Adds ECN handling managed by ROCCET. Use of srRTT as a hard condition after receiving an ECN.

tcp_roccet.c

@@ -14,8 +14,9 @@
  * CUBIC's window growth function and adds, based on RTT
  * and ACK rate, congestion events.
  *
- * NOTE: A paper for TCP ROCCET is currently under review.
+ * A peer-reviewed paper on TCP ROCCET will be presented at the WONS 2026 conference.
- * A draft of this paper can be found here:
+ * A draft of the paper is available here:
+ * 		https://arxiv.org/abs/2510.25281
  *
  *
  * Further information about CUBIC:
@@ -128,7 +129,8 @@ static inline void roccettcp_reset(struct roccettcp *ca)
 	ca->bw_limit.next_check = 0;
 	ca->curr_min_rtt_timed.rtt = ~0U;
 	ca->curr_min_rtt_timed.time = ~0U;
-	ca->last_rtt = 0;
+	ca->ece_srrtt = 0;
+	ca->ece_cwnd = 2;
 }
 
 static inline void update_min_rtt(struct sock *sk)
@@ -424,18 +426,32 @@ __bpf_kfunc static void roccettcp_cong_avoid(struct sock *sk, u32 ack,
 	update_min_rtt(sk);
 	update_srrtt(sk);
 
+	/* Reset ECE handling if we already have more bandwidth
+	 * than we received the last ECE.
+	 */
+	if(ca->ece_srrtt > 0){
+		if(tcp_snd_cwnd(tp) >= ca->ece_cwnd){
+			ca->ece_srrtt = 0;
+		}
+	}
+
 	/* ROCCET drain.
 	 * Do not increase the cwnd for 100ms after a roccet congestion event
 	 */
 	if (now - ca->roccet_last_event_time_us <= 100 * USEC_PER_MSEC)
 		return;
 
-	/* Lift off: Detect an exit point for tcp slow start
+	/* LAUNCH: Detect an exit point for tcp slow start
 	 * in networks with large buffers of multiple BDP
 	 * Like in cellular networks (5G, ...).
+	 * Or exit LAUNCH if cwnd is too large for application layer
+	 * data rate.
 	 */
-	if (tcp_in_slow_start(tp) && ca->curr_srRTT > sr_rtt_upper_bound &&
+
-	    get_ack_rate_diff(ca) >= ack_rate_diff_ss) {
+	if ((tcp_in_slow_start(tp) && ca->curr_srRTT > sr_rtt_upper_bound &&
+	    get_ack_rate_diff(ca) >= ack_rate_diff_ss) ||
+		(!tcp_is_cwnd_limited(sk) && tcp_in_slow_start(tp))
+		) {
 		ca->epoch_start = 0;
 
 		/* Handle inital slow start. Here we observe the most problems */
@@ -486,7 +502,13 @@ __bpf_kfunc static void roccettcp_cong_avoid(struct sock *sk, u32 ack,
 	if (roccet_xj < sr_rtt_upper_bound)
 		roccet_xj = sr_rtt_upper_bound;
 
-	if (ca->curr_srRTT > roccet_xj && bw_limit_detect) {
+	/* This is true if we recently received an ECE bit. 
+	 * Therefore we should respect the srRTT at this piont.
+	 */
+	if(ca->ece_srrtt < roccet_xj && ca->ece_srrtt > 0)
+		roccet_xj = ca->ece_srrtt;
+
+	if (ca->curr_srRTT > roccet_xj && (bw_limit_detect || ca->ece_srrtt > 0)) {
 		ca->epoch_start = 0;
 		ca->roccet_last_event_time_us = now;
 		ca->cnt = 100 * tcp_snd_cwnd(tp);
@@ -575,6 +597,20 @@ __bpf_kfunc static void roccettcp_acked(struct sock *sk,
 	}
 }
 
+__bpf_kfunc static void roccet_in_ack_event(struct sock *sk, u32 flags)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct roccettcp *ca = inet_csk_ca(sk);
+
+	/* Handle ECE bit. 
+	 * Pocessing of ECE events is done in roccettcp_cong_avoid()
+	 */
+	if (flags & CA_ACK_ECE) {
+		ca->ece_srrtt = ca->curr_srRTT;
+		ca->ece_cwnd = tcp_snd_cwnd(tp);
+	}
+}
+
 static struct tcp_congestion_ops roccet_tcp __read_mostly = {
 	.init = roccettcp_init,
 	.ssthresh = roccettcp_recalc_ssthresh,
@@ -583,6 +619,7 @@ static struct tcp_congestion_ops roccet_tcp __read_mostly = {
 	.undo_cwnd = tcp_reno_undo_cwnd,
 	.cwnd_event = roccettcp_cwnd_event,
 	.pkts_acked = roccettcp_acked,
+	.in_ack_event   = roccet_in_ack_event,
 	.owner = THIS_MODULE,
 	.name = "roccet",
 };

tcp_roccet.h

@@ -39,7 +39,9 @@ struct roccettcp {
 	u32 tcp_cwnd; 		/* estimated tcp cwnd */
 	u32 curr_rtt; 		/* the minimum rtt of current round */
 
-	u32 roccet_last_event_time_us; 	/* The last time ROCCETv2 was triggered */
+	u32 roccet_last_event_time_us; 	/* The last time ROCCET was triggered */
+	u32 ece_cwnd; 	/* The cwnd when a ECE bit was received */
+	u32 ece_srrtt; 	/* The srRTT whent the ECE was received */
 	u32 curr_min_rtt; 		/* The current observed minRTT */
 	struct TimedRTT curr_min_rtt_timed; /* The current observed minRTT with 
 						the timestamp when it was observed */