Second thoughts on Telnet Go-Ahead (RFC0596)

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

E.A. Taft: AUTHOR

Abstract

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

INTRODUCTION

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.

TERMINOLOGY

"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

character echoing.

"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

discip...