net/tcp: add a member to record the current receiving window

Change-Id: Ic4c46d643a905fdd3a828e563eab4814da70dbe5
Signed-off-by: chao.an <anchao@xiaomi.com>

net/sixlowpan/sixlowpan_tcpsend.c

@@ -263,6 +263,10 @@ static int sixlowpan_tcp_header(FAR struct tcp_conn_s *conn,
 
       ipv6tcp->tcp.wnd[0] = recvwndo >> 8;
       ipv6tcp->tcp.wnd[1] = recvwndo & 0xff;
+
+      /* Update the Receiver Window */
+
+      conn->rcv_wnd = recvwndo;
     }
 
   /* Calculate TCP checksum. */

net/tcp/tcp.h

@@ -194,6 +194,7 @@ struct tcp_conn_s
                            * connection */
   uint16_t snd_wnd;       /* Sequence and acknowledgement numbers of last
                            * window update */
+  uint16_t rcv_wnd;       /* Receiver window available */
 #ifdef CONFIG_NET_TCP_WRITE_BUFFERS
   uint32_t tx_unacked;    /* Number bytes sent but not yet ACKed */
 #else

net/tcp/tcp_send.c

@@ -366,6 +366,10 @@ static void tcp_sendcommon(FAR struct net_driver_s *dev,
 
       tcp->wnd[0] = recvwndo >> 8;
       tcp->wnd[1] = recvwndo & 0xff;
+
+      /* Update the Receiver Window */
+
+      conn->rcv_wnd = recvwndo;
     }
 
   /* Finish the IP portion of the message and calculate checksums */