Telnet Protocols(3)

　　The use of a standard, network-wide, intermediate representation of　terminal code between sites is intended to eliminate the need for　using and serving sites to keep information about the characteristics　of each other's terminals and terminal handling conventions. This　approach can be successful, but only if the user, the using site, and　the serving site assume certain responsibilities.

　　1. The serving site must specify how the intermediate code will be　mapped by it into the terminal codes that are expected at that　site.

　　2. The user must be familiar with that mapping.

　　3. The using site must provide some means for the user to enter all　of the intermediate codes, and as a convenience, special Telnet　signals, as well as specify for the user how the signals from the　serving site will be presented at the user terminal.

　　Other schemes were considered but rejected. For example, a proposal　that the using site be responsible for translating to and from the　code expected by the serving site was rejected since it required that　the using site keep tables of all serving site codes and provide a　mapping for each case. The information would require constant　maintenance as new hosts were added to the network.

　　Character Set

　　Since it is not known how the current or future sites will specify　the mapping between the network-wide standard code (7 bit ASCII in an　8 bit field) and the codes expected from their own terminals, it　seems necessary to permit the user to cause transmission of every one　of the 128 ASCII codes, plus (for full user power) selected signals　(either of a Telnet control nature, or of a special terminal nature　such as break or attention).

　　There was strong feeling about the importance of the user/system　interface at the using site, but equally strong feeling that this　problem is one of local implementation and should reflect the using　site installation philosophy rather than be subject to network-wide　standards. Some topics of consideration in this area are:

　　1. How to represent special graphics, not available at the using　site, at the user's terminal.

　　2. Treatment of upper/lower case problem on upper case only　devices.

　　a. Representing lower-case output.

　　b. Providing users with shift and shiftlock signals.

　　3. Incorporating editing capability in Telnet.

　　4. Extending user options in Network mode not available to local　users, e.g., hold output or kill print.

　　5. Permit users to specify how keyboard input is to the　translated, e.g., let a character from the terminal cause a　specified string to be sent by the user's Telnet.

　　The proposed solution to the Telnet Protocol problem seems to provide　a mechanism for a minimum implementation while providing a basis for　developing richer sets or protocol for present and future use in　terminal applications, process-process communications, and use by　other conventions to pass data or control information.

　　The understanding that ASCII be used as a network-wide code has been　established for some time. Its use in Telnet provided a problem with　respect to the limitation of a maximum character set of 128. Some　systems provide for more than this number in their operation, and　therefore, as serving sites, cannot map on a one-for-one basis.

　　Each such serving site could probably provide a reasonably useful　character set, including all system control signals, by mapping 128　of its codes and just not provide a network user accessto the other　codes. However, any character left out might later be used in a　major application at that site as a special control signal. This　could result in denying network users the facility offered by that　application. Serving sites are, therefore, encouraged to provide a　full mapping between the ASCII code and the code used on the serving　system. This may require that the server specify two character ASCII　sequences which map to single characters in the servers character　set.

　　Notice that there are some ASCII codes which have no effect on the　NVT printer. These codes must be transmitted over the network when　output by the serving process or by entered by the user.

　　End of Line Convention