Re: [re-ECN] implementations

[Matt Mathis <[e-mail]>]

Please explain "We’ve found packet loss to be unreliable means of measuring
congestion due to TCP rate control."  How do you define and measure
congestion?

Thanks,
--MM--
-------------------------------------------
Matt Mathis      http://staff.psc.edu/mathis
Work:412.268.3319   Home/Cell:412.654.7529
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Evil is defined by mortals who think they know
"The Truth" and use force to apply it to others.


On Sun, Oct 18, 2009 at 11:45 AM, Don Bowman <[e-mail]> wrote:

>  From: Leslie Daigle
>
> >
>
> > Hi,
>
> >
>
> > Following up an off-list suggestion -- we've heard passing references
>
> > to
>
> > re-ecn implementations, but it would probably be good to be able to be
>
> > a
>
> > bit more authoritative.
>
> >
>
> > If you've done such implementation work -- I'm happy to collect
>
> > references here and at least have a slide up in Hiroshima to list
>
> > them/provide further pointers for people.
>
>
>
> Sandvine has an 'implementation in spirit' we call fairshare traffic
> management.
>
>
>
> We detect the congestion in the forward path (implicitly, not explicitly),
>
> then act to reduce the priority of the users or applications which are
> disproportionately
>
> causing it.
>
>
>
> Although specific implementations @ customers & access technologies vary,
> those
>
> basic principles are always maintained. The most common implementation is
>
> to enforce a 'volume accounting fairness' per subscriber, enforced only
>
> during times of congestion. Variation on that theme include
> sub-prioritisation
>
> by application (so that the overall envelope of each user is balanced
>
> against each other, and then within that envelope their applications are
> balanced).
>
>
>
> The most common implementation is to reduce scheduling priority, without
>
> placing any fixed cap or capacity reduction. This has the affect in a
> network
>
> which is not dramatically under-provisioned of shifting the 'cost' to the
> heavier
>
> users, where the 'cost' is related to jitter, latency, and loss.
>
>
>
> This is deployed in a large (well, large to us) number of residential
> consumer
>
> Access networks world-wide.
>
>
>
> I understand it differs from the proposal for ‘ConEx’, i think that the
> explicit
>
> Exposure of congestion will be beneficial for us.
>
>
>
> I'd be happy to talk in much more detail about this in japan.
>
>
>
> Apologies for the HTML email and the image, but the below is an example of
> the impact.
>
> The image shows a cumulative distribution of the round-trip latency through
> the
>
> Access network for 2 classes of user. The ‘PBE’ is the
> priority-best-effort, the
>
> Users who are not assigned any ‘cost’ for causing congestion.
>
> The ‘BE’ is the best-effort group, those who are assigned the cost of
> congestion.
>
> In this particular network, the packet loss of both groups was negligible
> (although
>
> We’ve found packet loss to be unreliable means of measuring congestion
>
> due to TCP rate control).
>
> As you can see from the chart, the PBE group enjoys a situation where
> almost
>
> 100% of their flows have the same latency. The BE group has a situation
>
> where there is more significant jitter and variation in latency.
>
>
>
>
>
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>