RFC 357 (rfc357) - Page 3 of 13

Echoing strategy for satellite links

RFC 357         An Echoing Strategy For Satellite Links        June 1972


   As far as implementation, 1. and 2. could be communicated by allowing
   the user process to specify a 128-bit (for an ASCII device) table
   with 1's set for each wakeup character, and another table with 1's
   set for each break character.  This approach becomes fairly expensive
   in terms of core memory as the number of terminals becomes large; the
   system must store these bit tables itself since in most cases the
   user process will not be in core while echoing is being done by the
   Terminal Handler.

   To reduce the storage requirements, the system can make known to all
   its programmers a limited number, say 4, of supported break
   characters for his process from, for example:

      a. alphanumeric characters,
      b. punctuation characters,
      c. echoable control characters (including the bell and CR, etc.),
         or
      d. non-echoable control characters (Control-C, etc.),

   by specifying in a system call which break set(s) should be used.
   This requires no more than 4 bits of system storage per terminal, and
   a single table to identify the set(s) to which each of the 128
   possible ASCII characters belongs.

   For the user process to communicate (3) to the Terminal Handler
   (which break characters should and which should not have echoed), the
   process can specify another 4 bit field with 1's set for those break
   classes whose members should be echoed.  For the 4 classes above,
   only 3 bits would be required since members of class (d) are defined
   to be non-echoable.

   To communicate the completion of an output response (4), the user
   process could issue an explicit system call; or, the Terminal Handler
   could assume completion when the user process requests input of the
   first character following the break.

   "Hide your input" (5) would be communicated by a system call which
   specifies either:

     (a) "break on every character and don't echo any break characters",
         or, for example
     (b) "don't echo anything and break on punctuation, or any control
         character" for an alphanumeric password,

   depending on the syntax of the expression to be hidden.






Davidson