Application Layer Protocols: HTTP, SMTP, DNS

• Study notes for Application Layer fundamentals

Chapter Outline

• Overview of the principles of network applications, HTTP, Web, FTP, electronic mail (SMTP, POP3, IMAP), DNS, P2P applications and socket Programming with UDP and TCP

Goals of Application Layer

• Focus on application protocols and transport service models
• Explore client-server and P2P architectures
• Understand application interaction with transport layers
• Developing network applications using socket programming

Examples of Network Applications:

• Email (SMTP, IMAP, POP3)
• Web browsing (HTTP)
• Text messaging & VoIP
• P2P file sharing (BitTorrent, Skype)
• Online gaming
• Video streaming (YouTube, Netflix)
• Social networking (Facebook, Twitter)

How Applications Work:

• Applications run on end systems instead of network core devices
• Communication uses protocols like HTTP, FTP, and SMTP
• Application development does not require changes to network infrastructure

Application Architectures:

• Client-Server Model involves clients that request services from dedicated servers
• Peer-to-Peer (P2P) Model features peers communicating directly and sharing resources Client-Server relies on central servers, P2P is decentralized, scalable, but harder to manage

Client-Server Model:

• Server is always-on, has a fixed IP, and handles multiple clients
• Clients have intermittent connections and dynamic IPs and cannot communicate directly

P2P Model:

• There’s no central server; each peer acts as a client and a server
• Incorporates self-scalability where more peers mean more resources
• One challenge is that intermittent peers leads to difficult management

Concept of Process Communication:

• Process: A running program on a device
• Inter-process Communication:
  • Same host uses OS-defined
  • Different hosts use message mechanisms to exchange over a network
• Client Process initiates communication
• Server Process waits for requests
• Sockets are the interface between an application and the transport layer
• Processes send data through sockets and transport ensures delivery

Process Identification:

• A host is identified by a unique IP address
• A process is identified by a port number, such as HTTP: 80, SMTP: 25
• Multiple processes can run on a host, requiring port numbers for distinction

Application Layer Protocols:

• Define message types, syntax, semantics, and response rules
• Open Protocols are publicly available standards, such as HTTP and SMTP
• Proprietary Protocols have restricted access, such as Skype

Transport Service Needs & Application Requirements

• Different Applications have different needs
  • Data Integrity: File transfer requires 100% reliability
  • Timing: Low delay time is critical for VoIP and gaming
  • Throughput: Video streaming needs consistent bandwidth
  • Security: Encryption and authentication protect sensitive data

Application Transport Needs:

• File Transfer & Email → TCP for reliability
• Real-Time Communication (VoIP, Gaming) → UDP for low latency
• Video Streaming → Tolerates some loss but needs minimum throughput

TCP vs. UDP:

• TCP: Reliable, connection-oriented, controls flow and congestion
• UDP: Unreliable, connectionless, faster for real-time applications

Protocol Mappings:

• Email: SMTP uses TCP
• Web Browsing: HTTP uses TCP
• File Transfer: FTP uses TCP
• Streaming: RTP uses UDP/TCP
• VoIP: SIP uses UDP/TCP

Security Concerns:

• TCP and UDP provide no built-in encryption
• SSL/TLS secures TCP connections with encryption and authentication
• Used in HTTPS, secure email, and VPN services

Hypertext Transfer Protocol (HTTP):

• Web’s application-layer protocol, operates over TCP (port 80)
• Stateless means server does not store past client requests
• It uses request-response model
• Non-Persistent vs. Persistent connections:
  • Non-Persistent opens a new TCP connection per request
  • Persistent reuses a TCP connection, which reduces latency

Request Methods:

• Consists of operations like GET, POST, HEAD, PUT, DELETE

Response Codes:

• 200 OK means a successful request
• 301 Moved Permanently means resource relocated
• 400 Bad Request means there is a syntax issue
• 404 Not Found means resource unavailable
• 505 HTTP Version Not Supported

Cookies:

• Maintain user session states and preferences
• Used for authentication, tracking, and personalized experiences
• Cookies track user activity across multiple sites impacting user privacy

DNS (Domain Name System):

• Resolves domain names to IP addresses
• Shows a hierarchical structure of Root, TLD, and Authoritative servers
• Caching improves lookup speeds

Types of DNS Servers:

• Root DNS Servers handle top-level requests and refer to TLD servers
• TLD Servers store records for domain extensions (.com, .net, .org)
• Authoritative DNS Servers hold actual domain-to-IP mappings
• Local DNS Servers act as intermediaries between users and the DNS hierarchy
• Caching DNS Servers store recently resolved domain names to reduce lookup time

DNS Caching & TTL

• Once a DNS resolver finds an IP address, it stores the result in cache
• Cached entries have a TTL (Time-To-Live) value before expiration
• Reduces query times and relieves load on root DNS servers
• Outdated records are a risk if a domain's IP address changes before TTL expires

P2P Applications:

• Examples include BitTorrent and Skype
• Applications are scalable but complex to manage

Socket Programming:

• Applications use TCP and UDP sockets for network communication
  • Example: Simple web servers and chat applications

Web Caching Setup & Benefits:

• Web caches store frequently accessed web pages to improve performance
• Caching reduces load on origin servers and minimizes response time
• Caching lowers bandwidth usage and speeds up page loading
• Example:
  • Assume a cache hit rate of 40% (meaning 40% of requests are satisfied from cache)
  • Only 60% of requests go to the origin server, reducing network traffic
• Link utilization formula: Utilization = (0.6 × Data Rate) / Link Capacity

Optimizing Cache Performance

• The goal is to avoid sending redundant data if the cached version is still valid
• Clients send an If-Modified-Since request header
• Possible Server Responses:
  • 304 Not Modified means there’s no need to resend the object
  • 200 OK means the object is modified, so the new version is sent
• Reduces data transfer and speeds up web browsing

Components of Email Systems:

• User Agent (UA): Software for composing, reading, and sending emails such as Outlook and Thunderbird
• Mail Servers: Store incoming emails and queue outgoing messages
• SMTP (Simple Mail Transfer Protocol): Transfers emails between mail servers using TCP (port 25)
• Mailbox: Where received messages are stored until retrieved

SMTP Process:

• Alice writes an email using a User Agent (UA)
• Alice's mail server places it in the outgoing message queue
• SMTP client opens a TCP connection to Bob's mail server
• The email is sent and stored in Bob's mailbox
• Bob retrieves the message via POP3, IMAP, or webmail

SMTP Conversation Example:

• Uses command/response interaction (like HTTP and FTP)
• There are SMTP Commands:
  • HELO identifies client
  • MAIL FROM specifies sender
  • RCPT TO specifies recipient
  • DATA sends the email content
• SMTP is push-based, while HTTP is pull-based.

Mail Formatting (RFC 822):

• Email headers contain metadata
  • Metadata information includes recipient, sender and subject
• The body contains the message content in ASCII format

Mail Access Protocols:

• POP3 (Post Office Protocol):
  • Downloads emails from the server and optionally deletes them
  • Emails are not stored on the server after retrieval
• IMAP (Internet Mail Access Protocol):
  • Emails stay on the server, allowing organization into folders
  • Maintains message states across multiple devices
• HTTP-based Email:
  • Webmail services like Gmail and Yahoo Mail use HTTP for access

POP3 Features:

• Downloads emails to the local device
• Operates in download-and-delete or download-and-keep mode
• Does not maintain state across sessions.

IMAP Features:

• Emails remain stored on the mail server
• Supports folder organization and message state tracking
• Suitable for multi-device email access

Domain Name System (DNS):

• Translates human-readable domain names, such as “www.google.com” into IP addresses
• DNS is a distributed, hierarchical database
• Key challenges in centralizing DNS:
  • Single point of failure
  • High traffic volume
  • Latency issues
  • Difficult maintenance

How DNS Works:

• DNS operates in a hierarchical structure
• DNS Servers: Direct queries to TLD (Top-Level Domain) servers
• TLD Servers: These store records for domains like .com, .Edu, .org
• Authoritative Servers → Store records for individual domains
• The root servers handle requests when lower-level DNS servers cannot
• Caching improves DNS resolution efficiency

Explore application layer protocols, including HTTP for web browsing, SMTP for email, and DNS for domain name resolution. Understand client-server and P2P architectures. Learn how applications interact with transport layers and the basics of socket programming.