Re: [tsvwg] Switch testing at 25G with ECN --SCE Draft

"Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net> Sat, 10 August 2019 00:03 UTC

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From: "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net>
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To: Greg White <g.white@CableLabs.com>
Date: Fri, 9 Aug 2019 17:02:40 -0700 (PDT)
CC: Jonathan Morton <chromatix99@gmail.com>, "Scaglione, Giuseppe" <giuseppe.scaglione@hpe.com>, "rgrimes@freebsd.org" <rgrimes@freebsd.org>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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Subject: Re: [tsvwg] Switch testing at 25G with ECN --SCE Draft
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> Right.  Per the SCE method, the switch would mark ECT(1) using a ramp, and then when the ramp would exceed 100% marking, it would change to using CE.  
> 
> The implementation discussed here marks CE using a ramp, and when the ramp would exceed 100% marking, it changes to packet drop.  This, as you said, is simply RFC3168 ECN marking, not SCE ECN marking.

Purely due to a choice to an expedient experiment.  This is not "simply RFC3168 ECN marking" which would occur at the point the switch was about to start dropping, it is "linear ramp response proportionaly to queue depth SCE marking" using the CE bit pattern.  I can not get into the details as to why this was done this way.

RFC3168 ECN marking is already in the switch, the switch was modified to behave in a different manner, using the
RFC3168 CE bits.  This different manner was turned OFF to do the dctcp tests and turned ON to do the dctcp-sce tests.

> 
> I was just trying to set the record straight.  There was a claim made that a switch vendor had implemented SCE-style packet marking in hardware at 25Gbps, which wasn't accurate.

It was and still is accurate, the only inaccuracy is that the switch emmits CE rather than SCE, but the response form of the CE bits are driven by an SCE algorithm NOT the single threshold data center CE marking normally done by the switch.

> -Greg
> 
> 
> 
> ?On 8/9/19, 2:58 PM, "Jonathan Morton" <chromatix99@gmail.com> wrote:
> 
>     > On 9 Aug, 2019, at 11:44 pm, Scaglione, Giuseppe <giuseppe.scaglione@hpe.com> wrote:
>     > 
>     >>> Just to be super clear, this isn't a hardware implementation of SCE running at 25Gbps.
>     > 
>     > I am not sure I follow. The Test setup section of the paper clearly describes the hardware used -- severs, switch, cables. 
>     > What I cannot disclose at this point is the exact model and characteristics of the HPE Aruba Switch used. Yet, it is a "real" Ethernet Switch, providing 25Gbps connectivity, configured to do bridging across the four ports and implementing RFC3168 with the ECN remarking configuration described in my previous email and on the paper.
>     
>     I think the issue is with the way the switch itself marks with CE, not ECT(1).  It's a limitation I think is worth acknowledging and, hopefully, finding a way to remove.

Yes, that is the one non-SCE aspect to the implementation, use of CE instead of ECT(1), BUT the driving algorithm to create these marks is NOT classic RFC3168, but is linear ramp draft-morton-tsvwg-sce-00.

>      - Jonathan Morton
-- 
Rod Grimes                                                 rgrimes@freebsd.org