    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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