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Network Working Group R. Kahn Request for Comments: 17a Bolt Beranek and Newman Inc

                                                           August 1969

              Re: Some Questions Re: HOST-IMP Protocol

 THE FOLLOWING COMMENTS ARE IN RESPONSE TO JOHN KREZNAR'S QUESTIONS
 WHICH WERE RAISED IN NWG:- 17

 The deletion of a link entry from an IMP's link table will, in
 general, have no effect upon a Host transmission (or reception) at
 that IMP's site.  Let us distinguish between non-use of a link in-
 between messages and non-use of a link due to Host program delays in
 the middle of transmitting or receiving a message.  When the Host
 transmits a message on a link for which an entry is not in the link
 table, one will simply be inserted there.  There is no need for
 "dummy" Host messages to keep a link "open" since a link is
 effectively always open.  Only if the link table becomes full
 immediately after an entry is deleted (a situation we do not expect
 to occur) is there a possibility of resulting delay.

 Arbitrary delays introduced by Host programs are also not
 inconsistent with the link entry deletion procedure.  A link is
 blocked when the first access of the link table is made during
 transmission from the source IMP and is unblocked when the RFNM
 returns.  Only non-blocked transmit link entries are deleted after 30
 seconds of disuse.  The statement on page 23 referencing arbitrary
 delays was only intended to have hardware implications insofar as the
 Host/IMP interface is designed to transfer bits asynchronously
 between the Host and the IMP.

 A RFNM is returned from the destination IMP to the source IMP when a
 message reaches the head of the destination IMP's output queue to the
 Host (i.e. just before a message is sent to the Host).  If a
 destination IMP cannot then deliver that full message to the Host, at
 most one more message may possibly arrive at that IMP due to the
 premature release of the RFNM.  The new message will subsequently
 take its place at the end of the output queue to the Host thus
 guaranteeing the preservation of the proper message arrival sequence.

 The NOP message is a special control message which is available for
 use during initiation of communication between the Host and its IMP.
 The Host may, of course, decline to send NOP messages during this
 period, but the first received message after IMP startup or after the

Kahn [Page 1]  RFC 17a Re: Some Questions Re: HOST-IMP Protocol August 1969

 Host ready indicator has gone on, may be discarded by the IMP.  We do
 not require a Host to be prepared to repeat transmissions into the
 network.

 R.E. Kahn
 BOLT BERANEK AND NEWMAN INC.

       [ This RFC was put into machine readable form for entry ]
        [ into the online RFC archives by Marc Blanchett 3/00 ]

Kahn [Page 2]