[RFC,24/28] tcp: try to fit AccECN option with SACK

From: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>

As SACK blocks tend to eat all option space when there are
many holes, it is useful to compromise on sending many SACK
blocks in every ACK and try to fit AccECN option there
by reduction the number of SACK blocks. But never go below
two SACK blocks because of AccECN option.

As AccECN option is often not put to every ACK, the space
hijack is usually only temporary.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
---
 net/ipv4/tcp_output.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

On 3/18/20 2:43 AM, Ilpo Järvinen wrote:
> From: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
> 
> As SACK blocks tend to eat all option space when there are
> many holes, it is useful to compromise on sending many SACK
> blocks in every ACK and try to fit AccECN option there
> by reduction the number of SACK blocks. But never go below
> two SACK blocks because of AccECN option.
> 
> As AccECN option is often not put to every ACK, the space
> hijack is usually only temporary.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
> ---
>  net/ipv4/tcp_output.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 4cc590a47f43..0aec2c57a9cc 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -756,6 +756,21 @@ static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
>  	if (opts->num_ecn_bytes < required)
>  		return 0;

Have you tested this patch ?

(You forgot to remove the prior 2 lines)

>  
> +	if (opts->num_ecn_bytes < required) {
> +		if (opts->num_sack_blocks > 2) {
> +			/* Try to fit the option by removing one SACK block */
> +			opts->num_sack_blocks--;
> +			size = tcp_options_fit_accecn(opts, required,
> +						      remaining + TCPOLEN_SACK_PERBLOCK,
> +						      max_combine_saving);
> +			if (opts->options & OPTION_ACCECN)
> +				return size - TCPOLEN_SACK_PERBLOCK;
> +
> +			opts->num_sack_blocks++;
> +		}
> +		return 0;
> +	}
> +
>  	opts->options |= OPTION_ACCECN;
>  	return size;
>  }
>

On Wed, 18 Mar 2020, Eric Dumazet wrote:

> 
> 
> On 3/18/20 2:43 AM, Ilpo Järvinen wrote:
> > From: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
> > 
> > As SACK blocks tend to eat all option space when there are
> > many holes, it is useful to compromise on sending many SACK
> > blocks in every ACK and try to fit AccECN option there
> > by reduction the number of SACK blocks. But never go below
> > two SACK blocks because of AccECN option.
> > 
> > As AccECN option is often not put to every ACK, the space
> > hijack is usually only temporary.
> > 
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
> > ---
> >  net/ipv4/tcp_output.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index 4cc590a47f43..0aec2c57a9cc 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -756,6 +756,21 @@ static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
> >  	if (opts->num_ecn_bytes < required)
> >  		return 0;
> 
> Have you tested this patch ?
> 
> (You forgot to remove the prior 2 lines)
> 
> >  
> > +	if (opts->num_ecn_bytes < required) {

Yes and no. There was no unit test for this particular condition but
I added a few now (with and w/o timestamps). I also managed to find and 
fix a byte-order related bug related to non-fullsized option while making 
those tests.

(I didn't actually forget to remove it. I managed to add the problem 
during a botched conflict merge when I reorganized some of the code.)

Thanks for taking a look.