Network Working Group                                 A. McKenzie
RFC #312                                              BBN
NIC 9342                                              22 March 1972
Categories:  B.1

                Proposed Change in IMP-to-Host Protocol

        We are currently considering a redefinition of the IMP-to-Host
   error message types (type 1 and type 8) and the creation of addi-
   tional IMP-to-Host error message types.  We believe that these
   changes will assist the Hosts in determining appropriate recovery
   action, without causing any serious reprogramming problems.  Our
   current plans are to install these changes within a few months;
   therefore we should be informed of any strong negative reactions
   relatively quickly.

        The proposed changes fall into two general classes as de-
   scribed below:

        A)  General Error Message
            ---------------------

            Under certain circumstances, particularly when the Host
            has been unresponsive to queued input for a "long time"
            the IMP drops its ready line for a short period, causing
            the "error flip-flops" to be set (see RFC #270, NIC 7818).
            Under these conditions the IMP sends a few NOP's to the
            Host and then resumes normal operation.  We propose to
            send the Host a new message (message type 13) in addition
            to the NOPs; this message will tell the Host that the
            IMP's Ready Line was dropped, that the IMP's error flop
            was set, and that the IMP will respond to the next com-
            pletion of a Host-to-IMP message with a type 1 or type 8
            message (because of the setting of the IMP's error flop.

        B)  Error Messages which are Responses to Specific
            ----------------------------------------------
            Host-to-IMP Transmissions:
            --------------------------

            1)  IMP-to-Host message type 1 will be redefined to mean:
                "IMP's Error flip-flop was set on a message which
                the IMP cannot identify."

            2)  IMP-to-Host message type 8 will be redefined to mean:
                "IMP's Error flip-flop was set during receipt of the
                message identified by the 'source' and 'link number'
                bits of this error message."



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            3)  IMP-to-Host message type 10 will be defined to mean:
                "A Host-to-IMP message was too short (and cannot be
                identified)."

            4)  IMP-to-Host message type 11 will be defined to mean:
                "A Host-to-IMP message was too long; the message is
                identified by the 'source' and 'link number' bits
                of this error message."

            5)  IMP-to-Host message type 12 will be defined to mean:
                "A Host-to-IMP message with an illegal message type
                code was received; the message is identified by the
                'source' and 'link number' bits of this error message.
                (Note that the erroneous type code is not included in
                the error message.)"




   AAM/jm



       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by BBN Corp. under the   ]
       [ direction of Alex McKenzie.                   12/96   ]






















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