Second thoughts on Telnet Go-Ahead

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.


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