Re: [tcpm] WGLC: NCR draft-ietf-tcpm-tcp-dcr-06.txt ENDS 27 Jan 2006

On Mon, Jan 23, 2006 at 09:52:50AM -0500, Mark Allman wrote:
> 
> I am not sure there is a "requirement for fairness".  It's certainly a
> little bit of a fuzzy issue in a case like NCR tries to combat.  For
> instance, say over a given path two flows can get X bps given the
> current congestion and reordering state of the network.  But, if NCR is
> used on one flow and that flow gets 3*X bps and the first flow still
> gets X bps is that unfair?  For sure the flows have different rates.
> So, using something like Jain's fairness index (i.e., just measuring the
> spread of rates amongst all flows) might say this is unfair.  But, to
> me, neither flow is sending at an inappropriate rate for the given
> context.  I.e., the flow sending at X bps cannot send any faster and the
> flow sending at 3*X bps is just using what is available.
> 
> (Another example of this would be one flow using an advertised window of
> Y and another using 5*Y and obtaining better performance.)
> 
> Of course, it gets thornier if the flow that gets 3*X bps ends up
> stealing some of those bps from the other flow such that it sends at N*X
> (for some N < 1).  Now, the reordering robust flow is not just using
> spare capacity, but also getting some of its performance boost at the
> expense of the non-NCR flow.  We might say that is unfair and say we
> have to try to design something into NCR to avoid this - or, largely
> mitigate it.  On the other hand, we might decide that the disadvantage
> is not great and that this is part of the natural evolutionary path of
> TCP, whereby older TCPs don't compete as well as newer TCPs and
> therefore, we view this as an incentive to upgrade.  This would be a bit
> of engineering judgment after we have a solid set of experiments to look
> over, IMO.

More compactly, the question is how closely NCR can achieve a Pareto
optimum, or max-min fairness, in competition with other types of flows.

-Wes

-- 
Wesley M. Eddy
Verizon Federal Network Systems