Second thoughts on Telnet Go-Ahead(1)

　　Network Working Group E. Taft

　　Request for Comments: 596 PARC-MAXC

　　NIC: 15372 8 December 1973

　　Second Thoughts on TelnetGo-Ahead

　　INTRODUCTION

　　In this RFCwe 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 RFCis 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　disciplines.

　　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 think　the TIP does a very good job of turning the line around at the right　moments. (I am told this is also the case at Multics).

　　Given the trend toward more interactive communication, and given the　fact that terminals on the Network requiring a Go-Ahead mechanism are　a distinct minority of all terminals, I think we should be reluctant　to burden our protocols with kludges that are so clearly a concession　to obsolete design.

　　I have little doubt that before long somebody (if not IBM) will　produce a full-duplex 2741-like terminal (indeed, perhaps it has　already been done). There is an obvious need for a terminal with　Selectric quality keyboard and hard-copy better suited to　interactive applications (i.e. full-duplex).

　　As a more practical consideration, it makes little sense to have the　default state of the GA option be the one that benefits the least　number of hosts and terminals.

　　There is no question that most parties to Telnet communication　will immediately negotiate to suppress GA. To do otherwise will　double the amount of network traffic generated by character-at-a-time typein, and will increase it by a non-negligible amount even　for a line-at-a-time typein.

　　It strikes me as worthwhile to minimize the number of such　"necessary" option negotiations, especially in view of the large　number of TIPs and mini-hosts on the Network. Many such hosts　must, due to resource constraints, implement only a limited subset　of the available options. It follows, then, that the default　state of all options should be the one most hosts will be willing　to use.

　　WHY GA WON'T WORK

　　We now show that a server process's being "blocked on input" (as specified in the Protocol) is not itself a sufficient condition for　sending out GA.　

       This is due to the fact that the user Telnet has no control over the　packaging of a "line" of information sent to the server; rather, this　is a function of the NCP, which must observe constraints such as　allocation and buffering. Consider the following example:　A user types a line of text, which is buffered by his host's user　Telnet until he signals end-of-line. His keyboard then becomes　locked (this being the behavior of half-duplex terminals while the　computer has control of the line), and stays locked in　anticipation of the server's eventual response and subsequent GA command.