Application Layer Protocols | Computer Science Fundamentals

Learning Objectives

By the end of this reading, you will be able to:

Understand the role of application layer protocols
Explain how HTTP and HTTPS work
Describe the DNS resolution process
Compare email protocols (SMTP, POP3, IMAP)
Understand FTP and modern alternatives
Work with REST APIs and HTTP methods
Analyze real-world application layer communication

Introduction

The application layer is where network-aware software operates. This is the layer users interact with directly through web browsers, email clients, file transfer programs, and countless other applications.

Application layer protocols define how applications exchange data over a network. Unlike lower layers that focus on getting data from point A to point B, the application layer focuses on the meaning and structure of that data.

HTTP (Hypertext Transfer Protocol)

HTTP is the foundation of the World Wide Web, used for transferring web pages, images, videos, and other resources.

HTTP Characteristics

Request-Response Protocol: Client sends request, server sends response
Stateless: Each request is independent (no memory of previous requests)
Text-Based: Human-readable format
TCP-Based: Uses TCP port 80
Flexible: Can transfer any type of data

HTTP Request Structure

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64)
Accept: text/html,application/xhtml+xml
Accept-Language: en-US,en;q=0.9
Connection: keep-alive

[Optional Body]

Components:

Request Line: Method, URI, HTTP version
Headers: Metadata about the request
Blank Line: Separates headers from body
Body: Optional data (used in POST, PUT)

HTTP Methods

Method	Purpose	Idempotent	Safe
GET	Retrieve resource	Yes	Yes
POST	Submit data, create resource	No	No
PUT	Update/replace resource	Yes	No
PATCH	Partially update resource	No	No
DELETE	Delete resource	Yes	No
HEAD	Get headers only (no body)	Yes	Yes
OPTIONS	Get supported methods	Yes	Yes
TRACE	Echo request (debugging)	Yes	Yes

Idempotent: Multiple identical requests have the same effect as one request Safe: Does not modify resources

HTTP Response Structure

HTTP/1.1 200 OK
Date: Mon, 23 Dec 2024 10:00:00 GMT
Server: Apache/2.4.41
Content-Type: text/html; charset=UTF-8
Content-Length: 1234
Last-Modified: Mon, 23 Dec 2024 09:00:00 GMT
ETag: "3f80f-1b6-5d9a3e00"
Connection: keep-alive

<!DOCTYPE html>
<html>
<head><title>Example</title></head>
<body><h1>Hello, World!</h1></body>
</html>

Components:

Status Line: HTTP version, status code, status message
Headers: Metadata about the response
Blank Line: Separates headers from body
Body: The actual content (HTML, JSON, image data, etc.)

HTTP Status Codes

1xx: Informational
  100 Continue
  101 Switching Protocols

2xx: Success
  200 OK                    - Request succeeded
  201 Created               - Resource created
  204 No Content            - Success, but no body

3xx: Redirection
  301 Moved Permanently     - Resource permanently moved
  302 Found                 - Resource temporarily moved
  304 Not Modified          - Cached version is current

4xx: Client Error
  400 Bad Request           - Invalid syntax
  401 Unauthorized          - Authentication required
  403 Forbidden             - Permission denied
  404 Not Found             - Resource doesn't exist
  405 Method Not Allowed    - Method not supported
  429 Too Many Requests     - Rate limit exceeded

5xx: Server Error
  500 Internal Server Error - Generic server error
  502 Bad Gateway           - Invalid upstream response
  503 Service Unavailable   - Server overloaded/down
  504 Gateway Timeout       - Upstream timeout

HTTP Headers

Request Headers:

Host: www.example.com              (Required in HTTP/1.1)
User-Agent: Mozilla/5.0...          (Browser/client identification)
Accept: text/html                   (Acceptable content types)
Accept-Language: en-US              (Preferred languages)
Accept-Encoding: gzip, deflate      (Supported compression)
Cookie: session=abc123              (Session/tracking data)
Referer: https://google.com         (Previous page)
Authorization: Bearer token123      (Authentication credentials)

Response Headers:

Content-Type: text/html             (MIME type of content)
Content-Length: 1234                (Size in bytes)
Content-Encoding: gzip              (Compression applied)
Set-Cookie: session=abc123          (Set cookie on client)
Cache-Control: max-age=3600         (Caching instructions)
Location: https://example.com/new   (Redirect target)
Server: Apache/2.4.41               (Server software)
ETag: "3f80f-1b6"                   (Resource version identifier)

HTTP/1.1 vs HTTP/2 vs HTTP/3

HTTP/1.1 (1997):
  - Text-based
  - One request per TCP connection (or sequential with keep-alive)
  - Head-of-line blocking

HTTP/2 (2015):
  - Binary protocol
  - Multiplexing: Multiple requests over single TCP connection
  - Server push
  - Header compression (HPACK)

HTTP/3 (2022):
  - Based on QUIC (UDP-based)
  - Faster connection establishment
  - Better performance on lossy networks
  - No head-of-line blocking at transport layer

Example HTTP Exchange

Client                                      Server (www.example.com)
  |                                               |
  | TCP Connection (3-way handshake)              |
  |<==========================================>   |
  |                                               |
  | GET /api/users/123 HTTP/1.1                   |
  | Host: www.example.com                         |
  | Accept: application/json                      |
  |---------------------------------------------->|
  |                                               |
  |                     HTTP/1.1 200 OK           |
  |                     Content-Type: application/json |
  |                     {"id": 123, "name": "John"}    |
  |<----------------------------------------------|
  |                                               |
  | Connection closed or kept alive               |

HTTPS (HTTP Secure)

HTTPS is HTTP over TLS/SSL, providing encryption, authentication, and integrity.

HTTPS Characteristics

Encrypted: Data encrypted in transit
Authenticated: Server identity verified (client optionally verified)
Integrity: Data cannot be modified without detection
Port: TCP 443
Certificate-based: Uses X.509 certificates

HTTPS Connection Process

1. TCP Handshake (3 steps)
2. TLS Handshake (4 steps):
   a) Client Hello
      - Supported TLS versions
      - Supported cipher suites
      - Random number

   b) Server Hello
      - Selected TLS version
      - Selected cipher suite
      - Server certificate (contains public key)
      - Random number

   c) Client Key Exchange
      - Verifies certificate
      - Generates pre-master secret
      - Encrypts with server's public key
      - Sends encrypted pre-master secret

   d) Session Keys Generated
      - Both sides generate session keys from:
        * Client random
        * Server random
        * Pre-master secret
      - "Finished" messages exchanged (encrypted)

3. Encrypted HTTP communication begins

Certificate Chain

┌─────────────────────────┐
│  Root CA Certificate     │ (Trusted by browser/OS)
│  (e.g., DigiCert)       │
└────────────┬────────────┘
             │ signs
             ▼
┌─────────────────────────┐
│ Intermediate Certificate │
└────────────┬────────────┘
             │ signs
             ▼
┌─────────────────────────┐
│  Server Certificate      │
│  (www.example.com)      │
└─────────────────────────┘

Certificate contains:

Domain name
Public key
Issuer (CA)
Validity period
Digital signature

DNS (Domain Name System)

DNS translates human-readable domain names to IP addresses.

DNS Hierarchy

                    . (Root)
                    |
        ┌───────────┼───────────┐
        |           |           |
       com         org         net    (TLDs - Top Level Domains)
        |           |           |
    ┌───┴───┐   ┌───┴───┐   ┌───┴───┐
example     google   wikipedia
    |           |           |
   www         mail        www      (Subdomains)

DNS Record Types

Type	Purpose	Example
A	IPv4 address	example.com → 93.184.216.34
AAAA	IPv6 address	example.com → 2606:2800:220:1...
CNAME	Canonical name (alias)	www → example.com
MX	Mail server	example.com → mail.example.com
NS	Name server	example.com → ns1.example.com
TXT	Text (verification, SPF, etc.)	Various purposes
SOA	Start of Authority	Zone information
PTR	Reverse lookup (IP to name)	34.216.184.93 → example.com

DNS Resolution Process

User types: www.example.com

1. Browser cache
   └─> Not found

2. OS cache
   └─> Not found

3. Router cache (if configured)
   └─> Not found

4. ISP DNS Resolver (Recursive Resolver)
   └─> Not in cache, begins recursive query:

   a) Query Root DNS Server
      "Where is .com?"
      → Returns: "Ask TLD servers for .com"

   b) Query .com TLD Server
      "Where is example.com?"
      → Returns: "Ask ns1.example.com"

   c) Query Authoritative Name Server (ns1.example.com)
      "What is the IP for www.example.com?"
      → Returns: "93.184.216.34"

   d) ISP Resolver caches result (TTL: time-to-live)
      Returns IP to client

5. Browser connects to 93.184.216.34

DNS Query Types

Recursive Query:
- Client asks DNS server to fully resolve the name
- Server does all the work
Iterative Query:
- Server responds with best information it has
- May be a referral to another server
Non-Recursive Query:
- Answer already in cache

DNS Message Format

 0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                      ID                       |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    QDCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ANCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    NSCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ARCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                   Question                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    Answer                     |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  Authority                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  Additional                   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

DNS Security: DNSSEC

DNSSEC adds cryptographic signatures to DNS records to prevent spoofing.

Email Protocols

SMTP (Simple Mail Transfer Protocol)

SMTP is used for sending email.

Characteristics:

Port: 25 (unencrypted), 587 (STARTTLS), 465 (SSL/TLS)
Text-based: Human-readable commands
Push protocol: Sender pushes to receiver

SMTP Communication:

Client                                  Server
  |                                       |
  | TCP Connection to port 25/587         |
  |-------------------------------------->|
  |                                       |
  |        220 mail.example.com ESMTP     |
  |<--------------------------------------|
  |                                       |
  | HELO client.domain.com                |
  |-------------------------------------->|
  |                                       |
  |        250 Hello client.domain.com    |
  |<--------------------------------------|
  |                                       |
  | MAIL FROM:<sender@example.com>        |
  |-------------------------------------->|
  |                                       |
  |        250 OK                         |
  |<--------------------------------------|
  |                                       |
  | RCPT TO:<recipient@example.com>       |
  |-------------------------------------->|
  |                                       |
  |        250 OK                         |
  |<--------------------------------------|
  |                                       |
  | DATA                                  |
  |-------------------------------------->|
  |                                       |
  |        354 Start mail input           |
  |<--------------------------------------|
  |                                       |
  | From: sender@example.com              |
  | To: recipient@example.com             |
  | Subject: Test Email                   |
  |                                       |
  | This is the email body.               |
  | .                                     |
  |-------------------------------------->|
  |                                       |
  |        250 OK: Message accepted       |
  |<--------------------------------------|
  |                                       |
  | QUIT                                  |
  |-------------------------------------->|
  |                                       |
  |        221 Bye                        |
  |<--------------------------------------|

POP3 (Post Office Protocol v3)

POP3 is used for retrieving email.

Characteristics:

Port: 110 (unencrypted), 995 (SSL/TLS)
Download and Delete: Typically removes from server
Simple: Minimal server-side operations
Single Device: Not ideal for multiple devices

POP3 Session:

Client                              Server
  |                                   |
  | TCP Connection to port 110        |
  |---------------------------------->|
  |                                   |
  |    +OK POP3 server ready          |
  |<----------------------------------|
  |                                   |
  | USER john@example.com             |
  |---------------------------------->|
  |                                   |
  |    +OK Password required          |
  |<----------------------------------|
  |                                   |
  | PASS secretpassword               |
  |---------------------------------->|
  |                                   |
  |    +OK Logged in                  |
  |<----------------------------------|
  |                                   |
  | STAT                              |
  |---------------------------------->|
  |                                   |
  |    +OK 3 messages (1200 bytes)    |
  |<----------------------------------|
  |                                   |
  | RETR 1                            |
  |---------------------------------->|
  |                                   |
  |    [Message 1 content]            |
  |<----------------------------------|
  |                                   |
  | DELE 1                            |
  |---------------------------------->|
  |                                   |
  |    +OK Message deleted            |
  |<----------------------------------|
  |                                   |
  | QUIT                              |
  |---------------------------------->|
  |                                   |
  |    +OK Signing off                |
  |<----------------------------------|

IMAP (Internet Message Access Protocol)

IMAP is a more advanced email retrieval protocol.

Characteristics:

Port: 143 (unencrypted), 993 (SSL/TLS)
Server Storage: Messages stay on server
Folder Management: Create, delete, rename folders
Selective Download: Download parts of messages
Multiple Devices: Synchronized across devices
Search: Server-side search capabilities

IMAP Features:

- Message flags (Read, Deleted, Flagged, etc.)
- Folder hierarchies
- Partial message fetching (headers, body parts)
- Server-side search
- Multiple mailbox support
- Message state synchronization

Email Protocol Comparison

Feature	POP3	IMAP
Message Storage	Client (usually)	Server
Multiple Devices	Poor	Excellent
Offline Access	Good	Limited to cached
Server Storage	Minimal	All messages
Folder Management	No	Yes
Partial Download	No	Yes
Server Search	No	Yes
Complexity	Simple	Complex

FTP (File Transfer Protocol)

FTP transfers files between client and server.

FTP Characteristics

Two Connections:
1. Control connection (port 21): Commands
2. Data connection (port 20): File transfers
Authentication: Username/password
Modes: Active and Passive
Binary/ASCII: Transfer modes

FTP Session

Client                                  Server
  |                                       |
  | Control Connection (port 21)          |
  |-------------------------------------->|
  |                                       |
  |    220 FTP server ready               |
  |<--------------------------------------|
  |                                       |
  | USER username                         |
  |-------------------------------------->|
  |                                       |
  |    331 Password required              |
  |<--------------------------------------|
  |                                       |
  | PASS password                         |
  |-------------------------------------->|
  |                                       |
  |    230 User logged in                 |
  |<--------------------------------------|
  |                                       |
  | PWD                                   |
  |-------------------------------------->|
  |                                       |
  |    257 "/" is current directory       |
  |<--------------------------------------|
  |                                       |
  | PASV (Passive mode)                   |
  |-------------------------------------->|
  |                                       |
  |    227 Entering Passive Mode (ip,port)|
  |<--------------------------------------|
  |                                       |
  | RETR filename.txt                     |
  |-------------------------------------->|
  |                                       |
  | [Data connection established]         |
  |<=====================================>|
  |    [File data transferred]            |
  |<=====================================>|
  |                                       |
  |    226 Transfer complete              |
  |<--------------------------------------|
  |                                       |
  | QUIT                                  |
  |-------------------------------------->|
  |                                       |
  |    221 Goodbye                        |
  |<--------------------------------------|

Active vs Passive FTP

Active FTP:

Client                                  Server
  |  Control: Client → Server (port 21)  |
  |  Data: Server → Client (client port) |

Problem: Firewalls block incoming connections to client

Passive FTP:

Client                                  Server
  |  Control: Client → Server (port 21)  |
  |  Data: Client → Server (random port) |

Solution: Client initiates both connections (firewall-friendly)

Modern Alternatives to FTP

SFTP (SSH File Transfer Protocol):
- Uses SSH (port 22)
- Encrypted
- Single connection
FTPS (FTP Secure):
- FTP over SSL/TLS
- Encrypted
- More complex than SFTP
SCP (Secure Copy):
- Based on SSH
- Simple file copying
- No directory listing/management
HTTP/HTTPS:
- File uploads via POST/PUT
- Downloads via GET
- REST APIs for file management

REST APIs

REST (Representational State Transfer) is an architectural style for web services.

REST Principles

Client-Server: Separation of concerns
Stateless: Each request contains all needed information
Cacheable: Responses must define if cacheable
Uniform Interface: Standardized communication
Layered System: Client doesn't know if connected to end server
Code on Demand (optional): Server can send executable code

RESTful API Design

Resource-Based URLs:

Good:
  GET    /api/users              (Get all users)
  GET    /api/users/123          (Get user 123)
  POST   /api/users              (Create user)
  PUT    /api/users/123          (Update user 123)
  PATCH  /api/users/123          (Partially update user 123)
  DELETE /api/users/123          (Delete user 123)

Bad:
  GET    /api/getUser?id=123
  POST   /api/createUser
  POST   /api/updateUser
  POST   /api/deleteUser

HTTP Methods in REST

Method	CRUD	Idempotent	Safe	Use Case
GET	Read	Yes	Yes	Retrieve resource(s)
POST	Create	No	No	Create new resource
PUT	Update	Yes	No	Replace entire resource
PATCH	Update	No	No	Partially update resource
DELETE	Delete	Yes	No	Remove resource

REST API Example

Request:

POST /api/users HTTP/1.1
Host: api.example.com
Content-Type: application/json
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

{
  "name": "John Doe",
  "email": "john@example.com",
  "role": "user"
}

Response:

HTTP/1.1 201 Created
Content-Type: application/json
Location: /api/users/456

{
  "id": 456,
  "name": "John Doe",
  "email": "john@example.com",
  "role": "user",
  "created_at": "2024-12-23T10:00:00Z"
}

API Authentication

1. API Keys:

GET /api/data HTTP/1.1
X-API-Key: abc123def456ghi789

2. Bearer Token (JWT):

GET /api/data HTTP/1.1
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

3. OAuth 2.0:

User → Application → Authorization Server → Resource Server

4. Basic Auth:

GET /api/data HTTP/1.1
Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ=

JSON (JavaScript Object Notation)

The most common data format for REST APIs:

{
  "users": [
    {
      "id": 1,
      "name": "Alice",
      "email": "alice@example.com",
      "roles": ["admin", "user"],
      "active": true,
      "profile": {
        "age": 30,
        "city": "New York"
      }
    },
    {
      "id": 2,
      "name": "Bob",
      "email": "bob@example.com",
      "roles": ["user"],
      "active": false,
      "profile": null
    }
  ],
  "total": 2,
  "page": 1
}

REST API Status Codes

Success:
  200 OK              - Standard success
  201 Created         - Resource created
  204 No Content      - Success with no body

Client Errors:
  400 Bad Request     - Invalid syntax/data
  401 Unauthorized    - Authentication required
  403 Forbidden       - Authenticated but not authorized
  404 Not Found       - Resource doesn't exist
  409 Conflict        - Conflict with current state
  422 Unprocessable   - Validation failed
  429 Too Many Requests - Rate limit exceeded

Server Errors:
  500 Internal Server Error
  502 Bad Gateway
  503 Service Unavailable

WebSockets

WebSockets provide full-duplex communication over a single TCP connection.

WebSocket vs HTTP

HTTP:
  Client → Server: Request
  Server → Client: Response
  [Connection may close]

WebSocket:
  Client → Server: Upgrade request
  Server → Client: Upgrade acknowledgment
  Client ↔ Server: Bidirectional messages
  [Persistent connection]

WebSocket Handshake

Client Request:
GET /chat HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

Server Response:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

[WebSocket connection established]

Use Cases:

Real-time chat
Live notifications
Multiplayer games
Stock tickers
Collaborative editing

Exercises

Basic Exercises

HTTP Status Codes: What status code would you return for each scenario?
- a) User successfully logs in
- b) User tries to access admin page without permission
- c) Requested resource doesn't exist
- d) Server database is down
- e) Successfully deleted a user
HTTP Methods: Which HTTP method would you use for:
- a) Retrieving a list of products
- b) Creating a new blog post
- c) Updating a user's email address
- d) Deleting a comment
- e) Replacing an entire product record
DNS Resolution: List the steps in order when resolving "www.github.com":
- a) What servers are queried?
- b) What type of DNS record is returned?
- c) Where might the answer be cached?
Email Protocols: Which protocol (SMTP, POP3, IMAP) would you use for:
- a) Sending an email
- b) Checking email on multiple devices
- c) Downloading email to a single laptop with limited server storage

Intermediate Exercises

REST API Design: Design RESTful endpoints for a blog with posts and comments:
- a) Get all posts
- b) Get a specific post
- c) Create a new post
- d) Update a post
- e) Delete a post
- f) Get all comments for a post
- g) Add a comment to a post
- h) Delete a comment
HTTP Headers: Explain the purpose of these headers:
- a) Content-Type: application/json
- b) Authorization: Bearer token123
- c) Cache-Control: no-cache
- d) Accept-Encoding: gzip, deflate
- e) Set-Cookie: session=abc; HttpOnly; Secure
HTTPS Certificate: Explain what happens if:
- a) Certificate is expired
- b) Domain name doesn't match certificate
- c) Certificate is self-signed
- d) Certificate chain is broken
API Response Design: Design JSON responses for:
- a) Successful user creation
- b) Validation error (missing required fields)
- c) Paginated list of 100 users (10 per page)
- d) Resource not found error

Advanced Exercises

Complete API Implementation: Design a complete REST API for a library system:
- Resources: Books, Authors, Users, Loans
- Include: Endpoints, methods, request/response formats, status codes
- Relationships: Books have authors, users borrow books
- Authentication: Who can perform which operations?
DNS Analysis: Trace the complete DNS resolution for "mail.google.com":
- a) Show each query (recursive and iterative)
- b) Identify which DNS servers are contacted
- c) What type of record is mail.google.com?
- d) How long might this result be cached?
HTTP vs HTTPS Performance: Compare loading a web page over HTTP vs HTTPS:
- a) How many round trips for HTTP?
- b) How many round trips for HTTPS (including TLS handshake)?
- c) What are the performance trade-offs?
- d) When would HTTP/2 improve performance?
Email Flow: Trace the complete path of an email from alice@example.com to bob@different.com:
- a) What protocols are used?
- b) What DNS queries occur?
- c) How many servers are involved?
- d) What happens if the recipient server is down?
API Rate Limiting: Design a rate limiting strategy for an API:
- Different limits for: Anonymous users, authenticated users, premium users
- Response format when limit exceeded
- Headers to communicate rate limit status
- How to handle burst traffic

Summary

In this reading, we explored the major application layer protocols:

HTTP/HTTPS: The foundation of the web, with HTTPS adding security through TLS/SSL
DNS: Translates domain names to IP addresses through a hierarchical system
Email Protocols:
- SMTP for sending
- POP3 for simple retrieval
- IMAP for advanced, multi-device email access
FTP: Traditional file transfer (largely replaced by SFTP/HTTPS)
REST APIs: Modern web services using HTTP methods and JSON
WebSockets: Full-duplex, persistent connections for real-time communication

These protocols enable the applications we use daily: web browsing, email, file sharing, and API-driven services.

Key Takeaways

HTTP is stateless and request-response based; HTTPS adds encryption
Status codes communicate the outcome of HTTP requests (2xx success, 4xx client error, 5xx server error)
DNS uses a hierarchical system with caching at multiple levels
IMAP is generally preferred over POP3 for modern email access
REST APIs use HTTP methods (GET, POST, PUT, DELETE) to operate on resources
JSON has become the de facto standard for API data exchange
Authentication is critical for API security (API keys, tokens, OAuth)

Next Steps

Now that you understand application layer protocols, you're ready to explore network security. In the next reading, we'll cover:

Firewalls and network access control
Encryption fundamentals
TLS/SSL in depth
VPNs and secure tunneling
Common network attacks and defenses

Continue to: 04-network-security.md

Additional Resources

RFC 2616: HTTP/1.1
RFC 7540: HTTP/2
RFC 9114: HTTP/3
RFC 1035: Domain Names
RFC 5321: SMTP
RFC 1939: POP3
RFC 3501: IMAP
RFC 959: FTP
MDN Web Docs: HTTP
REST API Tutorial

This reading is part of Module 8: Networking