JONAS'S GARDEN

The OSI Reference Model

6 min read

Course: Computer Networks

Subject: Computer_network

Date: 11-06-2025

Topic: osi

Introduction

  • The OSI model has 7 layers, while the TCP/IP model has 4 layers.
  • The top three layers of the OSI model (Application, presentation, session) correspond to the Application layer in the TCP/IP model. Drawing 2025-06-11 08.45.46.excalidraw

Session layer (OSI layer 5)

key functions

  • controls conversation between computers
  • manages connection setup, maintenance and termination
  • organizes individual packets into coherent conversations

Communication methods

  1. Full-duplex: simultaneous two way communication (e.g telephones)
  2. Half-duplex: Alternating two way communication (e.g walkie-talkies)
  3. Simplex: one way communication only

Important Session Layer Protocols

  • H.245: Call control for multimedia communication
  • L2TP: Tunneling protocol for VPNs (operates at Layer 5 despite name)(layer 2 tunneling protocol)
  • NetBIOS: Enables LAN communication and file sharing(network basic input/output system)
  • RPC: Allows executing procedures on remote computers(Remote procedural call)
  • SMB: Provides shared access to files, printers, and ports(server message block)
  • SOCKS: Proxy protocol for bypassing firewalls (often uses port 1080)

Presentation Layer (OSI Layer 6)

Key Responsibilities

  1. Data Translation:
    • Converts between different encoding systems (e.g., ASCII to EBCDIC)
    • Ensures interoperability between different computer systems
    • Handles file format standards (TIFF, GIF, JPEG, MPEG, MIDI)
  2. Encryption/Decryption:
    • Secures sensitive information during transmission
    • Primarily encrypts Application Layer data
  3. Compression:
    • Reduces data size for efficient transmission
    • Especially important for multimedia content

Presentation Layer Protocols

  • IMAP: Email retrieval protocol (keeps emails on server)(internet message access protocol)
  • SSH: Secure remote command execution and shell access(secure shell)
  • SSL/TLS: Cryptographic protocols for secure connections(secure socket layer)(transport layer security)

Application Layer (OSI Layer 7)

Key Characteristics

  • Direct interface with users or application processes
  • Highest layer in both OSI and TCP/IP models
  • Where application software interacts with network services

Network Architectures

  1. Client-Server:
    • Dedicated servers with permanent IP addresses
    • Clients connect intermittently, may have dynamic IPs
    • Clients don’t communicate directly with each other
  2. Peer-to-Peer (P2P):
    • No central servers
    • All peers can act as both clients and servers
    • Highly scalable but harder to manage

Process Communication

  • Processes identified by IP address + port number
  • Common port examples: HTTP (80), SMTP (25)
  • Client process initiates communication
  • Server process waits for contact requests

Transport Service Requirements

Applications need different transport services based on:

  • Data loss tolerance (e.g., audio vs. file transfer)
  • Timing requirements (e.g., VoIP or online games need low delay)
  • Throughput needs (e.g., multimedia vs. elastic apps)
  • Security needs (encryption, data integrity)
  • TCP:
    • Connection-oriented
    • Reliable with flow and congestion control
    • No timing or throughput guarantees

Transport Protocol Options

  • UDP:
    • Connectionless
    • Unreliable but faster
    • No control mechanisms

Major Application Layer Protocols

  1. DNS (domain name system)
    • Translates domain names to IP addresses
    • Hierarchical structure (.com, .edu, .net domains)
  2. HTTPS:(hypertext transfer protocol)(hypertext transfer protocol secure)
    • Web page delivery protocol
    • Uses HTML for content markup
    • HTTPS adds security layer
  3. FTP: (file transfer protocol)
    • File transfer protocol
    • Uses port 21 for control, port 20 for data
    • Built into most OSes and browsers
  4. Email Protocols:
    • SMTP: Sending mail between servers (simple mail transfer protocol)
    • POP3: Download-and-delete email access (post office protocol 3)
    • IMAP: Server-based email management (internet message access protocol)
  5. Other Important Protocols:
    • DHCP: Automatic IP address assignment
    • NFS: Distributed file sharing system (network file system)
    • RTP: Streaming media protocol (real-time transfer protocol)
    • SNMP: Network management protocol (simple network management protocol)
    • Telnet: Remote terminal protocol (insecure)

Important Distinction

  • OSI Application Layer ≠ end-user applications
  • Provides services that applications use (e.g., browser uses HTTP)
  • Some applications may use network services transparently (e.g., word processor accessing network file)

Transport Layer (OSI Layer 4)

Introduction to Transport Layer

  • Position in OSI Model: 4th layer (between Network and Session layers)
  • Key Responsibility: Process-to-process delivery of complete messages (vs network layer’s packet-by-packet delivery)
  • Core Functions:
    • Multiplexing multiple applications on single devices
    • Ensuring reliable, in-order data delivery
    • Implementing error handling mechanisms

Key Concepts

Delivery Types Comparison

  1. Node-to-node: Handled by Data Link Layer (Layer 2)
  2. Host-to-host: Handled by Network Layer (Layer 3)
  3. Process-to-process: Handled by Transport Layer (Layer 4)

Transport Layer Responsibilities

  1. Service-point Addressing:
    • Uses port numbers to identify specific processes
    • While network layer delivers to host, transport layer delivers to correct application
  2. Segmentation and Reassembly:
    • Divides messages into transmittable segments
    • Uses sequence numbers for proper reassembly
    • Handles lost packet retransmission
  3. Flow Control:
    • End-to-end control (vs Data Link Layer’s single-link control)
    • Prevents sender from overwhelming receiver
  4. Error Control:
    • Process-to-process error checking
    • Ensures complete message arrives without errors
    • Uses acknowledgments and retransmissions
  5. Connection Control:
    • Connectionless (independent segments) vs Connection-oriented (established connection)

Addressing and Ports

Port Number System

  • Purpose: Identify specific processes on a host
  • Range: 16-bit integers (0-65,535)
  • Types:
    • Well-known ports: 0-1023 (assigned by IANA)
      • Examples: FTP (21), SSH (22), SMTP (25), HTTP (80)
    • Registered ports: 1024-49,151 (can be registered)
    • Dynamic/Ephemeral ports: 49,152-65,535 (temporary client ports)

Socket Address

  • Combination of IP address and port number
  • Uniquely identifies both ends of communication
  • Example: 200.23.56.8:69

Transport Layer Protocols

1. User Datagram Protocol (UDP)

  • Characteristics:
    • Connectionless
    • Unreliable (no guarantees)
    • Minimal overhead (8-byte header)
    • No flow/error/connection control
  • Header Fields:
    • Source port (16 bits)
    • Destination port (16 bits)
    • Length (16 bits)
    • Checksum (16 bits)
  • Use Cases:
    • Simple request-response applications
    • Processes with built-in error control (e.g., TFTP)
    • Multicasting
    • Management processes (SNMP)
    • Routing protocols (RIP)

2. Transmission Control Protocol (TCP)

  • Characteristics:
    • Connection-oriented (3-way handshake)
    • Reliable delivery
    • Stream-oriented (vs message-oriented)
    • Full-duplex communication
    • Implements flow and error control
  • Key Features:
    • Buffering: Uses send/receive buffers to handle speed mismatches
    • Segmentation: Groups bytes into segments for IP transmission
    • Sequence Numbers: Tracks every byte transmitted
    • Acknowledgments: Uses cumulative ACKs (ACK number = next expected byte)
  • Connection Management:
    • Establishment: 3-way handshake 1. SYN (client → server) 2. SYN-ACK (server → client) 3. ACK (client → server)
    • Termination: 4-way handshake (FIN/FIN-ACK sequence)
  • Segment Structure:
    • Sequence number (first byte in segment)
    • Acknowledgment number (next expected byte)
    • Control flags (SYN, ACK, FIN, PSH, RST, URG)
    • Window size (for flow control)

Comparison of UDP and TCP

FeatureUDPTCP
ConnectionConnectionlessConnection-oriented
ReliabilityUnreliableReliable
Flow ControlNoneImplemented
Error ControlMinimal checksumExtensive (ACKs, retransmissions)
Data OrganizationMessage-orientedStream-oriented
OverheadLow (8-byte header)Higher (20-byte header + control)
SpeedFasterSlower
Use CasesDNS, SNMP, VoIP, multicastingHTTP, FTP, email, file transfer

Graph View

Backlinks