From: Frederic Lecaille <flecaille@haproxy.com>
Date: Mon, 12 Feb 2024 10:37:17 +0000 (+0100)
Subject: MINOR: quic: Update K CUBIC calculation (RFC 9438)
X-Git-Tag: v3.0-dev4~61
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=2ed53ae4a02ba49f529addb9003b747c8ab281b1;p=thirdparty%2Fhaproxy.git

MINOR: quic: Update K CUBIC calculation (RFC 9438)

The new formula for K CUBIC which arrives with RFC 9438 is as follows:

       K = cubic_root((W_max - cwnd_epoch) / C)

Note that W_max is c->last_w_max, and cwnd_epoch is c->cwnd when entering
quic_cubic_update() just after a congestion event.

Must be backported as far as 2.6.
---

diff --git a/src/quic_cc_cubic.c b/src/quic_cc_cubic.c
index 7c478c6340..76a62acd06 100644
--- a/src/quic_cc_cubic.c
+++ b/src/quic_cc_cubic.c
@@ -241,8 +241,7 @@ static inline void quic_cubic_update(struct quic_cc *cc, uint32_t acked)
 			 * K = cubic_root((W_max - cwnd_epoch)/C) (Figure 2)
 			 * Note that K is stored in milliseconds.
 			 */
-			c->K = cubic_root((c->last_w_max - path->cwnd) *
-			                  (CUBIC_ONE_SCALED - CUBIC_BETA_SCALED) / (CUBIC_C_SCALED * path->mtu));
+			c->K = cubic_root(((c->last_w_max - path->cwnd) << CUBIC_SCALE_FACTOR_SHIFT) / (CUBIC_C_SCALED * path->mtu));
 			/* Convert to miliseconds. */
 			c->K *= 1000;
 			c->W_target = c->last_w_max;