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E-mail: Ouyang@china-pub.com
Translator: Gu Guofei (GGFEI GGFEI@263.net)
Translation time: 2001-4-2
Copyright: This Chinese translation copyright belongs to China Interactive Publishing Network. Can be used for non-commercial use free reprint, but the translation and copyright information of this document must be retained.
Network Working Group J. Postel
Request for Comments: 857 J. Reynolds
ISI
Obsoletes: NIC 15390 May 1983
RFC 857 Telnet Echo Options
(RFC 857-Telnet Echo Option)
This RFC specifies the standards on ARPA Internet Community. All hosts on the ARPA Internet should adopt and implement this standard.
table of Contents
Command and code 1
2. Command meaning 2
3. Default 2
4. The reason for the option 2
5. Option Description 2
6. Example 4
Command and code
Echo 1
2. Command meaning
* IAC WILL ECHO
The command sender request starts or determines that the data character will begin to reply to the transmitter.
* Iac Won't echo
Command sender requires stopping or rejecting to start replying to received data characters.
* Iac Do Echo
Command sender requires the receiver to start replying or determine the received data character to reply.
* Iac Don't echo
Command sender requires the recipient of this command not to start or stop replying to received data characters.
3. Default
Won't echo, don't echo is the default value, that is, no reply.
4. The reason for the option
NVT has a keyboard of a display device. In normal case, the keyboard input will be displayed directly on the display device, and the remote process to be controlled by the character to the remote process is suitable for the interaction. We can imagine that the situation in the chat room is understood, in this case, you need this option to look like a keyboard to control two display devices.
5. Option description
When this option is valid, the terminal terminal needs to return the received character to the sender. This option does not require returning and receiving exactly. When the receiver is not returned to the sender, the recipient does not say that the receiver is not to add the received data.
The usual connection is two-way, and the data stream in one direction is not related to the data stream in another aspect. Here are five possible situations:
This option provides the decision whether one end is returned to the other end data. If you do not return to the other end, it is not returned by yourself.
If the hosts of the two ends have an ECHO state, the infinite loop appears on the connection, so the implementation should pay attention to this situation when implementing, the data returned by one end, the other end does not return.
Both parties are not echo status when establishing a connection.
If one is decided to return the data issued by the other party, or want the other party to do so, it makes the corresponding command, and waits for a response. If the response is rejected, it still remains non-ECHO state; if the other party accepts the request, the connection enters the ECHO state, when in such a state, any party can release the ECHO state because the connection is two-way, so different aspects of ECHO The state should be released separately. Comply with cyclic prevention rules in the Telnet protocol when implementing. Because the switches in different states sometimes clear, pay special attention to the status of the corresponding switch. For example, one party responds to DO Echo by Will Echo, all characters after Do Echo are returned, and this should be kept in mind whether the recipient or sender should be kept.
The echo option is not enough to allow remote computers to understand the characters entered on the terminal, so use the Suppress-Go Ahead option to perform the appropriate processing.
6. Example
Here is a simple implementation called UHOST. The value for non-ECHO is less than the value used to represent the ECHO. For each user terminal, UHOST retains three status bits, whether it is echo yourself, whether the user wants to work in the ECHO state, whether the terminal is connected to the server on the ECHO state, these three we call P (physical) , D (Hope) and A (actual) bits.
When the terminal dials, set the P-bit and D bits, and the A bit is set to non-ECHO, the P-bit, and D bits can be used by the corresponding command.
When the UHOST and the server are connected, if the minimum value of the P-bit and D bit is less than a bit, then send a do echo command to the server. If you receive a Won't echo or a Will Echo response, UHOST sets a bit as receiving The minimum value of the following three values: The received value, the P bit value, and the D bit value. If you need to change the current state of the A bit, the UHOST wants to issue a corresponding determination information. If you do not change the current state of the A bit, the rejection is returned, indicating that you don't need to make changes.
If the UHOST terminal changes the value of the P-bit or D bit when the connection is open, UHOST repeats the above test. UHOST will restore the A bit value when the connection is closed. Since UHOST does not involve using the DO Echo and Don't echo commands when the connection is open or when the user explicitly changes the ECHO state, the large host will frequently switch. For example, when the Line-AT-A-TIME system is running, the server will try to set the user to a local Echo state via the won't echo command; however, when the Character-AT-A-Time system is running, the server needs to pass by Will echo command. Start the remote echo of the user. Moreover, because UHOST does not issue a WILL Echo command, only Won't echo commands are issued, and the server host often issues Will and Won't commands.
RFC857-Telnet Echo Option RFC857 Telnet Echo Options
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