Telnet services characteristic summary
Article summary: Now we come to summarize telnet service Telnet procedure and corresponding characteristic function for a while. It can be helpful to hope to understand the knowledge in this respect about everybody.
In remote management, a lot of controllers need to know this part of contents. Then we summarize some workflow that telnet serve and some basic characteristic and relevant function of now for everybody in the article. Let us have a look at the log-in procedure of this service at first.
Working phase of telnet service Telnet
Use telnet agreement to need to meet the following terms while carrying on telnet: Must be equipped with customer’s procedure including telnet agreement on the originally computer; Must know Ip address or domain name of the long-range host computer; Must know that log on label and password.
Telnet Telnet service divided into the following 4 pieces of course:
1)The local and long-range host computer sets up the connection. This course actually sets up one TCP to connect, users must know Ip address or domain name of the long-range host computer;
2)With NVT (Net Virtual Terminal) user name, password and any command or character that input afterwards that input at the local terminal The form is conveyed to the long-range host computer. This course actually sends a IP datagram to the long-range host computer;
3)The data of NVT form of outputting the long-range host computer transform the form accepted by here and send back to the local terminal, including inputting the order and going back for apparent and order and carrying out the result;
4)Finally, local terminal go on, cancel, connect to long-range host computer. This course is cancelled by one TCP and connected.
Has just discussed the most fundamental thing of telnet in content above, complicatedness and hardships of programming personnel among them are difficult for us to imagine, do not know whether you think of these while the comfortable one used telnet!
Telnet service agreement
We know telnet server software is our most frequently used Telnet server software, it is a service of a kind of typical client / server model, it employs telnet agreement to work. Then, what is telnet agreement What characteristics does it possess
A basic content
It is a member in TCP/IP agreement clan that telnet serve, is a standard agreement of Internet telnet service. Employing telnet agreement turn the computer that local users used into a terminal station of the long-range host computer systems. It has offered three kinds of basic services:
1)Telnet defines a network virtual terminal offers a standard interface for far system. The client procedure needn’t understand the system far in detail, they only need the structure to use the procedure of the standard interface;
2)Telnet includes one allows client and server to consult the mechanism of selecting, and it offers a series of standards to select;
3)Telnet is symmetrical to deal with both ends connected, namely telnet does not force the client to import from the keyboard, do not force the client to show and export on the screen either.
2 meets isomerism
In order to make it possible that a plurality of operation intersystem telnet is operated alternately, must understand isomerism computer and operating system in detail. For example, some operating systems need every style of writing to originally use ASCII carriage return command character (CR) Over, other systems need to use ASCII line feed character (LF) ,Some systems need array carriage return with two characters – enter a new line (CR-LF) ; Again for example, most operating systems have offered an accelerator to cut off program execution to user, but this accelerator is probably different (some systems in each system uses CTRL +C, and other systems use ESCAPE) . If does not consider intersystem isomerism, the character or order then sent out here, convey to remote and remote can inaccurate or will appear the mistake probably explain systematically. So, telnet agreement must solve this problem.
For meet the isomerism environment, telnet has served and defined the data and order transmission mode on Internet, this definition is called network virtual terminal NVT (Net Virtual Terminal) . Its Application Process is as follows:
To the data sent: The client software changes key set and command sequence from subscriber station into NVT form, erupt simultaneously and send to the server, data and order that the server software will be received, change from NVT form into the required form of the remote system;
To the data returning: The remote server changes the data into NVT form from the form of the remote machine, change the intersection of NVT and form data that receive into and then local form local client.
As to the detailed definition of NVT form, the interested friend can go to look for the related data.
3 conveys the ground command far
We know most operating systems offer various accelerators to realize the corresponding control command, when users typed in these accelerators in the local terminal, the local system would carry out the corresponding control command, do not regard these accelerators as input. Then to telnet, it to realize control command remote to convey with any
Telnet defines with NVT too how to convey control function to the server from the client. We know that can print 33 control codes of character sum in USASCII character repertoire including 95. When users type in the general character from here, NVT will be conveyed according to its primitive meaning; Type in the accelerator as users ‘ Key combination) When, NVT will turn it into the special ASCII character to convey in the network, and turn into the corresponding control command after it will reach remote machine. Distinguish and mainly have two reasons normal ASCII character repertoire and control command:
The editor recommends
TCP/IP agreement special topic
TCP/IP (transmission enter and discuss not controlling /internet protocol) It is a kind of network communication protocol, it standardizes all communicating device on the network, especially piece host computer and..
1)This kind of differentiation means telnet has greater flexibility: It can convey all possible ASCII character and all control function among client and server;
2)This kind of differentiation makes the client can not have appointed signalling of two justice, but will not produce the confusion of control function and general character.
4 data flow into
We mention, design, there is a shortcoming by software of application layer telnet, that is: Unefficient. Why The data now provided in telnet flow into:
Data information is typed in and passed from the local keyboard to the client procedure through the operating system by users, the client procedure returns to the operating system after dealing with it, and is passed the network by the operating system and conveyed to the remote machine, the remote operating system passes the receive data to the server procedure, and return to the false terminal entry point on the operating system after the server procedure will deal with again, finally, the remote operating system conveys the data to the utility program that users are operating, this is an intact input procedure; Export and convey from the server to the client according to the same thorough fare.
Because of every primary import and export, the computer will switch over process environment several times, this expenses are very expensive. It is a blessing users’ key input speed is not high, we can still accept this shortcoming.
5 pressure commands
We should consider such a situation: Suppose local users have operated a mistake command or procedure ceaselessly circulated of the remote machine, and the this command or procedure has already stopped reading input, then the buffer of the operating system may be taken because of this, if so, remote server unable again the intersection of data and the intersection of load and false terminal station, and result in stopping connecting and reading data fetch from TCP finally, the buffer that TCP connects will be taken finally, thus result in preventing the data flow from flowing into this connection. If the above thing has really happened, so local users will lose control on remote machine.
In order to solve this problem, telnet agreement must use the signalling in addition in order to force the server to read a control command. We know TCP is realized by urgent data mechanism signalling of the data in addition, then telnet only adds another and known as the data tagging (date mark) Keep eight bit set,and by being let by TCP and send last data the message section notice servers of the bitses urgent already, carrying the urgent machine format message section will wind flow control and reach the server directly. As corresponding to urgent signalling, the server will be read and abandoned all data, until finding a data tagging. Normal processing procedure of returning after meeting data tagging of the server.
6 select negotiation
Because of the intersection of telnet and machine and isomerism of operating system of both ends, make telnet should stipulate every telnet serve detailed disposition that connect strictly either while being impossible, otherwise influence the adaptation isomerism of telnet greatly. So, telnet adopts the consultation mechanism of selecting to solve this problem.
It is in a very large range that telnet selects: The functions of some option expansion general orientations, make and involve some minification details in and some options. For example: One can control telnet to work to select under half duplex or full duplex mode ‘ General orientation) ; Another piece select, allow the server at remote machine to determine the subscriber station type ‘ Small detail) .
The negotiation way that telnet selects is very interesting too, it is symmetric to each treatment selecting, namely any one end can be sent out and consulted the application; Any one end can accept or refuse this application. In addition, if the option not understood that one end attempts to consult another end, the refusal that it is accepted that one end asked can be simple is consulted. So, will probably upgrade, more complicated telnet client server edition and older one, the edition not very complicated operates alternately. If client and server understand new option, may to mutual to improve to some extent. Otherwise, they will turn into the way with lower but working efficiency and run together. All these designs, in order to strengthen meeting isomerism, it is obvious how important adaptation isomerism application and development of him of telnet are.
Tags: telnet, the network
