OSI Model

Fundamentals of Network Protocols

• In networking, a protocol is defined as a set of rules for formatting and processing data.

• Network protocols function as a common language for computers within a network.

• These protocols enable communication between systems that may use vastly different software and hardware, ensuring interoperability regardless of underlying technical differences.

• Examples of common protocols include:

  • HTTP (Hypertext Transfer Protocol)

  • HTTPS (Hypertext Transfer Protocol Secure)

  • SMTP (Simple Mail Transfer Protocol)

  • FTP (File Transfer Protocol)

  • TCP (Transmission Control Protocol)

  • UDP (User Datagram Protocol)

The Open System Interconnection (OSI) Model Overview

• Internetworking models are utilized to organize and describe specific network functions.

• The OSI Model is a conceptual framework consisting of seven abstraction layers.

• It was created to achieve interoperability among diverse devices from different vendors.

• The model partitions communication systems into distinct abstraction layers, where each layer describes a unique set of functions.

• Protocols serve as the standard terms that computers use to understand each other across these layers.

• Mnemonics for the OSI Layers (Top-to-Bottom: 7 to 1):

  • All People Seem To Need Data Processing

• Mnemonics for the OSI Layers (Bottom-to-Top: 1 to 7):

  • Please Do Not Throw Sausage Pizza Away

Layer 7: The Application Layer

• This layer serves as the entry door to network services at a lower level.

• User applications sit on top of this layer and use it to get services when they transmit ($TX$) or receive ($RX$) data over a network.

• Functions include generating data for transmission, processing received data, and interfacing user applications with network management services and remote access.

• It is specifically identified as the playground of hackers in modern networking.

• Specific Protocols and Port Numbers at Layer 7:

  • Simple Mail Transfer Protocol (SMTP): Port $25$

  • Simple Network Management Protocol (SNMP): Port $161$

  • Hypertext Transfer Protocol (HTTP): Port $80$

  • Line Printer Daemon (LPD)

  • File Transfer Protocol (FTP): Port $21$

  • Telnet: Port $23$

  • Trivial File Transfer Protocol (TFTP): Port $69$

  • Electronic Data Interchange (EDI)

  • Post Office Protocol version 3 (POP3): Port $110$

  • Internet Message Access Protocol (IMAP): Port $143$

  • Network News Transport Protocol (NNTP)

  • Secure Remote Procedure Call (S-RPC)

  • Secure Electronic Transaction (SET)

  • Domain Name System (DNS)

  • Peer-to-Peer (P2P)

Layer 6: The Presentation Layer

• The Presentation Layer is concerned with the format that applications must use for common understanding.

• It gets data ready for the application layer by performing the following core services:

  • Data Conversion

  • Character Code Translation

  • Compression

  • Encryption and Decryption

  • Encoding

• Standards and Data Formats at Layer 6:

  • American Standard Code For Information Interchange (ASCII)

  • Extended Binary-Coded Decimal Interchange Mode (EBCDIC)

  • Tagged Image File Format (TIFF)

  • Joint Photographic Experts Group (JPEG)

  • Motion Picture Experts Group (MPEG)

  • Musical Instrument Digital Interface (MIDI)

  • Hypertext Markup Language (HTML)

  • Word Documents (DOC)

  • MP3

  • AVI

  • Sockets

Layer 5: The Session Layer

• The Session Layer provides a logical persistent connection between peer hosts.

• It manages the "conversation" between applications exchanging information.

• Responsibilities include the creation, monitoring, and tearing down of sessions.

• It provides dialog control, allowing multiple persistent connections from different sources to be combined or synchronized.

• Communication Modes:

  • Full Duplex

  • Half Duplex

  • Simplex

• Specific Protocols and Services at Layer 5:

  • Network File System (NFS)

  • Structured Query Language (SQL)

  • NetBIOS

  • Remote Procedure Call (RPC)

  • Session establishment in TCP

  • Session Initiation Protocol (SIP)

  • Real-time Transport Protocol (RTP)

  • RPC-Named pipes

Layer 4: The Transport Layer

• This layer handles end-to-end communication and transport between peer hosts.

• Transmission Control Protocol (TCP):

  • Full-duplex, connection-oriented communication.

  • Reliable delivery using flags for session management.

• User Datagram Protocol (UDP):

  • Simplex, connectionless communication.

  • Best-effort delivery without guarantees.

• Additional Protocols:

  • Secure Sockets Layer (SSL)

  • Transport Layer Security (TLS)

  • Stream Control Transmission Protocol (SCTP)

  • Sequenced Packet Exchange (SPX)

• Port Classifications:

  • Well-Known Ports: Ports $0 - 1023$

  • Registered Ports: Ports $1024 - 49151$

  • Dynamic or Private Ports: Ports $49152 - 65535$

Layer 3: The Network Layer

• The primary role is to move data between two hosts that are not physically connected.

• It uses logical addresses, specifically IP addresses, for path determination.

• The Network Layer uses routing tables to find the best delivery route but does not guarantee delivery.

• Devices operating at this layer: Routers.

• Specific Protocols at Layer 3:

  • Internet Protocol (IP): Connectionless protocol for addressing.

  • Internet Control Message Protocol (ICMP)

  • Routing Information Protocol (RIP)

  • Open Shortest Path First (OSPF)

  • Border Gateway Protocol (BGP)

  • Internet Group Management Protocol (IGMP): Supports multicasting.

  • Internetwork Packet Exchange (IPX)

  • Internet Protocol Security (IPSec)

  • Network Address Translation (NAT)

  • Simple Key Management for Internet Protocols (SKIP)

  • Address Resolution Protocol (ARP): Operates at this level for path determination (also associated with Layer 2).

Layer 2: The Data Link Layer

• This layer provides communication within the same network using physical (MAC/Hardware) addresses.

• It receives packets from the Network Layer and encapsulates them into frames for transmission to the next physically connected device.

• Responsibilities include frame creation, error detection within frames, and converting information into bits.

• Devices operating at this layer: Switches and bridges.

• Data Link Sublayers:

  • Media Access Control (MAC): Manages protocol access to the physical network medium. It handles addressing and channel access control mechanisms.

  • Logical Link Control (LLC): The upper sublayer that provides flow control and error control.

• Protocols and Standards at Layer 2:

  • Address Resolution Protocol (ARP): Resolves IP into MAC.

  • Reverse Address Resolution Protocol (RARP): Resolves MAC into IP addresses.

  • Point-to-Point Protocol (PPP)

  • Serial Line Internet Protocol (SLIP)

  • Layer 2 Forwarding (L2F)

  • Layer 2 Tunneling Protocol (L2TP)

  • Point to Point Tunneling Protocol (PPTP)

  • Ethernet ($IEEE \, 802.3$): The only remaining standard in common use.

  • Token Ring ($IEEE \, 802.5$)

  • Wireless Local Area Network ($IEEE \, 802.11$)

  • Fiber Distributed Data Interface (FDDI)

  • Asynchronous Transfer Mode (ATM)

  • Copper Distributed Data Interface (CDDI)

  • Virtual LAN (VLAN)

  • HDP

  • Fibre Channel

  • Frame Relay

  • High-Level Data Link Control (HDLC)

  • Q.921

Layer 1: The Physical Layer

• The Physical Layer is responsible for media, signals, and binary transmission.

• It receives bits from the Data Link Layer and converts them into physical signals:

  • Electrical signals for copper cables.

  • Photons or beams of light for fiber optic cables.

  • Radiofrequency for wireless communications.

• Devices at Layer 1: Cables, connectors, wireless access points (WAPs), hubs, modems.

• Standards and Interfaces at Layer 1:

  • EIA/TIA-232 and EIA/TIA-449

  • RS-232, RJ45, V.34

  • X.21

  • High-Speed Serial Interface (HSSI)

  • Synchronous Optical Networking (SONET)

  • SDH

  • V.24 and V.35

  • Integrated Services Digital Network (ISDN)

  • Digital Subscriber Line (DSL)

  • 10BASE-T, 10BASE2, 10BASE5

  • 100BASE-TX, 100BASE-FX, 100BASE-T

  • 1000BASE-T, 1000BASE-SX

Encapsulation and Protocol Data Units (PDUs)

• Encapsulation is the process of data moving down the OSI model stack from the Application layer to the Physical layer.

• De-encapsulation is the process of data moving up the stack from the Physical layer back to the Application layer.

• Protocol Data Units (PDUs) by Layer:

  • Layers 7, 6, 5: Data Stream

  • Layer 4: Segment (TCP) or Datagram (UDP)

  • Layer 3: Packet

  • Layer 2: Frame

  • Layer 1: Bits

• Mnemonic for Data Units: Don’t Stop Pouring Free Beer (Data, Segment, Packet, Frame, Bits).