📡Systems Approach to Computer Networks Unit 5 – Protocol Layers and Service Models

Key Concepts

• Protocol layers divide complex network communication tasks into manageable pieces
• Each layer provides services to the layer above it and requests services from the layer below
• Well-defined interfaces between layers enable interoperability of protocols
• Encapsulation adds headers and trailers to data as it moves down the protocol stack
• Decapsulation removes headers and trailers as data moves up the protocol stack
• Service models define the services provided by each layer and the interfaces between them
• Protocol design principles guide the development of efficient and effective network protocols
• Real-world applications rely on the protocol stack to enable reliable communication over networks

Protocol Stack Overview

• A protocol stack is a hierarchical arrangement of protocols that work together to enable network communication
• Lower layers provide services to higher layers, allowing each layer to focus on specific tasks
• The OSI model and TCP/IP model are two widely-used protocol stack architectures
• Each layer in the stack has a specific role and communicates with its peer layer on other devices
• Data flows vertically through the layers on a single device and horizontally between peer layers on different devices
• As data moves down the stack, each layer adds a header (and sometimes a trailer) containing layer-specific information
• As data moves up the stack, each layer removes and processes its corresponding header (and trailer)

OSI Model vs. TCP/IP Model

• The OSI (Open Systems Interconnection) model is a conceptual framework with seven layers
  • Application, Presentation, Session, Transport, Network, Data Link, and Physical layers
• The TCP/IP model is a practical implementation with four layers
  • Application, Transport, Internet (Network), and Link (Network Interface) layers
• The OSI model provides a more granular and modular approach, while the TCP/IP model combines some layers for simplicity
• Both models share similar core concepts, such as encapsulation, decapsulation, and layer-specific protocols
• The TCP/IP model is more widely used in real-world network implementations (Internet)
• The OSI model serves as a valuable reference for understanding and designing network protocols

Layer-by-Layer Breakdown

• Application Layer: Provides high-level services to applications (HTTP, FTP, SMTP)
• Presentation Layer (OSI): Handles data representation, compression, and encryption
• Session Layer (OSI): Manages sessions between applications, synchronization, and checkpointing
• Transport Layer: Ensures reliable, end-to-end delivery of data segments (TCP, UDP)
  • Segments data, adds port numbers for multiplexing, and provides error checking
• Network Layer: Responsible for logical addressing and routing of packets (IP)
  • Adds source and destination IP addresses and handles fragmentation if necessary
• Data Link Layer: Provides reliable data transfer between adjacent network nodes (Ethernet, Wi-Fi)
  • Adds MAC addresses for local delivery and performs error detection (CRC)
• Physical Layer: Defines the physical characteristics of the network medium and the transmission of raw bits

Service Models and Interfaces

• Service models define the services provided by each layer and the interfaces between them
• Service primitives are the operations that a layer provides to the layer above it (request, indication, response, confirm)
• The service access point (SAP) is the interface between two adjacent layers, identified by a unique address
• The service data unit (SDU) is the data passed between two adjacent layers
• The protocol data unit (PDU) is the SDU with added layer-specific headers (and trailers)
• Well-defined interfaces enable the independent development and interoperability of protocols at each layer
• Service models help to maintain a modular and flexible protocol stack architecture

Encapsulation and Decapsulation

• Encapsulation is the process of adding layer-specific headers (and trailers) to data as it moves down the protocol stack
  • Each layer treats the data from the layer above as payload and adds its own header
• Decapsulation is the reverse process of removing headers (and trailers) as data moves up the protocol stack
  • Each layer removes its corresponding header and passes the remaining data to the layer above
• Encapsulation and decapsulation enable the separation of concerns and the independent operation of each layer
• The terms used for the PDU at each layer vary (segment, packet, frame)
• Encapsulation and decapsulation are essential for the proper functioning of the protocol stack and the delivery of data to its intended destination

Protocol Design Principles

• Layering: Divide complex tasks into manageable layers with well-defined interfaces
• Encapsulation: Each layer adds its own header (and trailer) to the data from the layer above
• Modularity: Protocols at each layer should be independent and interchangeable
• Efficiency: Minimize overhead and optimize performance through careful design choices
• Reliability: Ensure reliable data delivery through error detection, correction, and retransmission mechanisms
• Scalability: Design protocols that can accommodate growth in network size and complexity
• Simplicity: Keep protocols as simple as possible while meeting the required functionality
• Standardization: Adhere to widely-accepted standards to ensure interoperability between different implementations

Real-World Applications

• Web browsing: Uses HTTP (Application), TCP (Transport), IP (Network), and Ethernet (Data Link) protocols
• Email: Relies on SMTP (Application), TCP (Transport), IP (Network), and various link layer protocols
• Voice over IP (VoIP): Employs SIP or H.323 (Application), RTP/UDP (Transport), IP (Network), and link layer protocols
• File transfer: Uses FTP or SFTP (Application), TCP (Transport), IP (Network), and link layer protocols
• Remote access: Utilizes SSH or Telnet (Application), TCP (Transport), IP (Network), and link layer protocols
• Streaming media: Depends on HTTP or RTSP (Application), TCP or UDP (Transport), IP (Network), and link layer protocols
• Online gaming: Uses proprietary application protocols, UDP (Transport), IP (Network), and link layer protocols