Manav Goyal

Application Layer - Computer Networks

  • Services that directly provide user interfaces support user applications or services
    • It serves as a window through which application processes can access network services
  • The application layer enables the user to access the network
  • This would be the layer that a programmer uses to allow his application to access a network service, such as linking into a database
  • Ports
    • Error
    • 0 to 1023 => Well defined ports
  • Protocols
    • Domain Name Service (DNS)
      • Port no. 53
      • Uses UDP protocol
      • Searing DNS > Search in Local Cache/DNS Resolver
      • Request sent to Root server via Resolver => Total 13 root servers
      • Recursive
        • Request sent to Generic/Country
        • Request sent to Authoritative > Sent to Root server
        • Response sent to User via Resolver
      • Iterative
        • Request sent to Generic/Country server via Resolver
        • Request sent to Authoritative server via Resolver > Sent to Root server via Resolver
        • Response sent to User via Resolver
    • HTTP
      • Port no. 80
      • Port no. 443 for HTTPS
      • Uses TCP for reliability
      • Inband protocol => Commands and Data from same port
      • Stateless
      • Persistent (Server leaves connection open after sending response) & Non-persistent (More overhead)
        • RTT = 2*PD
    • FTP (File transfer protocol)
      • Port no. 20 (DATA) & 21 (Control)
      • Data connection is non-persistent
      • Control connection is persistent
      • Not inband
      • Reliable, Stateful
    • SMTP (Simple mail transfer protocol)
      • Error
      • Send mail (Push mail)
      • Uses TCP or UDP
    • POP (Post office protocol)
      • Receive mail (Pop mail)
      • Deletes from server while popping
    • IMAP (Internet message access protocol)
      • Retrieve mail (Pop mail)
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