[ih] why did CC happen at all?

[Detlef Bosau]

Am 31.08.2014 um 08:24 schrieb Vint Cerf:
> 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").
>

That's what I referred to.


> 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 IMP.

That's what I missed. And this point is important.

>
> 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.

And that's my concern. The only compelling reasons for this seem to me:
a) A concern about possible head of line blocking, b) a lack of
computing power at the nodes.

As far as I see, both problems can be overcome.
>
> 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.

However, this approach treated the "line" between sender and receiver,
may I say it extremely dense, as a "queueing system where Little's law
applies".
(Which is a bit a contradiction in terms, because EITHER Little's law
applies to a system EXCLUSIVE OR a system suffers from drops.)

However, one could take this as an approximation. (Which is sometimes
better, sometimes worse. As always in engineering. Basically, the world
is a perfect one  - unfortunately, what we actually have is only an
approximation.)
>
> 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.

That's the very trade off. And I don't advocate circuit switching as an
alternative. The strong shortcoming in circuit switching is the
"fragmentation loss" of resources: Resources are assigned to users who
don't really use them. What I have in mind is basically a flow layer
with flow control (in a sense, Ford and Iyengar had something similar in
mind in 2009) and - to exploit the flexibility of a packet switched
network - an on demand scheduling of resources.

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


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Detlef Bosau
Galileistraße 30   
70565 Stuttgart                            Tel.:   +49 711 5208031
                                           mobile: +49 172 6819937
                                           skype:     detlef.bosau
                                           ICQ:          566129673
detlef.bosau at web.de                     http://www.detlef-bosau.de

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