[ih] ARPANET uncontrolled packets (was: UDP Length Field?)

[Alex McKenzie]

 I was not an IMP programmer so I may have the details wrong, but as I recall an uncontrolled packet was treated just like any other packet, except:- if it ever arrived at an IMP that didn't have a buffer available it was ACKed but dropped, and- no RFNM was generated by the destination IMP.A host sending an uncontrolled packet could set the priority bit on or off; if priority was on the packet would go to the heads of queues just like any other packet.  It was retransmitted like a regular packet if it encountered errors on a circuit (to do otherwise would have been much more complicated).
If there were an uncontrolled packet resident in an IMP buffer and then that IMP ran out of buffers I don't believe that the IMP had a way of finding and deleting the uncontrolled packet to free up a buffer.  As I recall, that was the principal fear in allowing uncontrolled packets.
Cheers,Alex

    On Thursday, December 3, 2020, 3:37:16 PM EST, Jack Haverty via Internet-history <internet-history at elists.isoc.org> wrote:  
 
 Thanks, Steve, I vaguely remember that there were some highly restricted
experiments reluctantly authorized, but not anything widespread.   E.g.,
an obvious experiment would have been to use uncontrolled mode for all
the core gateways and other gateways attached to ARPANET, but AFAIK that
was never authorized.

Also, I don't remember ever knowing in detail exactly how "uncontrolled"
packets were actually handled by the IMP code, or what was experienced
in any experiments that were done.   E.g., did uncontrolled packets "go
to the front of the queue" bypassing normal traffic?  Did they interfere
with other traffic, e.g., by preempting buffer space, or just get
discarded when resources were unavailable?  In experiments, what were
latencies, drop rates, et al like for UDP datagrams?   How did the
ARPANET behave during such experiments?   Etc.

/Jack

On 12/2/20 4:33 PM, Stephen Casner wrote:
> On Wed, 2 Dec 2020, Jack Haverty via Internet-history wrote:
>
>> There was an "uncontrolled mode" of operation possible within the
>> ARPANET, which wouldallow a user computer to send packets bypassing all
>> of the reliability mechanisms.  That could be used to deliver datagram
>> Internet service.  However, the ARPANET managers (at DCA and BBN) were
>> extremely reluctant to permit hosts (e.g., gateways) to use that mode,
>> for fear that the uncontrolled traffic would crash the ARPANET.  I
>> recall being involved in several "discussions" about using ARPANET
>> uncontrolled mode for Internet experiments, but I don't remember any
>> permissions ever being granted.
> We definitely did get permission to use this for packet voice
> experiments between ISI and LL, for example.  What I recall is that
> initially we were required to arrange with BBN for a specific
> experiment time between specific hosts and the uncontrolled mode (Type
> 0, Subtype 3 -- BBN 1822 p. 3-17) would be enabled only for those
> hosts.  But later I believe our packet voice hosts were enabled all
> the time.
>
>                                                        -- Steve

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