Table of contents
The application layer is the top layer of the computer network architecture. It is the ultimate goal of designing and establishing a computer network. It is also the fastest growing part of the computer network.
- Early text-based applications (e-mail, remote login, file transfer, newsgroups)
- The World Wide Web (WWW) brought the Internet to millions of households in the 1990s.
- Today's popular instant messaging, P2P file sharing and various audio and video applications.
- The miniaturization and "ubiquity" of computing devices, and the increasing popularity and rapid development of broadband residential access and wireless access provide a broad stage for more new applications in the future.
1.1 Client/Server Model
Client/Server, in the C/S model, there is an always-open host called the server , which serves many requests from other hosts called clients . Clients cannot communicate directly with each other. The client is user-oriented and the server is task-oriented.
The C/S method is the traditional and most mature method on the Internet. Many network applications we are familiar with use the C/S method. Including World Wide Web WWW, email, file transfer FTP, etc.
Application services based on C/S mode are usually service centralized , that is, application services are concentrated on server computers that are much smaller than client computers in the network.
- Since a server computer has to provide services for multiple clients, in C/S applications, it often happens that the server computer cannot keep up with the requests of many clients.
- For this reason, in C/S applications, computer clusters (or server farms) are commonly used to build a powerful virtual server.
- After the connection is established , the server can also actively send data to the client for notification of some messages, such as some error information.
1.2 P2P model
Peer-to-Peer (P2P) method
- In the P2P method, there are no fixed service requesters and service providers . The application processes distributed in each end system at the edge of the network are peers and are called peers . Peers communicate directly with each other , and each peer is both a requester and provider of services.
Currently, popular P2P applications on the Internet mainly include P2P file sharing, instant messaging, P2P streaming media, distributed storage, etc.
Applications based on P2P are service decentralized , because the service is not concentrated in a few server computers, but is dispersed among a large number of peer computers. These computers are not owned by the service provider, but are desktop computers controlled by individuals. and laptops, which are typically located in residences, campuses, and offices.
One of the most outstanding features of the P2P approach is its scalability . Because every time a peer is added to the system, not only the service requesters are added, but also the service providers are added, and the system performance will not be reduced due to the increase in scale.