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[ih] why did CC happen at all?

ARPANET used an overly constrained system called RFNM (request for next
message). The mechanism was used to reserve space at the destination IMP
("get a block" "got a block").

however it was possible to send multiple messages over different "links"
(logical term) and overload the network that way. It was also possible to
overload an intermediate IMP simply by sending traffic between pairs
(source/destination) that happened to pass through the same intermediate

The Internet protocols did not use these methods and except for the
"congestion encountered" signal, all flow control was end/to/end which
still raised the possibility of intermediate router congestion.

The TCP flow control was an attempt to adjust to signals from the receiver
and signals (dropped packet, congestion encountered) from intermediate
nodes. Packet loss was treated as a flow control signal leading to backoff
of the retransmission mechanism of TCP. Slow start was a  crude way of
sensing where the limits of capacity lay.

your claim that there is no congestion with "proper" implementation may
result in lower resource utilization. Circuit switching dedicates capacity
so there is no congestion, except for the failure to get a circuit ("all
circuits busy" is a congestion signal). But dedicating capacity removes the
implicit statistical multiplexing advantage of packet switching.


On Sat, Aug 30, 2014 at 12:25 PM, Detlef Bosau <detlef.bosau at web.de> wrote:

> I'm yet to understand the sitch from the ARPAnet to the Internet in
> 1983, however, if this happened that way, that an Internet host sent a
> message to its peer using the "message switching system" (may I call it
> that way?) in the ARPAnet, CC would be an "impossible fact".
> (Some German readers might enjoy this little text here:
> http://ingeb.org/Lieder/palmstre.html)
> In the ARPAnet, congestion was avoided by flow control - and in fact,
> actually, there is nothing like "congestion" when networks are
> implemented correctly.
> To my understanding, "congestion" is an excuse for missing (or botched)
> flow control.
> So, what was the scenario, VJ describes in the congavoid paper? Up to
> know, I always thought, the ARPAnet infrastructure was still in use,
> although adopted by the Internet protocol stack, but I thought, IP
> datagrams were sent like ARPAnet messages?
> Detlef
> --
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> Detlef Bosau
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