OSI Model (& TCP Model Ports/Protocol) Objective 1.1: Explain concepts related to the Open Systems Interconnection (OSI) reference model ● Introduction ○ Open Systems Interconnect Model (OSI) ■ Developed in 1977 by the International Organization for Standardization ■ OSI is a reference model ● Used to categorize the functions of a network ● Useful for troubleshooting ○ Networks today operate under the TCP/IP mode ○ Layers ■ Physical - Layer 1 ■ Data Link - Layer 2 ■ Network - Layer 3 ■ Transport - Layer 4 ■ Session - Layer 5 ■ Presentation - Layer 6 ■ Application - Layer 7 ○ Networks are designed to make data flow across networks 21 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ Names of data as it flows through the OSI model ■ Bits - Layer 1 ■ Frames - Layer 2 ■ Packets - Layer 3 ■ Segments - Layer 4 ■ Data - Layer 5 ■ Data - Layer 6 ■ Data - Layer 7 ● Layer 1 (Physical) ○ Physical Layer Overview (Layer 1) ■ First layer of the OSI model where transmission of bits across the network occurs and includes physical and electrical network characteristics ■ Data type occurs as bits ● Binary bits represented as a series of 1s and 0s ○ Transition Modulation ■ Switching between levels to represent 1 or 0 ● Copper Wire (Cat5/Cat6) – Uses voltage (0V for 0, +5V/-5V for 1) ● Fiber Optic Cable – Uses light (on for 1, off for 0) ○ Connector Standards ■ RJ-45 Connector – Used in CAT5/CAT6 cables ■ Wiring Standards 22 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● TIA/EIA-568A ● TIA/EIA-568B ■ Crossover cables – TIA/EIA-568A on one end, and TIA/EIA-568B on the other end ■ Straight-through cables – TIA/EIA-568B on both ends ○ Physical Topology ■ Different physical network layouts ● Bus ● Ring ● Star ● Hub-and-Spoke ● Full Mesh ● Partial Mesh ■ Based on how cables are physically connected ○ Synchronization ■ Asynchronous Communication ● Start and stop bits for out-of-sync data transmission ■ Synchronous Communication ● Real-time communication using a common time source ○ Bandwidth Utilization ■ Broadband ● Divides bandwidth into separate channels (e.g., cable TV) 23 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ■ Baseband ● Uses all frequency of the cable all the time (e.g., telephone) ○ Multiplexing ■ Involves taking some limited amount of resource and using it more efficiently ● Allows multiple people to use a baseband connection at the same time ■ Time Division Multiplexing (TDM) ● Allocates dedicated time slots ■ Statistical Time Division Multiplexing (StatTDM) ● Dynamically allocates time slots based on when people need it ■ Frequency Division Multiplexing (FDM) ● Divides the medium into channels ○ Layer 1 Devices ■ Cables – media ● Fiber optic ● Ethernet ● Coaxial ■ Wireless Media ● Bluetooth ● Wi-Fi ● Near field communication 24 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ■ Infrastructure Devices ● Hubs ● Access points ● Media converters ○ Layer 1 Device Characteristics ■ Simply repeat whatever they receive ■ No logic or decision-making at Layer 1 ● Layer 2 (Data Link Layer) ○ Data Link Layer (Layer 2) ■ Responsible for packaging bits from Layer 1 into frames and transmitting them across the network ■ Performs error detection and correction, identifies devices using MAC addresses, and provides flow control ○ MAC Address (Media Access Control Address) ■ A means for identifying a device physically and allowing it to operate on a logical topology ■ A unique 48-bit physical addressing system is assigned to every network interface card (NIC) produced ● Written in hexadecimal numbers ● First 24 bits – identify the manufacturer ● Remaining 24 bits – identify the specific device 25 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ■ Crucial for logical topology – identifying devices on the network ○ Logical Link Control (LLC) ■ Provides connection services and acknowledges message receipt, ensuring controlled data flow ■ Most basic form of flow control ● Limits data sent by a sender and prevents receiver overwhelm ■ Uses a checksum to detect corrupted data frames ○ Synchronization Methods at Layer 2 ■ Isochronous Mode ● Common reference clock ● Time slots for transmissions ● Less overhead ■ Synchronous Method ● Devices use the same clock, with beginning and ending frames, and control characters for synchronization ■ Asynchronous ● Devices reference own clock cycles ● No strict control over communication timing ○ Layer 2 Devices ■ Network Interface Cards (NICs) ■ Bridges 26 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ■ Switches ● Intelligent use of logic to learn and send data to specific devices based on MAC addresses ○ Switch Operation ■ Switches use CAM tables with MAC addresses to identify physical ports connected to devices ■ Enables selective data transmission to specific areas in the network. ● Layer 3 (Network Layer) ○ Network Layer (Layer 3) ■ Concerned with routing and forwarding traffic using logical addresses ○ Logical Addressing ■ IP variants – common logical addressing schemes ● IPv4 – written in dotted octet notation which are four sets of numbers separated by dots (e.g., 172.16.254.1) ● IPv6 ■ Other protocols – these were replaced by IP (Internet Protocol) ● AppleTalk ● IPX (Internetwork Packet Exchange) ○ Switching/Routing Methods ■ Packet Switching (Routing) ● Data is divided into packets and then forwarded 27 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● Most commonly used method ■ Circuit Switching ● A dedicated communication link is established between two devices ■ Message Switching ● Data is divided into messages which may be stored and then forwarded ○ Route Discovery and Selection ■ Routers maintain routing tables for determining the best path ● Dynamic protocols (e.g., RIP, OSPF) enable routers to share and update route information ■ Routing protocols help decide how data is going to flow across the network and how the routers are going to communicate that information ○ Connection Services at Layer 3 ■ Augments Layer 2 services ■ Involves flow control ● Prevents sender from overwhelming the receiver ■ Packet reordering ● Ensures data packets arrive and are reassembled in the correct order 28 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ Internet Control Message Protocol (ICMP) ■ Used for sending error messages and operational information to an IP destination ■ PING ● Most commonly used ICMP ● Helps troubleshoot network issues by testing connectivity and response times ■ Traceroute ● Traces the route of a packet through the network ○ Devices and Protocols ■ Routers ■ Multi-layer switches ● Combines Layer 2 switch and Layer 3 router features ● A switch is always a Layer 2 device, unless specifically mentioned that it is a multi-layer switch, then it is considered as a Layer 3 device ■ Layer 3 protocols ● IPv4 ● IPv6 ● ICMP ○ IP and routers are commonly encountered Layer 3 devices in exams 29 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● Layer 4 (Transport Layer) ○ Transport Layer (Layer 4) ■ Dividing line between the upper layers and the lower layers of the OSI model ■ Upper Layers ● Transport ● Session ● Presentation ● Application ○ Segments ■ Data Type in Transport Layer ○ Protocols in Layer 4 ■ TCP (Transmission Control Protocol) ● Connection-oriented protocol that is a reliable way to transport segments across the network ● With acknowledgement ● Uses Three-Way Handshake ○ SYN – synchronization ○ SYN-ACK – synchronization - acknowledgement ○ ACK – acknowledgement ● Windowing for flow control 30 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● Used for all network data that needs to be assured to get to its final destination ■ UDP (User Datagram Protocol) ● A connectionless protocol that is an unreliable way to transport segments (datagram) ● Used for audio and visual streaming ● No three-way handshake and less overhead ● No acknowledgment or retransmission ○ Remember the data types in Layer 4 for the exam ■ Segment – data type for TCP ■ Datagram – data type for UDP ○ TCP vs. UDP ■ TCP ● Reliable ○ Uses Three-way Handshake ● Connection-oriented ● Segment retransmission and flow control through windowing ● Sequencing ● Acknowledgment of segments ■ UDP ● Unreliable ○ No Three-way Handshake 31 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● Connectionless ● No retransmission and no windowing ● No sequencing ● No acknowledgment of datagrams ○ Windowing ■ Allows clients to adjust the amount of data in each segments during transmission ■ Optimize throughput and bandwidth ■ Open or close window based on retransmissions ○ Buffering ■ Occurs when devices allocate memory to store segments if bandwidth is not readily available ■ Buffer ● Temporary storage for segments ■ Prevents overflow by clearing segments ○ Layer 4 Devices ■ Protocols ● TCP and UDP ■ Devices ● WAN accelerators ● Load balancers and firewalls 32 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● Layer 5 (Session Layer) ○ Session Layer (Layer 5) ■ Manages sessions, ensuring separate conversations to prevent data intermingling ○ Setting Up Session ■ Checking of user credentials and assigning numbers to sessions to help identify ○ Maintaining Session ■ Continuous data transfer between parties ■ If connection breaks, it will require re-establishment ■ Includes acknowledgement of data ○ Tearing Down a Session ■ Ending a session once communication goals are achieved ■ Mutual agreement or one party disconnects ○ Layer 5 Devices and Protocols ■ H.323 ● Used for setting up, maintaining, and tearing down voice and video connections ● Operates over the real-time transport protocol (RTP) ■ NetBIOS ● Utilized by computers for file sharing over a network ● Commonly associated with Windows file sharing 33 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ Layer 5 issues involve protocols and software rather than specific devices ● Layer 6 (Presentation Layer) ○ Presentation Layer (Layer 6) ■ Responsible for formatting data for exchange and securing it through encryption ○ Data Formatting ■ Formatting data by a computer to have compatibility between different devices ■ Formats ● American Standard Code for Information Interchange (ASCII) ○ Text-based language to use ○ Ensures data is readable by receiving system ○ Provides proper data structures ○ Negotiates data transfer syntax for the Application Layer (Layer 7) ● GIFs – motion pictures ● JPEG – photographs ● PNG – Internet images ■ Formats enable compatibility between different devices 34 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ Encryption ■ Used to scramble data in transit to keep it secure and provide data confidentiality ■ Transport Layer Security (TLS) ● Ensures secure data transfer ● Creates an encrypted tunnel, protecting sensitive information ○ Scripting languages in Layer 6 ■ Control how ASCII text is displayed on the screen ● HTML ● XML ● PHP ● JavaScript ○ Standard text formats ■ Different ways of displaying text using ones and zeros ● ASCII ● Unicode ● EBCDIC ○ Image formats ■ Different graphical representations of 1s and 0s ● GIFs ● JPEGs ● TIFFs 35 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● SVGs ● PNGs ○ Movie file formats ■ 1s and 0s formatted to create watchable videos ● MP4s ● MPEGs ● MOV ○ Encryption Algorithms ■ Scrambles data to provide confidentiality and security during transit and storage ● TLS ● SSL (Secure Sockets Layer) ■ Focus on Security ● Layer 7 (Application Layer) ○ Application Layer (Layer 7) ■ Provides application-level services where users communicate with the computer ■ Focus on lower-level applications ● File transfer ● Network transfer 36 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ Application Services ■ Unites components for more than one network application ● File transfer ● File sharing ● Email ○ Low-level protocols ■ POP3 (Post Office Protocol 3) ■ IMAP (Internet Message Access Protocol) ■ SMTP ( Simple Mail Transfer Protocol) ● Remote access ● Network management ● Client-server processes ○ Service Advertisement ■ Applications send announcements to other devices on the network ■ Devices advertise the services they offer ● Printers and file servers managed by Active Directory ● Self-advertising devices like wireless printers ○ Layer 7 Protocols ■ Email Applications ● POP3 ● IMAP ● SMTP 37 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ■ Web Browsing ● HTTP ● HTTPS ■ Domain Name Service (DNS) ■ File Transfer Protocols ● FTP ● FTPS ● SFTP ■ Remote Access ● Telnet ● SSH ● SNMP ● Encapsulation and Decapsulation ○ Encapsulation ■ Process of putting headers and sometimes trailers around data ○ Decapsulation ■ Removing the applied encapsulation to access the original data ○ OSI Model Layers ■ Moving down from Layer 7 to 1 – encapsulation ■ Moving up from Layer 1 to 7 – decapsulation 38 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ Protocol Data Units (PDUs) in OSI Model ■ A single unit of information transmitted in a computer network ● Terminology used for each layer is written as L(layer number) PDU ○ Example – L7 PDU for Layer 7 ■ There are special names for the PDUs for layers 1, 2, 3, and 4 ● Layer 1 – Bits ● Layer 2 – Frames ● Layer 3 – Packets ● Layer 4 – Segments (TCP) or Datagrams (UDP) ○ TCP Header (Layer 4) ■ 10 mandatory fields, totalling 20 bytes of information ● Source port ● Destination port ● Sequence number ● Acknowledgment numbers ● TCP data offset ● Reserved data – always set to zero ● Control flags ○ SYN – synchronize connection in three-way handshake ○ ACK – acknowledgment of the successful receipt of data ○ FIN (Finished) – tears down connections created by three-way handshake 39 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ○ RST (Reset) – used when an unexpected packet is received ○ PSH (Push) – ensures data is given priority ○ URG (Urgent) – identifies incoming data as urgent ● Window size ● TCP checksum ● Urgent pointer ● mTCP – optional ○ UDP Header (Layer 4) ■ 8-byte header ● Source port ● Destination port ● Length – indicates the total packet bytes ● Checksum – not mandatory ○ IP Header (Layer 3) ■ Contains various fields ● Version ● Length of IP header ● Type of service ● Total length of packet and header ● Identifier ● Flags ● Fragmented offset 40 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ● Time to live ● Protocol ● Header checksum ● Source IP Address ● Destination IP Address ● Options and Padding ○ Ethernet Header (Layer 2) ■ Features a few things ● Destination MAC Address ● Source MAC address ○ MAC Address ■ Physical address that is used to identify a network card on a local area network ■ Processed by switches ● EtherType field ○ Used to indicate which protocol is encapsulated in the payload of a frame ■ IPv4 or IPv6 ● VLAN Tag – optional ○ IEEE 802.1Q ○ IEEE 802.1AD 41 https://www.DionTraining.com CompTIA Network+ (N10-009) (Study Notes) ■ A frame being sent at Layer 2 will also contain a payload ● Payload ○ Data that being sent across the network ○ 42 bytes – using VLANs ○ 46 bytes – no VLANs ○ Maximum Transmission Unit (MTU) ■ Maximum size for payload ● 1500 bytes for Ethernet ● Jumbo Frames ○ Frames larger than 1500 bytes ○ Require reconfiguring MTU ○ Data Transmission ■ Encapsulation of data and adding header at each layer ● Layer 4 – source/destination ports ● Layer 3 – source/destination IP addresses ● Layer 2 – soure/destination MAC addresses ● Layer 1 – data transmitted as 1s and 0s ■ Decapsulation at each intermediate device until the final host is reached ■ Final host decapsulates to Layer 7 for application understanding