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Re: adjusted_rtt

Lars Eggert <lars@eggert.org> Thu, 08 April 2021 08:15 UTC

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From: Lars Eggert <lars@eggert.org>
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Subject: Re: adjusted_rtt
Date: Thu, 08 Apr 2021 11:15:22 +0300
In-Reply-To: <32445F64-13D6-4999-AE61-91350D06F02B@fh-muenster.de>
Cc: IETF QUIC WG <quic@ietf.org>
To: Timo Völker <timo.voelker@fh-muenster.de>
References: <32445F64-13D6-4999-AE61-91350D06F02B@fh-muenster.de>
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I created https://github.com/quicwg/base-drafts/issues/4860 to track this.

Thanks,
Lars


> On 2021-4-7, at 20:22, Timo Völker <timo.voelker@fh-muenster.de> wrote:
> 
> Hi,
> 
> I noticed in a simulation with a constant RTT that the smoothed_rtt value includes the ack_delay. It looks like my implementation follows the recovery draft. By looking closer, I believe I found a tiny error in the draft.
> 
> Section 5.3 contains this sentence.
> 
> "MUST NOT subtract the acknowledgment delay from the RTT sample if the resulting value is smaller than the min_rtt."
> 
> I interpret this as do not subtract ack_delay from latest_rtt if
> 
>  latest_rtt - ack_delay < min_rtt
> 
> Or to say the inverse, substract ack_delay from latest_rtt if
> 
>  latest_rtt - ack_delay >= min_rtt
>  <==> latest_rtt >= min_rtt + ack_delay
>  <==> min_rtt + ack_delay <= latest_rtt
> 
> The pseudo code in Section 5.3 and in the appendix use a smaller instead of an equal or smaller in the if statement.
> 
>  if (min_rtt + ack_delay < latest_rtt):
> 
> It should be <= or did I miss something?
> 
> Timo

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