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diff --git a/geekos/src/lwip/core/tcp_out.c b/geekos/src/lwip/core/tcp_out.c
new file mode 100644
index 0000000..c46e4b8
--- /dev/null
+++ b/geekos/src/lwip/core/tcp_out.c
@@ -0,0 +1,953 @@
+/**
+ * @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 "lwip/opt.h"
+
+#if LWIP_TCP /* don't build if not configured for use in lwipopts.h */
+
+#include "lwip/tcp.h"
+#include "lwip/def.h"
+#include "lwip/mem.h"
+#include "lwip/memp.h"
+#include "lwip/sys.h"
+#include "lwip/ip_addr.h"
+#include "lwip/netif.h"
+#include "lwip/inet.h"
+#include "lwip/inet_chksum.h"
+#include "lwip/stats.h"
+#include "lwip/snmp.h"
+
+#include <string.h>
+
+/* Forward declarations.*/
+static void tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb);
+
+/**
+ * Called by tcp_close() to send a segment including flags but not data.
+ *
+ * @param pcb the tcp_pcb over which to send a segment
+ * @param flags the flags to set in the segment header
+ * @return ERR_OK if sent, another err_t otherwise
+ */
+err_t
+tcp_send_ctrl(struct tcp_pcb *pcb, u8_t flags)
+{
+  /* no data, no length, flags, copy=1, no optdata, no optdatalen */
+  return tcp_enqueue(pcb, NULL, 0, flags, TCP_WRITE_FLAG_COPY, NULL, 0);
+}
+
+/**
+ * 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 of the TCP connection to enqueue data for.
+ * @param data pointer to the data to send
+ * @param len length (in bytes) of the data to send
+ * @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,
+ * @return ERR_OK if enqueued, another err_t on error
+ * 
+ * @see tcp_write()
+ */
+err_t
+tcp_write(struct tcp_pcb *pcb, const void *data, u16_t len, u8_t apiflags)
+{
+  LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_write(pcb=%p, data=%p, len=%"U16_F", apiflags=%"U16_F")\n", (void *)pcb,
+    data, len, (u16_t)apiflags));
+  /* connection is in valid state for data transmission? */
+  if (pcb->state == ESTABLISHED ||
+     pcb->state == CLOSE_WAIT ||
+     pcb->state == SYN_SENT ||
+     pcb->state == SYN_RCVD) {
+    if (len > 0) {
+      return tcp_enqueue(pcb, (void *)data, len, 0, apiflags, NULL, 0);
+    }
+    return ERR_OK;
+  } else {
+    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_STATE | 3, ("tcp_write() called in invalid state\n"));
+    return ERR_CONN;
+  }
+}
+
+/**
+ * Enqueue either data or TCP options (but not both) for tranmission
+ *
+ * Called by tcp_connect(), tcp_listen_input(), tcp_send_ctrl() and 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 flags tcp header flags to set in the outgoing segment
+ * @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,
+ * @param optdata
+ * @param optlen
+ */
+err_t
+tcp_enqueue(struct tcp_pcb *pcb, void *arg, u16_t len,
+  u8_t flags, u8_t apiflags,
+  u8_t *optdata, u8_t optlen)
+{
+  struct pbuf *p;
+  struct tcp_seg *seg, *useg, *queue;
+  u32_t seqno;
+  u16_t left, seglen;
+  void *ptr;
+  u16_t queuelen;
+
+  LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_enqueue(pcb=%p, arg=%p, len=%"U16_F", flags=%"X16_F", apiflags=%"U16_F")\n",
+    (void *)pcb, arg, len, (u16_t)flags, (u16_t)apiflags));
+  LWIP_ERROR("tcp_enqueue: len == 0 || optlen == 0 (programmer violates API)",
+      ((len == 0) || (optlen == 0)), return ERR_ARG;);
+  LWIP_ERROR("tcp_enqueue: arg == NULL || optdata == NULL (programmer violates API)",
+      ((arg == NULL) || (optdata == NULL)), return ERR_ARG;);
+  /* fail on too much data */
+  if (len > pcb->snd_buf) {
+    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_enqueue: too much data (len=%"U16_F" > snd_buf=%"U16_F")\n", len, pcb->snd_buf));
+    pcb->flags |= TF_NAGLEMEMERR;
+    return ERR_MEM;
+  }
+  left = len;
+  ptr = arg;
+
+  /* seqno will be the sequence number of the first segment enqueued
+   * by the call to this function. */
+  seqno = pcb->snd_lbb;
+
+  LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_enqueue: queuelen: %"U16_F"\n", (u16_t)pcb->snd_queuelen));
+
+  /* If total number of pbufs on the unsent/unacked queues exceeds the
+   * configured maximum, return an error */
+  queuelen = pcb->snd_queuelen;
+  /* check for configured max queuelen and possible overflow */
+  if ((queuelen >= TCP_SND_QUEUELEN) || (queuelen > TCP_SNDQUEUELEN_OVERFLOW)) {
+    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_enqueue: too long queue %"U16_F" (max %"U16_F")\n", queuelen, TCP_SND_QUEUELEN));
+    TCP_STATS_INC(tcp.memerr);
+    pcb->flags |= TF_NAGLEMEMERR;
+    return ERR_MEM;
+  }
+  if (queuelen != 0) {
+    LWIP_ASSERT("tcp_enqueue: pbufs on queue => at least one queue non-empty",
+      pcb->unacked != NULL || pcb->unsent != NULL);
+  } else {
+    LWIP_ASSERT("tcp_enqueue: no pbufs on queue => both queues empty",
+      pcb->unacked == NULL && pcb->unsent == NULL);
+  }
+
+  /* First, break up the data into segments and tuck them together in
+   * the local "queue" variable. */
+  useg = queue = seg = NULL;
+  seglen = 0;
+  while (queue == NULL || left > 0) {
+
+    /* The segment length should be the MSS if the data to be enqueued
+     * is larger than the MSS. */
+    seglen = left > pcb->mss? pcb->mss: left;
+
+    /* Allocate memory for tcp_seg, and fill in fields. */
+    seg = memp_malloc(MEMP_TCP_SEG);
+    if (seg == NULL) {
+      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue: could not allocate memory for tcp_seg\n"));
+      goto memerr;
+    }
+    seg->next = NULL;
+    seg->p = NULL;
+
+    /* first segment of to-be-queued data? */
+    if (queue == NULL) {
+      queue = seg;
+    }
+    /* subsequent segments of to-be-queued data */
+    else {
+      /* Attach the segment to the end of the queued segments */
+      LWIP_ASSERT("useg != NULL", useg != NULL);
+      useg->next = seg;
+    }
+    /* remember last segment of to-be-queued data for next iteration */
+    useg = seg;
+
+    /* If copy is set, memory should be allocated
+     * and data copied into pbuf, otherwise data comes from
+     * ROM or other static memory, and need not be copied. If
+     * optdata is != NULL, we have options instead of data. */
+     
+    /* options? */
+    if (optdata != NULL) {
+      if ((seg->p = pbuf_alloc(PBUF_TRANSPORT, optlen, PBUF_RAM)) == NULL) {
+        goto memerr;
+      }
+      LWIP_ASSERT("check that first pbuf can hold optlen",
+                  (seg->p->len >= optlen));
+      queuelen += pbuf_clen(seg->p);
+      seg->dataptr = seg->p->payload;
+    }
+    /* copy from volatile memory? */
+    else if (apiflags & TCP_WRITE_FLAG_COPY) {
+      if ((seg->p = pbuf_alloc(PBUF_TRANSPORT, seglen, PBUF_RAM)) == NULL) {
+        LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue : could not allocate memory for pbuf copy size %"U16_F"\n", seglen));
+        goto memerr;
+      }
+      LWIP_ASSERT("check that first pbuf can hold the complete seglen",
+                  (seg->p->len >= seglen));
+      queuelen += pbuf_clen(seg->p);
+      if (arg != NULL) {
+        MEMCPY(seg->p->payload, ptr, seglen);
+      }
+      seg->dataptr = seg->p->payload;
+    }
+    /* do not copy data */
+    else {
+      /* First, allocate a pbuf for holding the data.
+       * since the referenced data is available at least until it is sent out on the
+       * link (as it has to be ACKed by the remote party) we can safely use PBUF_ROM
+       * instead of PBUF_REF here.
+       */
+      if ((p = pbuf_alloc(PBUF_TRANSPORT, seglen, PBUF_ROM)) == NULL) {
+        LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue: could not allocate memory for zero-copy pbuf\n"));
+        goto memerr;
+      }
+      ++queuelen;
+      /* reference the non-volatile payload data */
+      p->payload = ptr;
+      seg->dataptr = ptr;
+
+      /* Second, allocate a pbuf for the headers. */
+      if ((seg->p = pbuf_alloc(PBUF_TRANSPORT, 0, PBUF_RAM)) == NULL) {
+        /* If allocation fails, we have to deallocate the data pbuf as
+         * well. */
+        pbuf_free(p);
+        LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue: could not allocate memory for header pbuf\n"));
+        goto memerr;
+      }
+      queuelen += pbuf_clen(seg->p);
+
+      /* Concatenate the headers and data pbufs together. */
+      pbuf_cat(seg->p/*header*/, p/*data*/);
+      p = NULL;
+    }
+
+    /* Now that there are more segments queued, we check again if the
+    length of the queue exceeds the configured maximum or overflows. */
+    if ((queuelen > TCP_SND_QUEUELEN) || (queuelen > TCP_SNDQUEUELEN_OVERFLOW)) {
+      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue: queue too long %"U16_F" (%"U16_F")\n", queuelen, TCP_SND_QUEUELEN));
+      goto memerr;
+    }
+
+    seg->len = seglen;
+
+    /* build TCP header */
+    if (pbuf_header(seg->p, TCP_HLEN)) {
+      LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue: no room for TCP header in pbuf.\n"));
+      TCP_STATS_INC(tcp.err);
+      goto memerr;
+    }
+    seg->tcphdr = seg->p->payload;
+    seg->tcphdr->src = htons(pcb->local_port);
+    seg->tcphdr->dest = htons(pcb->remote_port);
+    seg->tcphdr->seqno = htonl(seqno);
+    seg->tcphdr->urgp = 0;
+    TCPH_FLAGS_SET(seg->tcphdr, flags);
+    /* don't fill in tcphdr->ackno and tcphdr->wnd until later */
+
+    /* Copy the options into the header, if they are present. */
+    if (optdata == NULL) {
+      TCPH_HDRLEN_SET(seg->tcphdr, 5);
+    }
+    else {
+      TCPH_HDRLEN_SET(seg->tcphdr, (5 + optlen / 4));
+      /* Copy options into data portion of segment.
+       Options can thus only be sent in non data carrying
+       segments such as SYN|ACK. */
+      SMEMCPY(seg->dataptr, optdata, optlen);
+    }
+    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_TRACE, ("tcp_enqueue: queueing %"U32_F":%"U32_F" (0x%"X16_F")\n",
+      ntohl(seg->tcphdr->seqno),
+      ntohl(seg->tcphdr->seqno) + TCP_TCPLEN(seg),
+      (u16_t)flags));
+
+    left -= seglen;
+    seqno += seglen;
+    ptr = (void *)((u8_t *)ptr + seglen);
+  }
+
+  /* Now that the data to be enqueued has been broken up into TCP
+  segments in the queue variable, we add them to the end of the
+  pcb->unsent queue. */
+  if (pcb->unsent == NULL) {
+    useg = NULL;
+  }
+  else {
+    for (useg = pcb->unsent; useg->next != NULL; useg = useg->next);
+  }
+  /* { useg is last segment on the unsent queue, NULL if list is empty } */
+
+  /* If there is room in the last pbuf on the unsent queue,
+  chain the first pbuf on the queue together with that. */
+  if (useg != NULL &&
+    TCP_TCPLEN(useg) != 0 &&
+    !(TCPH_FLAGS(useg->tcphdr) & (TCP_SYN | TCP_FIN)) &&
+    !(flags & (TCP_SYN | TCP_FIN)) &&
+    /* fit within max seg size */
+    useg->len + queue->len <= pcb->mss) {
+    /* Remove TCP header from first segment of our to-be-queued list */
+    if(pbuf_header(queue->p, -TCP_HLEN)) {
+      /* Can we cope with this failing?  Just assert for now */
+      LWIP_ASSERT("pbuf_header failed\n", 0);
+      TCP_STATS_INC(tcp.err);
+      goto memerr;
+    }
+    pbuf_cat(useg->p, queue->p);
+    useg->len += queue->len;
+    useg->next = queue->next;
+
+    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("tcp_enqueue: chaining segments, new len %"U16_F"\n", useg->len));
+    if (seg == queue) {
+      seg = NULL;
+    }
+    memp_free(MEMP_TCP_SEG, queue);
+  }
+  else {
+    /* empty list */
+    if (useg == NULL) {
+      /* initialize list with this segment */
+      pcb->unsent = queue;
+    }
+    /* enqueue segment */
+    else {
+      useg->next = queue;
+    }
+  }
+  if ((flags & TCP_SYN) || (flags & TCP_FIN)) {
+    ++len;
+  }
+  if (flags & TCP_FIN) {
+    pcb->flags |= TF_FIN;
+  }
+  pcb->snd_lbb += len;
+
+  pcb->snd_buf -= len;
+
+  /* update number of segments on the queues */
+  pcb->snd_queuelen = queuelen;
+  LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_enqueue: %"S16_F" (after enqueued)\n", pcb->snd_queuelen));
+  if (pcb->snd_queuelen != 0) {
+    LWIP_ASSERT("tcp_enqueue: valid queue length",
+      pcb->unacked != NULL || pcb->unsent != NULL);
+  }
+
+  /* Set the PSH flag in the last segment that we enqueued, but only
+  if the segment has data (indicated by seglen > 0). */
+  if (seg != NULL && seglen > 0 && seg->tcphdr != NULL && ((apiflags & TCP_WRITE_FLAG_MORE)==0)) {
+    TCPH_SET_FLAG(seg->tcphdr, TCP_PSH);
+  }
+
+  return ERR_OK;
+memerr:
+  pcb->flags |= TF_NAGLEMEMERR;
+  TCP_STATS_INC(tcp.memerr);
+
+  if (queue != NULL) {
+    tcp_segs_free(queue);
+  }
+  if (pcb->snd_queuelen != 0) {
+    LWIP_ASSERT("tcp_enqueue: valid queue length", pcb->unacked != NULL ||
+      pcb->unsent != NULL);
+  }
+  LWIP_DEBUGF(TCP_QLEN_DEBUG | LWIP_DBG_STATE, ("tcp_enqueue: %"S16_F" (with mem err)\n", pcb->snd_queuelen));
+  return ERR_MEM;
+}
+
+/**
+ * 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 pbuf *p;
+  struct tcp_hdr *tcphdr;
+  struct tcp_seg *seg, *useg;
+  u32_t wnd;
+#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);
+
+  seg = pcb->unsent;
+
+  /* useg should point to last segment on unacked queue */
+  useg = pcb->unacked;
+  if (useg != NULL) {
+    for (; useg->next != NULL; useg = useg->next);
+  }                                                                             
+   
+  /* 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 ||
+      ntohl(seg->tcphdr->seqno) - pcb->lastack + seg->len > wnd)) {
+    p = pbuf_alloc(PBUF_IP, TCP_HLEN, PBUF_RAM);
+    if (p == NULL) {
+      LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_output: (ACK) could not allocate pbuf\n"));
+      return ERR_BUF;
+    }
+    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);
+
+    tcphdr = p->payload;
+    tcphdr->src = htons(pcb->local_port);
+    tcphdr->dest = htons(pcb->remote_port);
+    tcphdr->seqno = htonl(pcb->snd_nxt);
+    tcphdr->ackno = htonl(pcb->rcv_nxt);
+    TCPH_FLAGS_SET(tcphdr, TCP_ACK);
+    tcphdr->wnd = htons(pcb->rcv_ann_wnd);
+    tcphdr->urgp = 0;
+    TCPH_HDRLEN_SET(tcphdr, 5);
+
+    tcphdr->chksum = 0;
+#if CHECKSUM_GEN_TCP
+    tcphdr->chksum = inet_chksum_pseudo(p, &(pcb->local_ip), &(pcb->remote_ip),
+          IP_PROTO_TCP, p->tot_len);
+#endif
+#if LWIP_NETIF_HWADDRHINT
+    {
+      struct netif *netif;
+      netif = ip_route(&pcb->remote_ip);
+      if(netif != NULL){
+        netif->addr_hint = &(pcb->addr_hint);
+        ip_output_if(p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl,
+                     pcb->tos, IP_PROTO_TCP, netif);
+        netif->addr_hint = NULL;
+      }
+    }
+#else /* LWIP_NETIF_HWADDRHINT*/
+    ip_output(p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl, pcb->tos,
+        IP_PROTO_TCP);
+#endif /* LWIP_NETIF_HWADDRHINT*/
+    pbuf_free(p);
+
+    return ERR_OK;
+  }
+
+#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 %"U16_F
+                                 ", cwnd %"U16_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 %"U16_F", cwnd %"U16_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 */
+  /* data available and window allows it to be sent? */
+  while (seg != NULL &&
+         ntohl(seg->tcphdr->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_enqueue had a memory error before (prevent delayed ACK timeout) or
+     * - 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_enqueue/tcp_output.
+     */
+    if((tcp_do_output_nagle(pcb) == 0) &&
+      ((pcb->flags & (TF_NAGLEMEMERR | TF_FIN)) == 0)){
+      break;
+    }
+#if TCP_CWND_DEBUG
+    LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_output: snd_wnd %"U16_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;
+
+    if (pcb->state != SYN_SENT) {
+      TCPH_SET_FLAG(seg->tcphdr, TCP_ACK);
+      pcb->flags &= ~(TF_ACK_DELAY | TF_ACK_NOW);
+    }
+
+    tcp_output_segment(seg, pcb);
+    pcb->snd_nxt = ntohl(seg->tcphdr->seqno) + TCP_TCPLEN(seg);
+    if (TCP_SEQ_LT(pcb->snd_max, pcb->snd_nxt)) {
+      pcb->snd_max = pcb->snd_nxt;
+    }
+    /* put segment on unacknowledged list if length > 0 */
+    if (TCP_TCPLEN(seg) > 0) {
+      seg->next = NULL;
+      /* unacked list is empty? */
+      if (pcb->unacked == NULL) {
+        pcb->unacked = seg;
+        useg = 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 at the head. We need to check for
+         * this case. -STJ Jul 27, 2004 */
+        if (TCP_SEQ_LT(ntohl(seg->tcphdr->seqno), ntohl(useg->tcphdr->seqno))){
+          /* add segment to head of unacked list */
+          seg->next = pcb->unacked;
+          pcb->unacked = seg;
+        } else {
+          /* add segment to tail of unacked list */
+          useg->next = seg;
+          useg = useg->next;
+        }
+      }
+    /* do not queue empty segments on the unacked list */
+    } else {
+      tcp_seg_free(seg);
+    }
+    seg = pcb->unsent;
+  }
+
+  if (seg != NULL && pcb->persist_backoff == 0 && 
+      ntohl(seg->tcphdr->seqno) - pcb->lastack + seg->len > pcb->snd_wnd) {
+    /* prepare for persist timer */
+    pcb->persist_cnt = 0;
+    pcb->persist_backoff = 1;
+  }
+
+  pcb->flags &= ~TF_NAGLEMEMERR;
+  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;
+  struct netif *netif;
+
+  /** @bug Exclude retransmitted segments from this count. */
+  snmp_inc_tcpoutsegs();
+
+  /* The TCP header has already been constructed, but the ackno and
+   wnd fields remain. */
+  seg->tcphdr->ackno = htonl(pcb->rcv_nxt);
+
+  /* advertise our receive window size in this TCP segment */
+  seg->tcphdr->wnd = htons(pcb->rcv_ann_wnd);
+
+  /* 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
+  {
+    struct netif *netif;
+    netif = ip_route(&pcb->remote_ip);
+    if(netif != NULL){
+      netif->addr_hint = &(pcb->addr_hint);
+      ip_output_if(seg->p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl,
+                   pcb->tos, IP_PROTO_TCP, netif);
+      netif->addr_hint = NULL;
+    }
+  }
+#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;
+
+  pcb->snd_nxt = ntohl(pcb->unsent->tcphdr->seqno);
+  /* 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;
+
+  if (pcb->unacked == NULL) {
+    return;
+  }
+
+  /* Move the first unacked segment to the unsent queue */
+  seg = pcb->unacked->next;
+  pcb->unacked->next = pcb->unsent;
+  pcb->unsent = pcb->unacked;
+  pcb->unacked = seg;
+
+  pcb->snd_nxt = ntohl(pcb->unsent->tcphdr->seqno);
+
+  ++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 = p->payload;
+  tcphdr->src = htons(pcb->local_port);
+  tcphdr->dest = htons(pcb->remote_port);
+  tcphdr->seqno = htonl(pcb->snd_nxt - 1);
+  tcphdr->ackno = htonl(pcb->rcv_nxt);
+  TCPH_FLAGS_SET(tcphdr, 0);
+  tcphdr->wnd = htons(pcb->rcv_ann_wnd);
+  tcphdr->urgp = 0;
+  TCPH_HDRLEN_SET(tcphdr, 5);
+
+  tcphdr->chksum = 0;
+#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
+  {
+    struct netif *netif;
+    netif = ip_route(&pcb->remote_ip);
+    if(netif != NULL){
+      netif->addr_hint = &(pcb->addr_hint);
+      ip_output_if(p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl,
+                   0, IP_PROTO_TCP, netif);
+      netif->addr_hint = NULL;
+    }
+  }
+#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 = p->payload;
+  tcphdr->src = htons(pcb->local_port);
+  tcphdr->dest = htons(pcb->remote_port);
+  tcphdr->seqno = seg->tcphdr->seqno;
+  tcphdr->ackno = htonl(pcb->rcv_nxt);
+  TCPH_FLAGS_SET(tcphdr, 0);
+  tcphdr->wnd = htons(pcb->rcv_ann_wnd);
+  tcphdr->urgp = 0;
+  TCPH_HDRLEN_SET(tcphdr, 5);
+
+  /* Copy in one byte from the head of the unacked queue */
+  *((char *)p->payload + sizeof(struct tcp_hdr)) = *(char *)seg->dataptr;
+
+  tcphdr->chksum = 0;
+#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
+  {
+    struct netif *netif;
+    netif = ip_route(&pcb->remote_ip);
+    if(netif != NULL){
+      netif->addr_hint = &(pcb->addr_hint);
+      ip_output_if(p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl,
+                   0, IP_PROTO_TCP, netif);
+      netif->addr_hint = NULL;
+    }
+  }
+#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 */