net/tcp(buffered): retransmit only one the earliest not acknowledged segment

Retransmit only one the earliest not acknowledged segment
(according to RFC 6298 (5.4)). The issue is the same as it was
in tcp_send_unbuffered.c and tcp_sendfile.c.

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

net/tcp/tcp.h

@@ -175,10 +175,7 @@ struct tcp_conn_s
   uint8_t  rcvseq[4];     /* The sequence number that we expect to
                            * receive next */
   uint8_t  sndseq[4];     /* The sequence number that was last sent by us */
-#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS) || \
-    defined(CONFIG_NET_SENDFILE)
   uint32_t rexmit_seq;    /* The sequence number to be retrasmitted */
-#endif
   uint8_t  crefs;         /* Reference counts on this instance */
 #if defined(CONFIG_NET_IPv4) && defined(CONFIG_NET_IPv6)
   uint8_t  domain;        /* IP domain: PF_INET or PF_INET6 */

net/tcp/tcp_appsend.c

@@ -322,11 +322,6 @@ void tcp_rexmit(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
    * new data in it, we must send out a packet.
    */
 
-#if defined(CONFIG_NET_TCP_WRITE_BUFFERS) && defined(CONFIG_NET_SENDFILE)
-  if (conn->sendfile)
-#endif
-    {
-#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS) || defined(CONFIG_NET_SENDFILE)
   if ((result & TCP_REXMIT) != 0 &&
       dev->d_sndlen > 0 && conn->tx_unacked > 0)
     {
@@ -345,8 +340,6 @@ void tcp_rexmit(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 
       return;
     }
-#endif
-    }
 
 #if defined(CONFIG_NET_TCP_WRITE_BUFFERS)
   if (dev->d_sndlen > 0)

net/tcp/tcp_send_buffered.c

@@ -323,7 +323,9 @@ static uint16_t psock_send_eventhandler(FAR struct net_driver_s *dev,
    */
 
   FAR struct tcp_conn_s *conn = pvpriv;
-  bool rexmit = false;
+#ifdef CONFIG_NET_TCP_FAST_RETRANSMIT
+  uint32_t rexmitno = 0;
+#endif
 
   /* Get the TCP connection pointer reliably from
    * the corresponding TCP socket.
@@ -501,12 +503,9 @@ static uint16_t psock_send_eventhandler(FAR struct net_driver_s *dev,
                 {
                   /* Do fast retransmit */
 
-                  rexmit = true;
+                  rexmitno = ackno;
-                }
+
-              else if ((TCP_WBNACK(wrb) > TCP_FAST_RETRANSMISSION_THRESH) &&
+                  /* Reset counter */
-                       TCP_WBNACK(wrb) == sq_count(&conn->unacked_q) - 1)
-                {
-                  /* Reset the duplicate ack counter */
 
                   TCP_WBNACK(wrb) = 0;
                 }
@@ -573,14 +572,77 @@ static uint16_t psock_send_eventhandler(FAR struct net_driver_s *dev,
       return flags;
     }
 
-  /* Check if we are being asked to retransmit data */
+#ifdef CONFIG_NET_TCP_FAST_RETRANSMIT
-
+  if (rexmitno != 0)
-  else if ((flags & TCP_REXMIT) != 0)
     {
-      rexmit = true;
+      FAR struct tcp_wrbuffer_s *wrb;
+      FAR sq_entry_t *entry;
+      FAR sq_entry_t *next;
+      size_t sndlen;
+
+      /* According to RFC 6298 (5.4), retransmit the earliest segment
+       * that has not been acknowledged by the TCP receiver.
+       */
+
+      for (entry = sq_peek(&conn->unacked_q); entry; entry = next)
+        {
+          wrb = (FAR struct tcp_wrbuffer_s *)entry;
+          next = sq_next(entry);
+
+          if (rexmitno != TCP_WBSEQNO(wrb))
+            {
+              continue;
             }
 
-  if (rexmit)
+          conn->rexmit_seq = rexmitno;
+
+          /* Reconstruct the length of the earliest segment to be
+           * retransmitted.
+           */
+
+          sndlen = TCP_WBPKTLEN(wrb);
+
+          if (sndlen > conn->mss)
+            {
+              sndlen = conn->mss;
+            }
+
+          /* As we are retransmitting, the sequence number is expected
+           * already set for this write buffer.
+           */
+
+          DEBUGASSERT(TCP_WBSEQNO(wrb) != (unsigned)-1);
+          conn->rexmit_seq = TCP_WBSEQNO(wrb);
+
+#ifdef NEED_IPDOMAIN_SUPPORT
+          /* If both IPv4 and IPv6 support are enabled, then we will need to
+           * select which one to use when generating the outgoing packet.
+           * If only one domain is selected, then the setup is already in
+           * place and we need do nothing.
+           */
+
+          send_ipselect(dev, conn);
+#endif
+          /* Then set-up to send that amount of data. (this won't actually
+           * happen until the polling cycle completes).
+           */
+
+          devif_iob_send(dev, TCP_WBIOB(wrb), sndlen, 0);
+
+          /* Reset the retransmission timer. */
+
+          tcp_update_retrantimer(conn, conn->rto);
+
+          /* Continue waiting */
+
+          return flags;
+        }
+    }
+#endif
+
+  /* Check if we are being asked to retransmit data */
+
+  if ((flags & TCP_REXMIT) != 0)
     {
       FAR struct tcp_wrbuffer_s *wrb;
       FAR sq_entry_t *entry;