Summary

Application layer (7)

• Programs that run on different end systems
• communicate over network

Layer protocol defines:

• Types of messages exchanged
  • Request, response
• Message syntax (format)
  • Fields, how fields are delineated
• Message semantics (meaning of fields)
  • Meaning of information in fields
• Rules
  • for when and how processes send and respond to messages
• Open protocols
  • Defined in RFCs, everyone can implement them
  • Web and HTTP, E-mail, SMTP, IMAP, DNS
• Proprietary protocols
  • Defined by a single organization, only that organization can implement them
  • Skype, Facebook messenger

Paradigm

Client-server paradigm

Server:

• always-on host
• permanent IP address (fixed)
• in data centers

Clients:

• communicate with the server
• Have dynamic IP address
• Do not communicate directly with each other
• HTTP, IMAP, FTP

Peer-peer Architecture

• No always-on server
• Arbitrary end systems (peers) communicate directly.
• Self-scalability: New peers bring new service capacity as well as new service demands.
• Challenges: Harder to manage, less secure
• BitTorrent, P2P file sharing