🧱 Tutorial 6: The Internet Protocol Stack and the OSI Reference Model

Expanded Edition – COMP347 (Revision 10) | TrustOpen University

📑 Table of Contents

Learning Objectives
Overview
1. The Internet Protocol Stack (TCP/IP Model)
2. The OSI Reference Model
3. Comparison: Internet Stack vs. OSI
4. Encapsulation and Decapsulation
5. The Hourglass Model
6. The End‑to‑End Principle
7. Service Access Points and Primitives
8. Protocol Header Examples
Quiz
Exercises
Homework
Summary

🎯 Learning Objectives

Upon completion of this expanded tutorial, students will be able to:

Describe each layer of the Internet protocol stack (TCP/IP model) in detail, including its functions, key protocols, and PDU names.
Describe each layer of the OSI reference model in detail, including its functions and examples.
Compare and contrast the Internet stack and OSI model in terms of number of layers, origin, and practical use.
Explain the encapsulation and decapsulation process with specific header formats.
Analyse the hourglass model and its significance.
Understand the end‑to‑end principle and its relationship to layering.
Identify key protocol header fields (TCP, IPv4, Ethernet) and their purposes.
Explain the concepts of service access points (SAPs) and service primitives.

🔭 Overview

This tutorial provides a detailed examination of the Internet protocol stack (TCP/IP model) and the OSI reference model. While the previous tutorial introduced the basic concepts of layering, this tutorial delves into the specific protocols, functions, and interactions at each layer. We explore the header formats of key protocols (TCP, IP, Ethernet), the encapsulation process in depth, and the service models. We also compare the two models and discuss the hourglass architecture and the end‑to‑end principle. Understanding both models is essential for network professionals, as the Internet stack is what we use in practice, while the OSI model provides a valuable conceptual framework.

1. The Internet Protocol Stack (TCP/IP Model)

The Internet protocol stack, also known as the TCP/IP model, consists of five layers. We will examine each layer in depth, including its functions, key protocols, and header formats.

1.1 Application Layer

Purpose: Provides services to user applications and defines the protocols used for specific applications.

Functions: Application‑specific protocol logic, data formatting, user authentication, and service discovery.
Key protocols: HTTP/HTTPS (web), SMTP/POP3/IMAP (email), DNS (domain name resolution), FTP (file transfer), SSH (secure remote access), DHCP (IP address assignment).
PDU: Message.
Port numbers: Used by the transport layer to identify the application process (e.g., HTTP uses port 80, HTTPS uses 443, DNS uses 53).

1.2 Transport Layer

Purpose: Provides end‑to‑end communication between processes on different hosts, with optional reliability.

Functions: Segmentation and reassembly, error detection, error recovery (TCP), flow control (TCP), congestion control (TCP), and multiplexing/demultiplexing using port numbers.
Key protocols:
- TCP (Transmission Control Protocol): Connection‑oriented, reliable, ordered delivery, flow and congestion control. Uses a three‑way handshake for connection establishment and a four‑way teardown.
- UDP (User Datagram Protocol): Connectionless, unreliable, best‑effort delivery, no flow or congestion control. Used for real‑time applications and DNS.
PDU: Segment (TCP) or Datagram (UDP).
TCP header format (20 bytes minimum): Source Port (16), Destination Port (16), Sequence Number (32), Acknowledgment Number (32), Data Offset (4), Reserved (3), Flags (9: NS, CWR, ECE, URG, ACK, PSH, RST, SYN, FIN), Window Size (16), Checksum (16), Urgent Pointer (16), Options (variable).
UDP header format (8 bytes): Source Port (16), Destination Port (16), Length (16), Checksum (16).

1.3 Network Layer

Purpose: Moves packets from source to destination across multiple networks, providing addressing and routing.

Functions: Routing (determining end‑to‑end paths via routing protocols), forwarding (moving packets from input to output ports at routers), IP addressing (IPv4 and IPv6), fragmentation and reassembly, and error reporting (ICMP).
Key protocols:
- IP (Internet Protocol): The core protocol, providing best‑effort datagram delivery. IPv4 uses 32‑bit addresses, IPv6 uses 128‑bit addresses.
- ICMP (Internet Control Message Protocol): Used for error reporting and diagnostics (ping, traceroute).
- Routing protocols: OSPF (intra‑domain), BGP (inter‑domain).
PDU: Datagram.
IPv4 header format (20 bytes minimum): Version (4), IHL (4), Type of Service (8), Total Length (16), Identification (16), Flags (3), Fragment Offset (13), Time to Live (8), Protocol (8), Header Checksum (16), Source Address (32), Destination Address (32), Options (variable).
IPv6 header format (40 bytes): Version (4), Traffic Class (8), Flow Label (20), Payload Length (16), Next Header (8), Hop Limit (8), Source Address (128), Destination Address (128).

1.4 Link Layer

Purpose: Moves frames from one node to the next on the same link, providing node‑to‑node delivery.

Functions: Framing (encapsulating datagrams), error detection (CRC), media access control (MAC) – determines which node transmits on a shared medium, reliable delivery on the link (in some protocols), and flow control.
Key protocols: Ethernet (IEEE 802.3), Wi‑Fi (IEEE 802.11), PPP (Point‑to‑Point Protocol).
PDU: Frame.
Ethernet frame format: Preamble (7 bytes), Start Frame Delimiter (1), Destination MAC (6), Source MAC (6), Type/Length (2), Data (46–1500), Frame Check Sequence (4).

1.5 Physical Layer

Purpose: Transmits individual bits across the physical medium, handling the electrical, optical, or radio characteristics.

Functions: Bit encoding (e.g., Manchester encoding, NRZ), signal generation and reception, physical interface specifications (connectors, voltage levels, timing), and synchronization.
Key standards: IEEE 802.3 PHY (Ethernet physical layer), IEEE 802.11 PHY (Wi‑Fi), DOCSIS PHY (cable), DSL PHY.
PDU: Bit.

2. The OSI Reference Model

The OSI (Open Systems Interconnection) model, developed by ISO, is a seven‑layer conceptual framework that standardises network communication.

Layer	Name	Function	Examples
7	Application	Provides network services to user applications; application‑specific protocols.	HTTP, FTP, SMTP, Telnet
6	Presentation	Data formatting, translation, encryption, and compression. Converts data between application and network formats.	JPEG, MPEG, TLS (partially), ASCII/EBCDIC conversion
5	Session	Establishes, manages, and terminates sessions between applications. Provides dialog control (half‑duplex/full‑duplex) and synchronization (checkpoints).	NetBIOS, RPC (partially), SIP (session control)
4	Transport	End‑to‑end communication, segmentation, error recovery, flow control, multiplexing.	TCP, UDP
3	Network	Routing, addressing, forwarding of packets across multiple networks.	IP, IPX, AppleTalk
2	Data Link	Node‑to‑node delivery, framing, error detection, media access control (MAC).	Ethernet, Wi‑Fi, PPP, HDLC
1	Physical	Bit transmission, signal encoding, physical interface characteristics.	Ethernet PHY, 802.11 PHY, RS‑232

The OSI model includes Presentation and Session layers, which in the Internet stack are handled by the Application layer (e.g., TLS for encryption, cookies or application‑level sessions).

3. Comparison: Internet Stack vs. OSI Model

Aspect	Internet Stack (TCP/IP)	OSI Model
Number of layers	5	7
Origin	Practical, from ARPANET and DARPA	Theoretical, from ISO
Adoption	Widely implemented and used	Primarily conceptual, not widely implemented
Presentation layer	Functionality within Application layer	Separate layer
Session layer	Functionality within Application layer and TCP	Separate layer
Connection model	Supports both connectionless (UDP) and connection‑oriented (TCP)	Connection‑oriented model
Protocol suite	TCP/IP suite	OSI protocols (rarely used; e.g., X.400, X.500)
Service model	Best‑effort at network layer, reliable at transport (TCP)	Variety of service models

4. Encapsulation and Decapsulation in Detail

Encapsulation is the process of adding headers (and sometimes trailers) as data passes down the layers. Decapsulation is the reverse process at the receiver.

Example: Sending an HTTP request

Application: "GET /index.html HTTP/1.1" (Message) ↓ Transport (TCP): Adds TCP header (source port, dest port=80, seq#, flags, checksum) → Segment ↓ Network (IP): Adds IP header (source IP, dest IP, TTL, protocol=6 for TCP) → Datagram ↓ Link (Ethernet): Adds Ethernet header (dest MAC, src MAC, type) and trailer (CRC) → Frame ↓ Physical: Bits transmitted

At the receiver, each layer strips its corresponding header and passes the payload up.

5. The Hourglass Model

The Internet architecture is often represented as an hourglass: the wide top is applications (HTTP, SMTP, etc.), the narrow waist is IP (the common protocol), and the wide bottom is the diverse link‑layer technologies (Ethernet, Wi‑Fi, fibre, etc.). This design allows many different applications to run over IP, and IP to run over many different link technologies, enabling interoperability and innovation.

6. The End‑to‑End Principle

The end‑to‑end principle (Saltzer, Reed, Clark, 1984) states that functions should be implemented at the communication endpoints whenever possible, rather than in the network core. The Internet follows this by keeping the core simple (best‑effort IP) and placing reliability, security, and other functions at the endpoints (TCP, TLS). This enables rapid innovation at the application layer without changing the core.

7. Service Access Points and Primitives

Service Access Point (SAP): A logical point where a layer provides its service to the layer above. In TCP/IP, sockets are SAPs.
Service primitives: Operations that define how layers interact. The four types are:
- Request: Upper layer asks lower layer for a service.
- Indication: Lower layer notifies upper layer of an event.
- Response: Upper layer responds to an indication.
- Confirm: Lower layer acknowledges a request.

8. Protocol Header Examples

TCP Header (20 bytes minimum)

Source Port (16 bits)
Destination Port (16 bits)
Sequence Number (32 bits)
Acknowledgment Number (32 bits)
Data Offset (4 bits), Reserved (3 bits), Flags (9 bits: NS, CWR, ECE, URG, ACK, PSH, RST, SYN, FIN)
Window Size (16 bits)
Checksum (16 bits)
Urgent Pointer (16 bits)
Options (variable)

IPv4 Header (20 bytes minimum)

Version (4 bits), IHL (4 bits), Type of Service (8 bits), Total Length (16 bits)
Identification (16 bits), Flags (3 bits), Fragment Offset (13 bits)
Time to Live (8 bits), Protocol (8 bits), Header Checksum (16 bits)
Source Address (32 bits), Destination Address (32 bits)
Options (variable)

Ethernet Frame (minimum 64 bytes)

Preamble (7 bytes), Start Frame Delimiter (1 byte)
Destination MAC (6 bytes), Source MAC (6 bytes)
Type/Length (2 bytes)
Data (46–1500 bytes)
Frame Check Sequence (4 bytes, CRC)

📝 Quiz: Tutorial 6

Q1: What are the five layers of the Internet protocol stack, in order from top to bottom?

Answer

Application, Transport, Network, Link, Physical.

Q2: What is the primary function of the transport layer?

Answer

End‑to‑end communication between processes, including segmentation, error control, flow control, and congestion control (in TCP).

Q3: Name three protocols at the application layer and their uses.

Answer

HTTP (web browsing), SMTP (email sending), DNS (domain name resolution). Other examples: FTP, SSH, DHCP.

Q4: What is the difference between TCP and UDP?

Answer

TCP is connection‑oriented, reliable, ordered, with flow and congestion control. UDP is connectionless, unreliable, and has no flow or congestion control.

Q5: What are the seven layers of the OSI reference model?

Answer

Application, Presentation, Session, Transport, Network, Data Link, Physical.

Q6: What is the purpose of the presentation layer in the OSI model?

Answer

Data formatting, translation, encryption/decryption, and compression/decompression.

Q7: How does the Internet stack handle the functions of the OSI presentation layer?

Answer

It handles them at the application layer, with protocols like TLS providing encryption and application‑specific formatting (e.g., JSON, XML).

Q8: What is the difference between routing and forwarding?

Answer

Routing is the process of determining the end‑to‑end path (using routing protocols). Forwarding is the per‑hop action of moving a packet from an input port to the appropriate output port at a router.

Q9: What is the purpose of the data link layer?

Answer

It moves frames from one node to the next on the same link, handling framing, error detection (CRC), and media access control (MAC).

Q10: Why is the Internet protocol stack sometimes called the TCP/IP model?

Answer

Because TCP (Transport) and IP (Network) are the two most important and defining protocols of the Internet stack.

Q11: What is the hourglass model of the Internet architecture?

Answer

It describes the Internet stack with IP at its narrow waist, allowing many applications above and many link technologies below, enabling interoperability and innovation.

Q12: What is the end‑to‑end principle and how does it relate to the Internet's architecture?

Answer

The end‑to‑end principle states that functions should be implemented at the endpoints whenever possible, not in the network core. The Internet follows this by keeping the core simple (best‑effort IP) and placing reliability (TCP) and security (TLS) at the endpoints.

✏️ Exercises: Tutorial 6

Exercise 1 – Layer identification

For each of the following functions, identify the corresponding layer in the Internet stack and the OSI model: (a) error detection using CRC, (b) routing, (c) encryption, (d) session establishment, (e) bit transmission.

Solution

(a) Link layer (Internet) / Data Link (OSI). (b) Network layer / Network. (c) Application layer (Internet) / Presentation (OSI). (d) Application layer (Internet) / Session (OSI). (e) Physical layer / Physical.

Exercise 2 – Encapsulation diagram

Draw a detailed encapsulation diagram for a DNS query (UDP) from a client to a DNS server. Show the headers at each layer and the resulting PDUs.

Solution

Application: DNS query (Message) ↓ Transport (UDP): UDP header (src port, dst port=53, length, checksum) → Datagram ↓ Network (IP): IP header (src IP, dst IP, protocol=17 for UDP) → Datagram ↓ Link (Ethernet): Ethernet header (dest MAC, src MAC, type) + trailer → Frame ↓ Physical: Bits

Exercise 3 – Compare models

Why does the Internet stack not have a separate presentation layer? What are the trade‑offs?

Solution

The Internet stack combines presentation functions into the application layer (e.g., TLS, JSON, XML). This simplifies the stack but puts more responsibility on applications. Trade‑off: less standardisation but more flexibility and efficiency.

Exercise 4 – Header fields

Explain the purpose of the following TCP header fields: Sequence Number, Acknowledgment Number, Flags, Window Size.

Solution

Sequence Number: orders bytes for reassembly and reliability. Acknowledgment Number: confirms received bytes. Flags: control connection (SYN, ACK, FIN, etc.). Window Size: flow control – tells sender how much data can be sent.

Exercise 5 – Service primitives

Map the TCP socket API calls (socket, bind, listen, accept, connect, send, recv, close) to the OSI service primitives (Request, Indication, Response, Confirm).

Solution

connect() → Request/Confirm; accept() → Indication/Response; send/recv → data transfer primitives; close() → Request/Confirm.

Exercise 6 – Hourglass model

Explain how the hourglass model enables the Internet to support new applications without changing the core.

Solution

Because IP provides a minimal, universal service, new applications can be built on top using TCP/UDP without modifying routers. For example, HTTP/3 uses QUIC over UDP, but IP remains unchanged.

📚 Homework: Tutorial 6

Homework 1 – OSI history

Research the history of the OSI reference model. Why was it developed? Why did TCP/IP succeed while OSI largely failed?

Guidance

OSI was comprehensive but complex and slow; TCP/IP was practical, had working implementations, and was incrementally deployable.

Homework 2 – IPv6 benefits

List the main differences between IPv4 and IPv6 headers. What are the benefits of IPv6?

Guidance

IPv6 has larger addresses (128-bit), simplified header (no checksum, no fragmentation), flow label, and built-in security (IPsec).

Homework 3 – MAC vs IP

Explain the relationship between MAC addresses and IP addresses. Which layers do they belong to and why are both needed?

Guidance

MAC addresses are link-layer (Layer 2) and used for local delivery; IP addresses are network-layer (Layer 3) and used for end‑to‑end routing.

Homework 4 – TLS and OSI

In which OSI layer does TLS (Transport Layer Security) operate? Justify your answer. How does this relate to the Internet stack?

Guidance

TLS operates between the application and transport layers, often considered part of the presentation or session layer in OSI; in the Internet stack it is an application-layer protocol.

Homework 5 – Microservices and layering

Explain how modern microservices architectures relate to the concept of protocol layering. Are they following the same principles?

Guidance

Microservices use lightweight protocols and APIs, similar to layering but at the application level; they emphasise modularity and independent deployability.

📌 Summary

This tutorial has provided a detailed examination of the Internet protocol stack and the OSI reference model:

Internet protocol stack: The five layers (Application, Transport, Network, Link, Physical) with their functions, key protocols, and PDU names.
OSI reference model: The seven‑layer conceptual model (Application, Presentation, Session, Transport, Network, Data Link, Physical).
Comparison: Similarities, differences, and trade‑offs between the two models.
Encapsulation and decapsulation: How headers are added and removed at each layer.
Hourglass model: The centrality of IP enabling interoperability and innovation.
End‑to‑end principle: Functions at endpoints, keeping the core simple.
Service access points and primitives: How layers interact.

Understanding both models is essential for network professionals. The Internet stack provides the practical implementation, while the OSI model offers a valuable conceptual framework. In the next tutorial, we will explore the World Wide Web and web technologies.