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