"Network application is the reason for the existence of a computer network, if we can not conceive of any useful application, there is no need to design a network protocol that supports them."
What is the application layer (Application Layer)?
The application layer is located in the top level Internet protocol stack, network applications and their application layer protocols remaining in place. The application layer includes many protocols, such as HTTP (Hypertext Transfer Protocol), SMTP (Simple Mail Transfer Protocol), FTP (File Transfer Protocol), DNS (Domain Name System) and so on. Application layer protocol distributed over multiple end systems, a terminal system applications using the application protocol to another end-system exchange information packets ( Packet ) - This packet information is located in the application layer, which we call message ( the message ).
In studying the application layer, we need to mainstream architecture of modern web applications used, as well as to run the program on multiple end system is to have a basic understanding of how to communicate with each other.
Application Architecture (Application Architecture)
We are familiar with many of the applications is based on multi-protocol application layer protocol above. Core Research network application is written to run on different end systems, and communicate with each other through a network procedure. Application architecture designed by the application developer specifies how to organize the application in a variety of end systems programs.
The main application architecture has the following two: 1 client - server architecture 2. P2P architecture
Client - server architecture (client - server architecture)
1. Server - always active hosts, with a fixed, well-known address (IP address), serves many clients request called host.
2. Customer - packets transmitted by the client contacted the server IP address, the client does not communicate directly with each other.
When the server receives a client request for an object (request), it sends the requested object as a response (response) to the client.
This architecture applications include FTP, Telnet and e-mail.
In a client - server applications, often there will be a separate server host can not meet the requests of all customers. To this end, and with a range of host data center (data center) is often used to create a powerful virtual server (virtual server). Data center may have many servers need maintenance, and enough bandwidth to meet the needs of data transmission.
P2P architecture (P2P architecture)
In a P2P architecture, the data center located in a dedicated server relies on little or no dependence, the host application process between the intermittently connected to the direct communication, which is called on the host peer (peers). This does not have to peer communications via a dedicated server.
Many popular, traffic-intensive applications are P2P architecture, including file sharing (such as BitTorrent), download assist device (such as Thunder), Internet telephony and video protocols (such as Skype) and so on.
The main advantage of this architecture is scalable self (self-scalability), and does not require a large server infrastructure and server bandwidth.