seg->tcphdr->wnd = htons(pcb->rcv_ann_wnd);

  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 */
  opts = (u32_t *)(seg->tcphdr + 1);
  if (seg->flags & TF_SEG_OPTS_MSS) {
    TCP_BUILD_MSS_OPTION(*opts);
    opts += 1;
  }
#if LWIP_TCP_TIMESTAMPS
  pcb->ts_lastacksent = pcb->rcv_nxt;

  if (seg->flags & TF_SEG_OPTS_TS) {
    tcp_build_timestamp_option(pcb, opts);
    opts += 3;
  }
#endif

  /* If we don't have a local IP address, we get one by
     calling ip_route(). */
  if (ip_addr_isany(&(pcb->local_ip))) {
    netif = ip_route(&(pcb->remote_ip));
    if (netif == NULL) {
      return;
    }
    ip_addr_set(&(pcb->local_ip), &(netif->ip_addr));
  }

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

  if (pcb->rttest == 0) {
    pcb->rttest = tcp_ticks;
    pcb->rtseq = ntohl(seg->tcphdr->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;
#if CHECKSUM_GEN_TCP
  seg->tcphdr->chksum = inet_chksum_pseudo(seg->p,
             &(pcb->local_ip),
             &(pcb->remote_ip),
             IP_PROTO_TCP, seg->p->tot_len);
#endif
  TCP_STATS_INC(tcp.xmit);

#if LWIP_NETIF_HWADDRHINT
  ip_output_hinted(seg->p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl, pcb->tos,
      IP_PROTO_TCP, &(pcb->addr_hint));
#else /* LWIP_NETIF_HWADDRHINT*/
  ip_output(seg->p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl, pcb->tos,
      IP_PROTO_TCP);
#endif /* LWIP_NETIF_HWADDRHINT*/
}

/**
 * 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.
 *
 * @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,
  struct ip_addr *local_ip, struct ip_addr *remote_ip,
  u16_t local_port, u16_t remote_port)
{
  struct pbuf *p;
  struct tcp_hdr *tcphdr;
  p = pbuf_alloc(PBUF_IP, TCP_HLEN, PBUF_RAM);
  if (p == NULL) {
      LWIP_DEBUGF(TCP_DEBUG, ("tcp_rst: could not allocate memory for pbuf\n"));
      return;
  }
  LWIP_ASSERT("check that first pbuf can hold struct tcp_hdr",
              (p->len >= sizeof(struct tcp_hdr)));

  tcphdr = p->payload;
  tcphdr->src = htons(local_port);
  tcphdr->dest = htons(remote_port);
  tcphdr->seqno = htonl(seqno);
  tcphdr->ackno = htonl(ackno);
  TCPH_FLAGS_SET(tcphdr, TCP_RST | TCP_ACK);
  tcphdr->wnd = htons(TCP_WND);
  tcphdr->urgp = 0;
  TCPH_HDRLEN_SET(tcphdr, 5);

  tcphdr->chksum = 0;
#if CHECKSUM_GEN_TCP
  tcphdr->chksum = inet_chksum_pseudo(p, local_ip, remote_ip,
              IP_PROTO_TCP, p->tot_len);
#endif
  TCP_STATS_INC(tcp.xmit);
  snmp_inc_tcpoutrsts();
   /* Send output with hardcoded TTL since we have no access to the pcb */
  ip_output(p, local_ip, remote_ip, TCP_TTL, 0, IP_PROTO_TCP);
  pbuf_free(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;
  /* 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(ntohl((*cur_seg)->tcphdr->seqno), ntohl(seg->tcphdr->seqno))) {
      cur_seg = &((*cur_seg)->next );
  }
  seg->next = *cur_seg;
  *cur_seg = seg;

  ++pcb->nrtx;

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

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

/**
 * 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;

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_keepalive: sending KEEPALIVE probe to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
                          ip4_addr1(&pcb->remote_ip), ip4_addr2(&pcb->remote_ip),
                          ip4_addr3(&pcb->remote_ip), ip4_addr4(&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));
   
  p = pbuf_alloc(PBUF_IP, TCP_HLEN, PBUF_RAM);
   
  if(p == NULL) {
    LWIP_DEBUGF(TCP_DEBUG, 
                ("tcp_keepalive: could not allocate memory for pbuf\n"));
    return;
  }
  LWIP_ASSERT("check that first pbuf can hold struct tcp_hdr",
              (p->len >= sizeof(struct tcp_hdr)));

  tcphdr = tcp_output_set_header(pcb, p, 0, htonl(pcb->snd_nxt - 1));

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

  /* Send output to IP */
#if LWIP_NETIF_HWADDRHINT
  ip_output_hinted(p, &pcb->local_ip, &pcb->remote_ip, pcb->ttl, 0, IP_PROTO_TCP,
    &(pcb->addr_hint));
#else /* LWIP_NETIF_HWADDRHINT*/
  ip_output(p, &pcb->local_ip, &pcb->remote_ip, pcb->ttl, 0, IP_PROTO_TCP);
#endif /* LWIP_NETIF_HWADDRHINT*/

  pbuf_free(p);

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_keepalive: seqno %"U32_F" ackno %"U32_F".\n",
                          pcb->snd_nxt - 1, pcb->rcv_nxt));
}


/**
 * 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;

  LWIP_DEBUGF(TCP_DEBUG, 
              ("tcp_zero_window_probe: sending ZERO WINDOW probe to %"
               U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
               ip4_addr1(&pcb->remote_ip), ip4_addr2(&pcb->remote_ip),
               ip4_addr3(&pcb->remote_ip), ip4_addr4(&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));

  seg = pcb->unacked;

  if(seg == NULL)
    seg = pcb->unsent;

  if(seg == NULL)
    return;

  p = pbuf_alloc(PBUF_IP, TCP_HLEN + 1, PBUF_RAM);
   
  if(p == NULL) {
    LWIP_DEBUGF(TCP_DEBUG, ("tcp_zero_window_probe: no memory for pbuf\n"));
    return;
  }
  LWIP_ASSERT("check that first pbuf can hold struct tcp_hdr",
              (p->len >= sizeof(struct tcp_hdr)));

  tcphdr = tcp_output_set_header(pcb, p, 0, seg->tcphdr->seqno);

  /* Copy in one byte from the head of the unacked queue */
  *((char *)p->payload + sizeof(struct tcp_hdr)) = *(char *)seg->dataptr;

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

  /* Send output to IP */
#if LWIP_NETIF_HWADDRHINT
  ip_output_hinted(p, &pcb->local_ip, &pcb->remote_ip, pcb->ttl, 0, IP_PROTO_TCP,
    &(pcb->addr_hint));
#else /* LWIP_NETIF_HWADDRHINT*/
  ip_output(p, &pcb->local_ip, &pcb->remote_ip, pcb->ttl, 0, IP_PROTO_TCP);
#endif /* LWIP_NETIF_HWADDRHINT*/

  pbuf_free(p);

  LWIP_DEBUGF(TCP_DEBUG, ("tcp_zero_window_probe: seqno %"U32_F
                          " ackno %"U32_F".\n",
                          pcb->snd_nxt - 1, pcb->rcv_nxt));
}
#endif /* LWIP_TCP */