net: tcp2: Adjust the send window according to avail bufs

We should have a max value for sending window so that application
is not able to use all our net_bufs for queueing packets.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

subsys/net/ip/Kconfig

@@ -377,6 +377,16 @@ config NET_TCP_RETRY_COUNT
 	  Should a retransmission timeout occur, the receive callback is
 	  called with -ECONNRESET error code and the context is dereferenced.
 
+config NET_TCP_MAX_SEND_WINDOW_SIZE
+	int "Maximum sending window size to use"
+	depends on NET_TCP2
+	default 0
+	range 0 65535
+	help
+	  This value affects how the TCP selects the maximum sending window
+	  size. The default value 0 lets the TCP stack select the value
+	  according to amount of network buffers configured in the system.
+
 choice
 	prompt "Select TCP stack"
 	depends on NET_TCP

subsys/net/ip/tcp2.c

@@ -1152,7 +1152,30 @@ static void tcp_in(struct tcp *conn, struct net_pkt *pkt)
 	}
 
 	if (th) {
+		size_t max_win;
+
 		conn->send_win = ntohs(th->th_win);
+
+#if IS_ENABLED(CONFIG_NET_TCP_MAX_SEND_WINDOW_SIZE)
+		if (CONFIG_NET_TCP_MAX_SEND_WINDOW_SIZE) {
+			max_win = CONFIG_NET_TCP_MAX_SEND_WINDOW_SIZE;
+		} else
+#endif
+		{
+			/* Adjust the window so that we do not run out of bufs
+			 * while waiting acks.
+			 */
+			max_win = (CONFIG_NET_BUF_TX_COUNT *
+				   CONFIG_NET_BUF_DATA_SIZE) / 3;
+		}
+
+		max_win = MAX(max_win, NET_IPV6_MTU);
+		if ((size_t)conn->send_win > max_win) {
+			NET_DBG("Lowering send window from %zd to %zd",
+				(size_t)conn->send_win, max_win);
+
+			conn->send_win = max_win;
+		}
 	}
 
 	if (FL(&fl, &, RST)) {