Second thoughts in defense of the Telnet Go-Ahead (RFC0595)

Network Working Group Wayne Hathaway Request for Comments # 595 AMES-67 NIC # 20617 12 Dec 1973 References: NIC # 20812

Some Thoughts in Defense of the TELNET Go-Ahead

This note is a reply to Edward Taft’s "Second Thoughts on TELNET Go- Ahead" (NIC #20812). Specifically, I will attempt to show the following about the three main directions of his objections:

1. It is the idea of line-at-a-time systems which are esthetically unappealing, not the GA mechanism. This may be a valid point, but given the large number of such systems on the net, it would seem a rather academic one.

2. The specified GA mechanism will in fact work very well between (reasonably implemented) line-at-a-time systems, and should provide significant help elsewhere.

3. While the GA mechanism may not be correct in all cases, it can provide significant advantages fro the line-at-a-time systems and users.

My comments will be arranged under the original headings from the subject RFC (NIC #20812).

"TECHNOLOGY"

The definitions of "half-duplex" and "reverse break" are satisfactory. Two points should be made regarding "reverse break", however. First: having reverse break on the terminal is of course not sufficient; the operating system must support it. As "support" is equivalent to "require" in this context, it is not too surprising that some systems do not in fact do this. That is, there are systems which will not type through an unlocked keyboard until the user manually turns the line around, and the operational problems with such systems are much less than might be assumed. Second, at least on IBM 2741’s and equivalent, the line turnaround takes a significant amount of time, during which user-typed characters may be missed or garbled. In fact, a fairly standard mode of operation with systems that use reverse break (including TIP’s) is to automatically enter a "line delete" character and start over every time the reverse break is used while typing, which can hardly be called esthetic. One solution to this problem would be for the system to not use reverse break once the user has begun typing (as suggested near the end of NIC #20812), but most systems (including TIP’s) do not do this.

Hathaway [Page 1]

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 typ...