[conex] Crediting [was: Re: Review of draft-ietf-conex-abstract-mech-06]

Mirja Kühlewind <mirja.kuehlewind@ikr.uni-stuttgart.de> Sun, 09 June 2013 15:03 UTC

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From: Mirja =?iso-8859-1?q?K=FChlewind?= <mirja.kuehlewind@ikr.uni-stuttgart.de>
To: Bob Briscoe <bob.briscoe@bt.com>
Date: Sun, 9 Jun 2013 17:03:33 +0200
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Subject: [conex] Crediting [was: Re: Review of draft-ietf-conex-abstract-mech-06]
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Hi,

so back on this one:

> >9) Section 5.5.1 introdues the credit concept. Not sure if the meaning of
> >credits is well enough specified here. My personal option is that credits
> >should somehow get invalid (at some point in time). This is left open in
> > the current text.
> >
> >
> >I think we need to agree before we can talk
> >about writing down what we agree...
> >
> >
> >I think that abstract-mech needs to embrace
> >*both*, explicitly if not implicitly.  I need to
> >think about this some more, but I suspect that
> >it means we have unnecessarily over constrained audit here.
>
> [BB]: We need to allow multiple experiments at
> this experimental stage. But ultimately, sources
> will need to unambiuously know what Credit means,
> so they know how much to introduce and when.

Yes, but we need to propose a mechanism when to send credits for the TCP mod 
draft and this means we need to have a common understanding how to handle 
credits in the endsystem and the audit. I guess that's what standards are 
good for. We might need a separate document for this. Not sure we are able to 
agree on this right now. As an alternative, I could also add some text in the 
TCP mod draft that the crediting is an open issue for experiments...?

>
> >Rather than thinking of Credit expiring after a
> >time, one can think of the combination of recent
> >Re-Echo signals and earlier Credit signals
> >keeping the Credit state fresh within a flow. As
> >long as you've sent Credit before a round of
> >congestion, then if you send Re-Echo afterwards
> >the Auditor can switch it round as if you sent
> >the Re-Echo before and the Credit after.

I don't think this would change anything. Maybe make it even worse. As you 
could also just send credit instead of ConEx marks and moreover there is 
still no incentive to send further marks when you have build up a large 
number of credits during Slow Start.

> >
> >So, when the Auditor holds Credit, it allows up
> >to that amount of Re-Echo to be considered as
> >having been sent before the congestion, rather
> >than after. Then, as it switches the Re-Echoes
> >back in time, it switches the Credits forward, so they always stay recent.
> >
> >Credit is primarily a mechanism to ensure the
> >sender has to suffer some cost before it is
> >trusted to pay back some cost. Credit doesn't
> >need to degrade over time if the cost to the
> >sender of introducing credit doesn't degrade over time.
> >
> >Does this move us forward, or do you still
> >disagree? If so, I suggest a new thread would be useful.

I have two concerns:
1) As mentioned above if a sender has sent a large number of credits during 
Slow Start and causes only few congestion during the rest of the transmission 
(as today's TCP usually does), there is no incentive to send further ConEx 
marks at all (neither credits nor loss/ECN ConEx marks).
2) When sufficient markings has been sent during Slow Start, no further 
credits should be needed. But if the audit for any reason will loose state 
(e.g. because of memory restriction or a new audit is used due to rerouting), 
the sender will still not send new credits as will and thus will cause the 
audit penalize the flow eventhough the sender did do nothing wrong.

> >
> >
> >This is probably correct, but I really don't think it belongs in A-M.

We might need an own document but there might also be some additional 
requirements that should be added to this document.

>
> [BB]: I don't think it should either. This is a
> discussion with Mirja, rather than a proposal for text.