OSI Model (1.1 & 5.3) Flashcards
OSI Model Overview & Objective
- OSI Model (Open Systems Interconnection)
- Purpose of Reference Model
- OSI Model Layers
- Data Types in the OSI Model
OSI Model (1977 ISO / 7 layers / troubleshooting / reference model)
- 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
Purpose of Reference Model (categorized in layers / comparing tech / learning and understanding to communicate)
▪ 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
OSI Model Layers (PDNTSPA)
7: Application
6: Presentation
5: Session
4: Transport
3: Network
2: Data Link
1: Physical
(Please / Do / Not / Throw / Sausage / Pizza / Away!)
Data Types in the OSI Model (DSPFB?)
7 / 6 / 5 = Data 4 = Segments 3 = Packets 2 = Frames 1 = Bits (Don't / Some / People / Fear / Birthdays?)
Layer 1 (Physical) ( Transmission / Physical and Electrical / bits)
Physical Layer (Layer 1)
▪ 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
How are bits represented on the medium? ( 1’s and 0’s / 0 = 0, +/- 5 = 1 / clock cycle)
▪ Electrical voltage (copper wiring) or light (fiber optics) represent 1’s and 0’s (bits)
▪ Current State
● If 0 volts, then 0 is represented
● If +/- 5 volts, then 1 is represented
▪ Transition Modulation
● If it changed during the clock cycle, then a 1 is represented, otherwise, a 0
How are the cables wired? (RJ45 standard = TIA/EIA-568-B / crossover = T568A or B / straight-thru = 68B on both ends or 68A)
▪ TIA/EIA-568-B is standard wiring for RJ-45 cables and ports
▪ Crossover cables use T-568A and T-568B
▪ Straight-thru cables typically use T-568B on both ends, but could use T-568A on both
How are the cables connected? (Layer 1 = B R S H-S FM PM)
▪ Layer 1 devices view networks from a physical topology perspective ▪ Includes: ● Bus ● Ring ● Star ● Hub-and-Spoke ● Full Mesh ● Partial Mesh
How is communication synchronized? (Asyn = Start and Stop to indicate (When) / Syn = Reference Clock to coordinate (Lead) )
▪ Asynchronous
● Uses start bits and stop bits to indicate when transmissions occur from sender to receiver
▪ Synchronous
● Uses a reference clock to coordinate the transmissions by both sender and receiver
How is bandwidth utilized? (Broadband = Channel / Baseband = Ethernet)
▪ Broadband ● Divides bandwidth into separate channels ● Example: o Cable TV ▪ Baseband ● Uses all available frequency on a medium (cable) to transmit data and uses a reference clock to coordinate the transmissions by both sender and receiver ● Example: o Ethernet
How can we get more out of a limited network? (TDM = take turns / STDM = needed basis / FDM = combined transmission on single line/channel)
▪ Time-Division Multiplexing (TDM)
● Each session takes turns, using time slots, to share the medium between all users
▪ Statistical Time-Division Multiplexing (StatTDM)
● More efficient version of TDM, it dynamically allocates time slots on an as-needed basis instead of statically assigning
▪ Frequency-Division Multiplexing (FDM)
● Medium is divided into various channels based on frequencies and each session is transmitted over a different channel
o Broadband
Examples at Layer 1 (Cables / Radio Frequencies / Infrastructure)
▪ Cables ● Ethernet ● Fiber optic ▪ Radio frequencies ● Wi-Fi ● Bluetooth ▪ Infrastructure devices ● Hubs ● Wireless Access Points ● Media Converters
Layer 2 (Data Link) (Data into frames / correction / identify / flow) (MAC, addressing, logical top, transmission, LLC)
Data Link Layer (Layer 2) ▪ 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
Media Access Control (MAC) (Physical Addressing / Logical Topology / Transmission)
▪ Physical addressing
● Uses 48-bit address assigned to a network interface card (NIC) by manufacturer
● First 24-bits is the vendor code
● Second 24-bits is a unique value
▪ Logical topology
● Layer 2 devices view networks logically
● Ring, bus, star, mesh, hub-and-spoke, …
▪ Method of transmission
● Many devices are interconnected
● Determines whose turn it is to transmit to prevent interference with other devices
Logical Link Control (LLC) (Connection services / acknowledgement of receipt / flow control / error control)
▪ 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
How is communication synchronized? (Iso = ref clock for time slots / Syn = clocking for beginning to end / Asyn = internal clocks for start/stop bits)
▪ Isochronous
● Network devices use a common reference clock source and create time slots for transmission
● Less overhead than synchronous or asynchronous
▪ Synchronous
● Network devices agree on clocking method to indicate beginning and end of frames
● Uses control characters or separate timing channel
▪ Asynchronous
● Network devices reference their own internal clocks and use start/stop bits
Examples at Layer 2 (NICs, Bridges, Switches)
▪ Network Interface Cards (NIC)
▪ Bridges
▪ Switches
Layer 3 (Network) (Forwards traffic / IPv4 or IPv6)
Network Layer (Layer 3) ▪ Forwards traffic (routing) with logical address ● Example: IP Address (IPv4 or IPv6) ▪ Logical addressing ▪ Switching ▪ Route discovery and selection ▪ Connection services ▪ Bandwidth usage ▪ Multiplexing strategy
Logical Address (protocols over the years / AT / IPX / IP / dominance)
▪ 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
How should data be forwarded or routed? (Packet / Circuit / Message Switching)
▪ 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 switching, except these messages may be stored then forwarded
Route Discovery and Selection (Routing table –> understand –> dest. IP / configured static or dynamic via routing protocol)
▪ Routers maintain a routing table to understand how to forward a packet based on destination IP address
▪ Manually configured as a static route or dynamically through a routing protocol
● RIP
● OSPF
● EIGRP
Connection Services (Flow Control / Packet Reordering = packets to multiple links/routes for faster service)
▪ Layer 3 augment Layer 2 to improve reliability
▪ Flow control
● Prevents sender from sending data faster than receiver can get it
▪ Packet reordering
● Allows packets to be sent over multiple links and across multiple routes for faster service
Internet Control Message Protocol (ICMP) (sends error messages / troubleshooting / ping and traceroute)
▪ 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)
Examples at Layer 3
▪ Routers ▪ Multilayer switches ▪ IPv4 protocol ▪ IPv6 protocol ▪ Internet Control Message Protocol (ICMP)
Layer 4 (Transport) (Transfer data TCP/UDP / End-to-End connections)
Transport Layer (Layer 4) ▪ Dividing line between upper and lower layers of the OSI model ▪ Data is sent as segments ▪ TCP/UDP ▪ Windowing ▪ Buffering
TCP (Transmission Control Protocol) (Connection-oriented / resends segments / successful communication / assures delivery)
▪ 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
UDP (User Datagram Protocol) (UDP = unreliable / no handshake / etc)
▪ Connectionless protocol ▪ Unreliable transport of segments ● If dropped, sender is unaware ▪ No retransmission ▪ Good for audio/video streaming ▪ Lower overhead for increased performance
Examples of UDP: Video Streaming (Youtube)
TCP vs UDP
TCP:
- Reliable
- Connection-oriented
- Segment retransmission and flow control through windowing
- Segment sequencing
- Acknowledge segments
UDP:
- Unreliable
- Connectionless
- No windowing or retransmission
- No sequencing
- No acknowledgement
Windowing (Adjusts data in each segment / adjusts to send more or less / low = more retransmissions / high = less retransmissions)
▪ Allows the clients to adjust the amount of data sent in each segment
▪ Continually adjusts to send more or less data per segment transmitted
● Adjusts lower as number of retransmissions occur
● Adjusts upwards as retransmissions are eliminated
Buffering (bandwidth that is not available / when available it transmits / segments drop if overflow)
▪ Devices, such as routers, allocate memory to store segments if bandwidth isn’t readily available
▪ When available, it transmits the contents of the buffer
▪ If the buffer overflows, segments will be dropped
Examples at Layer 4
▪ TCP ▪ UDP ▪ WAN Accelerators ▪ Load Balancers ▪ Firewalls
Layer 5 (Session)
Session Layer (Layer 5) ▪ 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 ▪ Tearing down sessions
Setting up a Session (Check user / assign numbers / services / who sends)
▪ Check user credentials
▪ Assign numbers to session to identify them
▪ Negotiate services needed for session
▪ Negotiate who begins sending data
Maintaining a Session ( Transfer / Reestablish / Acknowledge)
▪ Transfer the data
▪ Reestablish a disconnected session
▪ Acknowledging receipt of data
Tearing Down a Session (Mutual agreement (done) / other party disconnects )
▪ Due to mutual agreement
● After the transfer is done
▪ Due to other party disconnecting
Examples at Layer 5 ( H.323 = setup/maintain/tear voice/video / NetBIOS = computers sharing files over a network)
▪ 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) (Formatting the data –> securing the data with proper encryption)
Presentation Layer (Layer 6) ▪ Responsible for formatting the data exchanged and securing that data with proper encryption ▪ Functions ▪ Data formatting ▪ Encryption
Data Formatting (Ensures data structure and readability / data transfer syntax for layer 7)
▪ Formats data for proper compatibility between devices
● ASCII
● GIF
● JPG
▪ Ensures data is readable by receiving system
▪ Provides proper data structures
▪ Negotiates data transfer syntax for the Application Layer (Layer 7)
Encryption (scrambles data to hide it from prying eyes / confidentiality )
▪ Used to scramble the data in transit to keep it secure from prying eyes
▪ Provides confidentiality of data
▪ Example:
● TLS to secure data between your PC and website
Examples at Layer 6
▪ HTML, XML, PHP, JavaScript, … ▪ ASCII, EBCDIC, UNICODE, … ▪ GIF, JPG, TIF, SVG, PNG, … ▪ MPG, MOV, … ▪ TLS, SSL, …
Layer 7 (Application)
Application Layer (Layer 7) ▪ Provides application-level services ● Not Microsoft Word or Notepad ▪ Layer where the users communicate with the computer ▪ Functions: ● Application services ● Service advertisement
Application Services
▪ Application services unite communicating components from more than one network application ▪ Examples: ● File transfers and file sharing ● E-mail ● Remote access ● Network management activities ● Client/server processes
Service Advertisement
▪ Some applications send out announcements
▪ States the services they offer on the network
▪ Some centrally register with the Active Directory server instead
▪ Example:
● Printers
● File servers
Examples at Layer 7
▪ 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
o The process of putting headers (and sometimes trailers) around some data
Decapsulation (Layer 1 to 7 = decapsulation / 7 to 1 = encapsulation / process of decapsulation (READ))
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
o SYN (or synchronization) flag
▪ The most well-known flag in TCP communications because it is used to synchronize the connection during the three-way handshake
o ACK (or acknowledgement) flag
▪ Used during the three-way handshake, but it is also used to acknowledge the successful receipt of packets
o FIN (or finished) packet
▪ 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
o RST (or reset) flag
▪ Used when a client or server receives a packet that it was not expecting during the current connection
o PSH (or PUSH) flag
▪ Used to ensure that the data is given priority and is processed at the sending or receiving ends
o URG (or 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
MAC address (address to identify network card on LAN / finding the source )
▪ A physical address that is used to identify a network card on the local area network
▪ Allows the source to find the destination by using this type of addressing
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
