[e2e] link between Kelly's control and TCP's AIMD

Alex Cannara cannara at attglobal.net
Sat Feb 19 12:24:42 PST 2005

Right.  I was objecting to the use of "fluid" as a common term, because even 
conservation of fluid elements isn't obeyed by a protocol like TCP, or its 
sublayers.  And, a good model has to be aware of the effects of state changes 
within subflows, in order to determine, even statistically, what each end will 
do.  This particularly violates the fluid model (linear or not) when we 
realize the vast difference in how feedback to the originator can occur from 
the receiver.  The bottom line in modelling is always to respect the 
engineer's motto:  "The electrons (or whatever) know what they're doing.  It's 
up to us to figure that out."


Vishal Misra wrote:

> Cannara wrote:
>> I don't know "ignored" -- some friends (NetPredict) have a patent on 
>> just such
>> statistical analyses applied to TCP performance products.  It's the
>> nonlinearity that makes any linear differential modelling grossly 
>> approximate.
>> We have to remember some fundamentals -- at least that fluids are 
>> variously
>> compressible & reorderable, unlike packets, and loss of molecules is 
>> not an
>> option.  :]
> "fluid" modeling doesn't imply an application of Hydraulics or 
> Bernoulloi's laws for protocol analysis. The "fluid" simply implies that 
> the quantity that is being analyzed is continuous, and not discrete. 
> Fluid models of TCP themselves come in two flavors
>     - one that analyze some mean value, and hence are intrinsically 
> continuous in nature (e.g., the Sigcomm 2000 paper mentioned earlier in 
> the thread), and,
>     - the other kind where the "fluid limit" or continuous property is 
> obtained from a scaling of some parameters to infinity, e.g., the number 
> of flows (the Performance 2002 paper mentioned in the same posting).
> The differential equations that are obtained are non-linear, simply 
> being "fluid" does not constrain them to linearity. Additionally, 
> stochastic differential equation based "fluid" models of TCP allow jump 
> discontinuities.
> -Vishal

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