Re: [AVTCORE] Mirja Kühlewind's Discuss on draft-ietf-avtcore-rtp-circuit-breakers-15: (with DISCUSS and COMMENT)

"Mirja Kuehlewind (IETF)" <ietf@kuehlewind.net> Tue, 10 May 2016 15:18 UTC

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From: "Mirja Kuehlewind (IETF)" <ietf@kuehlewind.net>
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Date: Tue, 10 May 2016 11:18:41 -0400
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To: Varun Singh <varun@callstats.io>
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Cc: avtcore-chairs@ietf.org, Magnus Westerlund <magnus.westerlund@ericsson.com>, draft-ietf-avtcore-rtp-circuit-breakers@ietf.org, avt@ietf.org, The IESG <iesg@ietf.org>, Colin Perkins <csp@csperkins.org>
Subject: Re: [AVTCORE] =?utf-8?q?Mirja_K=C3=BChlewind=27s_Discuss_on_draft-iet?= =?utf-8?q?f-avtcore-rtp-circuit-breakers-15=3A_=28with_DISCUSS_and_COMMEN?= =?utf-8?q?T=29?=
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Hi Varun,

I would say that parts of tsvg-circuit-breaker are so general that they also apply for this doc. 

However, in any case my point it that (while congestion control may or may not threat ECN-CE marks similar as loss), circuit breaker MUST NOT do that because only ECN-CE marks without loss clearly shows that everything is working right and there is not need for a circuit breaker to react. That is really important to get right!

Mirja


> Am 05.05.2016 um 16:43 schrieb Varun Singh <varun@callstats.io>io>:
> 
> Hi Mirja,
> 
> see inline.
> 
> Regards,
> Varun
> 
> On Wed, May 4, 2016 at 1:52 PM, Mirja Kuehlewind (IETF)
> <ietf@kuehlewind.net> wrote:
>> 
>>> Not sure I agree. The circuit breaker needs to treat lost and CE-marked packets the same as a congestion control algorithm, and the ECN specifications say that the response to CE marks needs to be the same as the response to loss.
>> 
>> I disagree, given the explanation above that ECN-CE marking indicate that all traffic have been transmitted correctly. It’s only an input for a control mechanism/loop and should not be used as input for a circuit breaker, at least not the same way as loss it used.
>> 
>> You could calculate the loss and ecn ratio separately and e.g. lower the k value if there is also a high level of CE marks. However, only CE marks without loss should not trigger a circuit breaker. This is also what ietf-tsvwg-circuit-breaker says:
>> "If Explicit
>>      Congestion Notification (ECN) is enabled [RFC3168], an egress
>>      meter MAY also count the number of ECN congestion marks/event per
>>      measurement interval, but even if ECN is used, loss MUST still be
>>      measured, since this better reflects the impact of persistent
>>      congestion.“
>> 
> 
> First, I believe the draft assumes ECN-CE should be considered as lost
> packets in addition to the lost packets.
> 
> Second, I am not sure the text from the tsvg-circuit-breaker applies
> in this case. Because the calculation in section 4.3 takes place at
> the sender. And the  sender receives the ECN-CE counter from
> the receiver in an RTCP XR report (RFC6679).
> 
> In RFC6679: the receiver reports the cumulative lost packets and
> cumulative ECN counters. So at the sender, there may not be
> sufficient information to say if the ECN-CE packets were
> accompanied by loss or not.
> 
> 
>>> 
>>>> ----------------------------------------------------------------------
>>>> COMMENT:
>>>> ----------------------------------------------------------------------
>>>> 
>>>> Few more minor comments:
>>>> 
>>>> 1) reference [I-D.ietf-tsvwg-circuit-breaker] should be normative
>>> 
>>> This was discussed in relation to Ben’s AD review. I think you can implement the RTP circuit breaker without reading the TSV draft, so informative seems correct. However, I don’t much care either way.
>> 
>> I will double-check this; was not aware of any previous discussions here. I still think it should be normative...
>> 
>>> 
>>>> 2) How is the loss rate in 4.3 calculated if some (but no all) RR are
>>>> lost?
>>> 
>>> There’s no obvious way for the receiver of the RRs to know that some were lost, so the calculation will proceed as if the reporting interval was longer.
>> 
>> Hm… what’s about using the (difference of )total number of losses instead?
>> 
> 
> As Colin indicated there is no way to know that the report was lost.
> 
> In (RFC3550), the fraction lost is defined to be the number of packets
> lost divided by the number of packets expected since the last report.
> And the formula in Section 4.3 does not rely on all receiving all
> reports. Furthermore, the RTT and fraction lost plugged into the
> formula are based on the same report.
> 
> Is the concern that the fraction lost may not be sufficiently
> indicative if the preceding RTCP RR was lost?
> 
> 
> 
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