02 Application Layer

Overview

The application layer is the topmost layer in the TCP/IP and OSI models, providing network services directly to end-user applications. It enables user interactions with network resources and facilitates communication between software applications.

Key Characteristics

Aspect

Description

Position

Top layer (Layer 7 OSI, Layer 4 TCP/IP)

Purpose

Provide services to end-user applications

Interface

APIs, protocols (HTTP, SMTP, DNS, FTP)

Data Unit

Messages

Core Functions

End-User Services: Direct interaction with users and applications
Data Presentation: Format conversion, encryption, compression
Session Management: Establish, maintain, terminate connections
Application Protocols: Implement specific application services
Port-Based Identification: Use port numbers to identify services

Common Application Protocols

Protocol

Port

Purpose

Transport

HTTP

Web browsing

TCP

HTTPS

443

Secure web browsing

TCP

SMTP

Send email

TCP

POP3

110

Retrieve email

TCP

IMAP

143

Retrieve email (advanced)

TCP

FTP

20/21

File transfer

TCP

DNS

Name resolution

UDP/TCP

SSH

Secure remote access

TCP

Telnet

Remote access (insecure)

TCP

HTTP

HyperText Transfer Protocol is the foundation of data communication on the World Wide Web.

HTTP Characteristics

1. Client-Server Model:

┌──────────┐                      ┌──────────┐
│  Client  │ ─── HTTP Request ──→ │  Server  │
│(Browser) │ ←── HTTP Response ─  │ (Apache) │
└──────────┘                      └──────────┘

2. Stateless Protocol:

Each request is independent
Server doesn't retain information between requests
State maintained via cookies, sessions, or tokens

3. Text-Based:

Human-readable protocol
Headers and methods are plain text
Supports various content types in body

HTTP Request Structure

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html
Accept-Language: en-US
Connection: keep-alive

[Optional Request Body]

Components:

Request Line:
- Method (GET, POST, PUT, DELETE, HEAD, OPTIONS, PATCH)
- URL/URI
- HTTP Version (HTTP/1.0, HTTP/1.1, HTTP/2, HTTP/3)
Headers:
- Host: Domain name
- User-Agent: Client information
- Accept: Acceptable content types
- Content-Type: Body format (for POST/PUT)
- Authorization: Authentication credentials
- Cookie: Session/state information
Body (optional): Data payload for POST/PUT requests

HTTP Response Structure

HTTP/1.1 200 OK
Date: Mon, 01 Nov 2025 12:00:00 GMT
Server: Apache/2.4.1
Content-Type: text/html; charset=UTF-8
Content-Length: 1234
Connection: keep-alive

<!DOCTYPE html>
<html>...

Components:

Status Line:
- HTTP Version
- Status Code
- Reason Phrase
Response Headers:
- Date, Server, Content-Type, Content-Length
- Set-Cookie: Set client cookies
- Cache-Control: Caching directives
- Location: Redirect URL (for 3xx)
Body: Response data (HTML, JSON, XML, images, etc.)

HTTP Status Codes

Code Range

HTTP Methods

Method

Purpose

Idempotent

Safe

Body

GET

Retrieve resource

Yes

POST

Create resource

Yes

PUT

Update/replace resource

Yes

DELETE

Delete resource

Yes

HEAD

Retrieve headers only

Yes

OPTIONS

Get supported methods

Yes

PATCH

Partial update

Yes

Idempotent: Multiple identical requests have same effect as single request Safe: Read-only, doesn't modify server state

Cookies and Sessions

Cookies: Small data stored on client to maintain state

Server Response:
Set-Cookie: sessionId=abc123; Path=/; HttpOnly; Secure

Subsequent Client Request:
Cookie: sessionId=abc123

Cookie Attributes:

Domain: Which domain can access cookie
Path: Which paths can access cookie
Expires/Max-Age: Cookie lifetime
HttpOnly: Not accessible via JavaScript (XSS protection)
Secure: Only sent over HTTPS

Sessions: Server-side state management

Server creates session, generates session ID
Session ID sent to client in cookie
Client sends session ID with each request
Server retrieves session data using session ID

HTTP Caching

Reduces latency and server load by storing responses locally.

Client          Proxy Cache          Origin Server
  │                  │                     │
  ├───Request──────→ │                     │
  │                  ├──Check Cache────→   │
  │                  │  (Cache Miss)       │
  │                  ├───Forward Request──→│
  │                  │←──Response──────────┤
  │←──Response───────┤  (Store in cache)   │
  │                  │                     │

Cache-Control Directives:

max-age=3600: Cache for 3600 seconds
no-cache: Revalidate with server before using
no-store: Don't cache at all
private: Cache only in browser, not proxies
public: Cache in shared caches

HTTP Versions

Version

Year

Key Features

HTTP/1.0

1996

Basic functionality, one request per connection

HTTP/1.1

1997

Persistent connections, chunked transfer, pipelining

HTTP/2

2015

Binary protocol, multiplexing, server push, header compression

HTTP/3

2020

QUIC (UDP-based), improved performance over unreliable networks

HTTP/1.1 vs HTTP/2:

HTTP/1.1 (Sequential):
Conn ═══[Req1]═══[Res1]═══[Req2]═══[Res2]═══[Req3]═══[Res3]═══

HTTP/2 (Multiplexed):
Conn ═══[Req1][Req2][Req3]═══[Res1][Res3][Res2]═══
         All interleaved on single connection

HTTPS (HTTP Secure)

HTTP over TLS/SSL providing encryption and authentication.

Client                                    Server
  │                                         │
  ├────ClientHello─────────────────────────→│
  │                                         │
  │←───ServerHello, Certificate─────────────┤
  │                                         │
  ├────Key Exchange────────────────────────→│
  │                                         │
  │←───Finished──────────────────────────────┤
  │                                         │
  ├────Encrypted HTTP Request──────────────→│
  │                                         │
  │←───Encrypted HTTP Response──────────────┤

Benefits:

Confidentiality: Data encrypted in transit
Integrity: Detect tampering
Authentication: Verify server identity (certificates)

SMTP and Email Protocols

SMTP (Simple Mail Transfer Protocol)

SMTP is used for sending email from client to server and between servers.

┌─────────┐     SMTP      ┌──────────┐    SMTP     ┌─────────┐
│  Sender │  ────────→    │  Sender  │  ────────→  │Receiver │
│  (MUA)  │   Port 25     │   Mail   │   Port 25   │  Mail   │
│         │               │  Server  │             │ Server  │
└─────────┘               └──────────┘             └─────────┘
                                                         ↑
                                                         │
                                            ┌─────────┐  │ POP3/IMAP
                                            │Receiver │  │
                                            │  (MUA)  │←─┘
                                            └─────────┘

SMTP Session Example:

S: 220 mail.example.com SMTP Server ready
C: HELO client.example.com
S: 250 Hello client.example.com
C: MAIL FROM:<sender@example.com>
S: 250 OK
C: RCPT TO:<recipient@example.com>
S: 250 OK
C: DATA
S: 354 Start mail input
C: From: sender@example.com
C: To: recipient@example.com
C: Subject: Test Email
C:
C: This is the email body.
C: .
S: 250 OK Message accepted
C: QUIT
S: 221 Closing connection

Email Retrieval Protocols

POP3 (Post Office Protocol version 3):

Downloads emails to local device
Typically deletes from server after download
Stateful protocol with three phases:
1. Authorization (username/password)
2. Transaction (retrieve, delete messages)
3. Update (commit changes)

C: USER alice
S: +OK
C: PASS secret123
S: +OK User successfully logged in
C: LIST
S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
C: RETR 1
S: +OK 120 octets
[message content]
C: DELE 1
S: +OK message 1 deleted
C: QUIT
S: +OK Goodbye

IMAP (Internet Message Access Protocol):

Keeps emails on server
Allows folder management
Supports multiple devices
More features than POP3

C: A001 LOGIN alice secret123
S: A001 OK LOGIN completed
C: A002 SELECT INBOX
S: * 5 EXISTS
S: A002 OK SELECT completed
C: A003 FETCH 1 BODY[]
S: * 1 FETCH (BODY[] {320}
[message content]
S: A003 OK FETCH completed
C: A004 LOGOUT
S: A004 OK LOGOUT completed

POP3 vs IMAP:

Feature

POP3

IMAP

Email Storage

Downloads to client

Kept on server

Multiple Devices

Difficult

Easy

Folder Management

Yes

Offline Access

Yes (local copy)

Partial (cached)

Server Storage

Minimal (deleted)

All emails stored

Complexity

Simple

More complex

DNS

Domain Name System translates human-readable domain names to IP addresses.

DNS Hierarchy

                    Root (.)
                       │
      ┌────────────────┼────────────────┐
      │                │                │
     com              org              net
      │                │                │
   example          example          example
      │                │                │
   www,mail        www,mail         www,mail

DNS Query Process:

1. User types "www.example.com"

2. Client → Local DNS Resolver

3. Resolver → Root DNS Server
   Response: "Try .com TLD server"

4. Resolver → .com TLD Server
   Response: "Try example.com authoritative server"

5. Resolver → example.com Authoritative Server
   Response: "www.example.com = 93.184.216.34"

6. Resolver → Client
   Response: "93.184.216.34"

7. Client → 93.184.216.34 (HTTP request)

DNS Record Types

Type

Purpose

Example

IPv4 address

example.com → 93.184.216.34

AAAA

IPv6 address

example.com → 2606:2800:220:1:248:1893:25c8:1946

CNAME

Alias (canonical name)

www → example.com

Mail server

Mail → mail.example.com (priority 10)

Name server

NS → ns1.example.com

TXT

Text information

SPF, DKIM records

PTR

Reverse DNS (IP→name)

34.216.184.93.in-addr.arpa → example.com

SOA

Start of authority

Zone information

DNS Query Types

1. Recursive Query:

Client expects complete answer
DNS resolver does all the work

2. Iterative Query:

Server returns referral to next server
Client queries each server in chain

DNS Caching

Reduces query load and latency
Each record has TTL (Time To Live)
Cached at multiple levels: browser, OS, resolver, ISP

DNS Security (DNSSEC)

Adds cryptographic signatures to DNS records to prevent spoofing and cache poisoning.

P2P Networks

Peer-to-Peer networks distribute workload among peers rather than relying on centralized servers.

P2P vs Client-Server

Client-Server:              P2P:
    [Server]              [Peer] ←→ [Peer]
      ↗ ↓ ↖                 ↕  ↖  ↗  ↕
  [C] [C] [C]            [Peer] ←→ [Peer]

Aspect

Client-Server

P2P

Scalability

Limited by server

High (more peers = more capacity)

Cost

High (server infrastructure)

Low (distributed)

Reliability

Single point of failure

Redundant

Control

Centralized

Decentralized

Examples

Web, email

BitTorrent, Bitcoin

BitTorrent Protocol

Efficient P2P file sharing protocol.

Key Concepts:

Torrent File (.torrent): Metadata about file and trackers
Tracker: Coordinates peers (who has what pieces)
Peer: Downloads and uploads file pieces
Seeder: Has complete file, only uploads
Leecher: Downloading file, may also upload
Pieces: File divided into fixed-size chunks
Swarm: All peers sharing a specific file

BitTorrent Workflow:

1. User downloads .torrent file
2. Client contacts tracker(s)
3. Tracker returns list of peers
4. Client connects to peers
5. Client requests rarest pieces first
6. Upload to other peers while downloading (tit-for-tat)
7. Once complete, becomes seeder

Advantages:

Efficient for popular files (many seeders)
Distributed load
Resilient to failures

Challenges:

Slow for unpopular files (few seeders)
Incentive issues (free-riding)
Legal concerns (copyright infringement)

Content Delivery Networks

CDNs distribute content across geographically dispersed servers to improve performance and availability.

CDN Architecture

┌──────────────────────────────────────────────┐
│           Origin Server                      │
│        (Master content)                      │
└──────────┬───────────────────────────────────┘
           │
     ┌─────┴─────┬─────────┬─────────┐
     │           │         │         │
  ┌──┴───┐   ┌──┴───┐  ┌──┴───┐  ┌──┴───┐
  │ Edge │   │ Edge │  │ Edge │  │ Edge │
  │Server│   │Server│  │Server│  │Server│
  │(NYC) │   │(LON) │  │(TKO) │  │(SYD) │
  └───┬──┘   └───┬──┘  └───┬──┘  └───┬──┘
      │          │         │          │
   [Users]    [Users]   [Users]    [Users]

CDN Benefits

Benefit

Description

Reduced Latency

Content served from nearby edge server

Load Distribution

Offload traffic from origin server

High Availability

Redundant servers, failover capability

DDoS Protection

Absorb and filter malicious traffic

Bandwidth Savings

Reduce origin server bandwidth costs

CDN Request Flow

1. User requests www.example.com/image.jpg
2. DNS returns IP of nearest CDN edge server
3. User connects to edge server
4. If cached: Edge server returns content
   If not cached:
   a. Edge server requests from origin
   b. Origin returns content
   c. Edge caches content
   d. Edge returns content to user
5. Subsequent requests served from cache (if not expired)

Cache Strategies

1. Push CDN:

Content uploaded to CDN manually
Full control over what's cached
Good for static, infrequently updated content

2. Pull CDN:

Content fetched from origin on first request
Automatic, lazy caching
Good for dynamic content

Popular CDNs

Cloudflare
Akamai
Amazon CloudFront
Fastly
Google Cloud CDN

Summary

The application layer provides network services directly to end users. Key protocols:

HTTP/HTTPS:

Foundation of web communication
Stateless, request-response model
Versions: HTTP/1.1, HTTP/2, HTTP/3
HTTPS adds encryption and authentication

Email (SMTP, POP3, IMAP):

SMTP: Send mail (client→server, server→server)
POP3: Download mail (simple)
IMAP: Manage mail on server (advanced)

DNS:

Hierarchical system for name resolution
Distributed database
Recursive and iterative queries
Various record types (A, AAAA, CNAME, MX, etc.)

P2P:

Decentralized architecture
BitTorrent: Efficient file distribution
Scalable but faces incentive challenges

CDN:

Distributed content delivery
Reduces latency, improves availability
Essential for high-traffic websites

References

Course Materials:

CSEE 4119: An Introduction to Computer Networks - Columbia University

Textbooks:

Kurose, James F., and Keith W. Ross. Computer Networking: A Top-Down Approach. 8th Edition, Pearson, 2021.

RFCs:

RFC 2616: HTTP/1.1
RFC 7540: HTTP/2
RFC 9114: HTTP/3
RFC 5321: SMTP
RFC 1939: POP3
RFC 3501: IMAP
RFC 1035: DNS
RFC 4034: DNSSEC

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Last updated 3 months ago

hashtagTable of Contents

hashtagOverview

hashtagKey Characteristics

hashtagCore Functions

hashtagCommon Application Protocols

hashtagHTTP

hashtagHTTP Characteristics

hashtagHTTP Request Structure

hashtagHTTP Response Structure

hashtagHTTP Status Codes

hashtagHTTP Methods

hashtagCookies and Sessions

hashtagHTTP Caching

hashtagHTTP Versions

hashtagHTTPS (HTTP Secure)

hashtagSMTP and Email Protocols

hashtagSMTP (Simple Mail Transfer Protocol)

hashtagEmail Retrieval Protocols

hashtagDNS

hashtagDNS Hierarchy

hashtagDNS Record Types

hashtagDNS Query Types

hashtagDNS Caching

hashtagDNS Security (DNSSEC)

hashtagP2P Networks

hashtagP2P vs Client-Server

hashtagBitTorrent Protocol

hashtagContent Delivery Networks

hashtagCDN Architecture

hashtagCDN Benefits

hashtagCDN Request Flow

hashtagCache Strategies

hashtagPopular CDNs

hashtagSummary

hashtagReferences

Table of Contents

Overview

Key Characteristics

Core Functions

Common Application Protocols

HTTP

HTTP Characteristics

HTTP Request Structure

HTTP Response Structure

HTTP Status Codes

HTTP Methods

Cookies and Sessions

HTTP Caching

HTTP Versions

HTTPS (HTTP Secure)

SMTP and Email Protocols

SMTP (Simple Mail Transfer Protocol)

Email Retrieval Protocols

DNS

DNS Hierarchy

DNS Record Types

DNS Query Types

DNS Caching

DNS Security (DNSSEC)

P2P Networks

P2P vs Client-Server

BitTorrent Protocol

Content Delivery Networks

CDN Architecture

CDN Benefits

CDN Request Flow

Cache Strategies

Popular CDNs

Summary

References