Second thoughts on Telnet Go-Ahead (RFC0596)
Original Publication Date: 1973-Dec-08
Included in the Prior Art Database: 2000-Sep-13
Internet Society Requests For Comment (RFCs)
In this RFC we present objections to the requirement that hosts implement the Telnet Go-Ahead (GA) command, as specified in the Telnet Protocol Specification (NIC #15372). The thrust of these objections is in three major directions:
Network Working Group E. Taft
Request for Comments: 596 PARC-MAXC
NIC: 15372 8 December 1973
Second Thoughts on Telnet Go-Ahead
In this RFC we present objections to the requirement that hosts
implement the Telnet Go-Ahead (GA) command, as specified in the
Telnet Protocol Specification (NIC #15372). The thrust of these
objections is in three major directions:
1. The GA mechanism is esthetically unappealing, both to myself
and to many other people I have talked to. I shall attempt to
describe why this is so.
2. As specified in the Protocol, GA will not, in general, work;
i.e. it will not serve its intended purpose unless hosts make
various unwarranted assumptions about how other hosts operate.
3. GA is impossible for most hosts to implement correctly in all
cases. This is certainly true of the PDP-10 operating systems
with which I am familiar (10/50 and Tenex).
The purpose of this RFC is to advocate either complete removal of the
GA mechanism or relegating it to the status of a negotiated option
whose default state is that it be suppressed.
"Half-duplex" is a two-way communication discipline in which
transmission takes place in only one direction at a time and the
receiving party is constrained not to transmit until the transmitting
party has explicitly given up control of the communication path
("turned the line around").
This definition is distinct from a common (but incorrect) use of the
terms "half-duplex" and "full-duplex" to designate local and remote
"Reverse break" is a means by which a computer connected to a
terminal by a half-duplex path may regain control of the path for
further typeout after previously having relinquished it.
This is the complement of the "break" or "attention" mechanism,
implemented by all half-duplex terminals, by means of which the user
may gain control of the line while it is in use by the computer.
ESTHETIC OBJECTIONS TO GA
One assumption that permeates the Telnet Protocol specification (and
is explicitly stated on Page 7) is that the "normal" mode of
communication between computers and terminals is half-duplex, line-
at-a-time. While historically this is partially true, it is also
clear, both within the ARPA Network community and elsewhere, that the
trend is toward highly interactive man-machine communication systems
which are difficult to implement under half-duplex communication
The GA mechanism is an attempt to solve a specific problem, that of
switching control between computer and user in a subset of those
hosts utilizing IBM 2741 or equivalent terminals. I say "a subset"
because in fact the problem arises only in the case of TIPs from
2741s (with reverse break); from what experience I have had, I th...