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

README.md

@@ -11,10 +11,11 @@ The congestion avoidance phase, called ORBITER, uses
 CUBIC's window growth function and adds, based on RTT
 and ACK rate, congestion events.
 
-A peer-reviewed paper on TCP ROCCET will be presented at the WONS 2026 conference.
+NOTE: A paper for TCP ROCCET is currently under review.
-A draft of the paper is available here:
+Paper draft:
-        https://arxiv.org/abs/2510.25281
+            https://arxiv.org/abs/2510.25281
-
+ 
+The oldest Linux kernel version we tested with ROCCET is 6.1.
 
 ## Setup
 

tcp_roccet.c

@@ -129,7 +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->ece_received = 0;
+	ca->ece_srrtt = 0;
+	ca->ece_cwnd = 2;
 }
 
 static inline void update_min_rtt(struct sock *sk)
@@ -425,6 +426,15 @@ __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
 	 */
@@ -492,9 +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 || ca->ece_received)) {
+	/* This is true if we recently received an ECE bit. 
-		if(ca->ece_received)
+	 * Therefore we should respect the srRTT at this piont.
-			ca->ece_received = 0;
+	 */
+	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);
@@ -585,13 +599,15 @@ __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_received = 1;
+		ca->ece_srrtt = ca->curr_srRTT;
+		ca->ece_cwnd = tcp_snd_cwnd(tp);
 	}
 }
 

tcp_roccet.h

@@ -40,7 +40,8 @@ struct roccettcp {
 	u32 curr_rtt; 		/* the minimum rtt of current round */
 
 	u32 roccet_last_event_time_us; 	/* The last time ROCCET was triggered */
-	u32 ece_received; 	/* Set to true if an ECE bit was received */
+	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 */