nuttx/net/tcp/tcp_send_buffered.c

/****************************************************************************
 * net/tcp/tcp_send_buffered.c
 *
 *   Copyright (C) 2007-2014, 2016-2018 Gregory Nutt. All rights reserved.
 *   Author: Gregory Nutt <gnutt@nuttx.org>
 *           Jason Jiang  <jasonj@live.cn>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name NuttX nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>

#if defined(CONFIG_NET) && defined(CONFIG_NET_TCP) && \
    defined(CONFIG_NET_TCP_WRITE_BUFFERS)

#if defined(CONFIG_DEBUG_FEATURES) && defined(CONFIG_NET_TCP_WRBUFFER_DEBUG)
/* Force debug output (from this file only) */

#  undef  CONFIG_DEBUG_NET
#  define CONFIG_DEBUG_NET 1
#endif

#include <sys/types.h>
#include <sys/socket.h>

#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <debug.h>
#include <debug.h>

#include <arch/irq.h>
#include <nuttx/clock.h>
#include <nuttx/net/net.h>
#include <nuttx/mm/iob.h>
#include <nuttx/net/netdev.h>
#include <nuttx/net/arp.h>
#include <nuttx/net/tcp.h>

#include "netdev/netdev.h"
#include "devif/devif.h"
#include "socket/socket.h"
#include "inet/inet.h"
#include "arp/arp.h"
#include "icmpv6/icmpv6.h"
#include "neighbor/neighbor.h"
#include "route/route.h"
#include "tcp/tcp.h"

/****************************************************************************
 * Pre-processor Definitions
 ****************************************************************************/
/* If both IPv4 and IPv6 support are both enabled, then we will need to build
 * in some additional domain selection support.
 */

#if defined(CONFIG_NET_IPv4) && defined(CONFIG_NET_IPv6)
#  define NEED_IPDOMAIN_SUPPORT 1
#endif

#define TCPIPv4BUF ((struct tcp_hdr_s *)&dev->d_buf[NET_LL_HDRLEN(dev) + IPv4_HDRLEN])
#define TCPIPv6BUF ((struct tcp_hdr_s *)&dev->d_buf[NET_LL_HDRLEN(dev) + IPv6_HDRLEN])

/* Debug */

#ifdef CONFIG_NET_TCP_WRBUFFER_DUMP
#  define BUF_DUMP(msg,buf,len) lib_dumpbuffer(msg,buf,len)
#else
#  define BUF_DUMP(msg,buf,len)
#  undef  TCP_WBDUMP
#  define TCP_WBDUMP(msg,wrb,len,offset)
#endif

/****************************************************************************
 * Private Functions
 ****************************************************************************/

/****************************************************************************
 * Name: psock_insert_segment
 *
 * Description:
 *   Insert a new segment in a write buffer queue, keep the segment queue in
 *   ascending order of sequence number.
 *
 * Input Parameters:
 *   wrb   The segment to be inserted
 *   q     The write buffer queue in which to insert the segment
 *
 * Returned Value:
 *   None
 *
 * Assumptions:
 *   The network is locked
 *
 ****************************************************************************/

static void psock_insert_segment(FAR struct tcp_wrbuffer_s *wrb,
                                 FAR sq_queue_t *q)
{
  FAR sq_entry_t *entry = (FAR sq_entry_t *)wrb;
  FAR sq_entry_t *insert = NULL;

  FAR sq_entry_t *itr;
  for (itr = sq_peek(q); itr; itr = sq_next(itr))
    {
      FAR struct tcp_wrbuffer_s *wrb0 = (FAR struct tcp_wrbuffer_s *)itr;
      if (TCP_WBSEQNO(wrb0) < TCP_WBSEQNO(wrb))
        {
          insert = itr;
        }
      else
        {
          break;
        }
    }

  if (insert)
    {
      sq_addafter(insert, entry, q);
    }
  else
    {
      sq_addfirst(entry, q);
    }
}

/****************************************************************************
 * Name: psock_lost_connection
 *
 * Description:
 *   The TCP connection has been lost.  Free all write buffers.
 *
 * Input Parameters:
 *   psock    The socket structure
 *   conn     The connection structure associated with the socket
 *
 * Returned Value:
 *   None
 *
 ****************************************************************************/

static inline void psock_lost_connection(FAR struct socket *psock,
                                         FAR struct tcp_conn_s *conn)
{
  FAR sq_entry_t *entry;
  FAR sq_entry_t *next;

  /* Do not allow any further callbacks */

  if (psock->s_sndcb != NULL)
    {
      psock->s_sndcb->flags = 0;
      psock->s_sndcb->event = NULL;
    }

  if (conn != NULL)
    {
      /* Free all queued write buffers */

      for (entry = sq_peek(&conn->unacked_q); entry; entry = next)
        {
          next = sq_next(entry);
          tcp_wrbuffer_release((FAR struct tcp_wrbuffer_s *)entry);
        }

      for (entry = sq_peek(&conn->write_q); entry; entry = next)
        {
          next = sq_next(entry);
          tcp_wrbuffer_release((FAR struct tcp_wrbuffer_s *)entry);
        }

      /* Reset write buffering variables */

      sq_init(&conn->unacked_q);
      sq_init(&conn->write_q);
      conn->sent       = 0;
      conn->sndseq_max = 0;
    }
}

/****************************************************************************
 * Name: send_ipselect
 *
 * Description:
 *   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.
 *
 * Input Parameters:
 *   dev   - The structure of the network driver that caused the event
 *   psock - Socket state structure
 *
 * Returned Value:
 *   None
 *
 * Assumptions:
 *   The network is locked
 *
 ****************************************************************************/

#ifdef NEED_IPDOMAIN_SUPPORT
static inline void send_ipselect(FAR struct net_driver_s *dev,
                                 FAR struct tcp_conn_s *conn)
{
  /* Which domain the socket support */

  if (conn->domain == PF_INET)
    {
      /* Select the IPv4 domain */

      tcp_ipv4_select(dev);
    }
  else /* if (conn->domain == PF_INET6) */
    {
      /* Select the IPv6 domain */

      DEBUGASSERT(conn->domain == PF_INET6);
      tcp_ipv6_select(dev);
    }
}
#endif

/****************************************************************************
 * Name: psock_send_addrchck
 *
 * Description:
 *   Check if the destination IP address is in the IPv4 ARP or IPv6 Neighbor
 *   tables.  If not, then the send won't actually make it out... it will be
 *   replaced with an ARP request (IPv4) or a Neighbor Solicitation (IPv6).
 *
 *   NOTE 1: This could be an expensive check if there are a lot of
 *   entries in the ARP or Neighbor tables.
 *
 *   NOTE 2: If we are actually harvesting IP addresses on incoming IP
 *   packets, then this check should not be necessary; the MAC mapping
 *   should already be in the ARP table in many cases (IPv4 only).
 *
 *   NOTE 3: If CONFIG_NET_ARP_SEND then we can be assured that the IP
 *   address mapping is already in the ARP table.
 *
 * Input Parameters:
 *   conn  - The TCP connection structure
 *
 * Returned Value:
 *   true - The Ethernet MAC address is in the ARP or Neighbor table (OR
 *          the network device is not Ethernet).
 *
 * Assumptions:
 *   The network is locked
 *
 ****************************************************************************/

#ifdef CONFIG_NET_ETHERNET
static inline bool psock_send_addrchck(FAR struct tcp_conn_s *conn)
{
  /* Only Ethernet drivers are supported by this function.
   *
   * REVISIT: Could the MAC address not also be in a routing table?
   */

  if (conn->dev->d_lltype != NET_LL_ETHERNET)
    {
      return true;
    }

#ifdef CONFIG_NET_IPv4
#ifdef CONFIG_NET_IPv6
  if (conn->domain == PF_INET)
#endif
    {
      /* For historical reasons, we will return true if both the ARP and the
       * routing table are disabled.
       */

      bool ret = true;
#ifdef CONFIG_NET_ROUTE
      in_addr_t router;
#endif

#if !defined(CONFIG_NET_ARP_IPIN) && !defined(CONFIG_NET_ARP_SEND)
      if (arp_find(conn->u.ipv4.raddr, NULL) >= 0)
        {
          /* Return true if the address was found in the ARP table */

          return true;
        }

      /* Otherwise, return false */

      ret = false;
#endif
#ifdef CONFIG_NET_ROUTE
      if (net_ipv4_router(conn->u.ipv4.raddr, &router) == OK)
        {
          /* Return true if the address was found in the routing table */

          return true;
        }

      /* Otherwise, return false */

      ret = false;
#endif

      return ret;
    }
#endif /* CONFIG_NET_IPv4 */

#ifdef CONFIG_NET_IPv6
#ifdef CONFIG_NET_IPv4
  else
#endif
    {
      /* For historical reasons, we will return true if both the ICMPv6
       * neighbor support and the routing table are disabled.
       */

      bool ret = true;
#ifdef CONFIG_NET_ROUTE
      net_ipv6addr_t router;
#endif

#if !defined(CONFIG_NET_ICMPv6_NEIGHBOR)
      if (neighbor_lookup(conn->u.ipv6.raddr, NULL) >= 0)
        {
          /* Return true if the address was found in the neighbor table */

          return true;
        }

      /* Otherwise, return false */

      ret = false;
#endif
#ifdef CONFIG_NET_ROUTE
      if (net_ipv6_router(conn->u.ipv6.raddr, router) == OK)
        {
          /* Return true if the address was found in the routing table */

          return true;
        }

      /* Otherwise, return false */

      ret = false;
#endif

      return ret;
    }
#endif /* CONFIG_NET_IPv6 */
}

#else /* CONFIG_NET_ETHERNET */
#  define psock_send_addrchck(r) (true)
#endif /* CONFIG_NET_ETHERNET */

/****************************************************************************
 * Name: psock_send_eventhandler
 *
 * Description:
 *   This function is called to perform the actual send operation when
 *   polled by the lower, device interfacing layer.
 *
 * Input Parameters:
 *   dev      The structure of the network driver that caused the event
 *   conn     The connection structure associated with the socket
 *   flags    Set of events describing why the callback was invoked
 *
 * Returned Value:
 *   None
 *
 * Assumptions:
 *   The network is locked
 *
 ****************************************************************************/

static uint16_t psock_send_eventhandler(FAR struct net_driver_s *dev,
                                        FAR void *pvconn, FAR void *pvpriv,
                                        uint16_t flags)
{
  FAR struct tcp_conn_s *conn = (FAR struct tcp_conn_s *)pvconn;
  FAR struct socket *psock = (FAR struct socket *)pvpriv;

  /* The TCP socket is connected and, hence, should be bound to a device.
   * Make sure that the polling device is the one that we are bound to.
   */

  DEBUGASSERT(conn->dev != NULL);
  if (dev != conn->dev)
    {
      return flags;
    }

  ninfo("flags: %04x\n", flags);

  /* If this packet contains an acknowledgment, then update the count of
   * acknowledged bytes.
   */

  if ((flags & TCP_ACKDATA) != 0)
    {
      FAR struct tcp_wrbuffer_s *wrb;
      FAR struct tcp_hdr_s *tcp;
      FAR sq_entry_t *entry;
      FAR sq_entry_t *next;
      uint32_t ackno;

      /* Get the offset address of the TCP header */

#ifdef CONFIG_NET_IPv4
#ifdef CONFIG_NET_IPv6
      if (conn->domain == PF_INET)
#endif
        {
          DEBUGASSERT(IFF_IS_IPv4(dev->d_flags));
          tcp = TCPIPv4BUF;
        }
#endif /* CONFIG_NET_IPv4 */

#ifdef CONFIG_NET_IPv6
#ifdef CONFIG_NET_IPv4
      else
#endif
        {
          DEBUGASSERT(IFF_IS_IPv6(dev->d_flags));
          tcp = TCPIPv6BUF;
        }
#endif /* CONFIG_NET_IPv6 */

      /* Get the ACK number from the TCP header */

      ackno = tcp_getsequence(tcp->ackno);
      ninfo("ACK: ackno=%u flags=%04x\n", ackno, flags);

      /* Look at every write buffer in the unacked_q.  The unacked_q
       * holds write buffers that have been entirely sent, but which
       * have not yet been ACKed.
       */

      for (entry = sq_peek(&conn->unacked_q); entry; entry = next)
        {
          uint32_t lastseq;

          /* Check of some or all of this write buffer has been ACKed. */

          next = sq_next(entry);
          wrb = (FAR struct tcp_wrbuffer_s *)entry;

          /* If the ACKed sequence number is greater than the start
           * sequence number of the write buffer, then some or all of
           * the write buffer has been ACKed.
           */

          if (ackno > TCP_WBSEQNO(wrb))
            {
              /* Get the sequence number at the end of the data */

              lastseq = TCP_WBSEQNO(wrb) + TCP_WBPKTLEN(wrb);
              ninfo("ACK: wrb=%p seqno=%u lastseq=%u pktlen=%u ackno=%u\n",
                    wrb, TCP_WBSEQNO(wrb), lastseq, TCP_WBPKTLEN(wrb),
                    ackno);

              /* Has the entire buffer been ACKed? */

              if (ackno >= lastseq)
                {
                  ninfo("ACK: wrb=%p Freeing write buffer\n", wrb);

                  /* Yes... Remove the write buffer from ACK waiting queue */

                  sq_rem(entry, &conn->unacked_q);

                  /* And return the write buffer to the pool of free buffers */

                  tcp_wrbuffer_release(wrb);
                }
              else
                {
                  unsigned int trimlen;

                  /* No, then just trim the ACKed bytes from the beginning
                   * of the write buffer.  This will free up some I/O buffers
                   * that can be reused while are still sending the last
                   * buffers in the chain.
                   */

                  trimlen = ackno - TCP_WBSEQNO(wrb);
                  if (trimlen > TCP_WBSENT(wrb))
                    {
                      /* More data has been ACKed then we have sent? */

                      trimlen = TCP_WBSENT(wrb);
                    }

                  ninfo("ACK: wrb=%p trim %u bytes\n", wrb, trimlen);

                  TCP_WBTRIM(wrb, trimlen);
                  TCP_WBSEQNO(wrb) = ackno;
                  TCP_WBSENT(wrb) -= trimlen;

                  /* Set the new sequence number for what remains */

                  ninfo("ACK: wrb=%p seqno=%u pktlen=%u\n",
                          wrb, TCP_WBSEQNO(wrb), TCP_WBPKTLEN(wrb));
                }
            }
        }

      /* A special case is the head of the write_q which may be partially
       * sent and so can still have un-ACKed bytes that could get ACKed
       * before the entire write buffer has even been sent.
       */

      wrb = (FAR struct tcp_wrbuffer_s *)sq_peek(&conn->write_q);
      if (wrb && TCP_WBSENT(wrb) > 0 && ackno > TCP_WBSEQNO(wrb))
        {
          uint32_t nacked;

          /* Number of bytes that were ACKed */

          nacked = ackno - TCP_WBSEQNO(wrb);
          if (nacked > TCP_WBSENT(wrb))
            {
              /* More data has been ACKed then we have sent? ASSERT? */

              nacked = TCP_WBSENT(wrb);
            }

          ninfo("ACK: wrb=%p seqno=%u nacked=%u sent=%u ackno=%u\n",
                wrb, TCP_WBSEQNO(wrb), nacked, TCP_WBSENT(wrb), ackno);

          /* Trim the ACKed bytes from the beginning of the write buffer. */

          TCP_WBTRIM(wrb, nacked);
          TCP_WBSEQNO(wrb) = ackno;
          TCP_WBSENT(wrb) -= nacked;

          ninfo("ACK: wrb=%p seqno=%u pktlen=%u sent=%u\n",
                wrb, TCP_WBSEQNO(wrb), TCP_WBPKTLEN(wrb), TCP_WBSENT(wrb));
        }
    }

  /* Check for a loss of connection */

  else if ((flags & TCP_DISCONN_EVENTS) != 0)
    {
      ninfo("Lost connection: %04x\n", flags);

      /* We could get here recursively through the callback actions of
       * tcp_lost_connection().  So don't repeat that action if we have
       * already been disconnected.
       */

      if (psock->s_conn != NULL && _SS_ISCONNECTED(psock->s_flags))
        {
          /* Report not connected */

          tcp_lost_connection(psock, psock->s_sndcb, flags);
        }

      /* Free write buffers and terminate polling */

      psock_lost_connection(psock, conn);
      return flags;
    }

   /* Check if we are being asked to retransmit data */

   else if ((flags & TCP_REXMIT) != 0)
    {
      FAR struct tcp_wrbuffer_s *wrb;
      FAR sq_entry_t *entry;

      ninfo("REXMIT: %04x\n", flags);

      /* If there is a partially sent write buffer at the head of the
       * write_q?  Has anything been sent from that write buffer?
       */

      wrb = (FAR struct tcp_wrbuffer_s *)sq_peek(&conn->write_q);
      ninfo("REXMIT: wrb=%p sent=%u\n", wrb, wrb ? TCP_WBSENT(wrb) : 0);

      if (wrb != NULL && TCP_WBSENT(wrb) > 0)
        {
          FAR struct tcp_wrbuffer_s *tmp;
          uint16_t sent;

          /* Yes.. Reset the number of bytes sent sent from the write buffer */

          sent = TCP_WBSENT(wrb);
          if (conn->unacked > sent)
            {
              conn->unacked -= sent;
            }
          else
            {
              conn->unacked = 0;
            }

          if (conn->sent > sent)
            {
              conn->sent -= sent;
            }
          else
            {
              conn->sent = 0;
            }

          TCP_WBSENT(wrb) = 0;
          ninfo("REXMIT: wrb=%p sent=%u, conn unacked=%d sent=%d\n",
                wrb, TCP_WBSENT(wrb), conn->unacked, conn->sent);

          /* Increment the retransmit count on this write buffer. */

          if (++TCP_WBNRTX(wrb) >= TCP_MAXRTX)
            {
              nwarn("WARNING: Expiring wrb=%p nrtx=%u\n",
                    wrb, TCP_WBNRTX(wrb));

              /* The maximum retry count as been exhausted. Remove the write
               * buffer at the head of the queue.
               */

              tmp = (FAR struct tcp_wrbuffer_s *)sq_remfirst(&conn->write_q);
              DEBUGASSERT(tmp == wrb);
              UNUSED(tmp);

              /* And return the write buffer to the free list */

              tcp_wrbuffer_release(wrb);

              /* NOTE expired is different from un-ACKed, it is designed to
               * represent the number of segments that have been sent,
               * retransmitted, and un-ACKed, if expired is not zero, the
               * connection will be closed.
               *
               * field expired can only be updated at TCP_ESTABLISHED state
               */

              conn->expired++;
            }
        }

      /* Move all segments that have been sent but not ACKed to the write
       * queue again note, the un-ACKed segments are put at the head of the
       * write_q so they can be resent as soon as possible.
       */

      while ((entry = sq_remlast(&conn->unacked_q)) != NULL)
        {
          wrb = (FAR struct tcp_wrbuffer_s *)entry;
          uint16_t sent;

          /* Reset the number of bytes sent sent from the write buffer */

          sent = TCP_WBSENT(wrb);
          if (conn->unacked > sent)
            {
              conn->unacked -= sent;
            }
          else
            {
              conn->unacked = 0;
            }

          if (conn->sent > sent)
            {
              conn->sent -= sent;
            }
          else
            {
              conn->sent = 0;
            }

          TCP_WBSENT(wrb) = 0;
          ninfo("REXMIT: wrb=%p sent=%u, conn unacked=%d sent=%d\n",
                wrb, TCP_WBSENT(wrb), conn->unacked, conn->sent);

          /* Free any write buffers that have exceed the retry count */

          if (++TCP_WBNRTX(wrb) >= TCP_MAXRTX)
            {
              nwarn("WARNING: Expiring wrb=%p nrtx=%u\n",
                    wrb, TCP_WBNRTX(wrb));

              /* Return the write buffer to the free list */

              tcp_wrbuffer_release(wrb);

              /* NOTE expired is different from un-ACKed, it is designed to
               * represent the number of segments that have been sent,
               * retransmitted, and un-ACKed, if expired is not zero, the
               * connection will be closed.
               *
               * field expired can only be updated at TCP_ESTABLISHED state
               */

              conn->expired++;
              continue;
            }
          else
            {
              /* Insert the write buffer into the write_q (in sequence
               * number order).  The retransmission will occur below
               * when the write buffer with the lowest sequence number
               * is pulled from the write_q again.
               */

              ninfo("REXMIT: Moving wrb=%p nrtx=%u\n", wrb, TCP_WBNRTX(wrb));

              psock_insert_segment(wrb, &conn->write_q);
            }
        }
    }

  /* Check if the outgoing packet is available (it may have been claimed
   * by a sendto event serving a different thread).
   */

  if (dev->d_sndlen > 0)
    {
      /* Another thread has beat us sending data, wait for the next poll */

      return flags;
    }

  /* We get here if (1) not all of the data has been ACKed, (2) we have been
   * asked to retransmit data, (3) the connection is still healthy, and (4)
   * the outgoing packet is available for our use.  In this case, we are
   * now free to send more data to receiver -- UNLESS the buffer contains
   * unprocessed incoming data or window size is zero.  In that event, we
   * will have to wait for the next polling cycle.
   */

  if ((conn->tcpstateflags & TCP_ESTABLISHED) &&
      (flags & (TCP_POLL | TCP_REXMIT)) &&
      !(sq_empty(&conn->write_q)) &&
      conn->winsize > 0)
    {
      /* Check if the destination IP address is in the ARP  or Neighbor
       * table.  If not, then the send won't actually make it out... it
       * will be replaced with an ARP request or Neighbor Solicitation.
       */

      if (psock_send_addrchck(conn))
        {
          FAR struct tcp_wrbuffer_s *wrb;
          uint32_t predicted_seqno;
          size_t sndlen;

          /* Peek at the head of the write queue (but don't remove anything
           * from the write queue yet).  We know from the above test that
           * the write_q is not empty.
           */

          wrb = (FAR struct tcp_wrbuffer_s *)sq_peek(&conn->write_q);
          DEBUGASSERT(wrb);

          /* Get the amount of data that we can send in the next packet.
           * We will send either the remaining data in the buffer I/O
           * buffer chain, or as much as will fit given the MSS and current
           * window size.
           */

          sndlen = TCP_WBPKTLEN(wrb) - TCP_WBSENT(wrb);
          if (sndlen > conn->mss)
            {
              sndlen = conn->mss;
            }

          if (sndlen > conn->winsize)
            {
              sndlen = conn->winsize;
            }

          ninfo("SEND: wrb=%p pktlen=%u sent=%u sndlen=%u mss=%u "
                "winsize=%u\n",
                wrb, TCP_WBPKTLEN(wrb), TCP_WBSENT(wrb), sndlen, conn->mss,
                conn->winsize);

          /* Set the sequence number for this segment.  If we are
           * retransmitting, then the sequence number will already
           * be set for this write buffer.
           */

           if (TCP_WBSEQNO(wrb) == (unsigned)-1)
            {
              TCP_WBSEQNO(wrb) = conn->isn + conn->sent;
            }

          /* The TCP stack updates sndseq on receipt of ACK *before*
           * this function is called. In that case sndseq will point
           * to the next unacknowledged byte (which might have already
           * been sent). We will overwrite the value of sndseq here
           * before the packet is sent.
           */

          tcp_setsequence(conn->sndseq, TCP_WBSEQNO(wrb) + TCP_WBSENT(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 with the offset
           * corresponding to the amount of data already sent. (this
           * won't actually happen until the polling cycle completes).
           */

          devif_iob_send(dev, TCP_WBIOB(wrb), sndlen, TCP_WBSENT(wrb));

          /* Remember how much data we send out now so that we know
           * when everything has been acknowledged.  Just increment
           * the amount of data sent. This will be needed in sequence
           * number calculations.
           */

          conn->unacked += sndlen;
          conn->sent    += sndlen;

          /* Below prediction will become true, unless retransmission occurrence */

          predicted_seqno = tcp_getsequence(conn->sndseq) + sndlen;

          if ((predicted_seqno > conn->sndseq_max) ||
              (tcp_getsequence(conn->sndseq) > predicted_seqno)) /* overflow */
            {
               conn->sndseq_max = predicted_seqno;
            }

          ninfo("SEND: wrb=%p nrtx=%u unacked=%u sent=%u\n",
                wrb, TCP_WBNRTX(wrb), conn->unacked, conn->sent);

          /* Increment the count of bytes sent from this write buffer */

          TCP_WBSENT(wrb) += sndlen;

          ninfo("SEND: wrb=%p sent=%u pktlen=%u\n",
                wrb, TCP_WBSENT(wrb), TCP_WBPKTLEN(wrb));

          /* Remove the write buffer from the write queue if the
           * last of the data has been sent from the buffer.
           */

          DEBUGASSERT(TCP_WBSENT(wrb) <= TCP_WBPKTLEN(wrb));
          if (TCP_WBSENT(wrb) >= TCP_WBPKTLEN(wrb))
            {
              FAR struct tcp_wrbuffer_s *tmp;

              ninfo("SEND: wrb=%p Move to unacked_q\n", wrb);

              tmp = (FAR struct tcp_wrbuffer_s *)sq_remfirst(&conn->write_q);
              DEBUGASSERT(tmp == wrb);
              UNUSED(tmp);

              /* Put the I/O buffer chain in the un-acked queue; the
               * segment is waiting for ACK again
               */

              psock_insert_segment(wrb, &conn->unacked_q);
            }

          /* Only one data can be sent by low level driver at once,
           * tell the caller stop polling the other connection.
           */

          flags &= ~TCP_POLL;
        }
    }

  /* Continue waiting */

  return flags;
}

/****************************************************************************
 * Name: send_txnotify
 *
 * Description:
 *   Notify the appropriate device driver that we are have data ready to
 *   be send (TCP)
 *
 * Input Parameters:
 *   psock - Socket state structure
 *   conn  - The TCP connection structure
 *
 * Returned Value:
 *   None
 *
 ****************************************************************************/

static inline void send_txnotify(FAR struct socket *psock,
                                 FAR struct tcp_conn_s *conn)
{
#ifdef CONFIG_NET_IPv4
#ifdef CONFIG_NET_IPv6
  /* If both IPv4 and IPv6 support are enabled, then we will need to select
   * the device driver using the appropriate IP domain.
   */

  if (psock->s_domain == PF_INET)
#endif
    {
      /* Notify the device driver that send data is available */

      netdev_ipv4_txnotify(conn->u.ipv4.laddr, conn->u.ipv4.raddr);
    }
#endif /* CONFIG_NET_IPv4 */

#ifdef CONFIG_NET_IPv6
#ifdef CONFIG_NET_IPv4
  else /* if (psock->s_domain == PF_INET6) */
#endif /* CONFIG_NET_IPv4 */
    {
      /* Notify the device driver that send data is available */

      DEBUGASSERT(psock->s_domain == PF_INET6);
      netdev_ipv6_txnotify(conn->u.ipv6.laddr, conn->u.ipv6.raddr);
    }
#endif /* CONFIG_NET_IPv6 */
}

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name: psock_tcp_send
 *
 * Description:
 *   psock_tcp_send() call may be used only when the TCP socket is in a
 *   connected state (so that the intended recipient is known).
 *
 * Input Parameters:
 *   psock    An instance of the internal socket structure.
 *   buf      Data to send
 *   len      Length of data to send
 *
 * Returned Value:
 *   On success, returns the number of characters sent.  On  error,
 *   -1 is returned, and errno is set appropriately:
 *
 *   EAGAIN or EWOULDBLOCK
 *     The socket is marked non-blocking and the requested operation
 *     would block.
 *   EBADF
 *     An invalid descriptor was specified.
 *   ECONNRESET
 *     Connection reset by peer.
 *   EDESTADDRREQ
 *     The socket is not connection-mode, and no peer address is set.
 *   EFAULT
 *      An invalid user space address was specified for a parameter.
 *   EINTR
 *      A signal occurred before any data was transmitted.
 *   EINVAL
 *      Invalid argument passed.
 *   EISCONN
 *     The connection-mode socket was connected already but a recipient
 *     was specified. (Now either this error is returned, or the recipient
 *     specification is ignored.)
 *   EMSGSIZE
 *     The socket type requires that message be sent atomically, and the
 *     size of the message to be sent made this impossible.
 *   ENOBUFS
 *     The output queue for a network interface was full. This generally
 *     indicates that the interface has stopped sending, but may be
 *     caused by transient congestion.
 *   ENOMEM
 *     No memory available.
 *   ENOTCONN
 *     The socket is not connected, and no target has been given.
 *   ENOTSOCK
 *     The argument s is not a socket.
 *   EPIPE
 *     The local end has been shut down on a connection oriented socket.
 *     In this case the process will also receive a SIGPIPE unless
 *     MSG_NOSIGNAL is set.
 *
 ****************************************************************************/

ssize_t psock_tcp_send(FAR struct socket *psock, FAR const void *buf,
                       size_t len)
{
  FAR struct tcp_conn_s *conn;
  FAR struct tcp_wrbuffer_s *wrb;
  ssize_t    result = 0;
  int        ret = OK;

  if (psock == NULL || psock->s_crefs <= 0)
    {
      nerr("ERROR: Invalid socket\n");
      ret = -EBADF;
      goto errout;
    }

  if (psock->s_type != SOCK_STREAM || !_SS_ISCONNECTED(psock->s_flags))
    {
      nerr("ERROR: Not connected\n");
      ret = -ENOTCONN;
      goto errout;
    }

  /* Make sure that we have the IP address mapping */

  conn = (FAR struct tcp_conn_s *)psock->s_conn;
  DEBUGASSERT(conn);

#if defined(CONFIG_NET_ARP_SEND) || defined(CONFIG_NET_ICMPv6_NEIGHBOR)
#ifdef CONFIG_NET_ARP_SEND
#ifdef CONFIG_NET_ICMPv6_NEIGHBOR
  if (psock->s_domain == PF_INET)
#endif
    {
      /* Make sure that the IP address mapping is in the ARP table */

      ret = arp_send(conn->u.ipv4.raddr);
    }
#endif /* CONFIG_NET_ARP_SEND */

#ifdef CONFIG_NET_ICMPv6_NEIGHBOR
#ifdef CONFIG_NET_ARP_SEND
  else
#endif
    {
      /* Make sure that the IP address mapping is in the Neighbor Table */

      ret = icmpv6_neighbor(conn->u.ipv6.raddr);
    }
#endif /* CONFIG_NET_ICMPv6_NEIGHBOR */

  /* Did we successfully get the address mapping? */

  if (ret < 0)
    {
      nerr("ERROR: Not reachable\n");
      ret = -ENETUNREACH;
      goto errout;
    }
#endif /* CONFIG_NET_ARP_SEND || CONFIG_NET_ICMPv6_NEIGHBOR */

  /* Dump the incoming buffer */

  BUF_DUMP("psock_tcp_send", buf, len);

  /* Set the socket state to sending */

  psock->s_flags = _SS_SETSTATE(psock->s_flags, _SF_SEND);

  if (len > 0)
    {
      /* Allocate a write buffer.  Careful, the network will be momentarily
       * unlocked here.
       */

      net_lock();
      if (_SS_ISNONBLOCK(psock->s_flags))
        {
          wrb = tcp_wrbuffer_tryalloc();
        }
      else
        {
          wrb = tcp_wrbuffer_alloc();
        }

      if (wrb == NULL)
        {
          /* A buffer allocation error occurred */

          nerr("ERROR: Failed to allocate write buffer\n");
          ret = _SS_ISNONBLOCK(psock->s_flags) ? -EAGAIN : -ENOMEM;
          goto errout_with_lock;
        }

      /* Allocate resources to receive a callback */

      if (psock->s_sndcb == NULL)
        {
          psock->s_sndcb = tcp_callback_alloc(conn);
        }

      /* Test if the callback has been allocated */

      if (psock->s_sndcb == NULL)
        {
          /* A buffer allocation error occurred */

          nerr("ERROR: Failed to allocate callback\n");
          ret = _SS_ISNONBLOCK(psock->s_flags) ? -EAGAIN : -ENOMEM;
          goto errout_with_wrb;
        }

      /* Set up the callback in the connection */

      psock->s_sndcb->flags = (TCP_ACKDATA | TCP_REXMIT | TCP_POLL |
                               TCP_DISCONN_EVENTS);
      psock->s_sndcb->priv  = (FAR void *)psock;
      psock->s_sndcb->event = psock_send_eventhandler;

      /* Initialize the write buffer */

      TCP_WBSEQNO(wrb) = (unsigned)-1;
      TCP_WBNRTX(wrb)  = 0;

      /* Copy the user data into the write buffer.  We cannot wait for
       * buffer space if the socket was opened non-blocking.
       */

      if (_SS_ISNONBLOCK(psock->s_flags))
        {
          /* The return value from TCP_WBTRYCOPYIN is either OK or
           * -ENOMEM if less than the entire data chunk could be allocated.
           * If -ENOMEM is returned, check if at least a part of the data
           * chunk was allocated. If more than zero bytes were sent
           * we return that number and let the caller deal with sending the
           * remaining data.
           */

          result = TCP_WBTRYCOPYIN(wrb, (FAR uint8_t *)buf, len);
          if (result == -ENOMEM)
            {
              if (TCP_WBPKTLEN(wrb) > 0)
                {
                  ninfo("INFO: Allocated part of the requested data\n");
                  result = TCP_WBPKTLEN(wrb);
                }
              else
                {
                  nerr("ERROR: Failed to add data to the I/O buffer chain\n");
                  ret = -EWOULDBLOCK;
                  goto errout_with_wrb;
                }
            }
          else
            {
              result = len;
            }
        }
      else
        {
          result = TCP_WBCOPYIN(wrb, (FAR uint8_t *)buf, len);
        }

      /* Dump I/O buffer chain */

      TCP_WBDUMP("I/O buffer chain", wrb, TCP_WBPKTLEN(wrb), 0);

      /* psock_send_eventhandler() will send data in FIFO order from the
       * conn->write_q
       */

      sq_addlast(&wrb->wb_node, &conn->write_q);
      ninfo("Queued WRB=%p pktlen=%u write_q(%p,%p)\n",
            wrb, TCP_WBPKTLEN(wrb),
            conn->write_q.head, conn->write_q.tail);

      /* Notify the device driver of the availability of TX data */

      send_txnotify(psock, conn);
      net_unlock();
    }

  /* Set the socket state to idle */

  psock->s_flags = _SS_SETSTATE(psock->s_flags, _SF_IDLE);

  /* Check for errors.  Errors are signaled by negative errno values
   * for the send length
   */

  if (result < 0)
    {
      ret = result;
      goto errout;
    }

  if (ret < 0)
    {
      goto errout;
    }

  /* Return the number of bytes actually sent */

  return result;

errout_with_wrb:
  tcp_wrbuffer_release(wrb);

errout_with_lock:
  net_unlock();

errout:
  return ret;
}

/****************************************************************************
 * Name: psock_tcp_cansend
 *
 * Description:
 *   psock_tcp_cansend() returns a value indicating if a write to the socket
 *   would block.  No space in the buffer is actually reserved, so it is
 *   possible that the write may still block if the buffer is filled by
 *   another means.
 *
 * Input Parameters:
 *   psock    An instance of the internal socket structure.
 *
 * Returned Value:
 *   OK
 *     At least one byte of data could be successfully written.
 *   -EWOULDBLOCK
 *     There is no room in the output buffer.
 *   -EBADF
 *     An invalid descriptor was specified.
 *   -ENOTCONN
 *     The socket is not connected.
 *
 ****************************************************************************/

int psock_tcp_cansend(FAR struct socket *psock)
{
  /* Verify that we received a valid socket */

  if (!psock || psock->s_crefs <= 0)
    {
      nerr("ERROR: Invalid socket\n");
      return -EBADF;
    }

  /* Verify that this is connected TCP socket */

  if (psock->s_type != SOCK_STREAM || !_SS_ISCONNECTED(psock->s_flags))
    {
      nerr("ERROR: Not connected\n");
      return -ENOTCONN;
    }

  /* In order to setup the send, we need to have at least one free write
   * buffer head and at least one free IOB to initialize the write buffer
   * head.
   *
   * REVISIT:  The send will still block if we are unable to buffer the entire
   * user-provided buffer which may be quite large.  We will almost certainly
   * need to have more than one free IOB, but we don't know how many more.
   */

  if (tcp_wrbuffer_test() < 0 || iob_navail(false) <= 0)
    {
      return -EWOULDBLOCK;
    }

  return OK;
}

#endif /* CONFIG_NET && CONFIG_NET_TCP && CONFIG_NET_TCP_WRITE_BUFFERS */