RFC 394 (rfc394) - Page 1 of 3

Two Proposed Changes to the IMP-Host Protocol

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Network Working Group                    John M. McQuillan
Request for Comments #394                Bolt Beranek and Newman Inc.
NIC 11856                                27 September 1972
Categories:  B.1
Updates:  RFC #381
Obsoletes:

             TWO PROPOSED CHANGES TO THE IMP-HOST PROTOCOL
---------------------------------------------
     This note describes two changes to the IMP-Host communication
protocol described in BBN Report 1822 and revised in RFC 381.  The
first deals with the IMP-to-Host interface and the 30-second timeout
mechanism on each IMP transmission to the Host.  The second deals with
the Host-to-IMP interface and proposes a new timeout mechanism.  These
changes are independent; in statement and in implementation.  We
invite comments on each proposal.  If no adverse comments are
received, they will be installed in the network on Tuesday, October 10
(if serious adverse comments are received, action will be delayed
until early November).

1)  Declaring an unresponsive Host as dead to the network
    -----------------------------------------------------
     Currently, a Host is given 30 seconds to accept each packet of a
regular message and is also given 30 seconds to accept non- regular
messages.  If the Host is unresponsive for this period of time, the
IMP takes the following actions:

     a)  All messages held for the Host are discarded.

     b)  The source Host for each discarded messages is sent
         a type 9, subtype 0 message (Incomplete Transmission -
         Destination Host Tardy).

     c)  The IMP ready line is dropped and raised again.

     d)  The Host is sent 3 type 4 messages (NOP).

     e)  The Host is sent a type 10 message (IMP-Host Interface
         Reset).

     We propose that in addition the IMP declare the Host dead to the
network.  Upon receipt of the next bit from the Host, the IMP will
declare the Host alive and begin the 30-second delay while the
information that the Host is now alive is propagated throughout the
network.
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