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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]