<i>
</i>Appendix B. The ASCII NVT Definition The main body of this specification is intended as an update to, and internationalized version of, the Net-ASCII definition. The specification is self-contained in that parts of the Net-ASCII definition that are no longer recommended are not included above. Because Net-ASCII evolved somewhat over time and there has been debate about which specification is the "official" Net-ASCII, it is appropriate to review the key elements of that definition here. This review is informal with regard to the contents of Net-ASCII and should not be considered as a normative update or summary of the earlier specifications (Section 2 does specify some normative updates to those specifications and some comments below are consistent with it). The first part of the section titled "THE NVT PRINTER AND KEYBOARD" in RFC 854 [RFC0854] is generally, although not universally, considered to be the normative definition of the (ASCII) Network Virtual Terminal and hence of Net-ASCII. It includes not only the graphic ASCII characters but a number of control characters. The latter are given Internet-specific meanings that are often more specific than the definitions in the ASCII specification. In today's usage, and for the present specification, the following clarifications and updates to that list should be noted. Each one is accompanied by a brief explanation of the reason why the original specification is no longer appropriate. 1. The "defined but not required" codes -- BEL (U+0007), BS (U+0008), HT (U+0009), VT (U+000B), and FF (U+000C) -- and the undefined control codes ("C0") SHOULD NOT be used unless required by exceptional circumstances. Either their original "network printer" definitions are no longer in general use, common practice has evolved away from the formats specified there, or their use to simulate characters that are better handled by Unicode is no longer appropriate. While the appearance of some of these characters on the list may seem surprising, BS now has an ambiguous interpretation in practice (erasing in some systems but not in others), the width associated with HT varies with the environment, and VT and FF do not have a uniform effect with regard to either vertical positioning or the associated horizontal position result. Of course, telnet escapes are not considered part of the data stream and hence are unaffected by this provision. 2. In Net-ASCII, CR MUST NOT appear except when immediately followed by either NUL or LF, with the latter (CR LF) designating the "new line" function. Today and as specified above, CR should generally appear only when followed by LF. Because page layout is better done in other ways, because NUL has a special interpretation in some programming languages, and to avoid other types of confusion, CR NUL should preferably be avoided as specified above. 3. LF CR SHOULD NOT appear except as a side-effect of multiple CR LF sequences (e.g., CR LF CR LF). 4. The historical NVT documents do not call out either "bare LF" (LF without CR) or HT for special treatment. Both have generally been understood to be problematic. In the case of LF, there is a difference in interpretation as to whether its semantics imply "go to same position on the next line" or "go to the first position on the next line" and interoperability considerations suggest not depending on which interpretation the receiver applies. At the same time, misinterpretation of LF is less harmful than misinterpretation of "bare" CR: in the CR case, text may be erased or made completely unreadable; in the LF one, the worst consequence is a very funny-looking display. Obviously, HT is problematic because there is no standard way to transmit intended tab position or width information in running text. Again, the harm is unlikely to be great if HT is simply interpreted as one or more spaces, but, in general, it cannot be relied upon to format information. It is worth noting that the telnet IAC character (an octet consisting of all ones, i.e., %xFF) itself is not a problem for UTF-8 since that particular octet cannot appear in a valid UTF-8 string. However, while few of them have been used, telnet permits other command- introducer characters whose bit sequences in an octet may be part of valid UTF-8 characters. While it causes no ambiguity in UTF-8, Unicode assigns a graphic character ("Latin Small Letter Y with Diaeresis") to U+00FF (octets C3 B0 in UTF-8). Some caution is clearly in order in this area.
