OSI Model (Section 2)

Protocol

A set of rules for formatting and processing data. Network protocols are like a common language for computers, allowing them to commuinicate

OSI Model

Open System Interconnection Model. A layered approach to how computers/devices communicate. Created to achieve interoperability of diverse vendor devices. Each layer describes a different set of unique functions, composed of various types of hardware and software

Partitions communication systems into abstraction layers.

Protocols are the standard terms that computers use to understand each other

• Layer 1: Physical (ex: cables)

• Layer 2: Data Link (ex: switch)

• Layer 3: Network (ex: router)

• Layer 4: Transport (ex: firewall)

• Layer 5: Session

• Layer 6: Presentation

• Layer 7: Application

(Layer 7-5 = all software, 1-3 = responsible for movement of data, Layer 4 = sits on top of the communication)

Please Do Not Throw Sausage Pizza Away (mnemonic device), or All People Seem To Need Data Processing (7-1)

Layer 1 (OSI Model)

Physical: Converts bits into electrical signal over copper cables, or pulses of light for fiber optic cables, or radiofrequency for wireless communication

Physical topologies

Devices: Cables, Connectors, wireless access point, hub, modems, etc

Common connections/cables:

• EIA/TIA-232 and EIA/TIA-449

• High-Speed Serial Interface (HSSI)

• Synchronous Optical Networking (SONET)

• 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

Layer 2 (OSI Model)

Data Link: Provides communication within the same network via MAC addresses

Gets packets from the Network Layer

Transmits frames

Detects errors within frames (but doesn’t correct them)

Converts information into bits

Uses MAC/Hardware addresses to communicate

Moves data to the next physically connected device

Two sublayers:

• Logical Link Layer (LLC): Upper sublayer, provides flow control and error control

• Media Access Control (MAC): Manages protocol access to the physical network medium. It’s responsible for the addressing and channel access control mechanisms that enable several nodes to communicate within a network, typically using a MAC address

Devices: Switches and bridges

Common Protocols:

• 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) - only remaining

• Fiber Distributed Data Interface (FFDI)

• Asynchronous Transfer Mode (ATM)

• Copper DDI (CDDI)

Layer 3 (OSI Model)

Network: Provides communication between different networks via IP addresses. Moves data between two hosts that are not generally physically connected. Uses IP

IP is at this layer. IP doesnt use guarantee delivery as its not a connection oriented communication (it only finds the best delivery route)

Uses routing tables to deliver info

Common protocols:

• Internet Protocol

• Internet Control Message Protocol (ICMP)

• Routing Information Protocol (RIP)

• Open Shortest Path First (OSPF)

• Border Gateway Protocol (BGP)

• Supports multicasting

• Internetwork Packet Exchange (IPX)

• Internet Protocol Security (IPSec)

• Network Address Translation (NAT)

• Simple Key Management for Internet Protocols (SKIP)

Layer 4 (OSI Model)

Transport: Handles the End-to-End communication either via TCP for connection-oriented communication or UDP for connectionless communication

One of the busiest layers. Creates an end-to-end transport between peer hosts. Responsible for checking the data. UDP is connectionless and known as “best effort”, TCP is connection oriented and known as reliable (TCP uses flags). Port numbers are assigned at this layer

Common protocols:

• Transmission Control Protocol (TCP): full-duplex connection-oriented

• User Datagram Protocol (UDP): simplex connectionless

• Secure Sockets Layer (SSL)

• Transport Layer Security (TLS)

• Sequenced Packet Exchange (SPX)

• TCP and UDP ports

• Well-known ports: Ports 0-1023

• Registered ports: Ports 1024-49151, registered by vendors

• Dynamic or Private Ports: Ports 49152-65535

Ports help tell the machine what kind of data it is

Layer 5 (OSI Model)

Session: Provides dialog control by allowing multiple persistent connections from different sources to be properly combined or synchronized.

Provides a logical persistent connection between peer hosts; basically manages conversation between applications exchanging data.

Creates, monitors, and tears down sessions

Full Duplex (send and receive data simultaneously - ex. switch), Half Duplex (only send or receive at a time - ex. hub), Simplex (a device that can only send data - ex. satellite)

Protocols:

• Network File System (NFS)

• Structured Query Language (SQL)

• NetBIOS

• Remote Procedure Call (RPC)

Layer 6 (OSI Model)

Presentation: Gets the data ready for the application layer by converting, translating, encoding, compressing, and encrypting data.

This layer is concerned about formatting the data. Main services:

• Data Conversion

• Character Code Translation

• Compression

• Encryption/Decryption

Format examples:

• 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)

Layer 7 (OSI Model)

Application: Generates data to be transmitted, processes data that is received. Application refers to application-level protocols and services used by end-user applications to communicate/send data over a network.

The entry door to network services at a lower level. Apps use this layer to get services when they transfer or receive data over a network.

Applications sit on top of this layer

Responsible for interfacing user applications, network mgmt. services, remote access, etc.

Prone to hackers

ex protocols in this layer:

• Simple Mail Transfer Protocol (SMTP) - 25

• Simple Network Management Protocol (SNMP) - 161

• Hypertext Transfer Protocol (HTTP) - 80

• Line Printer Daemon (LPD)

• File Transfer Protocol (FTP) - 21

• Telnet - 23

• Trivial File Transfer Protocol - 69

• Electronic Data Interchange (EDI)

• Post Office Protocol version 3 (POP3) - 110

• Internet Message Access Protocol (IMAP) - 143

• Network News Transport Protocol (NNTP)

• Secure Remote Procedure Call (S-RPC) RPC - Session

• Secure Electronic Transaction (SET)

Encapsulation/De-encapsulation

When data goes from one machine to another, data is encapsulated into different things (Data Stream>Segment/Datagram>Packet>Frame>Bits), and then de-encapsulated upon arrival in reverse order.

Ex: Segment section may add port number to the Data Stream, Packet adds IP address, Frame adds MAC address

Dont Stop Pouring Free Beer mnemonic

At which layer of the OSI model does the encapsulation process begin?

Answer: application