Core Network Protocols

HyperText Transfer Protocol (Secure)

HTTP is the stateless application-layer protocol powering web communications. HTTPS wraps raw HTTP streams inside an encrypted TLS (Transport Layer Security) tunnel to prevent eavesdropping and man-in-the-middle attacks.

HTTP Methods:

• GET: Retrieves resources without modifying server state (Idempotent).
• POST: Submits data payloads to create new server resources.
• PUT: Replaces target resources entirely with the request payload.
• PATCH: Applies partial modifications to existing resources.
• DELETE: Removes target resources from the server.

Status Code Ranges:

• 2xx (Success): 200 OK, 201 Created, 204 No Content
• 3xx (Redirection): 301 Moved Permanently, 304 Not Modified
• 4xx (Client Error): 400 Bad Request, 401 Unauthorized, 404 Not Found
• 5xx (Server Error): 500 Internal Server Error, 503 Service Unavailable

// HTTP/1.1 Request Payload Example
GET /api/v1/courses HTTP/1.1
Host: api.wcode.edu
User-Agent: WCodeClient/2.0
Authorization: Bearer eyJhbGciOiJIUzI1Ni...
Accept: application/json

// Server Response Payload
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: max-age=3600

{"status": "success", "courses": ["DSA", "System Design", "Networks"]}

The Internet's Phonebook

Computers communicate using numerical IP addresses (104.21.32.88), but humans prefer human-readable domain names (wcode.edu). DNS operates over Port 53 (primarily UDP for fast lookups, TCP for zone transfers) to resolve hostnames to IP addresses.

Essential DNS Record Types:

# Command-Line DNS Query via dig & nslookup
$ dig +noall +answer A wcode.edu
wcode.edu.   300   IN   A   104.21.32.88

$ nslookup -type=MX wcode.edu
Server:		8.8.8.8
Address:	8.8.8.8#53

wcode.edu	mail exchanger = 10 mail.wcode.edu.

Dynamic Host Configuration Protocol

Imagine plugging a new smartphone into a home Wi-Fi router. Instead of manually entering your IP address, subnet mask, and gateway IP, DHCP automatically assigns network configuration settings dynamically.

The DORA Process:

1. Discover (Client Broadcast): Client broadcasts DHCPDISCOVER on UDP Port 67 asking for an IP.
2. Offer (Server Unicast/Broadcast): DHCP Server offers an available IP address (DHCPOFFER).
3. Request (Client Broadcast): Client requests the offered IP configuration (DHCPREQUEST).
4. Acknowledge (Server Unicast): Server confirms the lease allocation (DHCPACK).

# Inspecting Client DHCP Lease via Linux dhclient
$ sudo dhclient -v eth0
Internet Systems Consortium DHCP Client 4.4.1
Listening on LPF/eth0/aa:bb:cc:11:22:33
Sending on   LPF/eth0/aa:bb:cc:11:22:33
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3
DHCPOFFER of 192.168.1.105 from 192.168.1.1
DHCPREQUEST for 192.168.1.105 on eth0 to 255.255.255.255 port 67
DHCPACK of 192.168.1.105 from 192.168.1.1
bound to 192.168.1.105 -- renewal in 41235 seconds.

Email Transfer & Retrieval Protocol Suite

Electronic mail delivery relies on a combination of push and pull protocols:

1. SMTP (Simple Mail Transfer Protocol - Port 25 / 587)

• Push Protocol: Used to dispatch emails from client to mail server, and between intermediate mail transfer agents (MTAs).

2. POP3 (Post Office Protocol v3 - Port 110 / 995)

• Download & Delete: Downloads emails from the server to local storage and removes them from the server inbox.

3. IMAP (Internet Message Access Protocol - Port 143 / 993)

• Sync & Persist: Keeps messages synchronized on the remote server across multiple devices (laptops, phones).

// Raw SMTP Telnet Communication Session
$ telnet mail.wcode.edu 25
250 mail.wcode.edu Simple Mail Transfer Service Ready
EHLO client.wcode.edu
250-mail.wcode.edu Hello client.wcode.edu
250-SIZE 52428800
250-AUTH LOGIN PLAIN
MAIL FROM:<user@wcode.edu>
250 2.1.0 Sender OK
RCPT TO:<dest@example.com>
250 2.1.5 Recipient OK
DATA
354 Start mail input; end with <CR><LF>.<CR><LF>
Subject: W Code System Notification
Hello, your build finished successfully.
.
250 2.0.0 Message accepted for delivery

File Transfer Protocols

FTP uses separate control (Port 21) and data channels (Port 20 or passive ports). SFTP runs entirely over SSH (Port 22), encrypting both authentication credentials and data transfers into a single secure channel.

Protocol Security & Architecture:

# Secure File Transfer via SFTP CLI
$ sftp -i ~/.ssh/id_ed25519 deploy@server.wcode.edu
Connected to server.wcode.edu.
sftp> put build.tar.gz /var/www/releases/
Uploading build.tar.gz to /var/www/releases/build.tar.gz
build.tar.gz                    100%   45MB  15.2MB/s   00:03

Encrypted Remote Shell vs Plaintext Terminal

SSH (Secure Shell) provides encrypted command-line access over Port 22 using public key cryptography. Telnet (Port 23) sends all keystrokes in unencrypted plaintext, making it obsolete for modern production environments.

Key SSH Capabilities:

• Public Key Authentication: Uses RSA, ED25519 keypairs (~/.ssh/id_ed25519) instead of passwords.
• Port Forwarding / Tunneling: Encapsulates local ports securely over encrypted SSH tunnels (ssh -L).

# Passwordless SSH Authentication & Remote Tunneling
$ ssh-keygen -t ed25519 -C "dev@wcode.edu"
$ ssh-copy-id -i ~/.ssh/id_ed25519.pub user@server.wcode.edu

# Local Port Forwarding (Connect local port 5432 to remote PostgreSQL)
$ ssh -L 5432:127.0.0.1:5432 user@db.wcode.edu

Reliable Stream vs Fast Datagram Transport

The two bedrock protocols of the Transport Layer dictate how data payloads move across networks:

TCP vs UDP Architecture Comparison:

// C POSIX Socket Instantiation for TCP vs UDP
#include <sys/socket.h>

// 1. TCP Socket (Byte Stream, Connection-Oriented)
int tcp_fd = socket(AF_INET, SOCK_STREAM, 0);

// 2. UDP Socket (Datagram Stream, Connectionless)
int udp_fd = socket(AF_INET, SOCK_DGRAM, 0);