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Network Working Group                                            E. TaftRequest for Comments: 596                                      PARC-MAXCNIC: 15372                                               8 December 1973Second Thoughts on Telnet Go-AheadINTRODUCTION   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.Taft                                                            [Page 1]

RFC 596            Second Thoughts on Telnet Go-Ahead      December 1973   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 hostsTaft                                                            [Page 2]

RFC 596            Second Thoughts on Telnet Go-Ahead      December 1973      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.Taft                                                            [Page 3]

RFC 596            Second Thoughts on Telnet Go-Ahead      December 1973      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.Taft                                                            [Page 4]

RFC 596            Second Thoughts on Telnet Go-Ahead      December 1973   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 ]Taft                                                            [Page 5]