net: tcp: Implement TCP new Reno collision avoidance

To avoid a TCP connection from collapsing a link, implement a collision
avoidance algorithm. Initially TCP new Reno is implemented for its
simplicity.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>

subsys/net/ip/Kconfig

@@ -456,6 +456,14 @@ config NET_TCP_FAST_RETRANSMIT
 	  In that case a retransmission is triggerd to avoid having to wait for
 	  the retransmit timer to elapse.
 
+config NET_TCP_CONGESTION_AVOIDANCE
+	bool "Implement a collision avoidance algorithm in TCP"
+	depends on NET_TCP
+	default y
+	help
+	  To avoid overstressing a link reduce the transmission rate as soon as
+	  packets are starting to drop.
+
 config NET_TCP_MAX_SEND_WINDOW_SIZE
 	int "Maximum sending window size to use"
 	depends on NET_TCP

subsys/net/ip/tcp.c

@@ -392,6 +392,44 @@ static void tcp_derive_rto(struct tcp *conn)
 #endif
 }
 
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+static void tcp_new_reno_init(struct tcp *conn)
+{
+	conn->ca.congestion_win = conn_mss(conn) * TCP_CONGESTION_INITIAL_WIN;
+	conn->ca.ssthresh = conn_mss(conn) * TCP_CONGESTION_INITIAL_SSTHRESH;
+}
+
+static void tcp_new_reno_fast_retransmit(struct tcp *conn)
+{
+	conn->ca.congestion_win = conn->ca.congestion_win / 2;
+	conn->ca.ssthresh = conn->ca.congestion_win;
+}
+
+static void tcp_new_reno_timeout(struct tcp *conn)
+{
+	conn->ca.congestion_win = conn_mss(conn) * TCP_CONGESTION_INITIAL_WIN;
+	conn->ca.ssthresh = conn->ca.ssthresh / 2;
+}
+
+static void tcp_new_reno_pkts_acked(struct tcp *conn)
+{
+	int32_t new_win = conn->ca.congestion_win;
+
+	if (conn->ca.state == TCP_NEW_RENO_RAMPUP) {
+		new_win += conn_mss(conn);
+		conn->ca.congestion_win += conn_mss(conn);
+	} else {
+		int32_t mss = conn_mss(conn);
+
+		new_win += (mss * mss) / conn->ca.congestion_win;
+	}
+	conn->ca.congestion_win = MIN(new_win, UINT16_MAX);
+	if (conn->ca.congestion_win > conn->ca.ssthresh) {
+		conn->ca.state = TCP_NEW_RENO_LINEAR;
+	}
+}
+#endif
+
 static void tcp_send_queue_flush(struct tcp *conn)
 {
 	struct net_pkt *pkt;
@@ -1140,6 +1178,9 @@ static int tcp_pkt_peek(struct net_pkt *to, struct net_pkt *from, size_t pos,
 static bool tcp_window_full(struct tcp *conn)
 {
 	bool window_full = (conn->send_data_total >= conn->send_win);
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+	window_full |= (conn->send_data_total >= conn->ca.congestion_win);
+#endif
 
 	NET_DBG("conn: %p window_full=%hu", conn, window_full);
 
@@ -1162,6 +1203,13 @@ static int tcp_unsent_len(struct tcp *conn)
 		unsent_len = 0;
 	} else {
 		unsent_len = MIN(unsent_len, conn->send_win - conn->unacked_len);
+#if CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+		if (conn->unacked_len >= conn->ca.congestion_win) {
+			unsent_len = 0;
+		} else {
+			unsent_len = MIN(unsent_len, conn->ca.congestion_win - conn->unacked_len);
+		}
+#endif
 	}
  out:
 	NET_DBG("unsent_len=%d", unsent_len);
@@ -1306,6 +1354,10 @@ static void tcp_resend_data(struct k_work *work)
 		goto out;
 	}
 
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+	tcp_new_reno_timeout(conn);
+#endif
+
 	conn->data_mode = TCP_DATA_MODE_RESEND;
 	conn->unacked_len = 0;
 
@@ -1486,6 +1538,11 @@ static struct tcp *tcp_conn_alloc(void)
 	conn->dup_ack_cnt = 0;
 #endif
 
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+	tcp_new_reno_init(conn);
+#endif
+
+
 	/* The ISN value will be set when we get the connection attempt or
 	 * when trying to create a connection.
 	 */
@@ -2211,6 +2268,10 @@ static enum net_verdict tcp_in(struct tcp *conn, struct net_pkt *pkt)
 			net_context_set_state(conn->context,
 					      NET_CONTEXT_CONNECTED);
 
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+			tcp_new_reno_init(conn);
+#endif
+
 			if (conn->accepted_conn) {
 				if (conn->accepted_conn->accept_cb) {
 					conn->accepted_conn->accept_cb(
@@ -2340,6 +2401,10 @@ static enum net_verdict tcp_in(struct tcp *conn, struct net_pkt *pkt)
 
 				/* Restore the current transmission */
 				conn->unacked_len = temp_unacked_len;
+
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+				tcp_new_reno_fast_retransmit(conn);
+#endif
 			}
 		}
 #endif
@@ -2366,6 +2431,9 @@ static enum net_verdict tcp_in(struct tcp *conn, struct net_pkt *pkt)
 			/* New segment, reset duplicate ack counter */
 			conn->dup_ack_cnt = 0;
 #endif
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+			tcp_new_reno_pkts_acked(conn);
+#endif
 
 			conn->send_data_total -= len_acked;
 			if (conn->unacked_len < len_acked) {

subsys/net/ip/tcp_private.h

@@ -7,6 +7,13 @@
 
 #include "tp.h"
 
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+/* Define the number of MSS sections the congestion window is initialized at */
+#define TCP_CONGESTION_INITIAL_WIN	1
+#define TCP_CONGESTION_INITIAL_SSTHRESH	3
+
+#endif
+
 #define is(_a, _b) (strcmp((_a), (_b)) == 0)
 
 #ifndef MIN3
@@ -223,6 +230,20 @@ struct tcp_options {
 	bool wnd_found : 1;
 };
 
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+
+enum tcp_new_reno_state {
+	TCP_NEW_RENO_RAMPUP = 1,
+	TCP_NEW_RENO_LINEAR
+};
+
+struct tcp_collision_avoidance_reno {
+	uint16_t congestion_win;
+	uint16_t ssthresh;
+	enum tcp_new_reno_state state;
+};
+#endif
+
 struct tcp { /* TCP connection */
 	sys_snode_t next;
 	struct net_context *context;
@@ -273,6 +294,9 @@ struct tcp { /* TCP connection */
 	uint16_t send_win;
 #ifdef CONFIG_NET_TCP_RANDOMIZED_RTO
 	uint16_t rto;
+#endif
+#ifdef CONFIG_NET_TCP_CONGESTION_AVOIDANCE
+	struct tcp_collision_avoidance_reno ca;
 #endif
 	uint8_t send_data_retries;
 #ifdef CONFIG_NET_TCP_FAST_RETRANSMIT