RFC596

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 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.

  The user Telnet transmits this text line over the connection;
  however, due to insufficient allocation or other conditions, the
  text actually gets packaged up and sent as two or more separate
  messages, which arrive at the server host in the correct order but
  separated by some amount of time.

  The server Telnet passes the contents of the first message to the
  appropriate process, which reads the partial text line and
  immediately blocks for further input.  At this moment (assuming
  the second message hasn't arrived yet), the server telnet, in
  accordance with the Protocol, sends back a GA command.

  The rest of the text then arrives in response, the server process
  may generate a large volume of output.  Meanwhile, however, the GA
  command has caused the user's keyboard to become unlocked and
  computer output thereby blocked.  Hence we have a deadlock, which
  will be resolved only when the user recognizes what has happened
  and (manually) gives control back to the computer.

Of course, this particular problem is avoided if the Telnet protocol is modified to specify that the server Telnet will transmit GA only if the server process is blocked for input AND the most recent character passed to that process was end-of-line.

  I claim that this solution is bad in principle because it assumes
  too much knowledge on the part of the serving host as to what
  constitutes "end-of-line" in the using host.

  Furthermore, the Protocol explicitly (and quite rightly) specifies
  that the user Telnet should provide some means by which a user may
  signal that all buffered text should be transmitted immediately,
  without its being terminated by end-of-line.

One must conclude, then, that in general the server Telnet has no precise way of knowing when it should send GA commands.

IMPLEMENTATION PROBLEMS

The foregoing analysis illustrates the problems that arise with the GA mechanism in communication between servers and users whose normal mode of operation is half-duplex, line-at-a-time. When we turn to hosts that provide full-duplex service, such as the PDP-10s and many other hosts on the Network, the problems are much more severe.

  This is particularly true of operating system such as Tenex that
  exercise such tight control over terminal behavior that they
  prefer to operate in server echoing, character-at-a-time mode.
  This will probably become less necessary as protocols such as
  Remote Controlled transmission and Echoing Option come into
  general use, enabling servers to regulate echoing and break
  character classes in user Telnets.

Even in hosts such as 10/50 systems that provide reasonable service to line-at-a-time users for most subsystems (e.g. excluding DDT and TECO), GA is impossible to implement correctly. This is true for several reasons.

First, there are a number of subsystems that never block for terminal input but rather poll for it or accept it on an interrupt basis. In the absence of typein, such processes go on to do other tasks, possibly generating terminal output.

  Processes of this sort come immediately to mind.  The user telnet,
  FTP, and RJE programs are implemented in this fashion by almost
  all hosts.  10/50 has a subsystem called OPSER, used to control
  multiple independent subjobs from a single terminal.

  Since these programs never block for input, GA commands will never
  be sent by the server Telnet in such cases even though the
  processes are prepared to accept terminal input at any time.

Second, there is not necessarily a one-to-one relationship between processes and terminals, as seems to be assumed by the Telnet Protocol specification.

  For example, in Tenex one process may be blocked for terminal
  input while another process is generating output to the same
  terminal.  (Such processes are typically parallel forks of the
  same job).

Third, there is the possibility of inter-terminal links, such as are provided in many systems.

  By this I do not mean special Telnet connections established
  between a pair of NVTs for the express purpose of terminal-to-
  terminal communication, as is suggested on page 9 of the Protocol
  specification.  Rather, I am referring to facilities such as the
  Tenex LINK facility, in which any number and any mixture of local
  and Network terminals and processes may have their input and
  output streams linked together in arbitrarily complex ways.
  Clearly the GA mechanism will fall flat on its face in this case.

  Also, the notion that one user of an inter-terminal link should
  have to "manually signal that it is time for a GA to be sent over
  the Telnet connection" in order to unblock another user's keyboard
  offends me to no end.

Finally, most systems provide means by which system personnel and processes may broadcast important messages to all terminals (e.g. SEND ALL in 10/50, NOTIFY in Tenex). Clearly such asynchronous messages will be blocked by a half-duplex terminal that has been irrevocably placed in the typein state by a previous GA.

  This strikes me as such an obvious problem that I am forced to
  wonder how half-duplex hosts handle it even for their local
  terminals.

  [ This RFC was put into machine readable form for entry ]
  [ into the online RFC archives by Mirsad Todorovac 5/98 ]