W Code - Pattern-Based DSA Learning Platform

Bhanu Bisht

OSI Model

Open Systems Interconnection - 7 Layer Architectural Model

Mnemonic (Bottom to Top): "Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application)

OSI Architectural Layer Stack

Application Layer(Data)

Gateways, Next-Gen Firewalls

Presentation Layer(Data)

Application Gateway

Session Layer(Data)

Session Border Controller, Gateway

Transport Layer(Segment (TCP) / Datagram (UDP))

Firewall, Load Balancer, Gateway

Network Layer(Packet)

Router, Layer 3 Switch

Data Link Layer(Frame)

Network Switch, Bridge, NIC

Physical Layer(Bits)

Network Hub, Repeater, Network Cables, Optical Transceivers

Application Layer

Data Unit: Data

Protocols

HTTP, HTTPS, FTP, SMTP, DNS, DHCP, SSH

Devices

Gateways, Next-Gen Firewalls

User-Facing Network Interface

Think of the Application Layer as an executive at a desk. The executive does not need to know how delivery trucks or cargo planes operate; they simply interact with their email client or web browser to initiate a transaction.

The Application Layer sits at the top of the OSI stack. It provides standard communication protocols directly to end-user software applications.

Key Responsibilities:

Network Service Access: Exposes standardized interfaces (e.g., REST APIs, web pages, email protocols) to application software.
User Authentication & Authorization: Validates user identity and application credentials before sending data onto the network.
Resource Sharing: Facilitates remote file management (FTP), domain resolution (DNS), and automated host configuration (DHCP).

Application Layer — Implementation Code / Header Spec

// HTTP/1.1 Request Payload Example
GET /api/v1/user/profile HTTP/1.1
Host: api.wcode.edu
User-Agent: WCodeClient/2.0 (Linux x86_64)
Authorization: Bearer eyJhbGciOiJIUzI1Ni...
Accept: application/json

// Server HTTP/1.1 Response Payload
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 84

{"status": "success", "user": {"id": 402, "role": "student", "enrolled": true}}

Presentation Layer

Data Unit: Data

Protocols

SSL/TLS, JPEG, PNG, MPEG, ASCII, UTF-8

Devices

Application Gateway

Data Translator & Cryptographer

Imagine a diplomatic translator and security agent. If a document arrives written in a foreign format, the translator converts it into the native language. If the message is confidential, the agent encrypts it before sending and decrypts it upon arrival.

The Presentation Layer acts as the network's syntax and formatting engine, ensuring data sent by one system is readable by another.

Key Responsibilities:

Data Translation: Converts heterogeneous character encodings (e.g., EBCDIC to ASCII or UTF-8).
Encryption & Decryption: Handles SSL/TLS cryptographic handshakes to protect sensitive information during transit.
Data Compression: Compresses raw payload sizes to minimize network bandwidth consumption.

Presentation Layer — Implementation Code / Header Spec

// SSL/TLS Session Cipher & Serialization Example (C / OpenSSL)
#include <openssl/ssl.h>
#include <openssl/err.h>

void initialize_tls_session() {
    SSL_library_init();
    OpenSSL_add_all_algorithms();
    SSL_load_error_strings();
    
    SSL_CTX *ctx = SSL_CTX_new(TLS_client_method());
    SSL *ssl = SSL_new(ctx);
    // Symmetric AES-256-GCM Session Established
}

Session Layer

Data Unit: Data

Protocols

NetBIOS, RPC, SOCKS, WebSockets, PPTP

Devices

Session Border Controller, Gateway

Connection Coordinator & Dialog Controller

Imagine a telephone operator managing a conference call. The operator places the call, ensures both callers can speak without cutting each other off, and automatically reconnects the line if there is a brief disconnect.

The Session Layer establishes, maintains, synchronizes, and terminates long-lived communication channels between applications.

Key Responsibilities:

Dialog Control: Regulates whether communication occurs in Simplex (one-way), Half-Duplex (one at a time), or Full-Duplex (simultaneous two-way).
Synchronization & Checkpointing: Inserts recovery points into large file transmissions so transfers can resume from the last valid checkpoint if interrupted.

Session Layer — Implementation Code / Header Spec

// POSIX Session Keep-Alive & RPC Control
#include <sys/socket.h>
#include <netinet/tcp.h>

int optval = 1;
// Enable TCP Keep-Alive probes at Session Layer boundary
setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &optval, sizeof(optval));

// Set Keep-Alive Probe Parameters (Idle: 60s, Interval: 10s, Count: 3)
int idle = 60, interval = 10, count = 3;
setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle));
setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPINTVL, &interval, sizeof(interval));

Transport Layer

Data Unit: Segment (TCP) / Datagram (UDP)

Protocols

TCP, UDP, SCTP, QUIC

Devices

Firewall, Load Balancer, Gateway

End-to-End Reliability & Port Dispatcher

Imagine a registered courier service vs. a fast flyer drop. The registered courier (TCP) numbers every parcel, requests a signed receipt for each one, and re-delivers lost packages. The flyer drop (UDP) tosses packets quickly without waiting for confirmation.

The Transport Layer manages process-to-process delivery across the network using port numbers.

Key Responsibilities:

Segmentation & Reassembly: Splits large application data streams into manageable chunks called segments.
Port Addressing: Directs incoming segments to the exact software application process (e.g., Port 80/443 for Web, Port 22 for SSH).
Flow & Error Control: Uses sliding windows and ACKs to prevent fast senders from overwhelming slow receivers.

Transport Layer — Implementation Code / Header Spec

// C POSIX TCP Socket Client Connection
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int client_fd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in serv_addr = {
    .sin_family = AF_INET,
    .sin_port = htons(443) // Destination Port 443 (HTTPS)
};
inet_pton(AF_INET, "104.21.32.88", &serv_addr.sin_addr);

// Executes TCP 3-Way Handshake (SYN -> SYN-ACK -> ACK)
connect(client_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr));

Network Layer

Data Unit: Packet

Protocols

IPv4, IPv6, ICMP, OSPF, BGP, IPsec

Devices

Router, Layer 3 Switch

Global Routing & Path Determination

Imagine the International Postal Routing System. When you post a letter to another country, local post offices inspect the destination country code and postal ZIP code (IP Address) to choose the best cargo flight or highway route.

The Network Layer handles logical addressing and packet routing across multiple interconnected networks.

Key Responsibilities:

Logical Addressing: Assigns unique IP addresses (IPv4 32-bit or IPv6 128-bit) to every network interface.
Routing Algorithms: Evaluates path costs (metric, hop count, bandwidth) via protocols like OSPF and BGP to determine optimal transit paths.
Packet Fragmentation: Breaks oversized packets down to fit within the Maximum Transmission Unit (MTU) of intermediate links.

IPv4 Packet Header Fields:

Field Name	Bit Width	Purpose
Version & IHL	8 bits	IP protocol version (IPv4=4) and Header Length
Total Length	16 bits	Complete packet size (Header + Payload) in bytes
Identification & Flags	32 bits	Packet fragmentation tracking and Don't Fragment (DF) flags
Time to Live (TTL)	8 bits	Hop counter decremented by routers to prevent infinite loops
Protocol	8 bits	Identifies upper layer protocol (6=TCP, 17=UDP, 1=ICMP)
Source / Destination IP	64 bits	32-bit Origin and Target IPv4 addresses

Network Layer — Implementation Code / Header Spec

// Linux Kernel IPv4 Header C Structure (netinet/ip.h)
struct iphdr {
    unsigned int   ihl:4;        // IP Header Length
    unsigned int   version:4;    // Version (IPv4)
    uint8_t        tos;          // Type of Service
    uint16_t       tot_len;      // Total Packet Length
    uint16_t       id;           // Packet Identification ID
    uint16_t       frag_off;     // Fragment Offset & Flags
    uint8_t        ttl;          // Time to Live (Hop limit)
    uint8_t        protocol;     // Transport Protocol (TCP=6, UDP=17)
    uint16_t       check;        // IP Header Checksum
    uint32_t       saddr;        // Source IP Address
    uint32_t       daddr;        // Destination IP Address
};

Data Link Layer

Data Unit: Frame

Protocols

Ethernet (802.3), Wi-Fi (802.11), ARP, PPP

Devices

Network Switch, Bridge, NIC

Local Node-to-Node Transfer & Hardware Addressing

Imagine a local courier delivery van. Once a parcel arrives at your city's regional hub, the local van uses your physical street door plate number (MAC Address) to hand-deliver the parcel directly to your doorstep.

The Data Link Layer manages error-free transmission of data frames between two directly connected nodes on the same physical local network (LAN).

Two Essential Sub-layers:

Logical Link Control (LLC): Manages link synchronization, multiplexing, and error checking.
Media Access Control (MAC): Appends hardware MAC addresses and controls physical medium access (e.g., CSMA/CD for Ethernet).

Ethernet II Frame Format:

Field	Preamble & SFD	Destination MAC	Source MAC	EtherType	Payload Data	CRC Checksum
Length	8 Bytes	6 Bytes	6 Bytes	2 Bytes	46 – 1500 Bytes	4 Bytes (FCS)

Data Link Layer — Implementation Code / Header Spec

// Linux Kernel Ethernet II Frame Header (net/ethernet.h)
struct ether_header {
    uint8_t  ether_dhost[6]; // Destination MAC (e.g. 00:1A:2B:3C:4D:5E)
    uint8_t  ether_shost[6]; // Source MAC (e.g. AA:BB:CC:11:22:33)
    uint16_t ether_type;     // EtherType (0x0800 = IPv4, 0x86DD = IPv6, 0x0806 = ARP)
} __attribute__((packed));

Physical Layer

Data Unit: Bits

Protocols

100BASE-TX, 1000BASE-T, USB, Bluetooth, RS-232

Devices

Network Hub, Repeater, Network Cables, Optical Transceivers

Raw Bitstream & Signal Transmission

Imagine the actual copper wire, optical glass fiber filaments, or radio frequency waves carrying electrical voltage pulses or light flashes down the highway.

The Physical Layer is the bottom layer of the OSI stack. It is responsible for transmitting raw, unformatted binary bits (0s and 1s) over physical communication media.

Key Responsibilities:

Bit Representation & Encoding: Defines how binary 0 and 1 are converted into electrical voltages, optical pulses, or radio frequencies (e.g., Manchester encoding).
Transmission Rate: Specifies the bit rate (number of bits transmitted per second).
Physical Topology & Cabling: Standardizes connector types (RJ-45, LC fiber) and physical network arrangements (Star, Mesh, Bus).

Physical Media Performance Metrics:

Cable Type	Max Throughput	Max Segment Distance	Electromagnetic Immunity
Cat6a Twisted Pair	10 Gbps	100 Meters	Moderate (Copper Wire)
Single-Mode Fiber	100+ Gbps	40 Kilometers	Immune (Optical Light)
Wi-Fi 6E (802.11ax)	9.6 Gbps	~50 Meters	High (Radio Frequency)

Physical Layer — Implementation Code / Header Spec

# Querying Physical Hardware Link Properties via ethtool
$ ethtool eth0
Settings for eth0:
	Supported ports: [ TP ]
	Supported link modes:   1000baseT/Full 10000baseT/Full
	Speed: 10000Mb/s
	Duplex: Full
	Auto-negotiation: on
	Link detected: yes

🎯 Key Interview Concepts — OSI Model

Data Encapsulation Flow: Data (L7-L5) → Segment (L4) → Packet (L3) → Frame (L2) → Bits (L1)

Hardware Mapping: Router = Network Layer (L3), Switch = Data Link Layer (L2), Hub/Cable = Physical Layer (L1)

OSI vs TCP/IP: OSI is a 7-layer reference model; TCP/IP is a practical 4-layer protocol suite powering the Internet.

OSI Architectural Layer Stack

Application Layer(Data)

Gateways, Next-Gen Firewalls

Presentation Layer(Data)

Application Gateway

Session Layer(Data)

Session Border Controller, Gateway

Transport Layer(Segment (TCP) / Datagram (UDP))

Firewall, Load Balancer, Gateway

Network Layer(Packet)

Router, Layer 3 Switch

Data Link Layer(Frame)

Network Switch, Bridge, NIC

Physical Layer(Bits)

Network Hub, Repeater, Network Cables, Optical Transceivers

// HTTP/1.1 Request Payload Example GET /api/v1/user/profile HTTP/1.1 Host: api.wcode.edu User-Agent: WCodeClient/2.0 (Linux x86_64) Authorization: Bearer eyJhbGciOiJIUzI1Ni... Accept: application/json // Server HTTP/1.1 Response Payload HTTP/1.1 200 OK Content-Type: application/json; charset=utf-8 Content-Length: 84 {"status": "success", "user": {"id": 402, "role": "student", "enrolled": true}}

// SSL/TLS Session Cipher & Serialization Example (C / OpenSSL) #include <openssl/ssl.h> #include <openssl/err.h> void initialize_tls_session() { SSL_library_init(); OpenSSL_add_all_algorithms(); SSL_load_error_strings(); SSL_CTX *ctx = SSL_CTX_new(TLS_client_method()); SSL *ssl = SSL_new(ctx); // Symmetric AES-256-GCM Session Established }

// POSIX Session Keep-Alive & RPC Control #include <sys/socket.h> #include <netinet/tcp.h> int optval = 1; // Enable TCP Keep-Alive probes at Session Layer boundary setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &optval, sizeof(optval)); // Set Keep-Alive Probe Parameters (Idle: 60s, Interval: 10s, Count: 3) int idle = 60, interval = 10, count = 3; setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle)); setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPINTVL, &interval, sizeof(interval));

End-to-End Reliability & Port Dispatcher

The Transport Layer manages process-to-process delivery across the network using port numbers.

Key Responsibilities:

Segmentation & Reassembly: Splits large application data streams into manageable chunks called segments.
Port Addressing: Directs incoming segments to the exact software application process (e.g., Port 80/443 for Web, Port 22 for SSH).
Flow & Error Control: Uses sliding windows and ACKs to prevent fast senders from overwhelming slow receivers.

// C POSIX TCP Socket Client Connection #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> int client_fd = socket(AF_INET, SOCK_STREAM, 0); struct sockaddr_in serv_addr = { .sin_family = AF_INET, .sin_port = htons(443) // Destination Port 443 (HTTPS) }; inet_pton(AF_INET, "104.21.32.88", &serv_addr.sin_addr); // Executes TCP 3-Way Handshake (SYN -> SYN-ACK -> ACK) connect(client_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr));

Field Name

Bit Width

Purpose

Version & IHL

8 bits

IP protocol version (IPv4=4) and Header Length

Total Length

16 bits

Complete packet size (Header + Payload) in bytes

Identification & Flags

32 bits

Packet fragmentation tracking and Don't Fragment (DF) flags

Time to Live (TTL)

8 bits

Hop counter decremented by routers to prevent infinite loops

Protocol

8 bits

Identifies upper layer protocol (6=TCP, 17=UDP, 1=ICMP)

Source / Destination IP

64 bits

32-bit Origin and Target IPv4 addresses

// Linux Kernel IPv4 Header C Structure (netinet/ip.h) struct iphdr { unsigned int ihl:4; // IP Header Length unsigned int version:4; // Version (IPv4) uint8_t tos; // Type of Service uint16_t tot_len; // Total Packet Length uint16_t id; // Packet Identification ID uint16_t frag_off; // Fragment Offset & Flags uint8_t ttl; // Time to Live (Hop limit) uint8_t protocol; // Transport Protocol (TCP=6, UDP=17) uint16_t check; // IP Header Checksum uint32_t saddr; // Source IP Address uint32_t daddr; // Destination IP Address };

Field

Preamble & SFD

Destination MAC

Source MAC

EtherType

Payload Data

CRC Checksum

Length

8 Bytes

6 Bytes

2 Bytes

46 – 1500 Bytes

4 Bytes (FCS)

// Linux Kernel Ethernet II Frame Header (net/ethernet.h) struct ether_header { uint8_t ether_dhost[6]; // Destination MAC (e.g. 00:1A:2B:3C:4D:5E) uint8_t ether_shost[6]; // Source MAC (e.g. AA:BB:CC:11:22:33) uint16_t ether_type; // EtherType (0x0800 = IPv4, 0x86DD = IPv6, 0x0806 = ARP) } __attribute__((packed));

Cable Type

Max Throughput

Max Segment Distance

Electromagnetic Immunity

Cat6a Twisted Pair

10 Gbps

100 Meters

Moderate (Copper Wire)

Single-Mode Fiber

100+ Gbps

40 Kilometers

Immune (Optical Light)

Wi-Fi 6E (802.11ax)

9.6 Gbps

~50 Meters

High (Radio Frequency)

# Querying Physical Hardware Link Properties via ethtool $ ethtool eth0 Settings for eth0: Supported ports: [ TP ] Supported link modes: 1000baseT/Full 10000baseT/Full Speed: 10000Mb/s Duplex: Full Auto-negotiation: on Link detected: yes

🎯 Key Interview Concepts — OSI Model

Data Encapsulation Flow: Data (L7-L5) → Segment (L4) → Packet (L3) → Frame (L2) → Bits (L1)

Hardware Mapping: Router = Network Layer (L3), Switch = Data Link Layer (L2), Hub/Cable = Physical Layer (L1)

OSI vs TCP/IP: OSI is a 7-layer reference model; TCP/IP is a practical 4-layer protocol suite powering the Internet.