Network Working Group J. Kreznar
Request for Comments: 17 SDC
Category: Informational 27 August 1969
Some Questions Re: HOST-IMP Protocol
1. 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) Busy HOST (many jobs)
ii) Much local I/O and/or CPU time between messages - is it
that, if a HOST's user fails 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 don'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 non-network 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
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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
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.
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