/**
 * @file
 * Transmission Control Protocol, outgoing traffic
 *
 * The output functions of TCP.
 *
 */

/*
 * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
 * All rights reserved.
 *
 * 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. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 AUTHOR 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.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 * Author: Adam Dunkels <adam@sics.se>
 *
 */

#include "vma/lwip/opt.h"

#if LWIP_TCP /* don't build if not configured for use in lwipopts.h */

#include "vma/lwip/tcp_impl.h"
#include "vma/lwip/stats.h"

#include <string.h>
#include <errno.h>

/* Define some copy-macros for checksum-on-copy so that the code looks
   nicer by preventing too many ifdef's. */
#if TCP_CHECKSUM_ON_COPY
#define TCP_DATA_COPY(dst, src, len, seg) do { \
  tcp_seg_add_chksum(LWIP_CHKSUM_COPY(dst, src, len), \
                     len, &seg->chksum, &seg->chksum_swapped); \
  seg->flags |= TF_SEG_DATA_CHECKSUMMED; } while(0)
#define TCP_DATA_COPY2(dst, src, len, chksum, chksum_swapped)  \
  tcp_seg_add_chksum(LWIP_CHKSUM_COPY(dst, src, len), len, chksum, chksum_swapped);
#else /* TCP_CHECKSUM_ON_COPY*/
#define TCP_DATA_COPY(dst, src, len, seg)                     MEMCPY(dst, src, len)
#define TCP_DATA_COPY2(dst, src, len, chksum, chksum_swapped) MEMCPY(dst, src, len)
#endif /* TCP_CHECKSUM_ON_COPY*/

/** Define this to 1 for an extra check that the output checksum is valid
 * (usefule when the checksum is generated by the application, not the stack) */
#ifndef TCP_CHECKSUM_ON_COPY_SANITY_CHECK
#define TCP_CHECKSUM_ON_COPY_SANITY_CHECK   0
#endif

#if LWIP_DEBUG_ENABLE
static char* _dump_seg(struct tcp_seg *seg)
{
	static __thread char _tcp_dump_buf[100];
	struct tcp_seg *cur_seg = NULL;
	struct pbuf *cur_pbuf = NULL;
	int seg_num = 0;
	int pbuf_num = 0;
	int seg_len = 0;
	int pbuf_len = 0;

	cur_seg = seg;
	while (cur_seg) {
		seg_len += cur_seg->len;
		seg_num++;
		cur_pbuf = cur_seg->p;
		while (cur_pbuf) {
			pbuf_len += cur_pbuf->len;
			pbuf_num++;
			cur_pbuf = cur_pbuf->next;
		}
		cur_seg = cur_seg->next;
	}

	snprintf(_tcp_dump_buf, sizeof(_tcp_dump_buf),
			"[seg] num: %-2d len: %-6d [pbuf] num: %-2d len: %-6d",
			seg_num, seg_len, pbuf_num, pbuf_len);
	return _tcp_dump_buf;
}
#endif /* LWIP_DEBUG_ENABLE */

sys_now_fn sys_now;
void register_sys_now(sys_now_fn fn)
{
	sys_now = fn;
}

#if LWIP_3RD_PARTY_L3
ip_output_fn external_ip_output;

void register_ip_output(ip_output_fn fn)
{
    external_ip_output = fn;
}

ip_route_mtu_fn external_ip_route_mtu;

void register_ip_route_mtu(ip_route_mtu_fn fn)
{
    external_ip_route_mtu = fn;
}

sys_readv_fn sys_readv;
void register_sys_readv(sys_readv_fn fn)
{
	sys_readv = fn;
}
#endif

/* Forward declarations.*/
static void tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb);

/** Allocate a pbuf and create a tcphdr at p->payload, used for output
 * functions other than the default tcp_output -> tcp_output_segment
 * (e.g. tcp_send_empty_ack, etc.)
 *
 * @param pcb tcp pcb for which to send a packet (used to initialize tcp_hdr)
 * @param optlen length of header-options
 * @param datalen length of tcp data to reserve in pbuf
 * @param seqno_be seqno in network byte order (big-endian)
 * @return pbuf with p->payload being the tcp_hdr
 */
static struct pbuf *
tcp_output_alloc_header(struct tcp_pcb *pcb, u16_t optlen, u16_t datalen,
                      u32_t seqno_be /* already in network byte order */)
{
  struct tcp_hdr *tcphdr;
  struct pbuf *p = tcp_tx_pbuf_alloc(pcb, optlen + datalen, PBUF_RAM);
  if (p != NULL) {
    pbuf_header(p, TCP_HLEN);
    LWIP_ASSERT("check that first pbuf can hold struct tcp_hdr",
                 (p->len >= TCP_HLEN + optlen));
    tcphdr = (struct tcp_hdr *)p->payload;
    tcphdr->src = htons(pcb->local_port);
    tcphdr->dest = htons(pcb->remote_port);
    tcphdr->seqno = seqno_be;
    tcphdr->ackno = htonl(pcb->rcv_nxt);
    TCPH_HDRLEN_FLAGS_SET(tcphdr, (5 + optlen / 4), TCP_ACK);
    tcphdr->wnd = htons(TCPWND_MIN16(RCV_WND_SCALE(pcb, pcb->rcv_ann_wnd)));
    tcphdr->chksum = 0;
    tcphdr->urgp = 0;

    /* If we're sending a packet, update the announced right window edge */
    pcb->rcv_ann_right_edge = pcb->rcv_nxt + pcb->rcv_ann_wnd;
  }
  return p;
}

/**
 * Called by tcp_close() to send a segment including FIN flag but not data.
 *
 * @param pcb the tcp_pcb over which to send a segment
 * @return ERR_OK if sent, another err_t otherwise
 */
err_t
tcp_send_fin(struct tcp_pcb *pcb)
{
  /* first, try to add the fin to the last unsent segment */
  if (pcb->unsent != NULL) {
    struct tcp_seg *last_unsent;
    for (last_unsent = pcb->unsent; last_unsent->next != NULL;
         last_unsent = last_unsent->next);

    if ((TCPH_FLAGS(last_unsent->tcphdr) & (TCP_SYN | TCP_FIN | TCP_RST)) == 0) {
      /* no SYN/FIN/RST flag in the header, we can add the FIN flag */
      TCPH_SET_FLAG(last_unsent->tcphdr, TCP_FIN);
      pcb->flags |= TF_FIN;
      return ERR_OK;
    }
  }
  /* no data, no length, flags, copy=1, no optdata */
  return tcp_enqueue_flags(pcb, TCP_FIN);
}

/**
 * Create a TCP segment with prefilled header.
 *
 * Called by tcp_write and tcp_enqueue_flags.
 *
 * @param pcb Protocol control block for the TCP connection.
 * @param p pbuf that is used to hold the TCP header.
 * @param flags TCP flags for header.
 * @param seqno TCP sequence number of this packet
 * @param optflags options to include in TCP header
 * @return a new tcp_seg pointing to p, or NULL.
 * The TCP header is filled in except ackno and wnd.
 * p is freed on failure.
 */
static struct tcp_seg *
tcp_create_segment(struct tcp_pcb *pcb, struct pbuf *p, u8_t flags, u32_t seqno, u8_t optflags)
{
  struct tcp_seg *seg;
  u8_t optlen = LWIP_TCP_OPT_LENGTH(optflags);

  if (!pcb->seg_alloc) {
    // seg_alloc is not valid, we should allocate a new segment.
    if ((seg = external_tcp_seg_alloc(pcb)) == NULL) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_create_segment: no memory.\n"));
      tcp_tx_pbuf_free(pcb, p);
      return NULL;
    }

    seg->next = NULL;
#if TCP_OVERSIZE_DBGCHECK
    seg->oversize_left = 0;
#endif /* TCP_OVERSIZE_DBGCHECK */
#if TCP_CHECKSUM_ON_COPY
    seg->chksum = 0;
    seg->chksum_swapped = 0;
    /* check optflags */
    LWIP_ASSERT("invalid optflags passed: TF_SEG_DATA_CHECKSUMMED",
              (optflags & TF_SEG_DATA_CHECKSUMMED) == 0);
#endif /* TCP_CHECKSUM_ON_COPY */

    if (p == NULL) {
      // Request a new segment in order to update seg_alloc for the next packet.
      seg->p = NULL;
      return seg;
    }
  } else {
    // seg_alloc is valid, we dont need to allocate a new segment element.
    seg = pcb->seg_alloc;
    pcb->seg_alloc = NULL;
  }

  seg->flags = optflags;
  seg->p = p;
#if LWIP_TSO
#else
  seg->dataptr = p->payload;
#endif /* LWIP_TSO */
  seg->len = p->tot_len - optlen;
  seg->seqno = seqno;

  /* build TCP header */
  if (pbuf_header(p, TCP_HLEN)) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_create_segment: no room for TCP header in pbuf.\n"));
    TCP_STATS_INC(tcp.err);
    tcp_tx_seg_free(pcb, seg);
    return NULL;
  }
  seg->tcphdr = (struct tcp_hdr *)seg->p->payload;
  seg->tcphdr->src = htons(pcb->local_port);
  seg->tcphdr->dest = htons(pcb->remote_port);
  seg->tcphdr->seqno = htonl(seqno);
  /* ackno is set in tcp_output */
  TCPH_HDRLEN_FLAGS_SET(seg->tcphdr, (5 + optlen / 4), flags);
  /* wnd and chksum are set in tcp_output */
  seg->tcphdr->urgp = 0;
  return seg;
} 

/**
 * Allocate a PBUF_RAM pbuf, perhaps with extra space at the end.
 *
 * This function is like pbuf_alloc(layer, length, PBUF_RAM) except
 * there may be extra bytes available at the end.
 *
 * @param length size of the pbuf's payload.
 * @param max_length maximum usable size of payload+oversize.
 * @param oversize pointer to a u16_t that will receive the number of usable tail bytes.
 * @param pcb The TCP connection that willo enqueue the pbuf.
 * @param tcp_write_flag_more true if TCP_WRITE_FLAG_MORE flag was enabled.
 * @param first_seg true when this pbuf will be used in the first enqueued segment.
 * @param 
 */
static struct pbuf *
tcp_pbuf_prealloc(u16_t length, u16_t max_length,
                  u16_t *oversize, struct tcp_pcb *pcb, u8_t tcp_write_flag_more,
                  u8_t first_seg)
{
  struct pbuf *p;
  u16_t alloc = length;

  if (length < max_length) {
    /* Should we allocate an oversized pbuf, or just the minimum
     * length required? If tcp_write is going to be called again
     * before this segment is transmitted, we want the oversized
     * buffer. If the segment will be transmitted immediately, we can
     * save memory by allocating only length. We use a simple
     * heuristic based on the following information:
     *
     * Will the Nagle algorithm defer transmission of this segment?
     */
    if (tcp_write_flag_more ||
        (!(pcb->flags & TF_NODELAY) &&
         (!first_seg ||
          pcb->unsent != NULL ||
          pcb->unacked != NULL))) {
          alloc = LWIP_MIN(max_length, LWIP_MEM_ALIGN_SIZE(length + pcb->tcp_oversize_val));
    }
  }
  p = tcp_tx_pbuf_alloc(pcb, alloc, PBUF_RAM);
  if (p == NULL) {
    return NULL;
  }
  LWIP_ASSERT("need unchained pbuf", p->next == NULL);
  *oversize = p->len - length;
  /* trim p->len to the currently used size */
  p->len = p->tot_len = length;
  return p;
}

/** Checks if tcp_write is allowed or not (checks state, snd_buf and snd_queuelen).
 *
 * @param pcb the tcp pcb to check for
 * @param len length of data to send (checked agains snd_buf)
 * @return ERR_OK if tcp_write is allowed to proceed, another err_t otherwise
 */
static err_t
tcp_write_checks(struct tcp_pcb *pcb, u32_t len)
{
  /* connection is in invalid state for data transmission? */
  if ((get_tcp_state(pcb) != ESTABLISHED) &&
      (get_tcp_state(pcb) != CLOSE_WAIT) &&
      (get_tcp_state(pcb) != SYN_SENT) &&
      (get_tcp_state(pcb) != SYN_RCVD)) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_STATE | LWIP_DBG_LEVEL_SEVERE, ("tcp_write() called in invalid state\n"));
    return ERR_CONN;
  } else if (len == 0) {
    return ERR_OK;
  }

  /* fail on too much data */
  if (len > pcb->snd_buf) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_write: too much data (len=%"U32_F" > snd_buf=%"U32_F")\n",
      len, pcb->snd_buf));
    pcb->flags |= TF_NAGLEMEMERR;
    return ERR_MEM;
  }
  LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_write: queuelen: %"U32_F"\n", (u32_t)pcb->snd_queuelen));

  /* If total number of pbufs on the unsent/unacked queues exceeds the
   * configured maximum, return an error */
  /* check for configured max queuelen and possible overflow */
  if ((pcb->snd_queuelen >= pcb->max_unsent_len) || (pcb->snd_queuelen > TCP_SNDQUEUELEN_OVERFLOW)) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_write: too long queue %"U32_F" (max %"U32_F")\n",
      pcb->snd_queuelen, pcb->max_unsent_len));
    TCP_STATS_INC(tcp.memerr);
    pcb->flags |= TF_NAGLEMEMERR;
    return ERR_MEM;
  }
  if (pcb->snd_queuelen != 0) {
  } else {
    LWIP_ASSERT("tcp_write: no pbufs on queue => both queues empty",
      pcb->unacked == NULL && pcb->unsent == NULL);
  }
  return ERR_OK;
}

#if LWIP_TSO
static inline u16_t tcp_xmit_size_goal(struct tcp_pcb *pcb, int use_max)
{
  u16_t size = pcb->mss;

#if LWIP_TCP_TIMESTAMPS
  if ((pcb->flags & TF_TIMESTAMP)) {
    /* ensure that segments can hold at least one data byte... */
    size = LWIP_MAX(size, LWIP_TCP_OPT_LEN_TS + 1);
  }
#endif /* LWIP_TCP_TIMESTAMPS */

#if LWIP_TSO
  if (use_max && tcp_tso(pcb) && pcb->tso.max_buf_sz) {
    /* use maximum buffer size in case TSO */
    size = LWIP_MAX(size, pcb->tso.max_buf_sz);
  }
#endif /* LWIP_TSO */

  /* don't allocate segments bigger than half the maximum window we ever received */
  size = LWIP_MIN(size, (pcb->snd_wnd_max >> 1));

  return size;
}
#endif /* LWIP_TSO */

/**
 * Write data for sending (but does not send it immediately).
 *
 * It waits in the expectation of more data being sent soon (as
 * it can send them more efficiently by combining them together).
 * To prompt the system to send data now, call tcp_output() after
 * calling tcp_write().
 *
 * @param pcb Protocol control block for the TCP connection to enqueue data for.
 * @param arg Pointer to the data to be enqueued for sending.
 * @param len Data length in bytes
 * @param apiflags combination of following flags :
 * - TCP_WRITE_FLAG_COPY (0x01) data will be copied into memory belonging to the stack
 * - TCP_WRITE_FLAG_MORE (0x02) for TCP connection, PSH flag will be set on last segment sent
 * - TCP_WRITE_DUMMY (0x10) indicates if the packet is a dummy packet
 * - TCP_WRITE_FILE (0x40) data should be taken from file
 * @return ERR_OK if enqueued, another err_t on error
 */
err_t
tcp_write(struct tcp_pcb *pcb, const void *arg, u32_t len, u8_t apiflags)
{
  struct pbuf *concat_p = NULL;
  struct tcp_seg *seg = NULL, *prev_seg = NULL, *queue = NULL;
  u32_t pos = 0; /* position in 'arg' data */
  u32_t queuelen;
  u8_t optlen = 0;
  u8_t optflags = 0;
#if TCP_OVERSIZE
  u16_t oversize = 0;
  u16_t oversize_used = 0;
#endif /* TCP_OVERSIZE */
#if TCP_CHECKSUM_ON_COPY
  u16_t concat_chksum = 0;
  u8_t concat_chksum_swapped = 0;
  u16_t concat_chksummed = 0;
#endif /* TCP_CHECKSUM_ON_COPY */
  err_t err;
  u16_t mss_local = 0;
#if LWIP_TSO
  int tot_p = 0;
#endif /* LWIP_TSO */
  const int piov_max_size = 512;
  const int piov_max_len = 65536;
  struct iovec piov[piov_max_size];
  int piov_cur_index = 0;
  int piov_cur_len = 0;

  int byte_queued = pcb->snd_nxt - pcb->lastack;
  if ( len < pcb->mss && !(apiflags & TCP_WRITE_DUMMY))
          pcb->snd_sml_add = (pcb->unacked ? pcb->unacked->len : 0) + byte_queued;

  LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_write(pcb=%p, data=%p, len=%"U16_F", apiflags=%"U16_F")\n",
    (void *)pcb, arg, len, (u16_t)apiflags));
  LWIP_ERROR("tcp_write: arg == NULL (programmer violates API)", 
             arg != NULL, return ERR_ARG;);

  err = tcp_write_checks(pcb, len);
  if (err != ERR_OK) {
    return err;
  }
  queuelen = pcb->snd_queuelen;

#if LWIP_TSO
  mss_local = tcp_xmit_size_goal(pcb, 1);
#else
  mss_local = LWIP_MIN(pcb->mss, pcb->snd_wnd_max/2);
  mss_local = mss_local ? mss_local : pcb->mss;
#endif /* LWIP_TSO */

  optflags |= (apiflags & TCP_WRITE_DUMMY ? TF_SEG_OPTS_DUMMY_MSG : 0);

#if LWIP_TCP_TIMESTAMPS
  if ((pcb->flags & TF_TIMESTAMP)) {
    optflags |= TF_SEG_OPTS_TS;
#if LWIP_TSO
#else
    /* ensure that segments can hold at least one data byte... */
    mss_local = LWIP_MAX(mss_local, LWIP_TCP_OPT_LEN_TS + 1);
#endif /* LWIP_TSO */
  }
#endif /* LWIP_TCP_TIMESTAMPS */

  optlen = LWIP_TCP_OPT_LENGTH( optflags );

  /*
   * TCP segmentation is done in three phases with increasing complexity:
   *
   * 1. Copy data directly into an oversized pbuf.
   * 2. Chain a new pbuf to the end of pcb->unsent.
   * 3. Create new segments.
   *
   * We may run out of memory at any point. In that case we must
   * return ERR_MEM and not change anything in pcb. Therefore, all
   * changes are recorded in local variables and committed at the end
   * of the function. Some pcb fields are maintained in local copies:
   *
   * queuelen = pcb->snd_queuelen
   * oversize = pcb->unsent_oversize
   *
   * These variables are set consistently by the phases:
   *
   * seg points to the last segment tampered with.
   *
   * pos records progress as data is segmented.
   */

  /* Find the tail of the unsent queue. */
  if (pcb->unsent != NULL) {
    u16_t space;
    u16_t unsent_optlen;

    if (!pcb->last_unsent || pcb->last_unsent->next) {
      /* @todo: this could be sped up by keeping last_unsent in the pcb */
      for (pcb->last_unsent = pcb->unsent; pcb->last_unsent->next != NULL;
           pcb->last_unsent = pcb->last_unsent->next);
    }
    /* Usable space at the end of the last unsent segment */
    unsent_optlen = LWIP_TCP_OPT_LENGTH(pcb->last_unsent->flags);
    LWIP_ASSERT("mss_local is too small", mss_local >= pcb->last_unsent->len + unsent_optlen);
    space = mss_local - (pcb->last_unsent->len + unsent_optlen);
    seg = pcb->last_unsent;
#if LWIP_TSO
    tot_p = pbuf_clen(seg->p);
#endif /* LWIP_TSO */

    /*
     * Phase 1: Copy data directly into an oversized pbuf.
     *
     * The number of bytes copied is recorded in the oversize_used
     * variable. The actual copying is done at the bottom of the
     * function.
     */
#if TCP_OVERSIZE
#if TCP_OVERSIZE_DBGCHECK
    /* check that pcb->unsent_oversize matches last_unsent->unsent_oversize */
    LWIP_ASSERT("unsent_oversize mismatch (pcb vs. last_unsent)",
                pcb->unsent_oversize == pcb->last_unsent->oversize_left);
#endif /* TCP_OVERSIZE_DBGCHECK */

    if (pcb->unsent_oversize > 0) {
      if (!(apiflags & TCP_WRITE_FILE)) {
        oversize = pcb->unsent_oversize;
        LWIP_ASSERT("inconsistent oversize vs. space", oversize_used <= space);
        oversize_used = oversize < len ? oversize : len;
        pos += oversize_used;
        oversize -= oversize_used;
        space -= oversize_used;
      }
    }
    /* now we are either finished or oversize is zero */
    LWIP_ASSERT("inconsistend oversize vs. len", (oversize == 0) || (pos == len));
#endif /* TCP_OVERSIZE */

    /*
     * Phase 2: Chain a new pbuf to the end of pcb->unsent.
     *
     * We don't extend segments containing SYN/FIN flags or options
     * (len==0). The new pbuf is kept in concat_p and pbuf_cat'ed at
     * the end.
     */
#if LWIP_TSO
    if (!(apiflags & TCP_WRITE_FILE) && (pos < len) && (space > 0) && (pcb->last_unsent->len > 0) &&
        (tot_p < (int)pcb->tso.max_send_sge)) {
#else
    if (!(apiflags & TCP_WRITE_FILE) && (pos < len) && (space > 0) && (pcb->last_unsent->len > 0)) {
#endif /* LWIP_TSO */

      u16_t seglen = space < len - pos ? space : len - pos;

      /* Create a pbuf with a copy or reference to seglen bytes. We
       * can use PBUF_RAW here since the data appears in the middle of
       * a segment. A header will never be prepended. */
      /* Data is copied */
      if ((concat_p = tcp_pbuf_prealloc(seglen, space, &oversize, pcb, TCP_WRITE_FLAG_MORE, 1)) == NULL) {
    	  LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2,
    			  ("tcp_write : could not allocate memory for pbuf copy size %"U16_F"\n",
    					  seglen));
    	  goto memerr;
      }
#if TCP_OVERSIZE_DBGCHECK
      pcb->last_unsent->oversize_left += oversize;
#endif /* TCP_OVERSIZE_DBGCHECK */
      TCP_DATA_COPY2(concat_p->payload, (u8_t*)arg + pos, seglen, &concat_chksum, &concat_chksum_swapped);
#if TCP_CHECKSUM_ON_COPY
      concat_chksummed += seglen;
#endif /* TCP_CHECKSUM_ON_COPY */

      pos += seglen;
      queuelen += pbuf_clen(concat_p);
    }
  } else {
#if TCP_OVERSIZE
    pcb->last_unsent = NULL;
    LWIP_ASSERT("unsent_oversize mismatch (pcb->unsent is NULL)",
                pcb->unsent_oversize == 0);
#endif /* TCP_OVERSIZE */
  }

  /*
   * Phase 3: Create new segments.
   *
   * The new segments are chained together in the local 'queue'
   * variable, ready to be appended to pcb->unsent.
   */
  while (pos < len) {
    struct pbuf *p;
    u32_t left = len - pos;
    u16_t max_len = mss_local - optlen;
    u16_t seglen = left > max_len ? max_len : left;

    /* If copy is set, memory should be allocated and data copied
     * into pbuf */
    if ((p = tcp_pbuf_prealloc(seglen + optlen, max_len, &oversize, pcb, TCP_WRITE_FLAG_MORE, queue == NULL)) == NULL) {
    	LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_write : could not allocate memory for pbuf copy size %"U16_F"\n", seglen));
    	goto memerr;
    }
    LWIP_ASSERT("tcp_write: check that first pbuf can hold the complete seglen",
    		(p->len >= seglen));
    if (apiflags & TCP_WRITE_FILE) {
       piov[piov_cur_index].iov_base = (void *)((char *)p->payload + optlen);
       piov[piov_cur_index].iov_len = seglen;

       piov_cur_index++;
       piov_cur_len += seglen;
       if ((left <= seglen ) || (piov_cur_index >= piov_max_size) || (piov_cur_len >= piov_max_len)) {
           int ret = 0;
           int fd = *(int *)arg;
           ret = sys_readv(fd, piov, piov_cur_index);
           /* Set as failure any unexpected return values because tcp_write() function
            * does not support partial write
            */
           if (ret != piov_cur_len) {
             goto memerr;
           }
           piov_cur_index = 0;
           piov_cur_len = 0;
       }
    } else {
       TCP_DATA_COPY2((char *)p->payload + optlen, (u8_t*)arg + pos, seglen, &chksum, &chksum_swapped);
    }

    queuelen += pbuf_clen(p);

    /* Now that there are more segments queued, we check again if the
     * length of the queue exceeds the configured maximum or
     * overflows. */
    if ((queuelen > pcb->max_unsent_len) || (queuelen > TCP_SNDQUEUELEN_OVERFLOW)) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_write: queue too long %"U32_F" (%"U32_F")\n", queuelen, pcb->max_unsent_len));
      tcp_tx_pbuf_free(pcb, p);
      goto memerr;
    }

    if ((seg = tcp_create_segment(pcb, p, 0, pcb->snd_lbb + pos, optflags)) == NULL) {
      goto memerr;
    }
#if TCP_OVERSIZE_DBGCHECK
    seg->oversize_left = oversize;
#endif /* TCP_OVERSIZE_DBGCHECK */
#if TCP_CHECKSUM_ON_COPY
    seg->chksum = chksum;
    seg->chksum_swapped = chksum_swapped;
    seg->flags |= TF_SEG_DATA_CHECKSUMMED;
#endif /* TCP_CHECKSUM_ON_COPY */

    /* first segment of to-be-queued data? */
    if (queue == NULL) {
      queue = seg;
    } else {
      /* Attach the segment to the end of the queued segments */
      LWIP_ASSERT("prev_seg != NULL", prev_seg != NULL);
      prev_seg->next = seg;
    }
    /* remember last segment of to-be-queued data for next iteration */
    prev_seg = seg;

    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_TRACE, ("tcp_write: queueing %"U32_F":%"U32_F"\n",
      ntohl(seg->tcphdr->seqno),
      ntohl(seg->tcphdr->seqno) + TCP_TCPLEN(seg)));

    pos += seglen;
  }

  /*
   * All three segmentation phases were successful. We can commit the
   * transaction.
   */

  /*
   * Phase 1: If data has been added to the preallocated tail of
   * last_unsent, we update the length fields of the pbuf chain.
   */
#if TCP_OVERSIZE
  if (oversize_used > 0) {
    struct pbuf *p;
    /* Bump tot_len of whole chain, len of tail */
    for (p = pcb->last_unsent->p; p; p = p->next) {
      p->tot_len += oversize_used;
      if (p->next == NULL) {
        TCP_DATA_COPY((char *)p->payload + p->len, arg, oversize_used, pcb->last_unsent);
        p->len += oversize_used;
      }
    }
    pcb->last_unsent->len += oversize_used;
#if TCP_OVERSIZE_DBGCHECK
    pcb->last_unsent->oversize_left -= oversize_used;
#endif /* TCP_OVERSIZE_DBGCHECK */
  }
  pcb->unsent_oversize = oversize;
#endif /* TCP_OVERSIZE */

  /*
   * Phase 2: concat_p can be concatenated onto pcb->last_unsent->p
   */
  if (concat_p != NULL) {
    LWIP_ASSERT("tcp_write: cannot concatenate when pcb->unsent is empty",
      (pcb->last_unsent != NULL));
    pbuf_cat(pcb->last_unsent->p, concat_p);
    pcb->last_unsent->len += concat_p->tot_len;
#if TCP_CHECKSUM_ON_COPY
    if (concat_chksummed) {
      tcp_seg_add_chksum(concat_chksum, concat_chksummed, &pcb->last_unsent->chksum,
        &pcb->last_unsent->chksum_swapped);
      pcb->last_unsent->flags |= TF_SEG_DATA_CHECKSUMMED;
    }
#endif /* TCP_CHECKSUM_ON_COPY */
  }

  /*
   * Phase 3: Append queue to pcb->unsent. Queue may be NULL, but that
   * is harmless
   */
  if (pcb->last_unsent == NULL) {
    pcb->unsent = queue;
  } else {
    pcb->last_unsent->next = queue;
  }
  pcb->last_unsent = seg;

  /*
   * Finally update the pcb state.
   */
  pcb->snd_lbb += len;
  pcb->snd_buf -= len;
  pcb->snd_queuelen = queuelen;

  LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_write: %"S16_F" (after enqueued)\n",
    pcb->snd_queuelen));
  if (pcb->snd_queuelen != 0) {
    LWIP_ASSERT("tcp_write: valid queue length",
                pcb->unacked != NULL || pcb->unsent != NULL);
  }

  /* Set the PSH flag in the last segment that we enqueued. */
  if (seg != NULL && seg->tcphdr != NULL) {
    TCPH_SET_FLAG(seg->tcphdr, TCP_PSH);
  }

  LWIP_DEBUGF(TCP_TSO_DEBUG | LWIP_DBG_TRACE,
              ("tcp_write:  mss: %-5d unsent %s\n", mss_local, _dump_seg(pcb->unsent)));

  return ERR_OK;
memerr:
  pcb->flags |= TF_NAGLEMEMERR;
  TCP_STATS_INC(tcp.memerr);

  if (concat_p != NULL) {
    tcp_tx_pbuf_free(pcb, concat_p);
  }
  if (queue != NULL) {
    tcp_tx_segs_free(pcb, queue);
  }
  if (pcb->snd_queuelen != 0) {
    LWIP_ASSERT("tcp_write: valid queue length", pcb->unacked != NULL ||
      pcb->unsent != NULL);
  }
  LWIP_DEBUGF(TCP_QLEN_DEBUG | LWIP_DBG_STATE, ("tcp_write: %"S16_F" (with mem err)\n", pcb->snd_queuelen));
  return ERR_MEM;
}

/**
 * Enqueue TCP options for transmission.
 *
 * Called by tcp_connect(), tcp_listen_input(), and tcp_send_ctrl().
 *
 * @param pcb Protocol control block for the TCP connection.
 * @param flags TCP header flags to set in the outgoing segment.
 * @param optdata pointer to TCP options, or NULL.
 * @param optlen length of TCP options in bytes.
 */
err_t
tcp_enqueue_flags(struct tcp_pcb *pcb, u8_t flags)
{
  struct pbuf *p;
  struct tcp_seg *seg;
  u8_t optflags = 0;
  u8_t optlen = 0;

  LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_enqueue_flags: queuelen: %"U16_F"\n", (u16_t)pcb->snd_queuelen));

  LWIP_ASSERT("tcp_enqueue_flags: need either TCP_SYN or TCP_FIN in flags (programmer violates API)",
              (flags & (TCP_SYN | TCP_FIN)) != 0);

  /* check for configured max queuelen and possible overflow (FIN flag should always come through!)*/
  if (((pcb->snd_queuelen >= pcb->max_unsent_len) || (pcb->snd_queuelen > TCP_SNDQUEUELEN_OVERFLOW)) &&
		  ((flags & TCP_FIN) == 0)) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_enqueue_flags: too long queue %"U16_F" (max %"U16_F")\n",
                                       pcb->snd_queuelen, pcb->max_unsent_len));
    TCP_STATS_INC(tcp.memerr);
    pcb->flags |= TF_NAGLEMEMERR;
    return ERR_MEM;
  }

  if (flags & TCP_SYN) {
    optflags = TF_SEG_OPTS_MSS;
    if(enable_wnd_scale && ((get_tcp_state(pcb) != SYN_RCVD) || (pcb->flags & TF_WND_SCALE))) {
    /* In a <SYN,ACK> (sent in state SYN_RCVD), the window scale option may only
        be sent if we received a window scale option from the remote host. */
        optflags |= TF_SEG_OPTS_WNDSCALE;
	}
	#if LWIP_TCP_TIMESTAMPS
    	if (pcb->enable_ts_opt && !(flags & TCP_ACK)) {
    		// enable initial timestamp announcement only for the connecting side. accepting side reply accordingly.
    		optflags |= TF_SEG_OPTS_TS;
    	}
	#endif
  }
#if LWIP_TCP_TIMESTAMPS
  if ((pcb->flags & TF_TIMESTAMP)) {
    optflags |= TF_SEG_OPTS_TS;
  }
#endif /* LWIP_TCP_TIMESTAMPS */
  optlen = LWIP_TCP_OPT_LENGTH(optflags);

  /* tcp_enqueue_flags is always called with either SYN or FIN in flags.
   * We need one available snd_buf byte to do that.
   * This means we can't send FIN while snd_buf==0. A better fix would be to
   * not include SYN and FIN sequence numbers in the snd_buf count. */

  /*if (pcb->snd_buf == 0) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_enqueue_flags: no send buffer available\n"));
    TCP_STATS_INC(tcp.memerr);
    return ERR_MEM;
  }*/ //to consider snd_buf for syn or fin, unmarked sections with SND_BUF_FOR_SYN_FIN

  /* Allocate pbuf with room for TCP header + options */
  if ((p = tcp_tx_pbuf_alloc(pcb, optlen, PBUF_RAM)) == NULL) {
    pcb->flags |= TF_NAGLEMEMERR;
    TCP_STATS_INC(tcp.memerr);
    return ERR_MEM;
  }
  LWIP_ASSERT("tcp_enqueue_flags: check that first pbuf can hold optlen",
              (p->len >= optlen));

  /* Allocate memory for tcp_seg, and fill in fields. */
  if ((seg = tcp_create_segment(pcb, p, flags, pcb->snd_lbb, optflags)) == NULL) {
    pcb->flags |= TF_NAGLEMEMERR;
    TCP_STATS_INC(tcp.memerr);
    return ERR_MEM;
  }
  LWIP_ASSERT("tcp_enqueue_flags: invalid segment length", seg->len == 0);

  LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_TRACE,
              ("tcp_enqueue_flags: queueing %"U32_F":%"U32_F" (0x%"X16_F")\n",
               ntohl(seg->tcphdr->seqno),
               ntohl(seg->tcphdr->seqno) + TCP_TCPLEN(seg),
               (u16_t)flags));

  /* Now append seg to pcb->unsent queue */
  if (pcb->unsent == NULL) {
    pcb->unsent = seg;
  } else {
    struct tcp_seg *useg;
    for (useg = pcb->unsent; useg->next != NULL; useg = useg->next);
    useg->next = seg;
  }
  pcb->last_unsent = seg;
#if TCP_OVERSIZE
  /* The new unsent tail has no space */
  pcb->unsent_oversize = 0;
#endif /* TCP_OVERSIZE */

  /* SYN and FIN bump the sequence number */
  if ((flags & TCP_SYN) || (flags & TCP_FIN)) {
    pcb->snd_lbb++;
    /* optlen does not influence snd_buf */
    // pcb->snd_buf--; SND_BUF_FOR_SYN_FIN
  }
  if (flags & TCP_FIN) {
    pcb->flags |= TF_FIN;
  }

  /* update number of segments on the queues */
  pcb->snd_queuelen += pbuf_clen(seg->p);
  LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_enqueue_flags: %"S16_F" (after enqueued)\n", pcb->snd_queuelen));
  if (pcb->snd_queuelen != 0) {
    LWIP_ASSERT("tcp_enqueue_flags: invalid queue length",
      pcb->unacked != NULL || pcb->unsent != NULL);
  }

  return ERR_OK;
}
 

#if LWIP_TCP_TIMESTAMPS
/* Build a timestamp option (12 bytes long) at the specified options pointer)
 *
 * @param pcb tcp_pcb
 * @param opts option pointer where to store the timestamp option
 */
static void
tcp_build_timestamp_option(struct tcp_pcb *pcb, u32_t *opts)
{
  /* Pad with two NOP options to make everything nicely aligned */
  opts[0] = PP_HTONL(0x0101080A);
  opts[1] = htonl(sys_now());
  opts[2] = htonl(pcb->ts_recent);
}
#endif

/** Send an ACK without data.
 *
 * @param pcb Protocol control block for the TCP connection to send the ACK
 */
err_t
tcp_send_empty_ack(struct tcp_pcb *pcb)
{
  struct pbuf *p;
  struct tcp_hdr *tcphdr;
  u8_t optlen = 0;
  u32_t *opts;

#if LWIP_TCP_TIMESTAMPS
  if (pcb->flags & TF_TIMESTAMP) {
    optlen = LWIP_TCP_OPT_LENGTH(TF_SEG_OPTS_TS);
  }
#endif

  p = tcp_output_alloc_header(pcb, optlen, 0, htonl(pcb->snd_nxt));
  if (p == NULL) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_output: (ACK) could not allocate pbuf\n"));
    return ERR_BUF;
  }
  tcphdr = (struct tcp_hdr *)p->payload;
  LWIP_DEBUGF(TCP_OUTPUT_DEBUG,
              ("tcp_output: sending ACK for %"U32_F"\n", pcb->rcv_nxt));
  /* remove ACK flags from the PCB, as we send an empty ACK now */
  pcb->flags &= ~(TF_ACK_DELAY | TF_ACK_NOW);

  opts = (u32_t *)(void *)(tcphdr + 1);

  /* NB. MSS option is only sent on SYNs, so ignore it here */
#if LWIP_TCP_TIMESTAMPS
  pcb->ts_lastacksent = pcb->rcv_nxt;

  if (pcb->flags & TF_TIMESTAMP) {
    tcp_build_timestamp_option(pcb, opts );
    opts += 3;
  }
#endif 
#if LWIP_TSO
  pcb->ip_output(p, pcb, 0);
#else
  pcb->ip_output(p, pcb, 0, 0);
#endif /* LWIP_TSO */
  tcp_tx_pbuf_free(pcb, p);

  (void)opts; /* Fix warning -Wunused-but-set-variable */

  return ERR_OK;
}

/* Used by split functions to move FIN/RST flags to the rightmost segment. */
static void tcp_seg_move_flags(struct tcp_seg *from, struct tcp_seg *to, u8_t flags)
{
  u16_t from_flags = TCPH_FLAGS(from->tcphdr) & flags;

  if ((from != to) && (to != NULL) && from_flags) {
    TCPH_SET_FLAG(to->tcphdr, from_flags);
    TCPH_UNSET_FLAG(from->tcphdr, flags);
  }
}

#if LWIP_TSO
/**
 * Called by tcp_output() to actually join few following TCP segments
 * in one to send a TCP segment over IP using Large Segment Offload method.
 *
 * @param pcb the tcp_pcb for the TCP connection used to send the segment
 * @param seg the tcp_seg to send
 * @param wnd current wnd
 * @return pbuf with p->payload being the tcp_hdr
 */
static void
tcp_tso_segment(struct tcp_pcb *pcb, struct tcp_seg *seg, u32_t wnd)
{
  struct tcp_seg *cur_seg = seg;
  u32_t max_payload_sz = LWIP_MIN(pcb->tso.max_payload_sz, (wnd - (seg->seqno - pcb->lastack)));
  u32_t tot_len = 0;
  u8_t flags = seg->flags;
  int tot_p = 0;

  /* Ignore retransmitted segments and special segments
   */
  if (TCP_SEQ_LT(seg->seqno, pcb->snd_nxt) ||
      (seg->flags & (TF_SEG_OPTS_TSO | TF_SEG_OPTS_DUMMY_MSG)) ||
      ((TCPH_FLAGS(seg->tcphdr) & (~(TCP_ACK | TCP_PSH))) != 0)) {
    goto err;
  }

  while (cur_seg && cur_seg->next &&
          (cur_seg->flags == flags) &&
          ((TCPH_FLAGS(cur_seg->tcphdr) & (~(TCP_ACK | TCP_PSH))) == 0)) {

    tot_len += cur_seg->len;
    if (tot_len > max_payload_sz) {
      goto err;
    }

    tot_p += pbuf_clen(cur_seg->p);
    if (tot_p > (int)pcb->tso.max_send_sge) {
      goto err;
    }

    if (seg != cur_seg) {
        /* Update the original segment with current segment details */
        seg->next = cur_seg->next;
        seg->len += cur_seg->len;

        /* Update the first pbuf of current segment */
        cur_seg->p->payload = (u8_t *)cur_seg->tcphdr + LWIP_TCP_HDRLEN(cur_seg->tcphdr);
        cur_seg->p->len = cur_seg->len - (cur_seg->p->tot_len - cur_seg->p->len);
        cur_seg->p->tot_len = cur_seg->len;

        /* Concatenate two pbufs (each may be a pbuf chain) and
         * update tot_len values for all pbuf in the chain
         */
        pbuf_cat(seg->p, cur_seg->p);

        /* Free joined segment w/o releasing pbuf
         * tcp_seg_free() and tcp_segs_free() release pbuf chain.
         * Note, this code doesn't join the last unsent segment and thus
         * pcb->last_unsent is left unchanged. Otherwise, we would have
         * to update the last_unsent pointer to keep it valid.
         */
        external_tcp_seg_free(pcb, cur_seg);
    }
    cur_seg = seg->next;
  }

err:

  /* All segments that greater than MSS must be processed as TSO segments
   * For example it can be actual for segments with large (more than MSS) buffer size
   */
  if (seg->len > pcb->mss) {
    seg->flags |= TF_SEG_OPTS_TSO;
  }

#if TCP_TSO_DEBUG
    LWIP_DEBUGF(TCP_TSO_DEBUG | LWIP_DBG_TRACE,
                ("tcp_join:   max: %-5d unsent %s\n",
                		max_payload_sz, _dump_seg(pcb->unsent)));
#endif /* TCP_TSO_DEBUG */

  return;
}

static struct tcp_seg *
tcp_split_one_segment(struct tcp_pcb *pcb, struct tcp_seg *seg, u32_t lentosend, u8_t optflags, u8_t optlen)
{
  struct tcp_seg *cur_seg = NULL;
  struct tcp_seg *new_seg = NULL;
  struct tcp_seg *result = NULL;
  struct pbuf *cur_p = NULL;
  u16_t max_length = 0;
  u16_t oversize = 0;

  cur_seg = seg;
  max_length = cur_seg->p->len;
  while ((cur_seg->p->len == cur_seg->p->tot_len) && (cur_seg->len > lentosend)) {

    u32_t lentoqueue = cur_seg->len - lentosend;

    /* Allocate memory for p_buf and fill in fields. */
    if (NULL == (cur_p = tcp_pbuf_prealloc(lentoqueue + optlen, max_length, &oversize, pcb, 0, 0))) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_one_segment: could not allocate memory for pbuf copy size %"U16_F"\n", (lentoqueue + optlen)));
      goto out;
    }

    /* Do prefetch to avoid no memory issue during segment creation with
     * predefined pbuf. It allows to avoid releasing pbuf during failure processing.
     */
    if (!pcb->seg_alloc) {
      if (NULL == (pcb->seg_alloc = tcp_create_segment(pcb, NULL, 0, 0, 0))) {
        LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_one_segment: could not allocate memory for segment\n"));
        tcp_tx_pbuf_free(pcb, cur_p);
        goto out;
      }
    }

    /* Copy the data from the original buffer */
    TCP_DATA_COPY2((char *)cur_p->payload + optlen, (u8_t *)cur_seg->tcphdr + LWIP_TCP_HDRLEN(cur_seg->tcphdr) + lentosend, lentoqueue , &chksum, &chksum_swapped);

    /* Update new buffer */
    cur_p->tot_len = cur_seg->p->tot_len - lentosend - TCP_HLEN ;
    cur_p->next = cur_seg->p->next;

    /* Fill in tcp_seg (allocation was done before).
     * Do not expect NULL but it is possible as far as pbuf_header(p, TCP_HLEN) can return NULL inside tcp_create_segment()
     */
    if (NULL == (new_seg = tcp_create_segment(pcb, cur_p, 0,  cur_seg->seqno + lentosend, optflags))) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_one_segment: could not allocate memory for segment\n"));
      goto out;
    }

    /* New segment update */
    new_seg->next = cur_seg->next;
    new_seg->flags = cur_seg->flags;

    /* Update original buffer */
    cur_seg->p->next = NULL;
    cur_seg->p->len = cur_seg->p->len - lentoqueue;
    cur_seg->p->tot_len = cur_seg->p->len;

    /* Original segment update */
    cur_seg->next = new_seg;
    cur_seg->len = cur_seg->p->len - (TCP_HLEN + optlen);

    cur_seg = new_seg;

    /* Update number of buffer to be send */
    pcb->snd_queuelen++;
  }

  result = seg;

out:
  if (cur_seg->len > pcb->mss) {
    cur_seg->flags |= TF_SEG_OPTS_TSO;
  }
  if (pcb->last_unsent == seg) {
    /* We have split the last unsent segment, update last_unsent */
    pcb->last_unsent = cur_seg;
#if TCP_OVERSIZE
    pcb->unsent_oversize = result ? oversize : 0;
#endif /* TCP_OVERSIZE */
  }
  tcp_seg_move_flags(seg, cur_seg, TCP_FIN | TCP_RST);
  return result;
}

 /**
 * Called by tcp_output() to process TCP segment with ref > 1.
 * This call should process retransmitted TSO segment.
 *
 * @param pcb the tcp_pcb for the TCP connection used to send the segment
 * @param seg the tcp_seg to send
 * @param wnd current window size
 * @return current segment to proceed
 */
static struct tcp_seg *
tcp_rexmit_segment(struct tcp_pcb *pcb, struct tcp_seg *seg, u32_t wnd)
{
  struct tcp_seg *cur_seg = NULL;
  struct tcp_seg *new_seg = NULL;
  struct pbuf *cur_p = NULL;
  u16_t mss_local = 0;
  u8_t optflags = 0;
  u8_t optlen = 0;

  LWIP_ASSERT("tcp_rexmit_segment: sanity check", (seg && seg->p));

  mss_local = tcp_xmit_size_goal(pcb, 0);

  /* TCP_SEQ_LT(seg->seqno, pcb->snd_nxt) can be used as
   * retransmission attribute but according current design
   * checking this condition is needless.
   * Following check makes a decision to retransmit TSO segment as is or
   * convert one into a sequence of none TSO segments
   * Keep TSO segment w/o change in case:
   * 1. current TSO segment was sent and send operation
   * was completed
   * 2. current wnd is enough to send this segment as is
   */
  if ((seg->p->ref == 1) && ((seg->len + seg->seqno - pcb->lastack) <= wnd)) {
    if (seg->len <= mss_local) {
      seg->flags &= (~TF_SEG_OPTS_TSO);
    }
    return seg;
  }

#if LWIP_TCP_TIMESTAMPS
  if ((pcb->flags & TF_TIMESTAMP)) {
    optflags |= TF_SEG_OPTS_TS;
  }
#endif /* LWIP_TCP_TIMESTAMPS */

  optlen += LWIP_TCP_OPT_LENGTH(optflags);

  cur_seg = seg;
  cur_seg->flags &= (~TF_SEG_OPTS_TSO);
  cur_p = cur_seg->p->next;
  while (cur_p) {
    /* Do prefetch to avoid no memory issue during segment creation with
     * predefined pbuf. It allows to avoid releasing pbuf inside tcp_create_segment()
     * during failure processing.
     */
    if (!pcb->seg_alloc) {
      if (NULL == (pcb->seg_alloc = tcp_create_segment(pcb, NULL, 0, 0, 0))) {
        LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_segment: could not allocate memory for segment\n"));
        return seg;
      }
    }

    cur_p->len += optlen;
    cur_p->tot_len = cur_p->len;
    cur_p->payload = (u8_t *)cur_p->payload - optlen;

    /* Fill in tcp_seg (allocation was done before).
     * Do not expect NULL but it is possible as far as pbuf_header(p, TCP_HLEN) can return NULL inside tcp_create_segment()
     */
    if (NULL == (new_seg = tcp_create_segment(pcb, cur_p, 0,  cur_seg->seqno + cur_seg->p->len - TCP_HLEN - optlen, optflags))) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_one_segment: could not allocate memory for segment\n"));
      if (cur_seg->len > pcb->mss) {
        cur_seg->flags |= TF_SEG_OPTS_TSO;
      }
      return seg;
    }

    /* New segment update */
    new_seg->next = cur_seg->next;
    new_seg->flags = cur_seg->flags;

    /* Original segment update */
    cur_seg->next = new_seg;
    cur_seg->len = cur_seg->p->len - TCP_HLEN - optlen;
    cur_seg->p->tot_len = cur_seg->p->len;
    cur_seg->p->next = NULL;

    if (pcb->last_unsent == cur_seg) {
      /* We have split the last unsent segment, update last_unsent */
      pcb->last_unsent = new_seg;
#if TCP_OVERSIZE
      pcb->unsent_oversize = 0;
#endif /* TCP_OVERSIZE */
    }

    tcp_seg_move_flags(cur_seg, new_seg, TCP_FIN | TCP_RST);

    if (NULL == tcp_split_one_segment(pcb, cur_seg, mss_local, optflags, optlen)) {
        LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_one_segment: could not allocate memory for segment\n"));
        if (new_seg->len > pcb->mss) {
          new_seg->flags |= TF_SEG_OPTS_TSO;
        }
        return seg;
    }
    cur_seg = new_seg;

    cur_p = cur_seg->p->next;
  }

  if (NULL == tcp_split_one_segment(pcb, cur_seg, mss_local, optflags, optlen)) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_one_segment: could not allocate memory for segment\n"));
      return seg;
  }

#if TCP_TSO_DEBUG
  LWIP_DEBUGF(TCP_TSO_DEBUG | LWIP_DBG_TRACE,
                ("tcp_rexmit: cwnd: %-5d unsent %s\n",
                		pcb->cwnd, _dump_seg(pcb->unsent)));
#endif /* TCP_TSO_DEBUG */

   return seg;
}
#endif /* LWIP_TSO */

void
tcp_split_segment(struct tcp_pcb *pcb, struct tcp_seg *seg, u32_t wnd)
{
  struct pbuf *p = NULL;
  struct tcp_seg *newseg = NULL;
  u32_t lentosend = 0;
  u16_t oversize = 0;
  u8_t  optlen = 0, optflags = 0;
  u16_t mss_local = 0;

  LWIP_ASSERT("tcp_split_segment: sanity check", (seg && seg->p));

  if (((seg->seqno - pcb->lastack) >= wnd) || (seg->p->ref > 1)) {
    return ;
  }

  lentosend = (wnd - (seg->seqno - pcb->lastack));

#if LWIP_TSO
  mss_local = tcp_xmit_size_goal(pcb, 0);
#else
  /* don't allocate segments bigger than half the maximum window we ever received */
  mss_local = LWIP_MIN(pcb->mss, pcb->snd_wnd_max / 2);
  mss_local = mss_local ? mss_local : pcb->mss;

#if LWIP_TCP_TIMESTAMPS
  if ((pcb->flags & TF_TIMESTAMP)) {
    optflags |= TF_SEG_OPTS_TS;
    /* ensure that segments can hold at least one data byte... */
    mss_local = LWIP_MAX(mss_local, LWIP_TCP_OPT_LEN_TS + 1);
  }
#endif /* LWIP_TCP_TIMESTAMPS */
#endif /* LWIP_TSO */

  optlen += LWIP_TCP_OPT_LENGTH( optflags );

  if (seg->p->len > ((TCP_HLEN + optlen) + lentosend)) {/* First buffer is too big, split it */
    u32_t lentoqueue = seg->p->len - (TCP_HLEN + optlen) - lentosend;

    if (NULL == (p = tcp_pbuf_prealloc(lentoqueue + optlen, mss_local, &oversize, pcb, 0, 0))) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_segment: could not allocate memory for pbuf copy size %"U16_F"\n", (lentoqueue + optlen)));
      return;
    }

    /* Copy the data from the original buffer */
#if LWIP_TSO
    TCP_DATA_COPY2((char *)p->payload + optlen, (u8_t *)seg->tcphdr + LWIP_TCP_HDRLEN(seg->tcphdr) + lentosend, lentoqueue , &chksum, &chksum_swapped);
#else
    TCP_DATA_COPY2((char *)p->payload + optlen, (u8_t *)seg->dataptr + lentosend, lentoqueue , &chksum, &chksum_swapped);
#endif /* LWIP_TSO */

    /* Update new buffer */
    p->tot_len = seg->p->tot_len - lentosend - TCP_HLEN ;
    p->next = seg->p->next;

    /* Allocate memory for tcp_seg and fill in fields. */
    if (NULL == (newseg = tcp_create_segment(pcb, p, 0,  seg->seqno + lentosend, optflags))) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_segment: could not allocate memory for segment\n"));
      return;
    }

    /* Update original buffer */
    seg->p->next = NULL;
    seg->p->len = seg->p->len - lentoqueue;
    seg->p->tot_len = seg->p->len;

    /* New segment update */
    newseg->next = seg->next;
    newseg->flags = seg->flags;

    /* Original segment update */
    seg->next = newseg;
    seg->len = seg->p->len - (TCP_HLEN + optlen);

    /* Set the PSH flag in the last segment that we enqueued. */
    TCPH_SET_FLAG(newseg->tcphdr, TCP_PSH);

    /* Update number of buffer to be send */
    pcb->snd_queuelen++;

    if (pcb->last_unsent == seg) {
      /* We have split the last unsent segment, update last_unsent */
      pcb->last_unsent = newseg;
#if TCP_OVERSIZE
      pcb->unsent_oversize = oversize;
#endif /* TCP_OVERSIZE */
    }
  }
  else if (seg->p->next) {
  	/* Segment with more than one pbuffer and seg->p->len <= lentosend
       split segment pbuff chain. At least one pBuffer will be sent */
    struct pbuf *pnewhead = seg->p->next;
    struct pbuf *pnewtail = seg->p;
    struct pbuf *ptmp = seg->p;
    u32_t headchainlen = seg->p->len;

    while ((headchainlen + pnewhead->len - (TCP_HLEN + optlen))<= lentosend) {
      if (pnewtail->ref > 1) {
        return;
      }

      headchainlen += pnewhead->len;
      pnewtail = pnewhead;
      pnewhead = pnewhead->next;

      if (NULL == pnewhead) {
        LWIP_ASSERT("tcp_split_segment: We should not be here",0);
      	return;
      }
    }

    /* Allocate memory for tcp_seg, and fill in fields. */
    if (NULL == (newseg = tcp_create_segment(pcb, pnewhead, 0,  seg->seqno + headchainlen - (TCP_HLEN + optlen), optflags))) {
      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_split_segment: could not allocate memory for segment\n"));
      return;
    }

    /* Update new tail */
    pnewtail->next = NULL;

    /* New segment update */
    newseg->next = seg->next;
    newseg->flags = seg->flags;

    /* Original segment update */
    seg->next = newseg;
    seg->len = headchainlen - (TCP_HLEN + optlen);

    /* Update original buffers */
    while (ptmp) {
      ptmp->tot_len = headchainlen;
      headchainlen -= ptmp->len;
      ptmp = ptmp->next;
    }

    /* Update last unsent segment */
    if (pcb->last_unsent == seg) {
      pcb->last_unsent = newseg;
    }
  }
  else {
    LWIP_ASSERT("tcp_split_segment: We should not be here [else]",0);
  }

  tcp_seg_move_flags(seg, newseg, TCP_FIN | TCP_RST);

#if TCP_TSO_DEBUG
  LWIP_DEBUGF(TCP_TSO_DEBUG | LWIP_DBG_TRACE,
                ("tcp_split:  max: %-5d unsent %s\n",
                		lentosend, _dump_seg(pcb->unsent)));
#endif /* TCP_TSO_DEBUG */

  return;
}

/**
 * Check whether the input data_len fits the window
 *
 * @param pcb Protocol control block for the TCP connection to send data
 * @parma data_len length to be checked
 * @return 1 if input size fits the window, else 0.
 */
s32_t
tcp_is_wnd_available(struct tcp_pcb *pcb, u32_t data_len)
{
	s32_t tot_unacked_len = 0;
	s32_t tot_unsent_len = 0;
	s32_t wnd = (s32_t)(LWIP_MIN(pcb->snd_wnd, pcb->cwnd));
	s32_t tot_opts_hdrs_len = 0;

#if LWIP_TCP_TIMESTAMPS
	if (pcb->flags & TF_TIMESTAMP) {
		/* The default TCP Maximum Segment Size is 536 (LWIP_TCP_MSS) - RFC-879 */
		u16_t mss = pcb->mss ? pcb->mss : LWIP_TCP_MSS;
		u16_t mss_local = LWIP_MIN(pcb->mss, pcb->snd_wnd_max / 2);
		mss_local = mss_local ? mss_local : mss;
		tot_opts_hdrs_len = ((LWIP_TCP_OPT_LENGTH(TF_SEG_OPTS_TS)) * (1 + ((data_len - 1) / (mss_local))));
	}
#endif

	if (pcb->unacked) {
		tot_unacked_len = pcb->last_unacked->seqno - pcb->unacked->seqno + pcb->last_unacked->len;
	}

	if (pcb->unsent) {
		tot_unsent_len = pcb->last_unsent->seqno - pcb->unsent->seqno + pcb->last_unsent->len;
	}

	return ((wnd - tot_unacked_len) >= (tot_unsent_len + (tot_opts_hdrs_len + (s32_t)data_len)));
}

/**
 * Find out what we can send and send it
 *
 * @param pcb Protocol control block for the TCP connection to send data
 * @return ERR_OK if data has been sent or nothing to send
 *         another err_t on error
 */
err_t
tcp_output(struct tcp_pcb *pcb)
{
  struct tcp_seg *seg, *useg;
  u32_t wnd, snd_nxt;
#if TCP_CWND_DEBUG
  s16_t i = 0;
#endif /* TCP_CWND_DEBUG */

  /* First, check if we are invoked by the TCP input processing
     code. If so, we do not output anything. Instead, we rely on the
     input processing code to call us when input processing is done
     with. */
  if (tcp_input_pcb == pcb) {
    return ERR_OK;
  }

  wnd = LWIP_MIN(pcb->snd_wnd, pcb->cwnd);

  LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_output: snd_wnd %"U32_F", cwnd %"U32_F
     ", wnd %"U32_F"\n",pcb->snd_wnd, pcb->cwnd, wnd ));
  seg = pcb->unsent;

  /* If the TF_ACK_NOW flag is set and no data will be sent (either
  * because the ->unsent queue is empty or because the window does
  * not allow it), construct an empty ACK segment and send it.
  *
  * If data is to be sent, we will just piggyback the ACK (see below).
  */
  if ((pcb->flags & TF_ACK_NOW) &&
    (seg == NULL ||
    seg->seqno - pcb->lastack + seg->len > wnd)) {
    return tcp_send_empty_ack(pcb);
  }

#if TCP_OUTPUT_DEBUG
  if (seg == NULL) {
    LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_output: nothing to send (%p)\n",
                                   (void*)pcb->unsent));
  }
#endif /* TCP_OUTPUT_DEBUG */
#if TCP_CWND_DEBUG
  if (seg == NULL) {
    LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_output: snd_wnd %"U32_F
                                 ", cwnd %"U32_F", wnd %"U32_F
                                 ", seg == NULL, ack %"U32_F"\n",
                                 pcb->snd_wnd, pcb->cwnd, wnd, pcb->lastack));
  } else {
    LWIP_DEBUGF(TCP_CWND_DEBUG,
                ("tcp_output: snd_wnd %"U32_F", cwnd %"U32_F", wnd %"U32_F
                 ", effwnd %"U32_F", seq %"U32_F", ack %"U32_F"\n",
                 pcb->snd_wnd, pcb->cwnd, wnd,
                 ntohl(seg->tcphdr->seqno) - pcb->lastack + seg->len,
                 ntohl(seg->tcphdr->seqno), pcb->lastack));
  }
#endif /* TCP_CWND_DEBUG */
#if TCP_TSO_DEBUG
  if (seg) {
    LWIP_DEBUGF(TCP_TSO_DEBUG | LWIP_DBG_TRACE,
                ("tcp_output: wnd: %-5d unsent %s\n",
                		wnd, _dump_seg(pcb->unsent)));
  }
#endif /* TCP_TSO_DEBUG */

  while (seg) {
#if LWIP_TSO
    /* TSO segment can be in unsent queue only in case retransmission
     * The purpose of this processing is to avoid to send TSO segment
     * during retransmition.
     */
    if (seg->flags & TF_SEG_OPTS_TSO) {
      seg = tcp_rexmit_segment(pcb, seg, wnd);
    }
#endif /* LWIP_TSO */

    /* Split the segment in case of a small window */
    if ((NULL == pcb->unacked) && (wnd) && ((seg->len + seg->seqno - pcb->lastack) > wnd)) {
      LWIP_ASSERT("tcp_output: no window for dummy packet", !LWIP_IS_DUMMY_SEGMENT(seg));
      tcp_split_segment(pcb, seg, wnd);
    }

    /* data available and window allows it to be sent? */
    if (((seg->seqno - pcb->lastack + seg->len) <= wnd)){
      LWIP_ASSERT("RST not expected here!",
      (TCPH_FLAGS(seg->tcphdr) & TCP_RST) == 0);
      
      /* Stop sending if the nagle algorithm would prevent it
       * Don't stop:
       * - if tcp_write had a memory error before (prevent delayed ACK timeout) or
       * - if this is not a dummy segment
       * - if FIN was already enqueued for this PCB (SYN is always alone in a segment -
       *   either seg->next != NULL or pcb->unacked == NULL;
       *   RST is no sent using tcp_write/tcp_output.
       */
       if((tcp_do_output_nagle(pcb) == 0) &&
          !LWIP_IS_DUMMY_SEGMENT(seg) &&
          ((pcb->flags & (TF_NAGLEMEMERR | TF_FIN)) == 0)){
         if ( pcb->snd_sml_snt > (pcb->unacked ? pcb->unacked->len : 0) ) {
           break;
         }
         else {
           if ( (u32_t)((seg->next ? seg->next->len : 0) + seg->len) <= pcb->snd_sml_add ) {
             pcb->snd_sml_snt = pcb->snd_sml_add;
           }
         }
       }

#if LWIP_TSO
       /* Use TSO send operation in case TSO is enabled
        * and current segment is not retransmitted
        */
       if (tcp_tso(pcb)) {
         tcp_tso_segment(pcb, seg, wnd);
       }
#endif /* LWIP_TSO */

       #if TCP_CWND_DEBUG
         LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_output: snd_wnd %"U32_F", cwnd %"U16_F", wnd %"U32_F", effwnd %"U32_F", seq %"U32_F", ack %"U32_F", i %"S16_F"\n",
         pcb->snd_wnd, pcb->cwnd, wnd,
         ntohl(seg->tcphdr->seqno) + seg->len -
         pcb->lastack,
         ntohl(seg->tcphdr->seqno), pcb->lastack, i));
         ++i;
       #endif /* TCP_CWND_DEBUG */

       pcb->unsent = seg->next;

       // Send ack now if the packet is a dummy packet
       if (LWIP_IS_DUMMY_SEGMENT(seg) && (pcb->flags & (TF_ACK_DELAY | TF_ACK_NOW))) {
         tcp_send_empty_ack(pcb);
       }

       if (get_tcp_state(pcb) != SYN_SENT) {
         TCPH_SET_FLAG(seg->tcphdr, TCP_ACK);
         pcb->flags &= ~(TF_ACK_DELAY | TF_ACK_NOW);
       }

       #if TCP_OVERSIZE_DBGCHECK
         seg->oversize_left = 0;
       #endif /* TCP_OVERSIZE_DBGCHECK */

       tcp_output_segment(seg, pcb);
       snd_nxt = seg->seqno + TCP_TCPLEN(seg);
       if (TCP_SEQ_LT(pcb->snd_nxt, snd_nxt) && !LWIP_IS_DUMMY_SEGMENT(seg)) {
         pcb->snd_nxt = snd_nxt;
       }
       /* put segment on unacknowledged list if length > 0 */
       if (TCP_TCPLEN(seg) > 0) {
         seg->next = NULL;
         // unroll dummy segment
         if (LWIP_IS_DUMMY_SEGMENT(seg)) {
           pcb->snd_lbb -= seg->len;
           pcb->snd_buf += seg->len;
           pcb->snd_queuelen -= pbuf_clen(seg->p);
           tcp_tx_seg_free(pcb, seg);
         } else {
           /* unacked list is empty? */
           if (pcb->unacked == NULL) {
             pcb->unacked = seg;
             pcb->last_unacked = seg;
             /* unacked list is not empty? */
           } else {
             /* In the case of fast retransmit, the packet should not go to the tail
             * of the unacked queue, but rather somewhere before it. We need to check for
             * this case. -STJ Jul 27, 2004 */
             useg =  pcb->last_unacked;
             if (TCP_SEQ_LT(seg->seqno, useg->seqno)) {
               /* add segment to before tail of unacked list, keeping the list sorted */
               struct tcp_seg **cur_seg = &(pcb->unacked);
               while (*cur_seg &&
                 TCP_SEQ_LT((*cur_seg)->seqno, seg->seqno)) {
                 cur_seg = &((*cur_seg)->next );
               }
               LWIP_ASSERT("Value of last_unacked is invalid",
                           *cur_seg != pcb->last_unacked->next);
               seg->next = (*cur_seg);
               (*cur_seg) = seg;
             } else {
               /* add segment to tail of unacked list */
               useg->next = seg;
               pcb->last_unacked = seg;
             }
           }
         }
         /* do not queue empty segments on the unacked list */
       } else {
         tcp_tx_seg_free(pcb, seg);
       }
       seg = pcb->unsent;
    }
    else {
      break;
    }
  }

  if (pcb->unsent == NULL) {
    /* We have sent all pending segments, reset last_unsent */
    pcb->last_unsent = NULL;
#if TCP_OVERSIZE
    pcb->unsent_oversize = 0;
#endif /* TCP_OVERSIZE */
  }

  pcb->flags &= ~TF_NAGLEMEMERR;

  // Fetch buffers for the next packet.
  if (!pcb->seg_alloc) {
	  // Fetch tcp segment for the next packet.
	  pcb->seg_alloc = tcp_create_segment(pcb, NULL, 0, 0, 0);
  }

  if (!pcb->pbuf_alloc) {
	  // Fetch pbuf for the next packet.
	  pcb->pbuf_alloc = tcp_tx_pbuf_alloc(pcb, 0, PBUF_RAM);
  }

  return ERR_OK;
}

/**
 * Called by tcp_output() to actually send a TCP segment over IP.
 *
 * @param seg the tcp_seg to send
 * @param pcb the tcp_pcb for the TCP connection used to send the segment
 */
static void
tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb)
{
  u16_t len;
  u32_t *opts;

  /* The TCP header has already been constructed, but the ackno and
   wnd fields remain. */
  seg->tcphdr->ackno = htonl(pcb->rcv_nxt);


  if (seg->flags & TF_SEG_OPTS_WNDSCALE) {
    /* The Window field in a SYN segment itself (the only type where we send
       the window scale option) is never scaled. */
	  seg->tcphdr->wnd = htons(TCPWND_MIN16(pcb->rcv_ann_wnd));
  } else {
  /* advertise our receive window size in this TCP segment */
	  seg->tcphdr->wnd = htons(TCPWND_MIN16(RCV_WND_SCALE(pcb, pcb->rcv_ann_wnd)));
  }

  if (!LWIP_IS_DUMMY_SEGMENT(seg)) {
    pcb->rcv_ann_right_edge = pcb->rcv_nxt + pcb->rcv_ann_wnd;
  }
  /* Add any requested options.  NB MSS option is only set on SYN
     packets, so ignore it here */
  LWIP_ASSERT("seg->tcphdr not aligned", ((mem_ptr_t)(seg->tcphdr + 1) % 4) == 0);
  opts = (u32_t *)(void *)(seg->tcphdr + 1);
  if (seg->flags & TF_SEG_OPTS_MSS) {
    /* coverity[result_independent_of_operands] */
    TCP_BUILD_MSS_OPTION(*opts, pcb->advtsd_mss);
    opts += 1; // Move to the next line (meaning next 32 bit) as this option is 4 bytes long
  }

  /* If RCV_SCALE is set then prepare segment for window scaling option */
  if (seg->flags & TF_SEG_OPTS_WNDSCALE) {
    TCP_BUILD_WNDSCALE_OPTION(*opts, rcv_wnd_scale);
    opts += 1;	// Move to the next line (meaning next 32 bit) as this option is 3 bytes long + we added 1 byte NOOP padding => total 4 bytes
  }

#if LWIP_TCP_TIMESTAMPS
  if (!LWIP_IS_DUMMY_SEGMENT(seg)) {
    pcb->ts_lastacksent = pcb->rcv_nxt;
  }

  if (seg->flags & TF_SEG_OPTS_TS) {
    tcp_build_timestamp_option(pcb, opts);
    /* opts += 3; */  /* Note: suppress warning 'opts' is never read */ // Move to the next line (meaning next 32 bit) as this option is 10 bytes long, 12 with padding (so jump 3 lines)
  }
#endif

  /* If we don't have a local IP address, we get one by
     calling ip_route(). */
  if (ip_addr_isany(&(pcb->local_ip))) {
	  LWIP_ASSERT("tcp_output_segment: need to find route to host", 0);
  }

  /* Set retransmission timer running if it is not currently enabled */
  if (!LWIP_IS_DUMMY_SEGMENT(seg)) {
    if(pcb->rtime == -1) {
      pcb->rtime = 0;
    }

    if (pcb->rttest == 0) {
      pcb->rttest = tcp_ticks;
      pcb->rtseq = seg->seqno;

      LWIP_DEBUGF(TCP_RTO_DEBUG, ("tcp_output_segment: rtseq %"U32_F"\n", pcb->rtseq));
    }
  }
  LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_output_segment: %"U32_F":%"U32_F"\n",
          htonl(seg->tcphdr->seqno), htonl(seg->tcphdr->seqno) +
          seg->len));

  len = (u16_t)((u8_t *)seg->tcphdr - (u8_t *)seg->p->payload);

  seg->p->len -= len;
  seg->p->tot_len -= len;

  seg->p->payload = seg->tcphdr;

  seg->tcphdr->chksum = 0;

  TCP_STATS_INC(tcp.xmit);

#if LWIP_TSO
  u16_t flags = 0;
  flags |= seg->flags & TF_SEG_OPTS_DUMMY_MSG;
  flags |= seg->flags & TF_SEG_OPTS_TSO;
  flags |= (TCP_SEQ_LT(seg->seqno, pcb->snd_nxt) ? TCP_WRITE_REXMIT : 0);
  pcb->ip_output(seg->p, pcb, flags);
#else
  pcb->ip_output(seg->p, pcb, seg->seqno < pcb->snd_nxt, LWIP_IS_DUMMY_SEGMENT(seg));
#endif /* LWIP_TSO */
}

/**
 * Send a TCP RESET packet (empty segment with RST flag set) either to
 * abort a connection or to show that there is no matching local connection
 * for a received segment.
 *
 * Called by tcp_abort() (to abort a local connection), tcp_input() (if no
 * matching local pcb was found), tcp_listen_input() (if incoming segment
 * has ACK flag set) and tcp_process() (received segment in the wrong state)
 *
 * Since a RST segment is in most cases not sent for an active connection,
 * tcp_rst() has a number of arguments that are taken from a tcp_pcb for
 * most other segment output functions.
 *
 * The pcb is given only when its valid and from an output context.
 * It is used with the external_ip_output function.
 *
 * @param seqno the sequence number to use for the outgoing segment
 * @param ackno the acknowledge number to use for the outgoing segment
 * @param local_ip the local IP address to send the segment from
 * @param remote_ip the remote IP address to send the segment to
 * @param local_port the local TCP port to send the segment from
 * @param remote_port the remote TCP port to send the segment to
 */
void
tcp_rst(u32_t seqno, u32_t ackno, u16_t local_port, u16_t remote_port, struct tcp_pcb *pcb)
{
  struct pbuf *p;
  struct tcp_hdr *tcphdr;
#if LWIP_3RD_PARTY_BUFS
  if (!pcb) return;
#endif
  p = tcp_tx_pbuf_alloc(pcb, 0, PBUF_RAM);
  if (p == NULL) {
      LWIP_DEBUGF(TCP_DEBUG, ("tcp_rst: could not allocate memory for pbuf\n"));
      return;
  }
  pbuf_header(p, TCP_HLEN);
  LWIP_ASSERT("check that first pbuf can hold struct tcp_hdr",
              (p->len >= sizeof(struct tcp_hdr)));

  tcphdr = (struct tcp_hdr *)p->payload;
  tcphdr->src = htons(local_port);
  tcphdr->dest = htons(remote_port);
  tcphdr->seqno = htonl(seqno);
  tcphdr->ackno = htonl(ackno);
  TCPH_HDRLEN_FLAGS_SET(tcphdr, TCP_HLEN/4, TCP_RST | TCP_ACK);
  tcphdr->wnd = PP_HTONS(( TCP_WND  & 0xFFFF ));
  tcphdr->chksum = 0;
  tcphdr->urgp = 0;

  TCP_STATS_INC(tcp.xmit);
   /* Send output with hardcoded TTL since we have no access to the pcb */
#if LWIP_TSO
  if(pcb) pcb->ip_output(p, pcb, 0);
#else
  if(pcb) pcb->ip_output(p, pcb, 0, 0);
#endif /* LWIP_TSO */
  /* external_ip_output(p, NULL, local_ip, remote_ip, TCP_TTL, 0, IP_PROTO_TCP) */;
  tcp_tx_pbuf_free(pcb, p);
  LWIP_DEBUGF(TCP_RST_DEBUG, ("tcp_rst: seqno %"U32_F" ackno %"U32_F".\n", seqno, ackno));
}

/**
 * Requeue all unacked segments for retransmission
 *
 * Called by tcp_slowtmr() for slow retransmission.
 *
 * @param pcb the tcp_pcb for which to re-enqueue all unacked segments
 */
void
tcp_rexmit_rto(struct tcp_pcb *pcb)
{
  struct tcp_seg *seg;

  if (pcb->unacked == NULL) {
    return;
  }

  /* Move all unacked segments to the head of the unsent queue */
  for (seg = pcb->unacked; seg->next != NULL; seg = seg->next);
  /* concatenate unsent queue after unacked queue */
  seg->next = pcb->unsent;
  if (pcb->unsent == NULL) {
    /* If there are no unsent segments, update last_unsent to the last unacked */
    pcb->last_unsent = seg;
#if TCP_OVERSIZE && TCP_OVERSIZE_DBGCHECK
    pcb->unsent_oversize = seg->oversize_left;
#endif /* TCP_OVERSIZE && TCP_OVERSIZE_DBGCHECK*/
  }
  /* unsent queue is the concatenated queue (of unacked, unsent) */
  pcb->unsent = pcb->unacked;
  /* unacked queue is now empty */
  pcb->unacked = NULL;

  /* increment number of retransmissions */
  ++pcb->nrtx;

  /* Don't take any RTT measurements after retransmitting. */
  pcb->rttest = 0;

  /* Do the actual retransmission */
  tcp_output(pcb);
}

/**
 * Requeue the first unacked segment for retransmission
 *
 * Called by tcp_receive() for fast retramsmit.
 *
 * @param pcb the tcp_pcb for which to retransmit the first unacked segment
 */
void
tcp_rexmit(struct tcp_pcb *pcb)
{
  struct tcp_seg *seg;
  struct tcp_seg **cur_seg;

  if (pcb->unacked == NULL) {
    return;
  }

  /* Move the first unacked segment to the unsent queue */
  /* Keep the unsent queue sorted. */
  seg = pcb->unacked;
  pcb->unacked = seg->next;

  cur_seg = &(pcb->unsent);
  while (*cur_seg &&
    TCP_SEQ_LT((*cur_seg)->seqno, seg->seqno)) {
      cur_seg = &((*cur_seg)->next );
  }
  seg->next = *cur_seg;
  *cur_seg = seg;
  if (seg->next == NULL) {
    /* The retransmitted segment is the last in the unsent queue, update last_unsent */
    pcb->last_unsent = seg;
#if TCP_OVERSIZE
    pcb->unsent_oversize = 0;
#endif /* TCP_OVERSIZE */
  }

  ++pcb->nrtx;

  /* Don't take any rtt measurements after retransmitting. */
  pcb->rttest = 0;
}


/**
 * Handle retransmission after three dupacks received
 *
 * @param pcb the tcp_pcb for which to retransmit the first unacked segment
 */
void 
tcp_rexmit_fast(struct tcp_pcb *pcb)
{
  if (pcb->unacked != NULL && !(pcb->flags & TF_INFR)) {
    /* This is fast retransmit. Retransmit the first unacked segment. */
    LWIP_DEBUGF(TCP_FR_DEBUG,
                ("tcp_receive: dupacks %"U16_F" (%"U32_F
                 "), fast retransmit %"U32_F"\n",
                 (u16_t)pcb->dupacks, pcb->lastack,
                 pcb->unacked->seqno));
    tcp_rexmit(pcb);
#if TCP_CC_ALGO_MOD
    cc_cong_signal(pcb, CC_NDUPACK);
#else
    /* Set ssthresh to half of the minimum of the current
     * cwnd and the advertised window */
    if (pcb->cwnd > pcb->snd_wnd) {
      pcb->ssthresh = pcb->snd_wnd / 2;
    } else {
      pcb->ssthresh = pcb->cwnd / 2;
    }

    /* The minimum value for ssthresh should be 2 MSS */
    if (pcb->ssthresh < (2U * pcb->mss)) {
      LWIP_DEBUGF(TCP_FR_DEBUG,
                  ("tcp_receive: The minimum value for ssthresh %"U16_F
                   " should be min 2 mss %"U16_F"...\n",
                   pcb->ssthresh, 2*pcb->mss));
      pcb->ssthresh = 2*pcb->mss;
    }

    pcb->cwnd = pcb->ssthresh + 3 * pcb->mss;
#endif
    pcb->flags |= TF_INFR;
  } 
}


/**
 * Send keepalive packets to keep a connection active although
 * no data is sent over it.
 *
 * Called by tcp_slowtmr()
 *
 * @param pcb the tcp_pcb for which to send a keepalive packet
 */
void
tcp_keepalive(struct tcp_pcb *pcb)
{
  struct pbuf *p;
  struct tcp_hdr *tcphdr;
  u8_t optlen = 0;
  u32_t *opts;

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_keepalive: sending KEEPALIVE probe to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
                          ip4_addr1_16(&pcb->remote_ip), ip4_addr2_16(&pcb->remote_ip),
                          ip4_addr3_16(&pcb->remote_ip), ip4_addr4_16(&pcb->remote_ip)));

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_keepalive: tcp_ticks %"U32_F"   pcb->tmr %"U32_F" pcb->keep_cnt_sent %"U16_F"\n",
                          tcp_ticks, pcb->tmr, pcb->keep_cnt_sent));

#if LWIP_TCP_TIMESTAMPS
  if (pcb->flags & TF_TIMESTAMP) {
    optlen = LWIP_TCP_OPT_LENGTH(TF_SEG_OPTS_TS);
  }
#endif

  p = tcp_output_alloc_header(pcb, optlen, 0, htonl(pcb->snd_nxt - 1));
  if(p == NULL) {
    LWIP_DEBUGF(TCP_DEBUG,
                ("tcp_keepalive: could not allocate memory for pbuf\n"));
    return;
  }
  tcphdr = (struct tcp_hdr *)p->payload;
  opts = (u32_t *)(void *)(tcphdr + 1);

#if LWIP_TCP_TIMESTAMPS
  pcb->ts_lastacksent = pcb->rcv_nxt;
  if (pcb->flags & TF_TIMESTAMP) {
    tcp_build_timestamp_option(pcb, opts );
    opts += 3;
  }
#endif

#if CHECKSUM_GEN_TCP
  tcphdr->chksum = inet_chksum_pseudo(p, &pcb->local_ip, &pcb->remote_ip,
                                      IP_PROTO_TCP, (u16_t)p->tot_len);
#endif
  TCP_STATS_INC(tcp.xmit);

  /* Send output to IP */
#if LWIP_TSO
  pcb->ip_output(p, pcb, 0);
#else
  pcb->ip_output(p, pcb, 0, 0);
#endif /* LWIP_TSO */

  tcp_tx_pbuf_free(pcb, p);

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_keepalive: seqno %"U32_F" ackno %"U32_F".\n",
                          pcb->snd_nxt - 1, pcb->rcv_nxt));
  (void)tcphdr; /* Fix warning -Wunused-but-set-variable*/
  (void)opts; /* Fix warning -Wunused-but-set-variable */
}


/**
 * Send persist timer zero-window probes to keep a connection active
 * when a window update is lost.
 *
 * Called by tcp_slowtmr()
 *
 * @param pcb the tcp_pcb for which to send a zero-window probe packet
 */
void
tcp_zero_window_probe(struct tcp_pcb *pcb)
{
  struct pbuf *p;
  struct tcp_hdr *tcphdr;
  struct tcp_seg *seg;
  u16_t len;
  u8_t is_fin;
  u32_t snd_nxt;
  u8_t optlen = 0;
  u32_t *opts;

  LWIP_DEBUGF(TCP_DEBUG,
              ("tcp_zero_window_probe: sending ZERO WINDOW probe to %"
               U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
               ip4_addr1_16(&pcb->remote_ip), ip4_addr2_16(&pcb->remote_ip),
               ip4_addr3_16(&pcb->remote_ip), ip4_addr4_16(&pcb->remote_ip)));

  LWIP_DEBUGF(TCP_DEBUG,
              ("tcp_zero_window_probe: tcp_ticks %"U32_F
               "   pcb->tmr %"U32_F" pcb->keep_cnt_sent %"U16_F"\n",
               tcp_ticks, pcb->tmr, pcb->keep_cnt_sent));

  /* Only consider unsent, persist timer should be off when there data is in-flight */
  seg = pcb->unsent;
  if(seg == NULL) {
    /* Not expected, persist timer should be off when the send buffer is empty */
    return;
  }

  is_fin = ((TCPH_FLAGS(seg->tcphdr) & TCP_FIN) != 0) && (seg->len == 0);
  /* we want to send one seqno: either FIN or data (no options) */
  len = is_fin ? 0 : 1;

#if LWIP_TCP_TIMESTAMPS
  if (pcb->flags & TF_TIMESTAMP) {
    optlen = LWIP_TCP_OPT_LENGTH(TF_SEG_OPTS_TS);
  }
#endif

 /**
  * While sending probe of 1 byte we must split the first unsent segment.
  * This change is commented out because tcp_zero_window_probe() was replaced
  * with tcp_keepalive().
  * if (len > 0 && seg->len != 1) {
  *   tcp_split_segment(pcb, seg, seg->seqno - pcb->lastack + 1);
  *   seg = pcb->unsent;
  * }
  */

  p = tcp_output_alloc_header(pcb, optlen, len, seg->tcphdr->seqno);
  if(p == NULL) {
    LWIP_DEBUGF(TCP_DEBUG, ("tcp_zero_window_probe: no memory for pbuf\n"));
    return;
  }
  tcphdr = (struct tcp_hdr *)p->payload;
  opts = (u32_t *)(void *)(tcphdr + 1);

#if LWIP_TCP_TIMESTAMPS
  pcb->ts_lastacksent = pcb->rcv_nxt;
  if (pcb->flags & TF_TIMESTAMP) {
    tcp_build_timestamp_option(pcb, opts );
    opts += 3;
  }
#endif

  if (is_fin) {
    /* FIN segment, no data */
    TCPH_FLAGS_SET(tcphdr, TCP_ACK | TCP_FIN);
  } else {
    /* Data segment, copy in one byte from the head of the unacked queue */
#if LWIP_TSO
    *((char *)p->payload + TCP_HLEN + optlen) = *(char *)((u8_t *)seg->tcphdr + LWIP_TCP_HDRLEN(seg->tcphdr));
#else
    *((char *)p->payload + TCP_HLEN + optlen) = *(char *)seg->dataptr;
#endif /* LWIP_TSO */
  }

   /* The byte may be acknowledged without the window being opened. */
   snd_nxt = lwip_ntohl(seg->tcphdr->seqno) + 1;
   if (TCP_SEQ_LT(pcb->snd_nxt, snd_nxt)) {
     pcb->snd_nxt = snd_nxt;
   }

#if CHECKSUM_GEN_TCP
  tcphdr->chksum = inet_chksum_pseudo(p, &pcb->local_ip, &pcb->remote_ip,
                                      IP_PROTO_TCP, (u16_t)p->tot_len);
#endif
  TCP_STATS_INC(tcp.xmit);

  /* Send output to IP */
#if LWIP_TSO
  pcb->ip_output(p, pcb, 0);
#else
  pcb->ip_output(p, pcb, 0, 0);
#endif /* LWIP_TSO */

  tcp_tx_pbuf_free(pcb, p);

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_zero_window_probe: seqno %"U32_F
                          " ackno %"U32_F".\n",
                          pcb->snd_nxt - 1, pcb->rcv_nxt));
  (void)opts; /* Fix warning -Wunused-but-set-variable */
}
#endif /* LWIP_TCP */
