain?
Why?
IP Packet
Offset Octet 0 1 2 3
Octet Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
0 0 Version HLen DSCP ECN Total Length
4 32 Identification Flags Fragment Offset
8 64 Time to Live Protocol Header Checksum
12 96 Source IP Address
16 128 Destination IP Address
20 160 Options
DATA (Payload)
Questions
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How is this coordinated?
We have lots of hardware, such as?
Computers,
Network Cards
Motherboards
Bridges
Switches
Cables
Routers
Firewalls
Satellites
The list goes on…
How is all this managed?
We have lots of Network Topologies
We have lots of data
In packets
Moving across lots of networks
All at the same time
With different protocols
TCP / UDP
FTP, TELNET
ICMP, ARP
Standards
However, the internet seems to ‘work’!
This is because we have ‘standards’ that define protocols and how we should communicate
What would happen if a ‘standard’ did not exist?
Big Endian
Little Endian
A standard is required to ensure full compatibility and interoperability between devices.
Big vs Little Endian
What is this number:
101011
Assume Base 2 (Binary)
53 in Decimal
Or is it 43?
Depending on the standard used…
Big Endian
The MSB is stored in 0Ah, the lowest address
The other bytes follow in decreasing order of significance.
Little Endian
The LSB is stored in 0Dh, the lowest address
The other bytes follow in increasing order of significance
Why do we have both?
What about VHS vs Betamax?
What about NTSC vs PAL?
Competing technologies. Simple as that.
OSI Model
Problem addressed by the International Standards Organisation (ISO) in 1978. This became an international standard in 1984
To create a model that addresses the following:
To interconnect equipment, made by different manufacturers, through a standard interface
To integrate software and hardware, and to be portable on differing systems
Create a model which will be adhered to by all countries of the world
OSI Model
To create an open standardised networking model that all vendors would support.
Headed up by the International Organization for Standardization (ISO) beginning work on what would be known as the Open System Interconnection (OSI) networking model.
ISO had a noble goal for the OSI model: to standardise data networking protocols to allow communication between all computers across the entire planet.
OSI Model
Problem: How does information move between computers and applications?
Problem divided into seven manageable problems (layers)
Each layer solves one of the seven problems
7-layer Model
OSI – Basic Overview
Physical Layer
Transmission of binary data via a medium
It defines the cabling, or method of data carriage (could be wireless), connectors, electrical characteristics of the communication channel and the transmitted signal.
Defines the protocol to establish and terminate a connection between two ‘directly’ connected nodes.
The NIC converts the binary data into electrical voltages (high and low). Devices that operate at the Physical layer include network interface cards, hubs, bridges, switches, routers (though some of these devices work at other layers too).
OSI – Basic Overview
Data Link Layer
Provides a reliable link between two directly connected nodes
By detecting and possibly correcting errors that may occur in Physical Layer
Ensures that all of the data from the sending computer has been received thereby providing flow, error control, and synchronisation of the Physical layer.
Error detection – prevents two computers accessing the media simultaneously (collision detection).
Point-to-Point Protocol (PPP) is an example of the Data Link layer in the TCP/IP stack
Any idea what devices used to use PPP?
Dial-up modems
OSI – Basic Overview
3. Network Layer
Manages the transfer of packets that are to be forwarded on to different networks, between nodes on the same network
Network Addressing and determining best path
Data is routed through a network and can also be routed through interconnected networks. Splitting of data for transmission and re-assembling upon reception. The IP part of TCP/IP operates at this level.
OSI – Basic Overview
4. Transport Layer
Manages end-to-end message delivery in the network
Provides reliable and sequential packet delivery through error recovery and flow control mechanisms
Can keep track of segments, providing the ability to:
Resend failed segments
Acknowledge successful delivery and send next data
OSI – Basic Overview
5. Session Layer
Manages communication between applications after a connection is made. Sets up the session, manages information exchanges, and then breaks it down when the session ends.
Session Inter-host Communication – Sets up, maintains, and closes down a session. This operation provides an open communication path with another system.
Winsock, Sockets, etc.
OSI- Basic Overview
6. Presentation Layer
Formats data for exchange between Application and Session layer.
Masks the differences of data formats between dissimilar systems
Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data.
SSL, MIME for example
Presentation Data Representation and Interpretation Translation may need to occur between two systems using different presentation standards e.g. different character sets or character codes. The Presentation layers allows data to be interpreted using a set of translations and can also add data encryption for security purposes.
OSI – Basic Overview
7. Application Layer
Provides the user interface for communication. Functions typically include: email, file transfer, file management, message handling and database query functions.
Application Network Services to Application Programs – Provides network services to application programs, such as electronic mail and file transfer.
Telnet, SSH, FTP, TFTP, etc
Accessing a Web Page
Problem: You open a web browser and access a web site. How does this propagate through the OSI Model?
In an abstract way?
Via the protocols in the OSI model layers?
OSI – Web page access
Email via OSI
http://dl.dropbox.com/u/11235544/osi.swf
OSI Summary
OSI Summary
Benefits of the OSI model
Reduced complexity – complex problem is decomposed into seven layers.
Increased evolution – Technology can advance and still integrate with old technology.
Guarantees interoperability – Ensures data can be transferred between differing computer types, software, operating systems, network and computer hardware.
Allows modular engineering – hardware and software can interface well with each other.
Standardised interfaces – products can be designed to easily plug into one or more layers of the model.
TCP/IP Model
Questions
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