netdev - Re: [PATCH net-next 22/44] tcp: accecn: AccECN option

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Message-ID: <20241015183216.6e0be5f2@kicinski-fedora-PC1C0HJN>
Date: Tue, 15 Oct 2024 18:32:21 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: chia-yu.chang@...ia-bell-labs.com
Cc: netdev@...r.kernel.org, ij@...nel.org, ncardwell@...gle.com,
 koen.de_schepper@...ia-bell-labs.com, g.white@...leLabs.com,
 ingemar.s.johansson@...csson.com, mirja.kuehlewind@...csson.com,
 cheshire@...le.com, rs.ietf@....at, Jason_Livingood@...cast.com,
 vidhi_goel@...le.com
Subject: Re: [PATCH net-next 22/44] tcp: accecn: AccECN option

On Tue, 15 Oct 2024 12:29:18 +0200 chia-yu.chang@...ia-bell-labs.com
wrote:
> From: Ilpo Järvinen <ij@...nel.org>
> 
> The Accurate ECN allows echoing back the sum of bytes for
> each IP ECN field value in the received packets using
> AccECN option. This change implements AccECN option tx & rx
> side processing without option send control related features
> that are added by a later change.
> 
> Based on specification:
>   https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt
> (Some features of the spec will be added in the later changes
> rather than in this one).
> 
> A full-length AccECN option is always attempted but if it does
> not fit, the minimum length is selected based on the counters
> that have changed since the last update. The AccECN option
> (with 24-bit fields) often ends in odd sizes so the option
> write code tries to take advantage of some nop used to pad
> the other TCP options.
> 
> The delivered_ecn_bytes pairs with received_ecn_bytes similar
> to how delivered_ce pairs with received_ce. In contrast to
> ACE field, however, the option is not always available to update
> delivered_ecn_bytes. For ACK w/o AccECN option, the delivered
> bytes calculated based on the cumulative ACK+SACK information
> are assigned to one of the counters using an estimation
> heuristic to select the most likely ECN byte counter. Any
> estimation error is corrected when the next AccECN option
> arrives. It may occur that the heuristic gets too confused
> when there are enough different byte counter deltas between
> ACKs with the AccECN option in which case the heuristic just
> gives up on updating the counters for a while.

net/ipv4/tcp_output.c:922:5: warning: symbol 'synack_ecn_bytes' was not declared. Should it be static?