👉🏻 If you want to enjoy the full experience exploring this pdf, check it out here Chapter 2

Principles of the Application Layer

• When building new applications, all you have to worry about is two things:
  1. The services provided by the transport layer
  2. How the API (application layer interface) look like to these services
• So, we have to write applications that:
  • run on different end systems
  • communicate over network
• The are two styles of interaction that describe how the pieces of the network application are going to interact with each other:
  1. Client-Server model
  2. Peer-To-Peer (P2P) model

The Client-Server Paradigm

• On the server side:
  • An always-on host
  • Generally has a permanent IP address so that the clients will know where to contact it
  • may be hosted in home, company, university or commercial data centers
• On the client side:
  • Clients are going to operate by contacting and communicating with a server
  • Clients will typically be intermittently (بشكل متقطع) connected to the internet → won’t have a permanent IP address
  • Clients do NOT connect with each others, they interact with servers
  • Ex. HTTP, IMAP, FTP

Peer-Peer architecture

• The is NO server
• We have peers -end systems- that are going to directly communicate with each other
  • They request service from other peers
  • They provide service in return to other peers