netdev - [PATCH v6 net-next 12/15] tcp: accecn: AccECN option ceb/cep heuristic

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Message-Id: <20250509211820.36880-13-chia-yu.chang@nokia-bell-labs.com>
Date: Fri,  9 May 2025 23:18:17 +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 12/15] tcp: accecn: AccECN option ceb/cep heuristic

From: Ilpo Järvinen <ij@...nel.org>

The heuristic algorithm from draft-11 Appendix A.2.2 to
mitigate against false ACE field overflows.

Signed-off-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 include/net/tcp.h    |  1 +
 net/ipv4/tcp_input.c | 18 ++++++++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5e4593e39de4..201326b10396 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -247,6 +247,7 @@ static_assert((1 << ATO_BITS) > TCP_DELACK_MAX);
 #define TCP_ACCECN_MAXSIZE		(TCPOLEN_ACCECN_BASE + \
 					 TCPOLEN_ACCECN_PERFIELD * \
 					 TCP_ACCECN_NUMFIELDS)
+#define TCP_ACCECN_SAFETY_SHIFT		1 /* SAFETY_FACTOR in accecn draft */
 
 /* tp->accecn_fail_mode */
 #define TCP_ACCECN_ACE_FAIL_SEND	BIT(0)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c93e4bffb23e..f206c1a93a56 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -709,16 +709,19 @@ static u32 __tcp_accecn_process(struct sock *sk, const struct sk_buff *skb,
 				u32 delivered_pkts, u32 delivered_bytes,
 				int flag)
 {
+	u32 old_ceb = tcp_sk(sk)->delivered_ecn_bytes[INET_ECN_CE - 1];
 	const struct tcphdr *th = tcp_hdr(skb);
 	struct tcp_sock *tp = tcp_sk(sk);
-	u32 delta, safe_delta;
+	u32 delta, safe_delta, d_ceb;
+	bool opt_deltas_valid;
 	u32 corrected_ace;
 
 	/* Reordered ACK or uncertain due to lack of data to send and ts */
 	if (!(flag & (FLAG_FORWARD_PROGRESS | FLAG_TS_PROGRESS)))
 		return 0;
 
-	tcp_accecn_process_option(tp, skb, delivered_bytes, flag);
+	opt_deltas_valid = tcp_accecn_process_option(tp, skb,
+						     delivered_bytes, flag);
 
 	if (!(flag & FLAG_SLOWPATH)) {
 		/* AccECN counter might overflow on large ACKs */
@@ -741,6 +744,17 @@ static u32 __tcp_accecn_process(struct sock *sk, const struct sk_buff *skb,
 	safe_delta = delivered_pkts -
 		     ((delivered_pkts - delta) & TCP_ACCECN_CEP_ACE_MASK);
 
+	if (opt_deltas_valid) {
+		d_ceb = tp->delivered_ecn_bytes[INET_ECN_CE - 1] - old_ceb;
+		if (!d_ceb)
+			return delta;
+		if (d_ceb > delta * tp->mss_cache)
+			return safe_delta;
+		if (d_ceb <
+		    safe_delta * tp->mss_cache >> TCP_ACCECN_SAFETY_SHIFT)
+			return delta;
+	}
+
 	return safe_delta;
 }
 
-- 
2.34.1