Computer network

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Computer Network – application layer

Application layer protocols are different from network applications. Application layer protocols are only part of network applications. For example, the World Wide Web application is a web application based on client/server architecture. Web applications consist of many components, including web browsers, web servers, web document formatting standards, and an application layer protocol. The application layer protocol of the World Wide Web is HTTP, which defines the rules for the type, format, and sequence of messages transmitted between web browsers and web servers. How a web browser displays a web page, and whether a web server uses multiple threads or processes, are not defined by HTTP.

Many protocols at the application layer are based on a client-server approach. Even peer-to-peer communication is essentially a special client server. To be clear, both client and server refer to two application processes involved in communication. The client-server approach describes the relationship between processes that serve and are served. The main characteristic here is that the client is the service requester and the server is the service provider.

Let’s start with the domain name system, which is used by many application protocols. After introducing file transfer protocol and remote login protocol, the working principle and main protocols of the World Wide Web are introduced emphatically. As the emergence of the World Wide Web has made the Rapid development of the Internet, so the World Wide Web occupies the largest space in this chapter, is also the focus of this chapter. Then we discuss the E-mail that users often use. Finally, it introduces some problems about network management and some basic concepts about network programming. For a more in-depth study of the application layer, see [COME15J] [COME06] [TANE11] and related standards.

The most important part of this chapter

  1. DNS – Resolves IP addresses from domain names.
  2. The World Wide Web and THE HTTP protocol, and two different information search engines of the World Wide Web.
  3. Email transmission process, SMTP protocol and POP3 protocol, IMAP protocol used occasions.
  4. Dynamic host configuration protocol DHCP features.
  5. The role of the three components of network management: SNMP itself, management information structure SMI, and management information base MIB.
  6. Basic concepts of system calls and application programming interfaces.
  7. P2P file system.

Mind mapping

Important concepts in this chapter

  • Application layer protocol is to solve a certain kind of application problem, and the problem is solved through the communication and cooperation between multiple application processes located in different hosts. The application layer defines the protocols that application processes follow when communicating. Many protocols at the application layer are based on the client-server approach. The client is the service requester and the server is the service provider.
  • The Domain name System (DNS) is the naming system used on the Internet to convert user-friendly machine names into IP addresses. DNS is an online distributed database system, which adopts client server mode.
  • The resolution of domain names to IP addresses is done by many domain name server programs (DNS) distributed across the Internet.
  • The Internet uses the naming method of hierarchical tree structure. Every host or router connected to the Internet has a unique hierarchical name, that is, a domain name. The dots in a domain name are irrelevant to the dots in a dotted decimal IP address.
  • DNS servers include the root DNS server, top-level DNS server, permission DNS server, and local DNS server.
  • File transfer protocol FTP uses TCP as a reliable transport service. FTP uses client server mode. An FTP server process can provide services for multiple client processes simultaneously. During file transfer, two parallel TCP connections are established between the FTP client and the server: the control connection and the data connection. It is the data connection that is actually used to transfer files.
  • The WWW is a large, online repository of information that can be easily linked from one site on the Internet to another.
  • A client of the World Wide Web makes a request to a server program on the Internet, and the server program sends back to the client the web document that the client wants. The World Wide Web documents that appear on the client’s main window are called pages.
  • The World Wide Web uses universal resource locator urls to mark various documents on the World Wide Web and make each document have a unique identifier URL across the entire Internet.
  • The protocol used for interaction between web clients and server programs is the hypertext transfer protocol HTTP. HTTP uses TCP connections for reliable transmission. But THE HTTP protocol itself is connectionless and stateless. The HTTP/L.1 protocol uses persistent connections (classified as non-pipelined and pipelined).
  • The Web uses the hypertext markup language HTML to display various Web pages.
  • Web static documents are stored in the web server after the creation of the document, and the content will not change during the browsing process by users. Dynamic documents are documents whose contents are created dynamically by the application when the browser accesses the World Wide Web server.
  • Active document technology allows the browser screen to be continuously updated. The active document program interacts directly with the user and continuously changes the display of the screen.
  • The tools used to search on the World Wide Web are called search engines. Search engines can be broadly divided into full-text search engines and classified directory search engines.
  • E-mail is the most used and most popular application on the Internet. An E-mail sends a message to the mail server used by the recipient and is placed in the recipient box. The recipient can access the mail server used by himself at any time. It is equivalent to an “E-mail”.
  • An E-mail system has three main components, namely, the user agent, the mail server, and the mail protocols (including the mail sending protocols, such as SMTP, and the part reading protocols, such as POP3 and MAP), which are run by both the user agent and the mail server.
  • E-mail user agent is the interface between users and E-mail system, it provides a very friendly window interface between users to send and receive mail.
  • The SMTP protocol is used to transfer mail from user agents to and between mail servers. But when the user agent reads messages from the mail server, it uses the POP3 (or IMAP) protocol.
  • Web-based E-mail enables users to send and receive E-mail using a browser. HTTP is used for mail transfer between user browser and mail server, while SMTP is used for mail transfer between mail server.
  • Simple network management protocol SNMP consists of three parts, namely, (1) SNMP itself, which is responsible for reading and changing object names and status values in each agent: (2) Management information structure SMI, general rules for defining named objects and defining object types (including range and length), and basic coding rules for encoding objects and their values BER: (3) Management information base MIB, which creates named objects in managed entities and specifies their types.
  • System call interface is an interface that transforms the control rights of application process and operating system, also known as application programming interface (API). API is the interface between application program and operating system.
  • A socket is an interface between an application process and a transport layer protocol. It is a mechanism used by an application process to interact with an operating system for obtaining network communication services.
  • Peer-to-peer (P2P) file sharing now accounts for the largest share of Internet traffic, much larger than web applications.
  • BT is a popular P2P application. BT uses “rarest first” technology to collect the rarest chunks of files as early as possible. In addition, if there are neighboring peers that are currently sending blocks to a peer at the highest data rate, that peer will preferentially transfer the requested blocks to those neighboring peers. The result is that the peers are able to exchange file blocks with each other at a satisfactory rate.
  • When the number of peers is large, P2P downloading of large files is much faster than the traditional client-server approach.
  • The widely used indexing and lookup technique in P2P applications is distributed hash table DHT.

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