netdev - Re: [PATCH v5 net-next 09/15] tcp: accecn: AccECN option

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Message-ID: <eaa2db1a-dc4f-55f6-c7ef-573acf46c214@kernel.org>
Date: Tue, 6 May 2025 20:49:41 +0300 (EEST)
From: Ilpo Järvinen <ij@...nel.org>
To: Paolo Abeni <pabeni@...hat.com>
cc: chia-yu.chang@...ia-bell-labs.com, horms@...nel.org, dsahern@...nel.org, 
    kuniyu@...zon.com, bpf@...r.kernel.org, netdev@...r.kernel.org, 
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Subject: Re: [PATCH v5 net-next 09/15] tcp: accecn: AccECN option

On Tue, 6 May 2025, Ilpo Järvinen wrote:
> On Tue, 29 Apr 2025, Paolo Abeni wrote:
> > On 4/22/25 5:35 PM, chia-yu.chang@...ia-bell-labs.com wrote:

> > > @@ -1117,6 +1235,17 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
> > >  		opts->num_sack_blocks = 0;
> > >  	}
> > >  
> > > +	if (tcp_ecn_mode_accecn(tp) &&
> > > +	    sock_net(sk)->ipv4.sysctl_tcp_ecn_option) {
> > > +		int saving = opts->num_sack_blocks > 0 ? 2 : 0;
> > > +		int remaining = MAX_TCP_OPTION_SPACE - size;
> > 
> > AFACS the above means tcp_options_fit_accecn() must clear any already
> > set options, but apparently it does not do so. Have you tested with
> > something adding largish options like mptcp?
> 
> This "fitting" for AccEcn option is not to make room for the option but to 
> check if AccECN option fits and in what length, and how it can take 
> advantage of some nop bytes when available to save option space.

A minor correction. SACK blocks will naturally fill the entire option 
space if there are enough holes which would "starve" AccECN from using 
option space during loss recovery. Thus, AccECN option is allowed allowed
grab some of that space from SACK. There's redundancy in SACK blocks 
anyway so it shouldn't usually impact SACK signal much.

-- 
 i.