RFC 595 (rfc595) - Page 2 of 5
Second thoughts in defense of the Telnet Go-Ahead
Alternative Format: Original Text Document
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