Computer Network Technology and Application

Chapter 19 Services Provided by the Internet (1)
This chapter introduces the services provided on the Internet, including services such as domain name, e-mail, file upload and download, and information browsing.These are all necessary services provided to users on the Internet. Through these services, we can send emails, download pictures and other materials, and the services are directly facing users.

【Main content of this chapter】

Client/server model.

domain name service.

email service.

File transfer service.

Web services.

6.1 Basic knowledge
The Internet network covers a wealth of information resources, and the stored information is mainly business, technology and entertainment information.Through the Internet we can learn about information from all over the world, send and receive e-mails, chat with friends, shop online, watch movies, enjoy music, read online magazines and so on.

There are various forms of information disseminated on the Internet. More and more organizations and individuals use it to disseminate information all over the world, and the content of information on the Internet is becoming more and more extensive and complicated. The Internet has become the world's largest advertising system, information network and news media.

There are many topic forums in the Internet network. People with the same profession, industry or similar interests can raise topics for discussion online, and papers can be stored online for a long time for people to read or supplement.

The Internet has become the largest database with the most data, the most complete categories, and the largest scale in the world. You can freely search for the information you need online.

So how does the Internet provide services?
6.1.1 Main Internet Application Layer Protocols
The Internet uses the TCP/IP system structure, which includes several layers of protocols, and the application layer of TCP/IP introduces different application layer protocols to different network applications.The main protocols of the application layer are as follows.

(1) Protocols that rely on connection-oriented TCP services: including Virtual Terminal Protocol (Virtual Terminal Protocol, VTP), File Transfer Protocol (File Transfer Protocol, FTP), Simple Mail Transport Protocol (Simple Mail Transport Protocol, SMTP), hypertext links Protocol (HyperText Transfer Protocol, HTTP).

(2) Protocols that rely on UDP services: including Trivial File Transfer Protocol (Trivial File Transfer Protocol, TFTP), Network File System (Network File System, NFS), Remote Procedure Call (Remote Procedure Call, RPC), Simple Network Management Protocol (Simple Network Management Protocol, SNMP).

(3) Protocols that rely on TCP and UDP services: including DNS, public management information services and protocols (Common Management Over TCP/IP, CMOT).

6.1.2 Client/Server Model

1. Know the client/server model

The client and the server are two application entities involved in communication. The client actively initiates a communication request, and the server passively waits for the establishment of communication.

Clients and servers are separate computer processes connected to a network.When a process of a computer provides various network services such as data and file sharing to other computers, it is called a server.The computer processes used to access server resources are called clients.

Strictly speaking, the client/server model is not defined from the perspective of physical distribution, it reflects a network service model.

The client/server model features are as follows.

(1) Each networked computer in the network not only provides services for local users, but also provides services for users of other hosts in the network.

(2) The hardware, software and data resources of each networked computer should not only be available to local users, but also shared by users of other hosts on the network.

(3) Each network service corresponds to a "service program" process.

(4) The "service program" process should perform a set of specified actions for each approved network user request, so as to meet the user's network resource sharing needs.

(5) Inter-process interaction mode to be solved by process communication in network environment.

(6) The client and the server respectively represent two application processes that communicate with each other; sometimes the computer running the client process is called the client, and the computer running the server process is called the server.

(7) The client computer sends a service request to the server, and the server responds to the client computer's request and provides the network service required by the client computer.

2. Reasons for using the client/server model
(1) Uneven distribution of network resources

The uneven distribution of network resources is manifested in hardware, software, and data.First, from the perspective of personnel who design and build applications, they hope that the distribution of network resources is uneven; second, there are gaps in hardware configuration, computing power, storage capacity, and information distribution between different nodes in the network .

Therefore, the party with strong ability and rich resources needs to act as the server, that is, the role of the server, and the party with weak ability or needs certain resources becomes the user of resources, that is, the role of the client.

(2) Asynchrony of process communication in network environment

When processes distributed in different host systems send communication requests, which process of which host they want to communicate with, and whether the other process can accept communication requests are all uncertain, without a high degree of scheduling and coordination.

Based on the above reasons, a system should be established, that is, a connection is established between the processes preparing to communicate, and the connection is maintained during the process of exchanging data between the processes, so as to provide a synchronization control mechanism for data exchange.

6.2 Domain name service
DNS (Domain Name System) is what we often call the domain name system.In order to make computers based on IP addresses easy to be identified by users during communication, the Internet began to adopt the domain name management system in 1985.Servers on the Internet are identified by IP addresses, but IP addresses are inconvenient to remember.Therefore, addressing is realized through the corresponding relationship between domain names and IP addresses. The role of DNS is to resolve domain names that are easy for people to remember into IP addresses that can be recognized by the Internet (assuming 202.204.222.3).

6.2.1 Naming Rules for Hierarchical Hosts
On the Internet, domain names have naming rules.The domain name of a computer is expressed as: computer host name. organization name. network name. top-level domain name.

This is a hierarchical management mode, and the domain name is expressed in words, which is easier to remember than the IP address expressed in numbers.All levels of networks joining the Internet manage the naming of computers in the network according to the naming rules of DNS, and are responsible for completing the conversion of domain names to corresponding IP addresses during communication.

The Defense Data Network Communications Center (DDNNIC), which belongs to the US Department of Defense, is responsible for the registration and management of the Internet's highest-level domain names, and it is also responsible for the allocation of IP addresses.

DNS provides directory services, by searching the name of the computer to find the corresponding IP address of the computer on the Internet network, and vice versa.

6.2.2 Hierarchy of Internet Domain Name Servers
A root server (root server) at the top of the hierarchy, it is the top-level domain controller.For example, in a company network or a campus network, all domain names can be resolved by one domain name server, and several domain name servers can also run at the same time, so the campus or company network can be called a region.

When there are multiple domain name servers in an area, the domain name servers are divided into primary domain name servers and secondary domain name servers.

(1) Primary domain name server

Directly load the information of the zone from the zone file, which contains the server host names and their corresponding IP addresses in the zone where the server is located.

(2) From the domain name server

At startup, it contacts the master domain name server in charge of the zone, and copies the database of the master server through a process of "intra-zone transfer".After that, it will periodically query whether the data of the main domain name server has been modified, so as to keep the data in its own database the latest version.

6.2.3 Domain Name Resolution Principle
The main task of the domain name resolution server is to complete the resolution process from the domain name to the IP address, and it also adopts the client/server mode.

Running a service process on the domain name server, which realizes the resolution of the domain name to the IP address, is called the domain name resolution server.The domain name server stores the information of hosts in one zone (which can be understood as the range that the DNS server is responsible for resolving) or multiple zones.Usually, multiple servers are set up in one area, the main purpose is to improve the reliability of the domain name resolution system.When one of the domain name servers fails, all domain name requests it is responsible for resolving can be forwarded to other domain name servers; and, domain name requests can be evenly distributed to multiple servers, thus improving the domain name resolution capabilities of the entire system and The efficiency of resolution can be improved, and multiple domain name servers can be placed in different places as needed to provide users with resolution services in the nearest geographical location.

1. Analysis algorithm
(1) Recursive analysis
The name server system is required to complete all name-to-address conversions at one time, and the task is mainly undertaken by the server software.

(2) Repeated analysis

Each time a server is requested, if it cannot be resolved, another server is requested. The task of repeated resolution is mainly undertaken by the domain name resolution server software.

2. The complete process case of analysis

We use a case to illustrate the domain name resolution process.

(1) The user makes a domain name resolution request and sends the request to the local domain name server.

(2) When the local domain name server receives the request, it queries the local cache.If there is this record item, the local domain name server will directly return the result of the query.

(3) If there is no such record in the local cache, the local domain name server will directly send the request to the root domain name server, and the root domain name server will return it to the local domain name server for a queried domain (subdomain of the root, such as cn) The address of the primary nameserver.

(4) The local server sends a request to the domain name server returned in step (3), and the server that receives the request queries its cache, and returns the record corresponding to the request or the address of the related lower-level domain name server, which is determined by the local The domain name server saves the returned results to the cache.

(5) Repeat step (4) until the correct record is found.

(6) The local domain name server saves the returned result to the cache for the next use, and returns the result to the client at the same time.

6.3 E-mail service
6.3.1 Know the email system
At present, e-mail has been widely used, and the concept of e-mail will not be described here.However, the following main functions of the e-mail system must be clear.

(1) The e-mail system for receiving and sending e-mails can not only transmit text information in various formats, but also transmit various information such as images, sounds, and videos.

(2) The core mail server of the mail system is responsible for receiving the mail sent by the user and sending it to the other party's mail server according to the recipient's address; at the same time, it is responsible for receiving the mail sent by other mail servers and distributing it according to the recipient's address to the corresponding email address.

6.3.2 Working mode
1. Architecture of email system

(1) Agent

An agent, also known as a mail reader, is the interface between the user and the email system, allowing the user to read and send emails, generally a user process.For example, Outlook and Foxmail are popular email user agents.

(2) Mail server
The mail server is the heart of the email system and acts as a post office.Receive user mail and transmit it according to the address.The mail server is a program that runs in the background and must run continuously, otherwise many incoming mails will be lost.

2. Mail Transfer Protocol
Protocols need to be configured in some mail agents, and mail systems need mail sending and receiving protocols to complete complete services.For example, the SMTP protocol is responsible for sending emails; protocols such as POP3 are responsible for receiving emails.

(1) SMTP protocol
Its features and functions are as follows.

① It runs on the TCP protocol and uses the public port number 25.

②Transfer emails with simple commands.

③ It stipulates 14 commands and 21 kinds of response information.

④ Using the client/server working mode, the SMTP process of the SMTP client sending mail, and the SMTP process of receiving mail from the SMTP server.

(2) POP3
Its features and functions are as follows.

① is a simple mail reading protocol.

②Use the working mode of client/server.

③ The user's host computer receiving mail runs the POP3 client program, and the mail server runs the POP3 server program.

④ Run on the TCP protocol, using the public port number 110.

⑤ stipulates 15 commands and 24 kinds of response information.

The server of the POP3 protocol is an intermediate server with a store-and-forward function. After the mail is delivered to the user, this type of server no longer saves the mail. The IMAP protocol can solve this problem.

(3) IMAP
Its features and functions are as follows.

①When the client program opens the mailbox of the IMAP server, you can see the header of the mail; if the user needs to open a certain mail, the mail can be sent to the user's computer; before the user sends the command to delete the mail, the IMAP server mailbox emails are kept.

② The POP3 protocol runs offline, while the IMAP protocol runs online. The function of the IMAP protocol is better than that of the POP3 protocol.

6.4 File transfer service
6.4.1 Know the file transfer protocol
File Transfer Protocol (FTP) provides file transfer services.The file transfer service can transfer data in any format and can access various FTP servers on the Internet.There are two modes of accessing the FTP server. One is to log in to the server system as a registered user, and the other is to access the server anonymously. Among them, the anonymous service is widely welcomed and has become an important means for people to obtain network resources. .

The file transfer service provides the function of uploading and downloading files from a computer. The computer that provides the file is called the FTP server, and the computer that needs the file is called the client computer.

1. Main content of file transfer service

(1) 1 or more files can be transferred between the client and the server.

(2) Transfer files of various types, structures, and formats.

(3) It has the directory operation function of local and remote systems, and can change the directory.

(4) It has functions such as changing the file name, displaying content, changing attributes, and deleting.

(End of this chapter)