netdev - [PATCH net-next 34/44] tcp: accecn: retransmit downgraded SYN in AccECN negotiation

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Message-Id: <20241015102940.26157-35-chia-yu.chang@nokia-bell-labs.com>
Date: Tue, 15 Oct 2024 12:29:30 +0200
From: chia-yu.chang@...ia-bell-labs.com
To: netdev@...r.kernel.org, ij@...nel.org, ncardwell@...gle.com,
        koen.de_schepper@...ia-bell-labs.com, g.white@...leLabs.com,
        ingemar.s.johansson@...csson.com, mirja.kuehlewind@...csson.com,
        cheshire@...le.com, rs.ietf@....at, Jason_Livingood@...cast.com,
        vidhi_goel@...le.com
Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH net-next 34/44] tcp: accecn: retransmit downgraded SYN in AccECN negotiation

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

Based on specification:
  https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt

3.1.4.1. Retransmitted SYNs - If the sender of an AccECN SYN (the TCP
Client) times out before receiving the SYN/ACK, it SHOULD attempt to
negotiate the use of AccECN at least one more time by continuing to
set all three TCP ECN flags (AE,CWR,ECE) = (1,1,1) on the first
retransmitted SYN (using the usual retransmission time-outs). If this
first retransmission also fails to be acknowledged, in deployment
scenarios where AccECN path traversal might be problematic, the TCP
Client SHOULD send subsequent retransmissions of the SYN with the
three TCP-ECN flags cleared (AE,CWR,ECE) = (0,0,0).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 net/ipv4/tcp_output.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index ec10785f6d00..ae78ff6784d3 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3617,12 +3617,14 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 			tcp_retrans_try_collapse(sk, skb, avail_wnd);
 	}
 
-	/* RFC3168, section 6.1.1.1. ECN fallback
-	 * As AccECN uses the same SYN flags (+ AE), this check covers both
-	 * cases.
-	 */
-	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
-		tcp_ecn_clear_syn(sk, skb);
+	if (!tcp_ecn_mode_pending(tp) || icsk->icsk_retransmits > 1) {
+		/* RFC3168, section 6.1.1.1. ECN fallback
+		 * As AccECN uses the same SYN flags (+ AE), this check covers both
+		 * cases.
+		 */
+		if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
+			tcp_ecn_clear_syn(sk, skb);
+	}
 
 	/* Update global and local TCP statistics. */
 	segs = tcp_skb_pcount(skb);
-- 
2.34.1