BUG/MINOR: quic: Missing application limitation detection for BBR

The ->app_limited member of the delivery rate struct (quic_cc_drs) aim is to
store the index of the last transmitted byte marked as application-limited
so that to track the application-limited phases. During these phases,
BBR must ignore delivery rate sample to properly estimate the delivery rate.

Without such a patch, the Startup state could be exited very quickly with
a very low estimated bottleneck bandwidth. This had a very bad impact
on little objects with download time smaller than the expected Startup phase
duration.

Author: haproxyFred

include/haproxy/quic_cc-t.h

@@ -147,6 +147,7 @@ struct quic_cc_algo {
 	void (*on_pkt_lost)(struct quic_cc *cc,
 	                    struct quic_tx_packet *pkt, uint32_t lost_bytes);
 	void (*congestion_event)(struct quic_cc *cc, uint32_t ts);
+	void (*check_app_limited)(const struct quic_cc *cc, int sent);
 };
 
 #endif /* USE_QUIC */

src/quic_cc_bbr.c

@@ -1481,6 +1481,23 @@ uint bbr_pacing_burst(const struct quic_cc *cc)
 	return p->send_quantum / p->mtu;
 }
 
+/* Update the delivery rate sampling state about the application limitation. */
+static void bbr_check_app_limited(const struct quic_cc *cc, int sent)
+{
+	struct bbr *bbr = quic_cc_priv(cc);
+	struct quic_cc_drs *drs = &bbr->drs;
+	struct quic_cc_path *p = container_of(cc, struct quic_cc_path, cc);
+
+	if (p->in_flight >= p->cwnd) {
+		drs->is_cwnd_limited = 1;
+	}
+	else if (!sent) {
+		drs->app_limited = drs->delivered + p->in_flight;
+		if (!drs->app_limited)
+			drs->app_limited = p->mtu;
+	}
+}
+
 static inline const char *bbr_state_str(struct bbr *bbr)
 {
 	switch (bbr->state) {
@@ -1525,6 +1542,7 @@ struct quic_cc_algo quic_cc_algo_bbr = {
 	.on_ack_rcvd = bbr_update_on_ack,
 	.congestion_event = bbr_congestion_event,
 	.on_pkt_lost = bbr_update_on_loss,
+	.check_app_limited = bbr_check_app_limited,
 	.state_cli   = bbr_state_cli,
 };
 

src/quic_tx.c

@@ -18,6 +18,7 @@
 
 #include <haproxy/pool.h>
 #include <haproxy/trace.h>
+#include <haproxy/quic_cc_drs.h>
 #include <haproxy/quic_cid.h>
 #include <haproxy/quic_conn.h>
 #include <haproxy/quic_pacing.h>
@@ -506,6 +507,10 @@ enum quic_tx_err qc_send_mux(struct quic_conn *qc, struct list *frms,
 	TRACE_STATE("preparing data (from MUX)", QUIC_EV_CONN_TXPKT, qc);
 	qel_register_send(&send_list, qc->ael, frms);
 	sent = qc_send(qc, 0, &send_list, max_dgram);
+
+	if (pacer && qc->path->cc.algo->check_app_limited)
+		qc->path->cc.algo->check_app_limited(&qc->path->cc, sent);
+
 	if (sent <= 0) {
 		ret = QUIC_TX_ERR_FATAL;
 	}