Subject: Re: [Tsvwg] HEADS UP: consensus call on draft-floyd-tsvwg-cc-alt

From: Bob Briscoe <[email-masked]>
Date: Fri, 02 Mar 2007 19:15:11 +0000

Sally, Mark,

At 23:12 22/02/2007, Sally Floyd wrote:
>..
>I would leave bullet (2) about "Using Spare Capacity" in.  It was 
>motivated by Quick-Start, whose entire purpose is to
>use spare capacity.  It was also motivated by High-Speed TCP, which had as 
>a motivation using spare capacity.
>I think it is worthwhile that  say that successfully using space capacity 
>is a good thing, but it doesn't eliminate the
>requirement for fairness.

I've been trying to come up with text on spare capacity that better matches 
the absolutely necessary essence in the design of Quickstart, without 
over-constraining future controls. How about this - inspired by Mark's new 
text on bullet (1) (offline)?

============================================================================
(2) Using Spare Capacity.

If alternate congestion controllers send at high rates during times when 
they believe there is no congestion, they always run the risk that traffic 
in flight will cause congestion. Proposed congestion control mechanisms 
that aim to use spare capacity should be evaluated on how they minimise the 
risk of causing severe congestion that will not be seen until at least a 
round trip later.

Similar to point (1), alternate congestion controllers using spare capacity 
that risk a significantly negative impact on traffic using standard 
congestion control may be suspect and this aspect should be part of the 
community's decision making with regards to the suitability of the 
alternate congestion control mechanism.
============================================================================

I wanted to avoid the IETF making any judgement on how spare capacity 
should be divided up. This is a completely open question, that we should 
avoid. The only concern is what might happen if you're driving on an open 
road while snatching short bursts of shut-eye.

Bob

Bob Briscoe,                           Networks Research Centre, BT Research
B54/77 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK.    +44 1473 645196

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From: Erblichs <[email-masked]>
Date: Fri, 02 Mar 2007 11:46:05 -0800

> in flight will cause congestion.

Interesting. All inflight traffic generates some form of congestion, by
minimally increasing latencies, generating higher rates of interrupts,
and filling input queues. IMO, The question is whether the increased 
data rate reaches a threshold at a address that causes RED 
or tail-drop or some other congestion response OR removes a excess
range of capacity as to allow a non congestion aware flow to effect
congestion flows OR a combination of the above..

However, all the above assume somewhat consistent traffic flows where
over short periods of time this may be true or averaged over longer
time periods again may be true, but don't we have to assume that 
normal bursts of traffic will be generated for a number of reasons?
And thus a certain excess amount of capacity must be kept within a
environment to absorb hopefully most of these bursts without generating
repeated congestion responses.

Mitchell Erblich
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