RFC 595 (rfc595) - Page 2 of 5

Second thoughts in defense of the Telnet Go-Ahead

RFC 595            In Defense of the TELNET Go-Ahead       December 1973


   Some discussion is also warranted at this point about line-at-a-time
   systems (hereafter abbreviated as LAAT systems).  One prime reason for
   LAAT operation is to avoid the overhead of interrupting the CPU (and
   possibly the user process) for every character typed.  Instead,
   characters are buffered (in a controller, a front-end computer, etc)
   until some "end-of-line" signal is received; they are then passed to
   the system in a group.  This means that the system is totally unaware
   that any typing has occurred until the "end-of-line" signal is sent;
   a partially completed line will literally never be recognized.

"ESTHETIC OBJECTIONS TO GA"

   From the above, I feel that one can see that it is the operating mode
   of a system rather than the type of features of its terminals which
   determines whether GA is useful or not.  For example, IBM front-ends
   handle Teletypes in LAAT mode, while the TIP attempts to run 2741's
   as full-duplex devices (with something less than "a very good job at
   turning the line around," from my experience).

   At any rate, the half-duplex/full-duplex debate can go on forever --
   the problem here is to try to smooth the way for users on local LAAT
   systems connected to foreign systems of varying characteristics.

"WHY GA WON'T WORK"

   As mentioned, in LAAT systems no terminal input is recognized until
   the specified "end-of-line" character is entered, preceding characters
   having been buffered in a front-end etc.  This can of course be
   carried over into server TELNET: incoming network messages can be
   buffered at a very low level in the NCP awaiting a TELNET end-of-line
   signal.  User processes wanting input would remain blocked until the
   end-of-line is received, rather than being handed each character as
   it is read.  In fact, this is the implementation in all of the LAAT
   systems with which I am familiar.  The reason for doing this is
   obvious: many hosts continue to send single characters even in LAAT
   systems, resulting in a significant increase in overhead.  Equally
   obvious is the fact that in this mode the GA mechanism will function
   quite well, in fact as well as turning the line around to unlock the
   keyboard of a local terminal.












Hathaway