NETWORKING (1)

Dr. Andy Wu

BCIS 4630 Fundamentals of IT Security

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OSI Model• Network communications protocols are developed using the

Open System Interconnection (OSI) model.• An ISO standard for worldwide communications. • The OSI model defines a framework for implementing

protocols in seven distinct layers.• Each layer makes use of the services of the layer below it to

provide a set of specific services to the layer above it. • Adjacent layers communicate with each other through a well-

defined interface, so that in principle, we could change one layer or even completely rewrite it without affecting the others.

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OSI LayersLayer Function Examples

Application User interfaceProgram level communication

HTTP, SMTP

Presentation Presentation format of dataData conversion if needed

ASCII, JPEG, MP3, Encryption

Session Establish and maintain communication channels OS, Application access scheduling

Transport Reliable or unreliable end-to-end communicationIdentifies upper layer processes that will receive data

TCP, UDP, SPX

Network Logical addressing and routingDelivering packets from source to destination

IP, IPX

Data Link Transforming physical layer into a reliable link and presenting it to upper layersIncludes MAC (802.3) and LLC (802.2) sublayers

Ethernet

Physical Transmission of bit stream over physical mediumSignaling

EIA/TIA 568B

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OSI Mnemonic

Away Application

Pizza Presentation

Sausage Session

Throw Transport

Not Network

Do Data Link

Please Physical

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Encapsulation• When data is passed down from higher

layers to lower ones, each layer creates its header and places the data given to it by the next-higher layer behind its own header, thereby encapsulating the higher layer’s data.

• The Data Link layer also creates a trailer.

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Encapsulation – Layer 4

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Encapsulation – Layer 3

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Encapsulation – Layer 2

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Network Communication

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Decomposition – Layer 2

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Decomposition – Layer 3

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Decomposition – Layer 4

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Protocol Data Unit• A term that describes a set of bytes that includes the layer’s header and

trailer (if any) and all data encapsulated.• From Layer X’s perspective, the higher-layer headers and the user data

form one large data field.

Layer(s) PDU

Application, Presentation, Session Data

TransportTCP SegmentUDP Datagram

Network IP Packet

Data Link Frame

Physical Bit