This post is based on the textbook Computer Networking: A Top-Down Approach (6th Edition) by James Kurose and Keith Ross.

1. The Need for Protocol Layering
- 1.1 Airline Travel Analogy
- 1.2 Advantages of Layering
2. The Internet Protocol Stack (5-Layer Model)
3. Comparison with the OSI 7-Layer Model
- Presentation Layer
- Session Layer
4. Encapsulation
- 4.1 The Encapsulation Process
- 4.2 Complete Encapsulation Flow
5. Network Security Basics
6. Summary
7. Review Questions

1. The Need for Protocol Layering

The Internet is an extremely complex system. Numerous applications, various end systems, packet switches, links, and protocols all coexist.

To manage this complexity, we use layering.

1.1 Airline Travel Analogy

Air travel can also be divided into layers:

Departure                               Arrival
---------                               -------
 Ticket purchase                         Baggage claim
   |                                       ^
 Baggage check                           Baggage pickup
   |                                       ^
 Boarding gate                           Deplaning gate
   |                                       ^
 Takeoff                                 Landing
   |                                       ^
 En-route flight ----------------------> En-route flight

Each layer performs its own function and uses the services of the layer below.

1.2 Advantages of Layering

Modularity: Internal implementation of each layer can be changed independently
Complexity management: The entire system is divided into smaller, understandable parts
Flexibility: Protocols at one layer can be replaced with alternatives

2. The Internet Protocol Stack (5-Layer Model)

+-------------------+
| Application Layer |  Application Layer
+-------------------+
| Transport Layer   |  Transport Layer
+-------------------+
| Network Layer     |  Network Layer
+-------------------+
| Link Layer        |  Link Layer
+-------------------+
| Physical Layer    |  Physical Layer
+-------------------+

2.1 Application Layer

The layer where network applications and application-layer protocols reside.

HTTP: Web document request and transfer
SMTP: Email transfer
FTP: File transfer
DNS: Translating domain names to IP addresses

The information packet at the application layer is called a message.

2.2 Transport Layer

Delivers application-layer messages between end systems.

TCP: Connection-oriented, reliable delivery, congestion control, flow control
UDP: Connectionless, unreliable delivery, minimal service

The packet at the transport layer is called a segment.

TCP services:
  + Reliable data delivery (retransmission)
  + Flow control (prevents receiver overload)
  + Congestion control (prevents network overload)

UDP services:
  + Fast transmission (minimal overhead)
  x No reliability guarantee
  x No flow/congestion control

2.3 Network Layer

Delivers datagrams from one host to another.

IP protocol: Defines datagram fields and addressing scheme
Routing protocols: Determine the route from source to destination

The packet at the network layer is called a datagram.

Two core functions of the network layer:
  1. Forwarding: Moving a packet to the appropriate output link
  2. Routing: Determining the source-to-destination path

Since there is only one IP protocol, the network layer is often called the IP layer.

2.4 Link Layer

While the network layer determines the path from source to destination, the link layer is responsible for actually delivering datagrams from one node to the next.

Ethernet
WiFi (802.11)
PPP

The packet at the link layer is called a frame.

Path: A -> R1 -> R2 -> R3 -> B

Link layer's role:
  A --[Ethernet]--> R1 --[WiFi]--> R2 --[PPP]--> R3 --[Ethernet]--> B

  Different link-layer protocols can be used on each segment (link).

2.5 Physical Layer

While the link layer delivers data in frame units, the physical layer moves individual bits from one node to the next.

Deals with actual electrical signals, light signals, and wireless signals
Protocols differ depending on the medium (copper, fiber, wireless)

3. Comparison with the OSI 7-Layer Model

The OSI (Open Systems Interconnection) model proposed by ISO consists of 7 layers.

OSI 7 Layers              Internet 5 Layers
-----------              -----------------
Application Layer  --+
Presentation Layer --+-- Application Layer
Session Layer      --+
Transport Layer    ---- Transport Layer
Network Layer      ---- Network Layer
Data Link Layer    ---- Link Layer
Physical Layer     ---- Physical Layer

Presentation Layer

Data compression, encryption, data format conversion
In the Internet model, handled directly by the application

Session Layer

Synchronization of data exchange, checkpointing, recovery
In the Internet model, handled directly by the application

In the Internet protocol stack, if presentation/session layer functions are needed, application developers implement them directly.

4. Encapsulation

4.1 The Encapsulation Process

Each layer adds its own header information to the data received from the layer above.

Application:  [Message]
                |
Transport:   [Ht|Message]              <- Segment
                |
Network:     [Hn|Ht|Message]           <- Datagram
                |
Link:        [Hl|Hn|Ht|Message]        <- Frame
                |
Physical:     01011001010110...         <- Bits

Layer	PDU (Protocol Data Unit)	Header Added
Application	Message	-
Transport	Segment	Transport header
Network	Datagram	Network header
Link	Frame	Link header

4.2 Complete Encapsulation Flow

Source Host            Router               Destination Host
+------------+    +--------------+    +------------+
| Application|    |              |    | Application|
+------------+    |              |    +------------+
| Transport  |    |              |    | Transport  |
+------------+    +--------------+    +------------+
| Network    |    |   Network    |    | Network    |
+------------+    +--------------+    +------------+
| Link       |    |   Link       |    | Link       |
+------------+    +--------------+    +------------+
| Physical   |    |   Physical   |    | Physical   |
+------------+    +--------------+    +------------+

Host: Implements all 5 layers
Router: Implements only the lower 3 layers (Network, Link, Physical)
Switch: Implements only the lower 2 layers (Link, Physical)

5. Network Security Basics

5.1 The Early Internet and Security

The early Internet was designed with the assumption that only a small number of trusted users would use it. Therefore, security was not included in the original design.

Today, network security is essential, and the following threats exist.

5.2 Major Network Security Threats

Malware

Types of malware:
  +-- Virus: Infects through user interaction
  +-- Worm: Automatically spreads through the network
  +-- Trojan horse: Disguised as legitimate software
  +-- Botnet: A network of infected computers

Denial of Service (DoS)

An attack that makes a network, host, or infrastructure unusable for legitimate users.

Three types of DoS attacks:
  1. Vulnerability attack: Exploits a specific vulnerability
  2. Bandwidth flooding: Saturates links with massive packets
  3. Connection flooding: Creates a large number of half-open connections

DDoS (Distributed DoS): Simultaneous attacks from multiple sources

  Botnet PC1 --+
  Botnet PC2 --+---> Target server (overloaded)
  Botnet PC3 --+
  ...          --+

Packet Sniffing

Intercepting copies of packets passing through a shared medium.

Shared medium (WiFi, etc.):
  A -------------------- B
       |
       +-- Sniffer (packet eavesdropper)
           Receives copies of all packets

IP Spoofing

Generating packets with a forged source IP address to impersonate another user.

Attacker forges source IP:
  Actual IP: 192.168.1.100
  Forged IP: 10.0.0.1 (trusted host)

  Server mistakenly believes the packet came from the trusted host

5.3 Key Security Principles

Core elements of network security:
  +-- Confidentiality: Only sender and receiver understand content
  +-- Integrity: Detecting data alteration during transmission
  +-- Authentication: Verifying the identity of the other party
  +-- Availability: Services are always accessible

6. Summary

Internet Protocol Stack Summary:

Layer          PDU        Key Protocols      Role
-----          ---        -------------      ----
Application    Message    HTTP,SMTP,DNS      Network applications
Transport      Segment    TCP, UDP           End-to-end data delivery
Network        Datagram   IP                 Host-to-host routing
Link           Frame      Ethernet, WiFi     Adjacent node delivery
Physical       Bit        -                  Physical bit transmission

Key encapsulation concepts:

Each layer treats the upper layer's PDU as a payload and adds its own header
On the receiving side, each layer removes its header and passes data to the upper layer (decapsulation)

7. Review Questions

Q1. What is the PDU name for each layer in the Internet 5-layer model?

Layer	PDU
Application	Message
Transport	Segment
Network	Datagram
Link	Frame
Physical	Bit

Q2. How many layers does a router implement?

A router implements the lower 3 layers (Physical, Link, Network). A router needs the network layer to examine IP datagram headers and make forwarding decisions, but it does not implement the transport or application layers.

Q3. What are the two layers in the OSI model that are absent from the Internet model?

The Presentation layer and the Session layer.

Presentation layer: Handles data compression, encryption, and format conversion
Session layer: Handles synchronization of data exchange and checkpointing

In the Internet, these functions are implemented directly by application developers when needed.