From e9ab3adf2382d2bdc632274e7b6fab1944eacb8c Mon Sep 17 00:00:00 2001
From: Alexander Lunev <alexanderlunev@mail.ru>
Date: Sun, 2 Jan 2022 18:53:43 +0300
Subject: [PATCH] net/tcp(unbuffered): advance sndseq by +1 because SYN and FIN
 occupy one sequence number (RFC 793)

---
 net/tcp/tcp_input.c |  5 +++++
 net/tcp/tcp_send.c  | 19 +++++++++++++++++++
 net/tcp/tcp_timer.c | 15 +++++++++++++++
 3 files changed, 39 insertions(+)

diff --git a/net/tcp/tcp_input.c b/net/tcp/tcp_input.c
index 63b4d33c63..b5546dde60 100644
--- a/net/tcp/tcp_input.c
+++ b/net/tcp/tcp_input.c
@@ -870,6 +870,11 @@ found:
 
         if ((tcp->flags & TCP_CTL) == TCP_SYN)
           {
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+            tcp_setsequence(conn->sndseq, conn->rexmit_seq);
+#else
+            /* REVISIT for the buffered mode */
+#endif
             tcp_synack(dev, conn, TCP_ACK | TCP_SYN);
             return;
           }
diff --git a/net/tcp/tcp_send.c b/net/tcp/tcp_send.c
index fd7eca15c0..13b89b7a81 100644
--- a/net/tcp/tcp_send.c
+++ b/net/tcp/tcp_send.c
@@ -382,6 +382,25 @@ static void tcp_sendcommon(FAR struct net_driver_s *dev,
   /* Finish the IP portion of the message and calculate checksums */
 
   tcp_sendcomplete(dev, tcp);
+
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+  if ((tcp->flags & (TCP_SYN | TCP_FIN)) != 0)
+    {
+      /* Remember sndseq that will be used in case of a possible
+       * SYN or FIN retransmission
+       */
+
+      conn->rexmit_seq = tcp_getsequence(conn->sndseq);
+
+      /* Advance sndseq by +1 because SYN and FIN occupy
+       * one sequence number (RFC 793)
+       */
+
+      net_incr32(conn->sndseq, 1);
+    }
+#else
+  /* REVISIT for the buffered mode */
+#endif
 }
 
 /****************************************************************************
diff --git a/net/tcp/tcp_timer.c b/net/tcp/tcp_timer.c
index 7ecb987283..a206a0928d 100644
--- a/net/tcp/tcp_timer.c
+++ b/net/tcp/tcp_timer.c
@@ -317,6 +317,11 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
                      * SYNACK.
                      */
 
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+                    tcp_setsequence(conn->sndseq, conn->rexmit_seq);
+#else
+                    /* REVISIT for the buffered mode */
+#endif
                     tcp_synack(dev, conn, TCP_ACK | TCP_SYN);
                     goto done;
 
@@ -324,6 +329,11 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 
                     /* In the SYN_SENT state, we retransmit out SYN. */
 
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+                    tcp_setsequence(conn->sndseq, conn->rexmit_seq);
+#else
+                    /* REVISIT for the buffered mode */
+#endif
                     tcp_synack(dev, conn, TCP_SYN);
                     goto done;
 
@@ -344,6 +354,11 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 
                     /* In all these states we should retransmit a FINACK. */
 
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+                    tcp_setsequence(conn->sndseq, conn->rexmit_seq);
+#else
+                    /* REVISIT for the buffered mode */
+#endif
                     tcp_send(dev, conn, TCP_FIN | TCP_ACK, hdrlen);
                     goto done;
                 }