[e2e] Historical question: Link layer flow control / silent discard

Bob Braden braden at isi.edu
Fri May 24 12:22:40 PDT 2013

On 5/24/2013 10:48 AM, Noel Chiappa wrote:
> Source quench turned out not to work (for reasons I don't recall clearly any
> more -


Jon Postel really wanted Source Quench to work. Walt Prue under Jon 
Postel's direction
  made an exhaustive study of possible source quench
algorithms  in their RFC 1016,  July 1987, with the hopeful title:

	"Something a Host Could Do with Source Quench:
         The Source Quench Introduced Delay (SQuID)

The following sentence from that document says it all;

   "All of our algorithms oscillate, some worse than others."

Dave Clark is fond of saying that the distinctive property of
research is that it is allowed to fail. Source Quench was a
part of the TCP/IP development that failed. Another indication
of its failure is that a SQ may go to a host that is not causing
the congestion.

> Google will probably turn some things up). Possibly we just didn't
> understand enough about congestion control at that early stage to make it
> work 'well'.
> I don't recall when we stopped trying to use it - I think it was a little
> later than the cutover, actually.

AFAIK,no one ever used Source Quench, because of its apparent
defects. Sending a packet when there is an overload must be a
losing strategy!

> We then ran without any congestion control at all for a while, and that
> caused massive problems. Finally Van Jacobsen turned up and saved the day.
> His approach turned out to only need packet drops as a congestion signal,
> so SQ was not needed any more (and IIRC it has been deprecated).

Yes, although we (and I think you were involved) did not have the 
courage to deprecate SQ
when we wrote  Host Requirements, a couple of years after 1016. I 
suspect SQ was finally
killed in Router Requirements, RFC 1812, in 1995.

Bob Braden

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