W Code - Pattern-Based DSA Learning Platform

Bhanu Bisht

TCP/IP Model

Internet Protocol Suite - 4 Layer Practical Architecture

TCP/IP Protocol Suite: The practical, open-standard architecture powering the global Internet today, developed by DARPA.

OSI vs TCP/IP Model Mapping

OSI Model (7 Layers)

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

TCP/IP Model (4 Layers)

4. Application

3. Transport

2. Internet

1. Network Access

TCP/IP Layer Stack Breakdown

Application Layer(Application + Presentation + Session (Layers 7, 6, 5))

Transport Layer(Transport Layer (Layer 4))

Internet Layer(Network Layer (Layer 3))

Network Access Layer(Data Link + Physical (Layers 2, 1))

Application Layer

Application + Presentation + Session (Layers 7, 6, 5)

Protocols

HTTP/1.1, HTTP/2, HTTP/3, HTTPS, SSH, DNS, DHCP, FTP, SMTP, WebSockets

Combined Software Communication Interface

In the TCP/IP architectural suite, the Application Layer collapses the top three layers of the OSI model into a single pragmatically streamlined layer. It handles everything from user-facing application protocols to data serialization and session persistence.

Core Mechanics:

Protocol Standardizations: Standardized message formats for web browsing (HTTP/HTTPS), file exchange (FTP), and email dispatch (SMTP).
Port Mapping: Maps network connections to specific local service daemons (e.g., Web Server on Port 443, SSH Daemon on Port 22).
Payload Formatting: Formats payloads in JSON, Protobuf, XML, or binary streams before passing them down the stack.

Application Layer — Implementation & Protocol Code

// DNS Lookup & Socket Connection Workflow in C
#include <netdb.h>
#include <stdio.h>

struct addrinfo hints = { .ai_family = AF_INET, .ai_socktype = SOCK_STREAM };
struct addrinfo *res;

// Resolves hostname "api.wcode.edu" to IP address
if (getaddrinfo("api.wcode.edu", "443", &hints, &res) == 0) {
    int sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
    connect(sockfd, res->ai_addr, res->ai_addrlen);
    freeaddrinfo(res);
}

Transport Layer

Transport Layer (Layer 4)

Protocols

TCP (Transmission Control Protocol), UDP (User Datagram Protocol), QUIC

Host-to-Host Process Communication

The Transport Layer delivers data between specific processes running on hosts across the network. It uses two fundamental transport paradigms:

1. TCP (Transmission Control Protocol)

Connection-Oriented: Establishes a formal virtual connection via a 3-Way Handshake before sending data.
Reliable & Ordered: Numbers every byte; unacknowledged segments are retransmitted.
Flow & Congestion Control: Dynamically adjusts transmission rate based on receiver buffer capacity and network congestion metrics.

2. UDP (User Datagram Protocol)

Connectionless & Lightweight: Sends datagrams instantly without connection setup or ACKs.
Ultra-Low Latency: Perfect for real-time video conferencing, multiplayer gaming, and DNS lookups.

Transport Layer — Implementation & Protocol Code

// Linux Kernel C TCP Header Structure (netinet/tcp.h)
struct tcphdr {
    uint16_t source;     // Source Port Number
    uint16_t dest;       // Destination Port Number
    uint32_t seq;        // Sequence Number
    uint32_t ack_seq;    // Acknowledgment Number
    uint16_t res1:4, doff:4, fin:1, syn:1, rst:1, psh:1, ack:1, urg:1;
    uint16_t window;     // Flow Control Receive Window Size
    uint16_t check;      // TCP Checksum
    uint16_t urg_ptr;    // Urgent Pointer
};

Internet Layer

Network Layer (Layer 3)

Protocols

IPv4, IPv6, ICMP, ARP, IGMP, BGP, OSPF

Packet Addressing & Universal Inter-Network Routing

The Internet Layer is the core engine of the global Internet. It accepts transport segments, wraps them into IP Packets, and routes them across heterogeneous networks to their final logical destination.

Key Mechanics:

IP Addressing: Logical network addressing scheme (IPv4 32-bit dotted-decimal vs. IPv6 128-bit hexadecimal).
Packet Routing: Autonomous systems and routers inspect the destination IP address to forward packets across internetwork hops.
Address Resolution (ARP): Translates logical IP addresses to physical network card MAC addresses within local subnets.

IPv4 Address Class Allocation Table:

Class	First Octet Range	Default Subnet Mask	Primary Assignment	Max Usable Hosts
Class A	1 – 126	255.0.0.0 (`/8`)	Enterprise Backbones	16,777,214
Class B	128 – 191	255.255.0.0 (`/16`)	Mid-sized Organizations	65,534
Class C	192 – 223	255.255.255.0 (`/24`)	Local LANs / Home Wi-Fi	254
Class D	224 – 239	N/A	Multicast Video / Routing	Reserved

Internet Layer — Implementation & Protocol Code

# ARP Request Query & Packet Capture (tcpdump)
$ sudo arp-scan --interface=eth0 192.168.1.0/24
Interface: eth0, datalink type: EN10MB (Ethernet)
192.168.1.1     00:1a:2b:3c:4d:5e    Router Gateway
192.168.1.105   aa:bb:cc:11:22:33    Workstation

Network Access Layer

Data Link + Physical (Layers 2, 1)

Protocols

Ethernet (802.3), Wi-Fi (802.11), PPP, MAC, DSL

Physical Hardware Drivers & Local Frame Transmission

The Network Access Layer (or Link Layer) encompasses both the physical media drivers and local framing logic required to move data over physical network hardware.

Key Mechanics:

Hardware Framing: Converts IP packets into physical frames bounded by preambles and Frame Check Sequences (FCS/CRC).
Media Access Control: Regulates how devices share wireless radio spectrum or physical copper/fiber links (e.g., CSMA/CA for Wi-Fi).
Bit Transmission: Converts binary frames into electrical pulses, optical light flashes, or RF waves.

Physical Media Performance Metrics:

Generation / Standard	Frequency Band	Maximum Physical Data Rate	Max Range
Wi-Fi 4 (802.11n)	2.4 / 5.0 GHz	600 Mbps	~70 Meters
Wi-Fi 5 (802.11ac)	5.0 GHz	3.5 Gbps	~35 Meters
Wi-Fi 6E (802.11ax)	2.4 / 5.0 / 6.0 GHz	9.6 Gbps	~25 Meters

Network Access Layer — Implementation & Protocol Code

# Inspecting Physical Network Card State via Linux ip CLI
$ ip link show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP
    link/ether aa:bb:cc:11:22:33 brd ff:ff:ff:ff:ff:ff

Combined Software Communication Interface

Core Mechanics:

Protocol Standardizations: Standardized message formats for web browsing (HTTP/HTTPS), file exchange (FTP), and email dispatch (SMTP).
Port Mapping: Maps network connections to specific local service daemons (e.g., Web Server on Port 443, SSH Daemon on Port 22).
Payload Formatting: Formats payloads in JSON, Protobuf, XML, or binary streams before passing them down the stack.

// DNS Lookup & Socket Connection Workflow in C #include <netdb.h> #include <stdio.h> struct addrinfo hints = { .ai_family = AF_INET, .ai_socktype = SOCK_STREAM }; struct addrinfo *res; // Resolves hostname "api.wcode.edu" to IP address if (getaddrinfo("api.wcode.edu", "443", &hints, &res) == 0) { int sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol); connect(sockfd, res->ai_addr, res->ai_addrlen); freeaddrinfo(res); }

// Linux Kernel C TCP Header Structure (netinet/tcp.h) struct tcphdr { uint16_t source; // Source Port Number uint16_t dest; // Destination Port Number uint32_t seq; // Sequence Number uint32_t ack_seq; // Acknowledgment Number uint16_t res1:4, doff:4, fin:1, syn:1, rst:1, psh:1, ack:1, urg:1; uint16_t window; // Flow Control Receive Window Size uint16_t check; // TCP Checksum uint16_t urg_ptr; // Urgent Pointer };

Class

First Octet Range

Default Subnet Mask

Primary Assignment

Max Usable Hosts

Class A

1 – 126

255.0.0.0 (/8)

Enterprise Backbones

16,777,214

Class B

128 – 191

255.255.0.0 (/16)

Mid-sized Organizations

65,534

Class C

192 – 223

255.255.255.0 (/24)

Local LANs / Home Wi-Fi

254

Class D

224 – 239

N/A

Multicast Video / Routing

Reserved

# ARP Request Query & Packet Capture (tcpdump) $ sudo arp-scan --interface=eth0 192.168.1.0/24 Interface: eth0, datalink type: EN10MB (Ethernet) 192.168.1.1 00:1a:2b:3c:4d:5e Router Gateway 192.168.1.105 aa:bb:cc:11:22:33 Workstation

Generation / Standard

Frequency Band

Maximum Physical Data Rate

Max Range

Wi-Fi 4 (802.11n)

2.4 / 5.0 GHz

600 Mbps

~70 Meters

Wi-Fi 5 (802.11ac)

5.0 GHz

3.5 Gbps

~35 Meters

Wi-Fi 6E (802.11ax)

2.4 / 5.0 / 6.0 GHz

9.6 Gbps

~25 Meters

# Inspecting Physical Network Card State via Linux ip CLI $ ip link show eth0 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP link/ether aa:bb:cc:11:22:33 brd ff:ff:ff:ff:ff:ff