Telnet Protocols(5)

　　The convention of using "ECHO," "NO ECHO" signals seems to fill both　the requirements for dynamic echo control and for a minimum　implementation of Telnet Protocol. Note that when the user request　ECHO or NO ECHO the server replies by switching to the desired mode　(and possibly returning the signal for the new mode), or by　continuing in the current mode and returning the signal for the　current mode. The server never spontaneously sends an ECHO or NO　ECHO signal. Except that a NOECHO may be used to cancel a HIDE YOUR　INPUT.

　　Hide Your Input

　　The HIDE YOUR INPUT signal presents some difficulty in that it is　unclear how much is to be hidden. The server site usually knows how　long the secret is but the user Telnet in general does not.

　　Furthermore, if the user site cannot suppress the local echoing,　there is a difficult implementation problem. One possibility is for　the using site to overprint a full line with a mask, then have the　user type his secret on the mask. If the secret were longer than one　line, the use of the mask should be repeated.

　　The use of HIDE YOUR INPUT can be avoided altogether by having the　serving site send a mask (which it knows to be just long enough) on　which the user is to type the secret information.

　　EXAMPLE:

　　1. Default assumption is user site is echoing

　　2. Server-to User: Password Please CR LF

　　3. Server-to-User: XXXXCRIIIIICRMMMMCR NUL

　　4. User-to-Server: "password" CR LF

　　5. Server-to-User: Ready CR LF

　　Breaks and Attentions

　　There is a special control signal on some terminals that has no　corresponding bit pattern in ASCII, but is transmitted by a special　electrical signal. This control signal is Attn on a 2741 and Break　on a Teletype. This signal is represented by the Telnet control　signal BREAK. There is a corresponding control signal for use from　serving sites to using sites for reverse break. Notice, however,　that the NVT is a bi-directional character device, thus there is no　need to "turn the line around".

　　Some systems treat the Break as an extra code available for use in　conjunction with the data stream. For example, one system uses Break　as a special editing code meaning "delete the current line to this　point." In these cases, the code may simply be inserted in the data　stream with no special additional action by the user.

　　Other systems use Break or Attn in special interrupt fashion, to mean　stop processing the application and give me the supervisor, or cancel　the present job, etc. (Other systems which inspect input on a　character at a time basis use normal characters for this purpose,　such as .) In these cases, because of differences in the ways　both serving and using sites operate, it is necessary to take a route　in addition to the normal Telnet data stream to indicate that the　special control signal is embedded in the data stream.

　　Example -- Problem:

　　The PDP-10 normally will, when it fills its input buffer, continue　to accept characters from a terminal examining each to see if it　is a control character, then act on it if it is or throw it away　if it is not.　Since the Telnet server at the serving site is at the mercy of the　NCP with respect to controlling the bunching, and therefore,　arrival at the Telnet of bursts of characters, Telnet　implementations might be expected to choke off flow to the buffers　until they are ready to accept characters without throwing them　away.

　　Under this condition, the serving process might be outputting to　the using terminal, the input buffers at the server fill up, (with　user generated characters) and get stuck (at the user's　site) in the data stream that has been choked off.