7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

OSI Model Flashcards by Mike Groves

OSI mOdel Overview

Developed in 1977 by International Organization for Standardization (ISO)
▪ Called the OSI model or OSI stack
▪ Consists of 7 layers
▪ Useful in troubleshooting networks
▪ Serves as a reference model in networks

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Purposes of Reference Model

Categorize functions of the network into particular layer(s)
▪ Compare technologies across different manufacturers
▪ By understanding its functions, you can understand how best
to communicate with that device

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OSI Model Layers

Application
Presentation
Session
Transport
Network
Data Link
Physical

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Data Types in OSI Model

Application —————- Data
Presentation—————-Data
Session————————Data
Transport———Segments
Network————–Packets
Data Link—————Frames
Physical—————Bits

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Layer 1 (Physical)

Transmission of bits across the network
▪ Physical and electrical characteristics
▪ Characteristics:
● How bits are represented on the medium
● Wiring standards for connectors and jacks
● Physical topology
● Synchronizing bits
● Bandwidth usage
● Multiplexing strategy

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Layer 1 Examples

Cables
● Ethernet
● Fiber optic
▪ Radio frequencies
● Wi-Fi
● Bluetooth
▪ Infrastructure devices
● Hubs
● Wireless Access Points
● Media Converters

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Layer 2 (Data link)

▪ Packages data into frames and transmitting those frames on the network,
performing error detection/correction, and uniquely identifying network
devices with an address (MAC), and flow control
● MAC
● Physical addressing
● Logical topology
● Method of Transmission
● Link Layer Control (LLC)
o Connection services
o Synchronizing transmissions

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Layer 2 Examples

Network Interface Cards (NIC)
▪ Bridges
▪ Switches

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Layer 3 (Network)

Forwards traffic (routing) with logical address
● Example: IP Address (IPv4 or IPv6)

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Logical Link Control (LLC)

Provides connection services
▪ Acknowledgement of receipt of a message
▪ Flow control
● Limits amount of data sender can send at one time to keep
receiver from becoming overwhelmed
▪ Error control
● Allows receiver to let sender know when an expected data frame
wasn’t received or was corrupted by using a checksum

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Logical Address

Numerous routed protocols were used for logical addressing over the
years:
● AppleTalk
● Internetwork Packet Exchange (IPX)
● Internet Protocol (IP)
▪ Only Internet Protocol (IP) remains dominant
● IP v4
● IP v6

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How should data be forwarded or routed? Layer 3

Packet switching (known as routing)
● Data is divided into packets and forwarded
▪ Circuit switching
● Dedicated communication link is established between two devices
▪ Message switching
● Data is divided into messages, similar to packet sw

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ICMP (Layer 3)

Used to send error messages and operational information about an IP
destination
▪ Not regularly used by end-user applications
▪ Used in troubleshooting (ping and traceroute)

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Layer 3 Examples

▪ Routers
▪ Multilayer switches
▪ IPv4 protocol
▪ IPv6 protocol
▪ Internet Control Message Protocol (ICMP)

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Layer 4 (Transport Layer)

▪ Dividing line between upper and lower layers of the OSI model
▪ Data is sent as segments
▪ TCP/UDP
▪ Windowing
▪ Buffering

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TCP (LAYER 4)

▪ Connection-oriented protocol
▪ Reliable transport of segments
● If segment is dropped, protocol detects it and resends segment
▪ Acknowledgements received for successful communications
▪ Used for all network data that needs to be assured to get to its
destination

Layer 4 Examples

▪ TCP
▪ UDP
▪ WAN Accelerators
▪ Load Balancers
▪ Firewalls

Layer 5( Session)

▪ Think of a session as a conversation that must be kept separate from
others to prevent intermingling of the data
▪ Setting up sessions
▪ Maintaining sessions

Layer 5 Examples

H.323
● Used to setup, maintain, and tear down a voice/video connection
▪ NetBIOS
● Used by computers to share files over a network

Layer 6 (Presentation)

▪ Responsible for formatting the data exchanged and securing that data
with proper encryption
▪ Functions
▪ Data formatting
▪ Encryption

Layer 6 Examples

HTML, XML, PHP, JavaScript, …
▪ ASCII, EBCDIC, UNICODE, …
▪ GIF, JPG, TIF, SVG, PNG, …
▪ MPG, MOV, …
▪ TLS, SSL, …

Layer 7 (Application)

Provides application-level services
● Not Microsoft Word or Notepad
▪ Layer where the users communicate with the computer
▪ Functions:
● Application services
● Service advertisement

Layer 7 Examples

E-mail (POP3, IMAP, SMTP)
▪ Web Browsing (HTTP, HTTPS)
▪ Domain Name Service (DNS)
▪ File Transfer Protocol (FTP, FTPS)
▪ Remote Access (TELNET, SSH)
▪ Simple Network Management Protocol (SNMP)

Encapsulation

The process of putting headers (and sometimes trailers) around some data

Decapsulation

o Action of removing the encapsulation that was applied o If we move down the OSI layers from 7 to 1, we encapsulate data o If we move upward from layers 1 to 7, we decapsulate data o A protocol data unit is a single unit of information transmitted within a computer network ▪ Layer 1 - bits ▪ Layer 2 - frames ▪ Layer 3 - packets ▪ Layer 4 - segments if TCP or datagrams if UDP

SYN (Synchronization flag)

▪ The most well-known flag in TCP communications because it is used to synchronize the connection during the three-way handshake

ACK (acknowledge flag)

Used during the three-way handshake, but it is also used to acknowledge the successful receipt of packets

FIN (Finished flag)

▪ Used to tear down the virtual connections created using the three-way handshake and the SYN flag ▪ The FIN flag always appears when the last packets are exchanged between a client and server and the host is ready to shutdown the connection

RST (reset flag)

Used when a client or server receives a packet that it was not expecting during the current connection

PSH (push flag)

▪ Used to ensure that the data is given priority and is processed at the sending or receiving ends

URG (urgent flag)

▪ It is like the Push flag and identifies incoming data as “urgent” ▪ The main difference is PSH is used by a sender to indicate data with a higher priority level where URG is sent to tell the recipient to process it immediately and ignore anything else in queue ● Source and Destination ports o are just like the ones used in UDP, they dictate where the data is coming from and where it is going to ● Length o Used to indicate how many bytes the UDP packet is, including its header and its data ● Checksum o Not a mandatory field, but it can be used to provide some validation that the UDP data being sent was received with some level of integrity

EtherType field

Used to indicate which protocol is encapsulated in the payload of the frame ▪ As data moves from layer 7 to layer 1, that data is encapsulated ● At layer 4, we add our source and destination ports ● At layer 3, we add our source and destination IP addresses ● At layer 2, we add our source and destination MAC addresses ▪ Once we get to layer 1, we are simply transmitting our layer 2 frames as a series of 1’s and 0’s over the medium ▪ Once that host is found, it will keep decapsulating the information all the way up to layer 7, where its application can read and understand the underlying data