Network Working Group						John E. Kreznar
RFC-17								SDC
								27 August 1969



Some Questions Re:  HOST-IMP Protocol


l.  Automatic deletion of links, as indicated in BBN 1822, page 11, seems
    bad:

    a)  Link use may be dependent upon human use of a time share terminal -
	indefinite time between messages.

    b)  Program using link may be slow due to:

	 i)  Bush HOST (many jobs)

	ii)  Much local I/O and/or CPU time between messages - is it that,
	     if a HOST's user failes to use a link for 15 seconds, the HOST
	     network program must generate a dummy message merely to keep
	     the link open?

2.  Steve Crocker, HOST Software, 1969 Apr 7, asks on page 2:  "Can a HOST,
    as opposed to its IMP, control RFNM's?"  BBN, Report No. 1837, 1969 Jul,
    says on page 2:  "The principal function of the (IMP) program...includes
    ...generating of RFNM's..."  What if an IMP generates an RFNM and then
    discovers it cannot, for some reason, complete timely delivery of the
    last received message to its HOST?  This seems especially pressing since
    I can't recall seeing anywhere an IMP constraint upon HOSTs that they
    must accept incoming messages within some specified maximum time.

3.  A HOST has to be prepared to repeat transmissions of a message into
    network (see, e.g., Page 17, BBN 1822) therefore why the special
    discardable NOP message (Page 12, BBN 1822).

4.  "Arbitrary delays," middle paragraph, page 23, BBN 1822, seems inconsistent
    with automatic link deletion questioned in 1 above.  Normally the times
    involved differ by many orders of magnitude but a high priority nonnetwork
    HOST responsibility could delay next bit for a long time.

	1.  Abhi Bhushan, Proj. MAC	10.  Sal Aranda, SDC
	2.  Steve Crocker, UCLA		11.  Jerry Cole,  "
	3.  Ron Stoughton, UCSB		12.  John Kreznar,"
	4.  Elmer Shapiro, SRI		13.  Dick Linde,  "
 	5.  Steve Carr, Utah		14.  Bob Long,    "
	6.  John Haefner, RAND		15.  Reg Martin,  "
	7.  Paul Rovner, LL		16.  Hal Sackman, "
	8.  Bob Kahn, BB&N		17.  C. Weissman, "
	9.  Larry Roberts, ARPA









     THE FOLLOWING COMMENTS ARE IN RESPONSE TO JOHN KREZNAR'S QUESTIONS
		    WHICH WERE RAISED  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 nonuse of a link in-between messages
and nonuse 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 delection 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 nonblocked 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
betweem 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 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.