294.htm

unix高级编程原吗

   meaning or even how to return to the ASCII set:  The Telnet signals <br>
   used to indicate a change of code set are: <br>
                      CODE          MEANING <br>
                      160           ASCII - Standard Telnet <br>
                      161           Transparent <br>
                      162           EBCDIC <br>
USER TELNET SIGNALS <br>
   The following signals are to be available to the human user to cause <br>
   the user Telnet to take the indicated action. <br>
   Transmit Now         -  Transmit all data entered and locally <br>
                           buffered now.  Intended to be used with line <br>
                           mode. <br>
   Suppress end-of-line -  Transmit all data entered and locally <br>
                           buffered now, and do not transmit the <br>
                           end-of-line immediately following this signal. <br>
Postel                                                          [Page 5] <br>

RFC 318                     Telnet Protocol                   April 1972 <br>
STANDARD TELNET IMPLEMENTATION <br>
   Using Site <br>
   1)  User is able to enter and transmit all ASCII codes <br>
   2)  User is able to cause the Telnet signals BREAK, SYNCH, ECHO and <br>
       NOECHO to be transmitted. <br>
   3)  Provides for the User Telnet signals, (e.g. Transmit Now). <br>
   4)  Implements the CR LF end-of-line convention. <br>
   5)  Provides local echo for local user terminals. <br>
   6)  Correctly processes the Telnet signals BREAK, SYNCH, NOP, ECHO, <br>
       NOECHO, and Hide Your Input received from the server. <br>
   Serving Site: <br>
   1)  Provides a mapping between ASCII and the local character set. <br>
   2)  Correctly processes the Telnet signals BREAK, SYNCH, NOP, NOECHO, <br>
       and ECHO. <br>
   3)  Implements the CR LF end-of-line convention. <br>
   4)  Assumes the using site provides echoing.  May provide a server <br>
       echo mode. <br>
MINIMUM TELNET IMPLEMENTATION <br>
   Using Site: <br>
   1)  User must be able to enter and transmit all ASCII codes. <br>
   2)  Ignore and delete all Telnet signals from the serving site. <br>

   3)  Provide local echo for local user terminals. <br>
   4)  Implements CR LF end-of-line convention. <br>
   5)  Provide for the User Telnet signals. <br>
Postel                                                          [Page 6] <br>
RFC 318                     Telnet Protocol                   April 1972 <br>
   Serving Site: <br>
   1)  Provide a mapping between ASCII and the local character set. <br>
   2)  Ignore and Delete all Telnet signals from the using site. <br>
   3)  Assume the using site provides echoing. <br>
   4)  Implements the CR LF end-of-line convention. <br>
III.  DISCUSSION OF TELNET PROTOCOL <br>
   The use of a standard, network-wide, intermediate representation of <br>
   terminal code between sites is intended to eliminate the need for <br>
   using and serving sites to keep information about the characteristics <br>
   of each other's terminals and terminal handling conventions.  This <br>
   approach can be successful, but only if the user, the using site, and <br>
   the serving site assume certain responsibilities. <br>
   1.  The serving site must specify how the intermediate code will be <br>
       mapped by it into the terminal codes that are expected at that <br>
       site. <br>
   2.  The user must be familiar with that mapping. <br>
   3.  The using site must provide some means for the user to enter all <br>

       of the intermediate codes, and as a convenience, special Telnet <br>
       signals, as well as specify for the user how the signals from the <br>
       serving site will be presented at the user terminal. <br>
   Other schemes were considered but rejected.  For example, a proposal <br>
   that the using site be responsible for translating to and from the <br>
   code expected by the serving site was rejected since it required that <br>
   the using site keep tables of all serving site codes and provide a <br>
   mapping for each case.  The information would require constant <br>
   maintenance as new hosts were added to the network. <br>
Character Set <br>
   Since it is not known how the current or future sites will specify <br>
   the mapping between the network-wide standard code (7 bit ASCII in an <br>
   8 bit field) and the codes expected from their own terminals, it <br>
   seems necessary to permit the user to cause transmission of every one <br>
   of the 128 ASCII codes, plus (for full user power) selected signals <br>
   (either of a Telnet control nature, or of a special terminal nature <br>
   such as break or attention). <br>
Postel                                                          [Page 7] <br>
RFC 318                     Telnet Protocol                   April 1972 <br>
   There was strong feeling about the importance of the user/system <br>
   interface at the using site, but equally strong feeling that this <br>
   problem is one of local implementation and should reflect the using <br>

   site installation philosophy rather than be subject to network-wide <br>
   standards.  Some topics of consideration in this area are: <br>
      1.  How to represent special graphics, not available at the using <br>
          site, at the user's terminal. <br>
      2.  Treatment of upper/lower case problem on upper case only <br>
          devices. <br>
          a.  Representing lower-case output. <br>
          b.  Providing users with shift and shiftlock signals. <br>
      3.  Incorporating editing capability in Telnet. <br>
      4.  Extending user options in Network mode not available to local <br>
          users, e.g., hold output or kill print. <br>
      5.  Permit users to specify how keyboard input is to the <br>
          translated, e.g., let a character from the terminal cause a <br>
          specified string to be sent by the user's Telnet. <br>
   The proposed solution to the Telnet Protocol problem seems to provide <br>
   a mechanism for a minimum implementation while providing a basis for <br>
   developing richer sets or protocol for present and future use in <br>
   terminal applications, process-process communications, and use by <br>
   other conventions to pass data or control information. <br>
   The understanding that ASCII be used as a network-wide code has been <br>
   established for some time.  Its use in Telnet provided a problem with <br>
   respect to the limitation of a maximum character set of 128.  Some <br>

   systems provide for more than this number in their operation, and <br>
   therefore, as serving sites, cannot map on a one-for-one basis. <br>
   Each such serving site could probably provide a reasonably useful <br>
   character set, including all system control signals, by mapping 128 <br>
   of its codes and just not provide a network user access to the other <br>
   codes.  However, any character left out might later be used in a <br>
   major application at that site as a special control signal.  This <br>
   could result in denying network users the facility offered by that <br>
   application.  Serving sites are, therefore, encouraged to provide a <br>
   full mapping between the ASCII code and the code used on the serving <br>
   system.  This may require that the server specify two character ASCII <br>
   sequences which map to single characters in the servers character <br>
   set. <br>
Postel                                                          [Page 8] <br>
RFC 318                     Telnet Protocol                   April 1972 <br>
   Notice that there are some ASCII codes which have no effect on the <br>
   NVT printer.  These codes must be transmitted over the network when <br>
   output by the serving process or by entered by the user. <br>
End of Line Convention <br>
   The representation of the end of a physical line at a terminal is <br>
   implemented differently on different network hosts.  For example, <br>
   some use a return (or new line) key, the terminal hardware both <br>

   returns the carriage or printer to start of line and feeds the paper <br>
   to the next line.  In other implementations, the user hits carriage <br>
   return and the hardware returns carriage while the software sends the <br>
   terminal a line feed.  The network-wide representation is carriage <br>
   return followed by line feed.  It represents the physical formatting <br>
   that is being attempted, and is to be interpreted and appropriately <br>
   translated by both using site and serving site. <br>
      EXAMPLE:  A Multics user is working, through the network, on some <br>
      serving site host.  In the course of the session the user has <br>
      numerous occasions to hit New Line on his Model 37 TTY.  Each time <br>
      the Multics system is awakened by a New Line interrupt, the line <br>
      of buffered characters is passed to Telnet where it is scanned for <br>
      special characters.  If none is found, carriage return followed by <br>
      line feed is inserted where New Line was entered, and the line is <br>
      turned over the NCP for transmission.  Correspondingly, when the <br>
      Multics Telnet finds the carriage return line feed sequence in the <br>
      data stream coming from the serving site, the two characters are <br>
      replaced with the appropriate New Line code which is sent to the <br>
      terminal. <br>
   Telnet defines the end of a line to be indicated by the ASCII <br>
   character pair CR LR.  Several of the real devices in the world have <br>
   only a single new line (NL) function.  Several of the computer <br>

   systems have in some programs used the CR and LF functions to have <br>
   semantic meaning larger than the format effect they provide. <br>
   Further, several computer systems allow the CR and LF functions to be <br>
   used separately (e.g., such that a line may be overprinted).  One <br>
   problem, for those Telnet (user) programs required to map the NVT <br>
   into a device which only has a NL function, is how is the CR LF to be <br>
   dealt with.  One solution is to examine the character following the <br>
   CR.  If an LF is found, then perform the NL function; if anything <br>
   else is found then back space to the beginning of the line.  Another <br>
   problem is the case of a computer system which locally uses period, <br>
   ".", to cause the new line function and which uses, in some programs, <br>
   CR and LF for semantically significant operations.  Suppose the user <br>
   Telnet sends the sequence CR LF.  Does this mean "new line" or the <br>
   "CR operation" followed by the "LF operation "?  A solution to this <br>
   problem it to require that Telnet programs send a CR NOT intended to <br>
  <br>
-- <br>
  <br>
-- <br>
※ 来源:．武汉白云黄鹤站 bbs.whnet.edu.cn．[FROM: 211.69.194.79] <br>
</small><hr>
<p align="center">[<a href="index.htm">回到开始</a>][<a href="288.htm">上一层</a>][<a href="295.htm">下一篇</a>]
<p align="center"><a href="http://cterm.163.net">欢迎访问Cterm主页</a></p>
</table>
</body>
</html>