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Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on ACKing ACKs with good cause (was: Possible error in accurate-ecn)

Praveen Balasubramanian <pravb@microsoft.com> Tue, 30 March 2021 18:51 UTC

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From: Praveen Balasubramanian <pravb@microsoft.com>
To: "chromatix99@gmail.com" <chromatix99@gmail.com>, "vidhi_goel@apple.com" <vidhi_goel@apple.com>
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Thread-Topic: [tcpm] [EXTERNAL] Re: Seeking WG opinions on ACKing ACKs with good cause (was: Possible error in accurate-ecn)
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Subject: Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on ACKing ACKs with good cause (was: Possible error in accurate-ecn)
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This is uncharted territory for congestion control. To date ACK packets have not been considered part of bytes in flight in TCP and QUIC. Congestion control itself impeding the feedback loop has consequences.

-----Original Message-----
From: Jonathan Morton <chromatix99@gmail.com> 
Sent: Tuesday, March 30, 2021 11:10 AM
To: Vidhi Goel <vidhi_goel@apple.com>
Cc: Praveen Balasubramanian <pravb@microsoft.com>; tcpm@ietf.org; nishida@sfc.wide.ad.jp; ietf@kuehlewind.net
Subject: Re: [tcpm] [EXTERNAL] Re: Seeking WG opinions on ACKing ACKs with good cause (was: Possible error in accurate-ecn)

> On 30 Mar, 2021, at 9:04 pm, Vidhi Goel <vidhi_goel=40apple.com@dmarc.ietf.org> wrote:
> 
>> How is the sender of the pure ACKs supposed to react upon receiving the feedback that ACKs themselves are causing congestion? Option B conveys this additional information but how is it supposed to be used? Can a pure receiver get into such a state and does it need to start throttling ACKs?
> 
> This was discussed briefly during the WG. The receiver will act by reducing the ssthresh and cwnd (if it is not already minimum). This wouldn’t impact the receiver’s ACKing, but it would have an impact when the receiver starts sending data.

The information could also be used to tune the Delayed Ack ratio, via (for example) Gomez' Ack Rate Request option.  That *would* have a congestion control effect on ack traffic.

 - Jonathan Morton

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