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Application Layer

Short Notes

The layer closest to the end user. It contains protocols for specific services.

Protocols

  • HTTP: For web browsing. (Port 80).
  • DNS: Resolves Domain Name \(\to\) IP. (Port 53).
  • SMTP: Sending email. (Port 25).
  • POP3/IMAP: Receiving email.
  • FTP: File transfer. (Ports 20/21).

Key Theories & Formulas

1. DNS Resolution

  • Recursive: The server finds the result for you.
  • Iterative: The server gives you the address of the next server to ask.

2. HTTP Types

  • Non-persistent: One TCP connection per object.
  • Persistent: Multiple objects over one TCP connection (can be pipelined or not).

Example Problems

Problem: How many RTTs to fetch 1 base HTML and 10 images over non-persistent HTTP?

  • 1 RTT for TCP + 1 RTT for Object = 2 RTTs per object.
  • Total = \(2 + (10 \times 2) = 22\) RTTs.

Hardest GATE Questions

Topic: Email Protocol Interactions Tricky Question (GATE 2011/2014): Which protocol is used between the Sender's Mail Server and the Receiver's Mail Server?

  • Analysis: SMTP.
  • The "Trap": Thinking POP3 is used for server-to-server transfer. POP3/IMAP are only for the Client to pull mail from its own server.
  • Hard Aspect: DNS Caching and TTL.
  • If a DNS record is cached for 1 hour, updates won't be visible until it expires.
  • Complexity: HTTP Conditional GET using If-Modified-Since.
  • Allows the browser to check if a cached copy is still valid without downloading the whole object again

References