OSI Model: Layers and Protocols

Understanding OSI Basics

• The Open Systems Interconnection (OSI) model serves as a reference for data communication processes on computer networks
• It uses layers to provide services
• The layers are associated with protocols and devices
• The International Organization for Standardization (ISO) created and ratified the OSI model
• The American National Standards Institute (ANSI) represents it in the United States

Defining the OSI Model Layers

• The OSI model has seven layers, each containing different protocols from protocol suites like TCP/IP
• It categorizes TCP/IP processes and is useful for installing, configuring, maintaining, and troubleshooting networks
• TCP/IP is a protocol suite or stack
• A protocol stack operates on different transmission levels

Layer 1: Physical Layer

• It is the physical and electrical medium for data transfer
• Components include cables, jacks, patch panels, punch blocks, hubs, and multi station access units (MAUs)
• The physical plant is another name for it
• Topologies, analog vs. digital encoding, bit synchronization, baseband vs. broadband, multiplexing, and serial (5-volt logic) data transfer are related concepts
• It is the easiest layer to understand as it involves tangible components
• The unit of measurement to use on this layer is bits

Layer 2: Data Link Layer

• Defines how transfer occurs over the Physical layer
• Network interface cards and bridges are devices on this layer
• It ensures error-free transmission over the Physical layer for LAN transmissions using physical (hexadecimal) or MAC addresses from the NIC's ROM
• MAC address is a device's address burned into the ROM of the NIC
• Any device that physically connects to and moves data on the network is on the Data Link layer
• The unit of measurement is frames

Layer 3: Network Layer

• Dedicated to routing and switching information between networks, LANs, or internetworks
• It exists on LANs or WANs
• Routers and IP switches are devices on this layer
• Logical addressing of hosts occurs at this layer, unlike the physical addressing of previous layers
• The addressing system is stored in the OS, like an IP address
• Computers have a physical (MAC) and a logical (IP) address
• The unit of measurement is packets

Layer 4: Transport Layer

• Ensures error-free transmission between hosts using logical addressing
• Message transmission is managed through Layers 1-3
• Protocols break up messages, send them through the subnet, and ensure reassembly to prevent duplicates or lost messages
• Connection-oriented and connectionless systems are present
• It controls inbound and outbound ports
• Ports are handled by the Transport layer
• Segments or messages are the unit of measurement
• Data and messages are terms to describe this layer

Layer 5: Session Layer

• Governs the establishment, termination, and synchronization of sessions within the OS over the network and between hosts, such as when logging on and off
• The name and address database for the OS or NOS is controlled by it
• NetBIOS (Network Basic Input/Output System) operates on this layer

Layer 6: Presentation Layer

• Translate data format between sender and receiver in various operating systems
• Code conversion, data compression, and file encryption are concepts include in this layer
• Redirectors such as mapped network drives are on this layer

Layer 7: Application Layer

• Location for message and packet creation to begin
• Database access is on this level
• Protocols like FTP, SMTP, Telnet, and RAS operate on this layer
• Protocols that applications initiate, such as HTTP or POP3 exist here
• It is not the application itself, but the protocols initiated

Defining the Communications Subnetwork

• The communications subnetwork comprises Layers 1-3 and is the core of OSI model transmissions
• It is employed regardless of the data transmission type
• The Physical layer deals with tangible elements and transmits bits of information
• A data transfer rate is measured, testing a computer's internet connection speed

Layer 2 Switching

• Layer 2 switches are common in LANs
• Layer 2 switches are hardware-based
• Use the MAC address of each host computer’s network adapter to direct data frames
• Each port on the switch maps to the specific MAC address of the connected computer
• Layer 2 switches generally do not modify frames
• Each port is its own segment, providing usable bandwidth equivalent to the switch's rating

Layer 2 Switching and Security

• MAC address storage happens in memory assigned to a port translation table, MAC table, or Content Addressable Memory table (CAM table)
• A MAC flood attack can compromise this table by sending many packets with differing source MAC addresses to fill the switch memory
• The switch enters fail open mode and broadcasts data on all ports like a hub
• Network bandwidth decreases dramatically
• A protocol analyzer in promiscuous mode can capture data from any computer

Layer 3 Switching

• Layer 3 switches differ from Layer 2 switches by determining data paths using logical (IP) addresses instead of physical (MAC) addresses
• Layer 3 switches forward packets and Layer 2 switches forward frames
• Layer 3 switches are often manageable via SNMP
• They allow network engineers to analyze all packets passing through the switch
• Layer 2 switch is an advanced bridge and Layer 3 switch is a router
• Layer 3 switches connect multiple IP networks

Characteristics of Switches

• Number and types of ports and their transmission speed
• Number and speed of uplink ports
• Expandability capabilities
• Managed or unmanaged
• VLAN capabilities
• Hardware redundancy
• Security options
• Routing/Layer 3 capabilities

More Characteristics of Switches

• Larger networks use larger switches to connect more devices
• Switches process packets to determine the destination port
• Faster backplane and processing are required with more devices
• Backplane bandwidth measures the internal architecture's bandwidth, depending on the switch type

Uplink Ports

• Connect different Ethernet devices, like connecting a small to a larger switch, or a switch to a router
• Special port reversing transmit and receive circuits

Switch Configurations

• Fixed: Low-end with a set amount of ports
• Modular: Chassis that can slide in modular line cards
• Stackable: Connect together using a back cable for high bandwidth

Managed and Unmanaged Switches

• Unmanaged switches: Inexpensive for homes and SOHO environments and only require AC power and network devices
• Managed switches: Include a command-line or web interface; more advanced features like Spanning Tree Protocol, port mirroring, setting port bandwidth, and creating and modifying virtual LANs

Spanning Tree Protocol (STP)

• Network protocol that prevents bridge loops
• Looping traffic can take down a local area network
• Enables redundant links between switches

Hardware Redundancy

• A method for cases where critical hardware components need a backup
• The network continues to function if one component fails
• Achieved is reached by using multiple copies of a hardware component
• Disadvantages include increased size, power consumption, and cost

Virtual Switch

• Software to communicate between virtual machines
• Directs network communication by checking the data packets before sending them
• Reduces network configuration complexity by reducing physical switches
• May be embedded in installed software or a server's hardware as firmware
• Completely virtual and connects to a network interface card (NIC)
• Merges physical switches into a single logical switch

Advantages of a Virtual Switch

• Eases deployment and migration of virtual servers
• Allows network administrators to manage virtual switches deployed by using a hypervisor
• Computer software that creates and runs virtual machines is known as a hypervisor or virtual machine monitor (VMM)
• Easier to set up a virtual switch compared to a physical switch

Defining the Upper OSI Layers (4-7)

• Transport, Session, Presentation, and Application layers
• Handles protocols like HTTP, FTP, and mail protocols
• Compression, encryption, and session creation are performed by these layers

Ways To Define The Upper OSI Layers (4-7)

• Define Transport layer via connections in Command Prompt and port descriptions
• Define Session layer by logging into websites/servers, Microsoft networks, and email programs
• Define Presentation layer via encryption in Windows and websites
• Define Application layer by capturing and analyzing web server packets

Defining the Transport Layer

• Layer 4 manages message transmission through the communications subnetwork
• Provides connection-oriented data stream support, reliability, flow control, and multiplexing
• TCP (Transmission Control Protocol) is a connection-oriented protocol
• UDP (User Datagram Protocol) is connectionless
• A web browser uses TCP and streaming media uses UDP
• TCP ensures data arrives at its destination without losing packets

Connection-Oriented (CO) Communications

• Requires devices to establish an end-to-end logical connection before sending data
• Considered reliable network services, also known as CO mode
• Re-sends individual packets not delivered by resending the packet

Connectionless (CL) Communications

• No end-to-end is made before data transfer
• Every packet includes the destination address located in its header
• Sufficient to move independent packets, for example, streaming media
• Lost packets not resent because the sending computer doesn’t know which connection to use

Ports

• Layer 4 manages the ports that transfer Data
• Logical communication endpoints for computers
• There are 65,536 ports, numbered 0-65,535
• Defined by IANA (Internet Assigned Numbers Authority)

IANA Categories

• Well-Known Ports (0-1023): Commonly used protocols, for example, FTP uses port 21 for client connections
• Registered Ports (1024-49,151): Used by vendors for proprietary applications and required registration with IANA, for example, Microsoft registered port 3389 used with Remote Desktop Protocol
• Dynamic and Private Ports (49,152-65,535): used by applications, and vendors cannot registered them

Inbound Ports

• For computers connecting to a service or application running on another computer
• Servers primarily use to accept incoming connections and serve data
• IP addresses and port numbers combine, for example, a server's IP/port 66.249.91.104:80 uses port 80 to accept incoming web page requests

Outbound Ports

• Used by your computer to connect to a service/application on another computer
• Client computers primarily use these
• Dynamically assigned by the operating system

Defining the Session Layer

• The Session layer is involved every time you connect to a website, mail server, or other computer
• The same is involved when logging on or off
• Make several connections to other computers; for example:
  • www.microsoft.com
  • A mail account with MSN, Gmail, Yahoo!, or others
  • A network share (if available)
  • An FTP server (if available)
• Go back to the Command Prompt window and execute netstat –a
• Analyze the various sessions that were created
• Compare the results of both commands
• Check one Command Prompt windows and the other windows’ IP addresses

Logging On And Log Off Processes

• Log off/log on your Microsoft network
• Log on to a website like Amazon
• It starts with the Session Layer
• The Session layer terminates sessions
• Notice that web sessions change from Established to either Time wait, or closed
• Finally, log off the computer and log back on

Defining Layers Of The Presentation Layer

• The Presentation layer modifies data, including code conversion, encryption, or compression
• Occurs when connecting to a mapped network drive
• Showing how information is modified before being sent across the network

1. Access File Explorer on a Windows client computer
2. Create a simple text file
3. Right-click the text file and choose Properties
4. Click the Advanced button
5. Select the “Encrypt contents to secure data” check box
6. Click OK (The file will display in blue), and from now on, if the file is sent across the network, the Presentation layer comes into effect due to the encryption,

The Application Layer

• Protocols like HTTP, FTP, and POP3 reside in the Layer 7
• The Application is not the applications themselves (Internet Explorer or Outlook)
• Applications initiate network services and protocols, such as HTTP or POP3
• When you open Internet Explorer, you open a new application
• The HTTP protocol is started by filling in http://www.microsoft.com and the OSI model is started