[PATCH net v4 2/3] tcp_cubic: fix to match Reno additive increment

[Mingrui Zhang <mrzhang97@gmail.com>]

The original code follows RFC 8312 (obsoleted CUBIC RFC).

The patched code follows RFC 9438 (new CUBIC RFC):
"Once _W_est_ has grown to reach the _cwnd_ at the time of most
recently setting _ssthresh_ -- that is, _W_est_ >= _cwnd_prior_ --
the sender SHOULD set α__cubic_ to 1 to ensure that it can achieve
the same congestion window increment rate as Reno, which uses AIMD
(1,0.5)."

Add new field 'cwnd_prior' in bictcp to hold cwnd before a loss event

Fixes: 89b3d9aaf467 ("[TCP] cubic: precompute constants")
Signed-off-by: Mingrui Zhang <mrzhang97@gmail.com>
Signed-off-by: Lisong Xu <xu@unl.edu>
---
v2->v3: Correct the "Fixes:" footer content
v1->v2: Add new field 'cwnd_prior' in bictcp to hold cwnd before a loss event
v1->v2: Separate patches

 net/ipv4/tcp_cubic.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 00da7d592032..03cfbad37dab 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -102,6 +102,7 @@ struct bictcp {
 	u32	end_seq;	/* end_seq of the round */
 	u32	last_ack;	/* last time when the ACK spacing is close */
 	u32	curr_rtt;	/* the minimum rtt of current round */
+	u32	cwnd_prior;	/* cwnd before a loss event */
 };
 
 static inline void bictcp_reset(struct bictcp *ca)
@@ -305,7 +306,10 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 	if (tcp_friendliness) {
 		u32 scale = beta_scale;
 
-		delta = (cwnd * scale) >> 3;
+		if (cwnd < ca->cwnd_prior)
+			delta = (cwnd * scale) >> 3;	/* CUBIC additive increment */
+		else
+			delta = cwnd;			/* Reno additive increment */
 		while (ca->ack_cnt > delta) {		/* update tcp cwnd */
 			ca->ack_cnt -= delta;
 			ca->tcp_cwnd++;
@@ -355,6 +359,7 @@ __bpf_kfunc static u32 cubictcp_recalc_ssthresh(struct sock *sk)
 			/ (2 * BICTCP_BETA_SCALE);
 	else
 		ca->last_max_cwnd = tcp_snd_cwnd(tp);
+	ca->cwnd_prior = tcp_snd_cwnd(tp);
 
 	return max((tcp_snd_cwnd(tp) * beta) / BICTCP_BETA_SCALE, 2U);
 }
-- 
2.34.1