GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1143

Network Working Group D. Bernstein Request for Comments: 1143 NYU

                                                         February 1990
       The Q Method of Implementing TELNET Option Negotiation

Status of This Memo

 This is RFC discusses an implementation approach to option
 negotiation in the Telnet protocol (RFC 854).  It does not propose
 any changes to the TELNET protocol.  Rather, it discusses the
 implementation of the protocol of one feature, only.  This is not a
 protocol specification.  This is an experimental method of
 implementing a protocol.  This memo is not a recommendation of the
 Telnet Working Group of the Internet Engineering Task Force (IETF).
 This RFC is Copyright 1990, Daniel J. Bernstein.  However,
 distribution of this memo in original form is unlimited.

1. Introduction

 This RFC amplifies, supplements, and extends the RFC 854 [7] option
 negotiation rules and guidelines, which are insufficient to prevent
 all option negotiation loops.  This RFC also presents an example of
 correct implementation.
 DISCUSSION:
 The two items in this RFC of the most interest to implementors are
 1. the examples of option negotiation loops given below; and 2. the
 example of a TELNET state machine preventing loops.
    1. Implementors of TELNET should read the examples of option
       negotiation loops and beware that preventing such loops is a
       nontrivial task.
    2. Section 7 of this RFC shows by example a working method
       of avoiding loops.  It prescribes the state information that
       you must keep about each side of each option; it shows what
       to do in each state when you receive WILL/WONT/DO/DONT from
       the network, and when the user or process requests that an
       option be enabled or disabled.  An implementor who uses the
       procedures given in that example need not worry about
       compliance with this RFC or with a large chunk of RFC 854.
 In short, all implementors should be familiar with TELNET loops, and
 some implementors may wish to use the pre-written example here in

Bernstein [Page 1] RFC 1143 Q Method February 1990

 writing a new TELNET implementation.
 NOTE: Reading This Document
    A TELNET implementation is not compliant with this RFC if it fails
    to satisfy all rules marked MUST.  It is compliant if it satisfies
    all rules marked MUST.  If it is compliant, it is unconditionally
    compliant if it also satisfies all rules marked SHOULD and
    conditionally compliant otherwise.  Rules marked MAY are optional.
    Options are in almost all cases negotiated separately for each
    side of the connection.  The option on one side is separate from
    the option on the other side. In this document, "the" option
    referred to by a DONT/WONT or DO/WILL is really two options,
    combined only for semantic convenience.  Each sentence could be
    split into two, one with the words before the slash and one with
    the words after the slash.
    An implementor should be able to determine whether or not an
    implementation complies with this RFC without reading any text
    marked DISCUSSION.  An implementor should be able to implement
    option negotiation machinery compliant with both this RFC and RFC
    854 using just the information in Section 7.

2. RFC 854 Option Negotiation Requirements

 As specified by RFC 854: A TELNET implementation MUST obey a refusal
 to enable an option; i.e., if it receives a DONT/WONT in response to
 a WILL/DO, it MUST NOT enable the option.
 DISCUSSION:
    Where RFC 854 implies that the other side may reject a request to
    enable an option, it means that you must accept such a rejection.
 It MUST therefore remember that it is negotiating a WILL/DO, and this
 negotiation state MUST be separate from the enabled state and from
 the disabled state.  During the negotiation state, any effects of
 having the option enabled MUST NOT be used.
 If it receives WONT/DONT and the option is enabled, it MUST respond
 DONT/WONT repectively and disable the option.  It MUST NOT initiate a
 DO/WILL negotiation for an already enabled option or a DONT/WONT
 negotiation for a disabled option.  It MUST NOT respond to receipt of
 such a negotiation.  It MUST respond to receipt of a negotiation that
 does propose to change the status quo.

Bernstein [Page 2] RFC 1143 Q Method February 1990

 DISCUSSION:
    Many existing implementations respond to rejection by confirming
    the rejection; i.e., if they send WILL and receive DONT, they send
    WONT.  This has been construed as acceptable behavior under a
    certain (strained) interpretation of RFC 854.  However, to allow
    this possibility severely complicates later rules; there seems to
    be no use for the wasted bandwidth and processing.  Note that an
    implementation compliant with this RFC will simply ignore the
    extra WONT if the other side sends it.
 The implementation MUST NOT automatically respond to the rejection of
 a request by submitting a new request.  As a rule of thumb, new
 requests should be sent either at the beginning of a connection or in
 response to an external stimulus, i.e., input from the human user or
 from the process behind the server.
 A TELNET implementation MUST refuse (DONT/WONT) a request to enable
 an option for which it does not comply with the appropriate protocol
 specification.
 DISCUSSION:
    This is not stated as strongly in RFC 854.  However, any other
    action would be counterproductive.  This rule appears in
    Requirements for Internet Hosts [6, Section 3.2.2]; it appears
    here for completeness.

3. Rule: Remember DONT/WONT requests

 A TELNET implementation MUST remember starting a DONT/WONT
 negotiation.
 DISCUSSION:
    It is not clear from RFC 854 whether or not TELNET must remember
    beginning a DONT/WONT negotiation.  There seem to be no reasons to
    remember starting a DONT/WONT negotiation: 1. The argument for
    remembering a DO/WILL negotiation (viz., the state of negotiating
    for enabling means different things for the data stream than the
    state of having the option enabled) does not apply.  2. There is
    no choice for the other side in responding to a DONT/WONT; the
    option is going to end up disabled.  3. If we simply disable the
    option immediately and forget negotiating, we will ignore the
    WONT/DONT response since the option is disabled.
    Unfortunately, that conclusion is wrong.  Consider the following
    TELNET conversation between two parties, "us" and "him".  (The

Bernstein [Page 3] RFC 1143 Q Method February 1990

    reader of this RFC may want to sort the steps into chronological
    order for a different view.)
    LOOP EXAMPLE 1
       Both sides know that the option is on.
       On his side:
     1 He decides to disable.  He sends DONT and disables the option.
     2 He decides to reenable.  He sends DO and remembers he is
       negotiating.
     5 He receives WONT and gives up on negotiation.
     6 He decides to try once again to reenable.  He sends DO and
       remembers he is negotiating.
     7 He receives WONT and gives up on negotiation.
       For whatever reason, he decides to agree with future requests.
    10 He receives WILL and agrees. He responds DO and enables the
       option.
    11 He receives WONT and sighs. He responds DONT and disables the
       option.
       (repeat 10 and then 11, forever)
       On our side:
     3 We receive DONT and sigh.  We respond WONT and disable the
       option.
     4 We receive DO but disagree.  We respond WONT.
     8 We receive DO and decide to agree.  We respond WILL and enable
       the option.
     9 We decide to disable.  We send WONT and disable the option.
       For whatever reason, we decide to agree with future requests.
    12 We receive DO and agree.  We send WILL and enable the option.
    13 We receive DONT and sigh.  We send WONT and disable the option.
       (repeat 12 and then 13, forever)
    Both sides have followed RFC 854; but we end in an option
    negotiation loop, as DONT DO DO and then DO DONT forever travel
    through the network one way, and WONT WONT followed by WILL WONT
    forever travel through the network the other way.  The behavior in
    steps 1 and 9 is responsible for this loop.  Hence this section's
    rule.  In Section 6 below is discussion of whether separate states
    are needed for "negotiate for disable" and "negotiate for enable"
    or whether a single "negotiate" state suffices.

4. Rule: Prohibit new requests before completing old negotiation

 A TELNET implementation MUST NOT initiate a new WILL/WONT/DO/DONT
 request about an option that is under negotiation, i.e., for which it
 has already made such a request and not yet received a response.

Bernstein [Page 4] RFC 1143 Q Method February 1990

 DISCUSSION:
    It is unclear from RFC 854 whether or not a TELNET implementation
    may allow new requests about an option that is currently under
    negotiation; it certainly seems limiting to prohibit "option
    typeahead".  Unfortunately, consider the following:
    LOOP EXAMPLE 2
       Suppose an option is disabled, and we decide in quick
       succession to enable it, disable it, and reenable it.  We send
       WILL WONT WILL and at the end remember that we are negotiating.
       The other side agrees with DO DONT DO. We receive the first DO,
       enable the option, and forget we have negotiated.  Now DONT DO
       are coming through the network and both sides have forgotten
       they are negotiating; consequently we loop.
    (All possible TELNET loops eventually degenerate into the same
    form, where WILL WONT [or WONT WILL, or WILL WONT WILL WONT, etc.]
    go through the network while both sides think negotiation is over;
    the response is DO DONT and we loop forever.  TELNET implementors
    are encouraged to implement any option that can detect such a loop
    and cut it off; e.g., a method of explicitly differentiating
    requests from acknowledgments would be sufficient.  No such option
    exists as of February 1990.)
    This particular case is of considerable practical importance: most
    combinations of existing user-server TELNET implementations do
    enter an infinite loop when asked quickly a few times to enable
    and then disable an option.  This has taken on an even greater
    importance with the advent of LINEMODE [4], because LINEMODE is
    the first option that tends to generate such rapidly changing
    requests in the normal course of communication.  It is clear that
    a new rule is needed.
    One might try to prevent the several-alternating-requests problem
    by maintaining a more elaborate state than YES/NO/WANTwhatever,
    e.g., a state that records all outstanding requests.  Dave Borman
    has proposed an apparently working scheme [2] that won't blow up
    if both sides initiate several requests at once, and that seems to
    prevent option negotiation loops; complete analysis of his
    solution is somewhat difficult since it means that TELNET can no
    longer be a finite-state automaton.  He has implemented his
    solution in the latest BSD telnet version [5]; as of May 1989, he
    does not intend to publish it for others to use [3].
    Here the author decided to preserve TELNET's finite-state
    property, for robustness and because the result can be easily

Bernstein [Page 5] RFC 1143 Q Method February 1990

    proven to work.  Hence the above rule.
    A more restrictive solution would be to buffer all data and do
    absolutely nothing until the response comes back.  There is no
    apparent reason for this, though some existing TELNET
    implementations do so anyway at the beginning of a connection,
    when most options are negotiated.

5. How to reallow the request queue

 DISCUSSION:
    The above rule prevents queueing of more than one request through
    the network.  However, it is possible to queue new requests within
    the TELNET implementation, so that "option typeahead" is
    effectively restored.
    An obvious possibility is to maintain a list of requests that have
    been made but not yet sent, so that when one negotiation finishes,
    the next can be started immediately.  So while negotiating for a
    WILL, TELNET could buffer the user's requests for WONT, then WILL
    again, then WONT, then WILL, then WONT, as far as desired.
    This requires a dynamic and potentially unmanageable buffer.
    However, the restrictions upon possible requests guarantee that
    the list of requests must simply alternate between WONT and WILL.
    It is wasteful to enable an option and then disable it, just to
    enable it again; we might as well just enable it in the first
    place.  The few possible exceptions to this rule do not outweigh
    the immense simplification afforded by remembering only the last
    few entries on the queue.
    To be more precise, during a WILL negotiation, the only sensible
    queues are WONT and WONT WILL, and similarly during a WONT
    negotiation.  In the interest of simplicity, the Q method does not
    allow the WONT WILL possibility.
    We are now left with a queue consisting of either nothing or the
    opposite of the current negotiation.  When we receive a reply to
    the negotiation, if the queue indicates that the option should be
    changed, we send the opposite request immediately and empty the
    queue.  Note that this does not conflict with the RFC 854 rule
    about automatic regeneration of requests, as these new requests
    are simply the delayed effects of user or process commands.
 An implementation SHOULD support the queue, where support is defined
 by the rules following.

Bernstein [Page 6] RFC 1143 Q Method February 1990

 If it does support the queue, and if an option is currently under
 negotiation, it MUST NOT handle a new request from the user or
 process to switch the state of that option by sending a new request
 through the network.  Instead, it MUST remember internally that the
 new request was made.
 If the user or process makes a second new request, for switching back
 again, while the original negotiation is still incomplete, the
 implementation SHOULD handle the request simply by forgetting the
 previous one.  The third request SHOULD be treated like the first,
 etc.  In any case, these further requests MUST NOT generate immediate
 requests through the network.
 When the option negotiation completes, if the implementation is
 remembering a request internally, and that request is for the
 opposite state to the result of the completed negotiation, then the
 implementation MUST act as if that request had been made after the
 completion of the negotiation.  It SHOULD thus immediately generate a
 new request through the network.
 The implementation MAY provide a method by which support for the
 queue may be turned off and back on.  In this case, it MUST default
 to having the support turned on.  Furthermore, when support is turned
 off, if the implementation is remembering a new request for an
 outstanding negotiation, it SHOULD continue remembering and then deal
 with it at the close of the outstanding negotiation, as if support
 were still turned on through that point.
 DISCUSSION:
    It is intended (and it is the author's belief) that this queue
    system restores the full functionality of TELNET.  Dave Borman has
    provided some informal analysis of this issue [1]; the most
    important possible problem of note is that certain options which
    may require buffering could be slowed by the queue.  The author
    believes that network delays caused by buffering are independent
    of the implementation method used, and that the Q Method does not
    cause any problems; this is borne out by examples.

6. Rule: Separate WANTNO and WANTYES

 Implementations SHOULD separate any states of negotiating WILL/DO
 from any states of negotiating WONT/DONT.
 DISCUSSION:
    It is possible to maintain a working TELNET implementation if the
    NO/YES/WANTNO/WANTYES states are simplified to NO/YES/WANT.

Bernstein [Page 7] RFC 1143 Q Method February 1990

    However, in a hostile environment this is a bad idea, as it means
    that handling a DO/WILL response to a WONT/DONT cannot be done
    correctly.  It does not greatly simplify code; and the simplicity
    gained is lost in the extra complexity needed to maintain the
    queue.

7. Example of Correct Implementation

 To ease the task of writing TELNET implementations, the author
 presents here a precise example of the response that a compliant
 TELNET implementation could give in each possible situation.  All
 TELNET implementations compliant with this RFC SHOULD follow the
 procedures shown here.
 EXAMPLE STATE MACHINE
 FOR THE Q METHOD OF IMPLEMENTING TELNET OPTION NEGOTIATION
    There are two sides, we (us) and he (him).  We keep four
    variables:
       us: state of option on our side (NO/WANTNO/WANTYES/YES)
       usq: a queue bit (EMPTY/OPPOSITE) if us is WANTNO or WANTYES
       him: state of option on his side
       himq: a queue bit if him is WANTNO or WANTYES
    An option is enabled if and only if its state is YES.  Note that
    us/usq and him/himq could be combined into two six-choice states.
    "Error" below means that producing diagnostic information may be a
    good idea, though it isn't required.
    Upon receipt of WILL, we choose based upon him and himq:
       NO            If we agree that he should enable, him=YES, send
                     DO; otherwise, send DONT.
       YES           Ignore.
       WANTNO  EMPTY Error: DONT answered by WILL. him=NO.
            OPPOSITE Error: DONT answered by WILL. him=YES*,
                     himq=EMPTY.
       WANTYES EMPTY him=YES.
            OPPOSITE him=WANTNO, himq=EMPTY, send DONT.
  • This behavior is debatable; DONT will never be answered by WILL

over a reliable connection between TELNETs compliant with this

      RFC, so this was chosen (1) not to generate further messages,
      because if we know we're dealing with a noncompliant TELNET we
      shouldn't trust it to be sensible; (2) to empty the queue
      sensibly.

Bernstein [Page 8] RFC 1143 Q Method February 1990

    Upon receipt of WONT, we choose based upon him and himq:
       NO            Ignore.
       YES           him=NO, send DONT.
       WANTNO  EMPTY him=NO.
            OPPOSITE him=WANTYES, himq=NONE, send DO.
       WANTYES EMPTY him=NO.*
            OPPOSITE him=NO, himq=NONE.**
  • Here is the only spot a length-two queue could be useful; after

a WILL negotiation was refused, a queue of WONT WILL would mean

      to request the option again. This seems of too little utility
      and too much potential waste; there is little chance that the
      other side will change its mind immediately.
  • * Here we don't have to generate another request because we've

been "refused into" the correct state anyway.

    If we decide to ask him to enable:
       NO            him=WANTYES, send DO.
       YES           Error: Already enabled.
       WANTNO  EMPTY If we are queueing requests, himq=OPPOSITE;
                     otherwise, Error: Cannot initiate new request
                     in the middle of negotiation.
            OPPOSITE Error: Already queued an enable request.
       WANTYES EMPTY Error: Already negotiating for enable.
            OPPOSITE himq=EMPTY.
    If we decide to ask him to disable:
       NO            Error: Already disabled.
       YES           him=WANTNO, send DONT.
       WANTNO  EMPTY Error: Already negotiating for disable.
            OPPOSITE himq=EMPTY.
       WANTYES EMPTY If we are queueing requests, himq=OPPOSITE;
                     otherwise, Error: Cannot initiate new request
                     in the middle of negotiation.
            OPPOSITE Error: Already queued a disable request.
    We handle the option on our side by the same procedures, with DO-
    WILL, DONT-WONT, him-us, himq-usq swapped.

8. References

 [1] Borman, D., private communication, April 1989.
 [2] Borman, D., private communication, May 1989.
 [3] Borman, D., private communication, May 1989.

Bernstein [Page 9] RFC 1143 Q Method February 1990

 [4] Borman, D., Editor, "Telnet Linemode Option", RFC 1116, Cray
     Research, August 1989.
 [5] Borman, D., BSD Telnet Source, November 1989.
 [6] Braden, R., Editor, "Requirements for Internet Hosts --
     Application and Support", RFC 1123, USC/Information Sciences
     Institute, October 1989.
 [7] Postel, J., and J. Reynolds, "Telnet Protocol Specification", RFC
     854, USC/Information Sciences Institute, May 1983.

9. Acknowledgments

 Thanks to Dave Borman, dab@opus.cray.com, for his helpful comments.

Author's Address

 Daniel J. Bernstein
 5 Brewster Lane
 Bellport, NY 11713
 Phone:  516-286-1339
 Email:  brnstnd@acf10.nyu.edu

Bernstein [Page 10]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1143.txt · Last modified: 1990/03/02 04:57 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki