Application Layer Protocols
Chapter 5 • Intermediate
Application Layer Protocols
Application Layer provides network services to applications.
HTTP (Hypertext Transfer Protocol)
HTTP is protocol for web communication.
HTTP Methods
GET: Retrieve resource
POST: Submit data
PUT: Update resource
DELETE: Delete resource
HEAD: Get headers only
HTTP Versions
HTTP/1.0:
- One request per connection
- Non-persistent
HTTP/1.1:
- Multiple requests per connection
- Persistent connections
- Pipelining
HTTP/2:
- Multiplexing
- Header compression
- Server push
HTTP/3:
- Uses QUIC (UDP-based)
- Faster connection
HTTP Status Codes
2xx Success:
- 200 OK
- 201 Created
- 204 No Content
3xx Redirection:
- 301 Moved Permanently
- 302 Found
- 304 Not Modified
4xx Client Error:
- 400 Bad Request
- 401 Unauthorized
- 403 Forbidden
- 404 Not Found
5xx Server Error:
- 500 Internal Server Error
- 502 Bad Gateway
- 503 Service Unavailable
DNS (Domain Name System)
DNS translates domain names to IP addresses.
DNS Hierarchy
Root Domain: .
Top-Level Domains: .com, .org, .edu
Second-Level Domains: example.com
Subdomains: www.example.com
DNS Records
A Record: IPv4 address
AAAA Record: IPv6 address
CNAME Record: Canonical name
MX Record: Mail exchange
NS Record: Name server
DNS Query Types
Recursive Query: Server resolves completely
Iterative Query: Server returns referral
FTP (File Transfer Protocol)
FTP transfers files.
FTP Modes
Active Mode:
- Client connects to server port 21
- Server connects to client for data
Passive Mode:
- Client connects to server port 21
- Client connects to server for data
FTP Commands
Commands:
- USER: Username
- PASS: Password
- LIST: List files
- RETR: Retrieve file
- STOR: Store file
- QUIT: Quit
SMTP (Simple Mail Transfer Protocol)
SMTP sends email.
Port: 25
Process:
- Client connects to server
- HELO/EHLO
- MAIL FROM
- RCPT TO
- DATA
- Message
- QUIT
POP3 and IMAP
POP3 (Post Office Protocol):
- Download emails
- Port 110
- Emails stored locally
IMAP (Internet Message Access Protocol):
- Access emails on server
- Port 143
- Emails stay on server
GATE CS Important Points
- HTTP: Methods, versions, status codes
- DNS: Hierarchy, record types, query types
- FTP: Modes, commands
- SMTP: Email sending process
- Port Numbers: Know common ports
Practice Tips
- HTTP: Understand methods and status codes
- DNS: Know hierarchy and record types
- FTP: Understand active vs passive mode
- Email Protocols: SMTP, POP3, IMAP
- Previous Year Questions: Solve GATE application layer questions