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+Network Working Group                                          B. Cosell
+Request for Comments: 209                        Bolt Beranek and Newman
+NIC: 7187                                                 13 August 1971
+
+
+                     HOST/IMP Interface Documentation
+
+   On 25 June 1971 we put a new version of the IMP system, Version 2500,
+   up in the network.  Among other changes, the new system corrected the
+   problem of Hosts receiving spurious "Destination Dead" [Type 7]
+   messages.  The correction involved making the IMP refuse to accept
+   any messages from its Host for about 30 seconds after the Host
+   indicated its intention to come alive.  This delay has caused trouble
+   to a number of NCPs because they had imposed an upper limit on the
+   time an IMP may take to accept a message, and when that time was
+   exceeded the IMP was declared inoperative and the NCP shut itself
+   down, leaving the site personnel annoyed and perplexed.
+
+   We're sorry to have caused so much trouble, but we thought that the
+   new version modified no advertised property of the system.  To
+   prevent misunderstandings of this type in the future, the IMP/Host
+   interface should be well documented, well understood and invariant.
+   The only way to set up and maintain such an interface is to presume
+   that BBN Report No. 1822 is complete and correct.  We understand that
+   it is neither, but operating on the basis of this assumption is the
+   best way to force 1822 to improve.  In our view, the Hosts should
+   proceed under the constraint that anything not specifically
+   guaranteed by 1822 does not exist and should not be used.  If there
+   are problems using this approach, please don't "code around" the
+   problem or treat your IMP as a "black box" and extrapolate its
+   characteristics from a series of experiments.  Instead, send your
+   comments and problems to the NCC at BBN, and we will fix the IMP
+   system, or 1822, whichever is necessary.
+
+   We are, at the moment, working n an update to 1822.  It will reflect
+   the deletion of the "cease" mechanism [as per RFC #107].
+
+   We are also considering augmenting the IMP/Host protocol with a
+   mechanism to allow a Host to detect a change in the "version" of the
+   IMP system, and to provide the Hosts with status information as to
+   who is alive in the net.
+
+
+         [ This RFC was put into machine readable form for entry ]
+             [ into the online RFC archives by Ryan Kato 6/01]
+
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+Cosell                                                          [Page 1]
+