[e2e] Lost Layer?

Detlef Bosau detlef.bosau at web.de
Tue Feb 11 14:05:08 PST 2014

Am 11.02.2014 22:11, schrieb Joe Touch:
> On 2/11/2014 12:49 PM, Detlef Bosau wrote:
>> However, don't you agree that the basic system model in a TCP connection
>> is sill
>> Endpoint 1          -------------------- Pipe
>> --------------------------             Endpoint 2
> Yes, TCP presents a service of a reliable ordered bytestream between
> two endpoints.

That's the view from the application layer. I refer to the "pipe"
between the sockets.
>> We can well discuss the nature of the "Pipe", particularly the
>> correspondence CWND = Pipe's capacity.
> The 'capacity' of the pipe is an ill-defined term; it can mean:
>     - amount of sustained throughput through the pipe

that would be even more ill-defined. In the general (best effort) case,
there is no such thing like sustained throughput in the pipe.

>     - amount of data stored temporarily inside the
>     pipe as a side-effect of efficient communication
>         i.e., the average sustained throughput *
>         average E2E delay

this ill-definition is not that obvious ;-)

The misconception is that the "pipe" is taken as some kind of
homogeneous storage here where data temporarily "resides".
The hard problem is: Buffers (at nodes) can accept data and can _keep_
data. Links can generally _carry_ data, they cannot _keep_ data.
(Some weeks ago, I compared this to the situation of passengers on an
escalator. You will of course object that an escalator well may keep
passengers, o.k., let's modify the model and remove the emergency brake ;-))
(Or take a vertical water pipe. Yes, it may carry water and so there is
an amount of water in the pipe. However, it cannot keep the water. When
there is water in the pipe, it will eventually run of the pipe and don't
stay there.)

In this respect, the "storage" along the pipe is heterogeneous.
> CWND-MAX is usually set to the latter of these as one optimization,
> but there are others for which that might not be true.
> CWND is a part of currently specified TCP congestion control, but not
> the only means nor necessarily the optimium.

The question is whether it provides a useful abstraction, whatever will
be modelled by CWND. (A quantitative interpretation of CWND
is always difficult, or should I say: most likely wrong. The qualitative
interpretation, which is actually being used in the congavoid-paper, is
certainly more appropriate.)


Detlef Bosau
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