    Network Working Group                      John M. McQuillan
    Request for Comments #410                  Bolt Beranek and Newman
    NIC #12402                                 10 November 1972
    Categories:  B-1

               Removal of the 30-Second Delay When Hosts Come Up
               -------------------------------------------------

         The IMP currently delays accepting input from a Host for 30
    seconds after the Host has come up.  This delay serves to allow
    the fact that the Host is up to propagate through the network.
    The fundamental problem is that a Host must not be permitted
    to communicate with a second Host until the second Host
    (actually its IMP) has been made aware that the first Host is
    up.  Otherwise, one Host may come up and send a "hello"
    message to another Host, whose reply is discarded by the IMP
    because it is for a dead destination.

         All this reasoning is based on a dead destination de-
    tection mechanism at the source IMP.  The 30-second delay is
    based on the worst-case propagation delay for routing information
    in the network, so that every potential source IMP can update
    its host up/down table.  There are several drawbacks to this
    scheme:

             1.  Hosts should not have to wait the worst-case time
                 of 30 seconds to send to Hosts at their IMP or
                 nearby in the network.

             2.  The operation of half-duplex interfaces is made
                 even more complicated because of the startup delay.

             3.  The timeout period of 30 seconds is really a
                 function of network topology and we would like to
                 be able to change it when necessary as the network
                 expands.

         We propose to eliminate the 30-second delay altogether.
    The IMP subnetwork will detect messages for a dead Host at the
    destination IMP instead of at the source IMP.  There is no delay

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    involved for an IMP to sense when its own Hosts come up, so
    that it can always make the correct decision about whether to
    give a message to one of its Hosts or to return a destination
    dead message to the source Host.  Under this new scheme, when-
    ever the IMP's ready line is up it is ready to accept input
    from its Hosts without delay.  Several comments on this change
    should be noted:

             1.  No change to Host software should be necessitated
                 by this change.  The Host may attempt to send a
                 message to the IMP as soon as it brings its
                 ready line up, or it may delay for a long time.  In
                 either case, the IMP will take the message.  As
                 before, as soon as the Host has brought up its
                 ready line, it must accept messages from the IMP.

             2.  The Hosts may wish to remove any delays _they_ have
                 programmed into their startup routines, since
                 such delays are no longer necessary.

             3.  Destination dead messages will be returned as
                 before with two differences.  There will be more
                 delay between the receipt of the message at the
                 IMP and the return of the dead destination message
                 because it must travel through the network.  For
                 the same reason, if many messages are sent to
                 dead Hosts, the dead destination messages may come
                 back out of order.

         The Host personnel responsible for the IMP software at
    each site should check that this proposed change will not ad-
    versely affect them.  If no adverse comments are received,
    this change will go into effect on Tuesday morning, December
    12 at the regular IMP software release time.

      [ This RFC was put into machine readable form for entry ]
      [ into the online RFC archives by BBN Corp. under the   ]
      [ direction of Alex McKenzie.                      1/97 ]

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