netdev - [PATCH v8 net-next 14/15] tcp: accecn: try to fit AccECN option with SACK

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Message-Id: <20250610125314.18557-15-chia-yu.chang@nokia-bell-labs.com>
Date: Tue, 10 Jun 2025 14:53:13 +0200
From: chia-yu.chang@...ia-bell-labs.com
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v8 net-next 14/15] tcp: accecn: try to fit AccECN option with SACK

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

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 attempt to fit the AccECN option
there by reducing the number of SACK blocks. However, it will
never go below two SACK blocks because of the AccECN option.

As the AccECN option is often not put to every ACK, the space
hijack is usually only temporary. Depending on the length of
AccECN option (can be either 11, 8, 5, or 2 bytes, cf. Table
5 in AccECN spec) and the NOPs used for alignment of other
TCP options, up to two SACK blocks will be reduced.

Co-developed-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

---
v8:
- Update tcp_options_fit_accecn() to avoid using recursion
---
 net/ipv4/tcp_output.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d7cdc6589a9c..fc67c6760bef 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -966,23 +966,41 @@ static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
 				  int remaining, int max_combine_saving)
 {
 	int size = TCP_ACCECN_MAXSIZE;
+	int sack_blocks_reduce = 0;
+	int rem = remaining;
 
 	opts->num_accecn_fields = TCP_ACCECN_NUMFIELDS;
-
 	while (opts->num_accecn_fields >= required) {
 		int leftover_size = size & 0x3;
 		/* Pad to dword if cannot combine */
 		if (leftover_size > max_combine_saving)
 			leftover_size = -((4 - leftover_size) & 0x3);
 
-		if (remaining >= size - leftover_size) {
+		if (rem >= size - leftover_size) {
 			size -= leftover_size;
 			break;
+		} else if (opts->num_accecn_fields == required &&
+			   opts->num_sack_blocks > 2 &&
+			   required > 0) {
+			/* Try to fit the option by removing one SACK block */
+			opts->num_sack_blocks--;
+			sack_blocks_reduce++;
+			rem = rem + TCPOLEN_SACK_PERBLOCK;
+
+			opts->num_accecn_fields = TCP_ACCECN_NUMFIELDS;
+			size = TCP_ACCECN_MAXSIZE;
+			continue;
 		}
 
 		opts->num_accecn_fields--;
 		size -= TCPOLEN_ACCECN_PERFIELD;
 	}
+	if (sack_blocks_reduce > 0) {
+		if (opts->num_accecn_fields >= required)
+			size -= sack_blocks_reduce * TCPOLEN_SACK_PERBLOCK;
+		else
+			opts->num_sack_blocks += sack_blocks_reduce;
+	}
 	if (opts->num_accecn_fields < required)
 		return 0;
 
-- 
2.34.1