netdev - [PATCH v6 net-next 14/15] tcp: accecn: try to fit AccECN option with SACK

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Message-Id: <20250509211820.36880-15-chia-yu.chang@nokia-bell-labs.com>
Date: Fri,  9 May 2025 23:18:19 +0200
From: chia-yu.chang@...ia-bell-labs.com
To: horms@...nel.org,
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v6 net-next 14/15] tcp: accecn: try to fit AccECN option with SACK

From: Ilpo Järvinen <ij@...nel.org>

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 <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 net/ipv4/tcp_output.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index af619233a6f3..a53eae6892b5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -981,8 +981,21 @@ static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
 		opts->num_accecn_fields--;
 		size -= TCPOLEN_ACCECN_PERFIELD;
 	}
-	if (opts->num_accecn_fields < required)
+	if (opts->num_accecn_fields < 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;
-- 
2.34.1