Either party may send an unsolicited command only to request a change in mode. A party only changes its echo mode when it receives an admissible request. When a command is received, the party replies with its echo mode, unless it did not have to change mode to honor the request. A connection will work in ECHO mode only when both parties agree to operate that way. The procedure cannot loop.
Servers are free to specify their preferred mode of operation.
Thus, human, or machine, users do not have to learn the proper mode for each server. Three Principles Let us mention the general principles we alluded to at the beginning of this note. The principle of default implementation merely states that for all options, defaults are declare which must be implemented. The principle of symmetry merely states that neither party should have to 'know' whether it is the server or the user. Our scheme, as described thus far, is not totally symmetrical we will consider this matter in a later section.