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 /**
  * @file
  * Transmission Control Protocol for IP
  *
  * This file contains common functions for the TCP implementation, such as functinos
  * for manipulating the data structures and the TCP timer functions. TCP functions
  * related to input and output is found in tcp_in.c and tcp_out.c respectively.
  *
  */
 
 /*
  * 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/def.h"
 #include "lwip/mem.h"
 #include "lwip/memp.h"
 #include "lwip/snmp.h"
 #include "lwip/tcp.h"
 #include "lwip/debug.h"
 #include "lwip/stats.h"
 
 #include <string.h>
 
 const char *tcp_state_str[] = {
   "CLOSED",      
   "LISTEN",      
   "SYN_SENT",    
   "SYN_RCVD",    
   "ESTABLISHED", 
   "FIN_WAIT_1",  
   "FIN_WAIT_2",  
   "CLOSE_WAIT",  
   "CLOSING",     
   "LAST_ACK",    
   "TIME_WAIT"   
 };
 
 /* Incremented every coarse grained timer shot (typically every 500 ms). */
 u32_t tcp_ticks;
 const u8_t tcp_backoff[13] =
     { 1, 2, 3, 4, 5, 6, 7, 7, 7, 7, 7, 7, 7};
  /* Times per slowtmr hits */
 const u8_t tcp_persist_backoff[7] = { 3, 6, 12, 24, 48, 96, 120 };
 
 /* The TCP PCB lists. */
 
 /** List of all TCP PCBs bound but not yet (connected || listening) */
 struct tcp_pcb *tcp_bound_pcbs;  
 /** List of all TCP PCBs in LISTEN state */
 union tcp_listen_pcbs_t tcp_listen_pcbs;
 /** List of all TCP PCBs that are in a state in which
  * they accept or send data. */
 struct tcp_pcb *tcp_active_pcbs;  
 /** List of all TCP PCBs in TIME-WAIT state */
 struct tcp_pcb *tcp_tw_pcbs;
 
 struct tcp_pcb *tcp_tmp_pcb;
 
 static u8_t tcp_timer;
 static u16_t tcp_new_port(void);
 
 /**
  * Called periodically to dispatch TCP timers.
  *
  */
 void
 tcp_tmr(void)
 {
   /* Call tcp_fasttmr() every 250 ms */
   tcp_fasttmr();
 
   if (++tcp_timer & 1) {
     /* Call tcp_tmr() every 500 ms, i.e., every other timer
        tcp_tmr() is called. */
     tcp_slowtmr();
   }
 }
 
 /**
  * Closes the connection held by the PCB.
  *
  * Listening pcbs are freed and may not be referenced any more.
  * Connection pcbs are freed if not yet connected and may not be referenced
  * any more. If a connection is established (at least SYN received or in
  * a closing state), the connection is closed, and put in a closing state.
  * The pcb is then automatically freed in tcp_slowtmr(). It is therefore
  * unsafe to reference it.
  *
  * @param pcb the tcp_pcb to close
  * @return ERR_OK if connection has been closed
  *         another err_t if closing failed and pcb is not freed
  */
 err_t
 tcp_close(struct tcp_pcb *pcb)
 {
   err_t err;
 
 #if TCP_DEBUG
   LWIP_DEBUGF(TCP_DEBUG, ("tcp_close: closing in "));
   tcp_debug_print_state(pcb->state);
 #endif /* TCP_DEBUG */
 
   switch (pcb->state) {
   case CLOSED:
     /* Closing a pcb in the CLOSED state might seem erroneous,
      * however, it is in this state once allocated and as yet unused
      * and the user needs some way to free it should the need arise.
      * Calling tcp_close() with a pcb that has already been closed, (i.e. twice)
      * or for a pcb that has been used and then entered the CLOSED state 
      * is erroneous, but this should never happen as the pcb has in those cases
      * been freed, and so any remaining handles are bogus. */
     err = ERR_OK;
     TCP_RMV(&tcp_bound_pcbs, pcb);
     memp_free(MEMP_TCP_PCB, pcb);
     pcb = NULL;
     break;
   case LISTEN:
     err = ERR_OK;
     tcp_pcb_remove((struct tcp_pcb **)&tcp_listen_pcbs.pcbs, pcb);
     memp_free(MEMP_TCP_PCB_LISTEN, pcb);
     pcb = NULL;
     break;
   case SYN_SENT:
     err = ERR_OK;
     tcp_pcb_remove(&tcp_active_pcbs, pcb);
     memp_free(MEMP_TCP_PCB, pcb);
     pcb = NULL;
     snmp_inc_tcpattemptfails();
     break;
   case SYN_RCVD:
     err = tcp_send_ctrl(pcb, TCP_FIN);
     if (err == ERR_OK) {
       snmp_inc_tcpattemptfails();
       pcb->state = FIN_WAIT_1;
     }
     break;
   case ESTABLISHED:
     err = tcp_send_ctrl(pcb, TCP_FIN);
     if (err == ERR_OK) {
       snmp_inc_tcpestabresets();
       pcb->state = FIN_WAIT_1;
     }
     break;
   case CLOSE_WAIT:
     err = tcp_send_ctrl(pcb, TCP_FIN);
     if (err == ERR_OK) {
       snmp_inc_tcpestabresets();
       pcb->state = LAST_ACK;
     }
     break;
   default:
     /* Has already been closed, do nothing. */
     err = ERR_OK;
     pcb = NULL;
     break;
   }
 
   if (pcb != NULL && err == ERR_OK) {
     /* To ensure all data has been sent when tcp_close returns, we have
        to make sure tcp_output doesn't fail.
        Since we don't really have to ensure all data has been sent when tcp_close
        returns (unsent data is sent from tcp timer functions, also), we don't care
        for the return value of tcp_output for now. */
     /* @todo: When implementing SO_LINGER, this must be changed somehow:
        If SOF_LINGER is set, the data should be sent when tcp_close returns. */
     tcp_output(pcb);
   }
   return err;
 }
 
 /**
  * Abandons a connection and optionally sends a RST to the remote
  * host.  Deletes the local protocol control block. This is done when
  * a connection is killed because of shortage of memory.
  *
  * @param pcb the tcp_pcb to abort
  * @param reset boolean to indicate whether a reset should be sent
  */
 void
 tcp_abandon(struct tcp_pcb *pcb, int reset)
 {
   u32_t seqno, ackno;
   u16_t remote_port, local_port;
   struct ip_addr remote_ip, local_ip;
 #if LWIP_CALLBACK_API  
   void (* errf)(void *arg, err_t err);
 #endif /* LWIP_CALLBACK_API */
   void *errf_arg;
 
   
   /* Figure out on which TCP PCB list we are, and remove us. If we
      are in an active state, call the receive function associated with
      the PCB with a NULL argument, and send an RST to the remote end. */
   if (pcb->state == TIME_WAIT) {
     tcp_pcb_remove(&tcp_tw_pcbs, pcb);
     memp_free(MEMP_TCP_PCB, pcb);
   } else {
     seqno = pcb->snd_nxt;
     ackno = pcb->rcv_nxt;
     ip_addr_set(&local_ip, &(pcb->local_ip));
     ip_addr_set(&remote_ip, &(pcb->remote_ip));
     local_port = pcb->local_port;
     remote_port = pcb->remote_port;
 #if LWIP_CALLBACK_API
     errf = pcb->errf;
 #endif /* LWIP_CALLBACK_API */
     errf_arg = pcb->callback_arg;
     tcp_pcb_remove(&tcp_active_pcbs, pcb);
     if (pcb->unacked != NULL) {
       tcp_segs_free(pcb->unacked);
     }
     if (pcb->unsent != NULL) {
       tcp_segs_free(pcb->unsent);
     }
 #if TCP_QUEUE_OOSEQ    
     if (pcb->ooseq != NULL) {
       tcp_segs_free(pcb->ooseq);
     }
 #endif /* TCP_QUEUE_OOSEQ */
     memp_free(MEMP_TCP_PCB, pcb);
     TCP_EVENT_ERR(errf, errf_arg, ERR_ABRT);
     if (reset) {
       LWIP_DEBUGF(TCP_RST_DEBUG, ("tcp_abandon: sending RST\n"));
       tcp_rst(seqno, ackno, &local_ip, &remote_ip, local_port, remote_port);
     }
   }
 }
 
 /**
  * Binds the connection to a local portnumber and IP address. If the
  * IP address is not given (i.e., ipaddr == NULL), the IP address of
  * the outgoing network interface is used instead.
  *
  * @param pcb the tcp_pcb to bind (no check is done whether this pcb is
  *        already bound!)
  * @param ipaddr the local ip address to bind to (use IP_ADDR_ANY to bind
  *        to any local address
  * @param port the local port to bind to
  * @return ERR_USE if the port is already in use
  *         ERR_OK if bound
  */
 err_t
 tcp_bind(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port)
 {
   struct tcp_pcb *cpcb;
 
   LWIP_ERROR("tcp_bind: can only bind in state CLOSED", pcb->state == CLOSED, return ERR_ISCONN);
 
   if (port == 0) {
     port = tcp_new_port();
   }
   /* Check if the address already is in use. */
   /* Check the listen pcbs. */
   for(cpcb = (struct tcp_pcb *)tcp_listen_pcbs.pcbs;
       cpcb != NULL; cpcb = cpcb->next) {
     if (cpcb->local_port == port) {
       if (ip_addr_isany(&(cpcb->local_ip)) ||
           ip_addr_isany(ipaddr) ||
           ip_addr_cmp(&(cpcb->local_ip), ipaddr)) {
         return ERR_USE;
       }
     }
   }
   /* Check the connected pcbs. */
   for(cpcb = tcp_active_pcbs;
       cpcb != NULL; cpcb = cpcb->next) {
     if (cpcb->local_port == port) {
       if (ip_addr_isany(&(cpcb->local_ip)) ||
           ip_addr_isany(ipaddr) ||
           ip_addr_cmp(&(cpcb->local_ip), ipaddr)) {
         return ERR_USE;
       }
     }
   }
   /* Check the bound, not yet connected pcbs. */
   for(cpcb = tcp_bound_pcbs; cpcb != NULL; cpcb = cpcb->next) {
     if (cpcb->local_port == port) {
       if (ip_addr_isany(&(cpcb->local_ip)) ||
           ip_addr_isany(ipaddr) ||
           ip_addr_cmp(&(cpcb->local_ip), ipaddr)) {
         return ERR_USE;
       }
     }
   }
   /* @todo: until SO_REUSEADDR is implemented (see task #6995 on savannah),
    * we have to check the pcbs in TIME-WAIT state, also: */
   for(cpcb = tcp_tw_pcbs; cpcb != NULL; cpcb = cpcb->next) {
     if (cpcb->local_port == port) {
       if (ip_addr_cmp(&(cpcb->local_ip), ipaddr)) {
         return ERR_USE;
       }
     }
   }
 
   if (!ip_addr_isany(ipaddr)) {
     pcb->local_ip = *ipaddr;
   }
   pcb->local_port = port;
   TCP_REG(&tcp_bound_pcbs, pcb);
   LWIP_DEBUGF(TCP_DEBUG, ("tcp_bind: bind to port %"U16_F"\n", port));
   return ERR_OK;
 }
 #if LWIP_CALLBACK_API
 /**
  * Default accept callback if no accept callback is specified by the user.
  */
 static err_t
 tcp_accept_null(void *arg, struct tcp_pcb *pcb, err_t err)
 {
   LWIP_UNUSED_ARG(arg);
   LWIP_UNUSED_ARG(pcb);
   LWIP_UNUSED_ARG(err);
 
   return ERR_ABRT;
 }
 #endif /* LWIP_CALLBACK_API */
 
 /**
  * Set the state of the connection to be LISTEN, which means that it
  * is able to accept incoming connections. The protocol control block
  * is reallocated in order to consume less memory. Setting the
  * connection to LISTEN is an irreversible process.
  *
  * @param pcb the original tcp_pcb
  * @param backlog the incoming connections queue limit
  * @return tcp_pcb used for listening, consumes less memory.
  *
  * @note The original tcp_pcb is freed. This function therefore has to be
  *       called like this:
  *             tpcb = tcp_listen(tpcb);
  */
 struct tcp_pcb *
 tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog)
 {
   struct tcp_pcb_listen *lpcb;
 
   LWIP_UNUSED_ARG(backlog);
   LWIP_ERROR("tcp_listen: pcb already connected", pcb->state == CLOSED, return NULL);
 
   /* already listening? */
   if (pcb->state == LISTEN) {
     return pcb;
   }
   lpcb = memp_malloc(MEMP_TCP_PCB_LISTEN);
   if (lpcb == NULL) {
     return NULL;
   }
   lpcb->callback_arg = pcb->callback_arg;
   lpcb->local_port = pcb->local_port;
   lpcb->state = LISTEN;
   lpcb->so_options = pcb->so_options;
   lpcb->so_options |= SOF_ACCEPTCONN;
   lpcb->ttl = pcb->ttl;
   lpcb->tos = pcb->tos;
   ip_addr_set(&lpcb->local_ip, &pcb->local_ip);
   TCP_RMV(&tcp_bound_pcbs, pcb);
   memp_free(MEMP_TCP_PCB, pcb);
 #if LWIP_CALLBACK_API
   lpcb->accept = tcp_accept_null;
 #endif /* LWIP_CALLBACK_API */
 #if TCP_LISTEN_BACKLOG
   lpcb->accepts_pending = 0;
   lpcb->backlog = (backlog ? backlog : 1);
 #endif /* TCP_LISTEN_BACKLOG */
   TCP_REG(&tcp_listen_pcbs.listen_pcbs, lpcb);
   return (struct tcp_pcb *)lpcb;
 }
 
 /** 
  * Update the state that tracks the available window space to advertise.
  *
  * Returns how much extra window would be advertised if we sent an
  * update now.
  */
 u32_t tcp_update_rcv_ann_wnd(struct tcp_pcb *pcb)
 {
   u32_t new_right_edge = pcb->rcv_nxt + pcb->rcv_wnd;
 
   if (TCP_SEQ_GEQ(new_right_edge, pcb->rcv_ann_right_edge + LWIP_MIN((TCP_WND / 2), pcb->mss))) {
     /* we can advertise more window */
     pcb->rcv_ann_wnd = pcb->rcv_wnd;
     return new_right_edge - pcb->rcv_ann_right_edge;
   } else {
     if (TCP_SEQ_GT(pcb->rcv_nxt, pcb->rcv_ann_right_edge)) {
       /* Can happen due to other end sending out of advertised window,
        * but within actual available (but not yet advertised) window */
       pcb->rcv_ann_wnd = 0;
     } else {
       /* keep the right edge of window constant */
       pcb->rcv_ann_wnd = pcb->rcv_ann_right_edge - pcb->rcv_nxt;
     }
     return 0;
   }
 }
 
 /**
  * This function should be called by the application when it has
  * processed the data. The purpose is to advertise a larger window
  * when the data has been processed.
  *
  * @param pcb the tcp_pcb for which data is read
  * @param len the amount of bytes that have been read by the application
  */
 void
 tcp_recved(struct tcp_pcb *pcb, u16_t len)
 {
   int wnd_inflation;
 
   LWIP_ASSERT("tcp_recved: len would wrap rcv_wnd\n",
               len <= 0xffff - pcb->rcv_wnd );
 
   pcb->rcv_wnd += len;
   if (pcb->rcv_wnd > TCP_WND)
     pcb->rcv_wnd = TCP_WND;
 
   wnd_inflation = tcp_update_rcv_ann_wnd(pcb);
 
   /* If the change in the right edge of window is significant (default
    * watermark is TCP_WND/2), then send an explicit update now.
    * Otherwise wait for a packet to be sent in the normal course of
    * events (or more window to be available later) */
   if (wnd_inflation >= TCP_WND_UPDATE_THRESHOLD) 
     tcp_ack_now(pcb);
 
   LWIP_DEBUGF(TCP_DEBUG, ("tcp_recved: recveived %"U16_F" bytes, wnd %"U16_F" (%"U16_F").\n",
          len, pcb->rcv_wnd, TCP_WND - pcb->rcv_wnd));
 }
 
 /**
  * A nastly hack featuring 'goto' statements that allocates a
  * new TCP local port.
  *
  * @return a new (free) local TCP port number
  */
 static u16_t
 tcp_new_port(void)
 {
   struct tcp_pcb *pcb;
 #ifndef TCP_LOCAL_PORT_RANGE_START
 #define TCP_LOCAL_PORT_RANGE_START 4096
 #define TCP_LOCAL_PORT_RANGE_END   0x7fff
 #endif
   static u16_t port = TCP_LOCAL_PORT_RANGE_START;
   
  again:
   if (++port > TCP_LOCAL_PORT_RANGE_END) {
     port = TCP_LOCAL_PORT_RANGE_START;
   }
   
   for(pcb = tcp_active_pcbs; pcb != NULL; pcb = pcb->next) {
     if (pcb->local_port == port) {
       goto again;
     }
   }
   for(pcb = tcp_tw_pcbs; pcb != NULL; pcb = pcb->next) {
     if (pcb->local_port == port) {
       goto again;
     }
   }
   for(pcb = (struct tcp_pcb *)tcp_listen_pcbs.pcbs; pcb != NULL; pcb = pcb->next) {
     if (pcb->local_port == port) {
       goto again;
     }
   }
   return port;
 }
 
 /**
  * Connects to another host. The function given as the "connected"
  * argument will be called when the connection has been established.
  *
  * @param pcb the tcp_pcb used to establish the connection
  * @param ipaddr the remote ip address to connect to
  * @param port the remote tcp port to connect to
  * @param connected callback function to call when connected (or on error)
  * @return ERR_VAL if invalid arguments are given
  *         ERR_OK if connect request has been sent
  *         other err_t values if connect request couldn't be sent
  */
 err_t
 tcp_connect(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port,
       err_t (* connected)(void *arg, struct tcp_pcb *tpcb, err_t err))
 {
   err_t ret;
   u32_t iss;
 
   LWIP_ERROR("tcp_connect: can only connected from state CLOSED", pcb->state == CLOSED, return ERR_ISCONN);
 
   LWIP_DEBUGF(TCP_DEBUG, ("tcp_connect to port %"U16_F"\n", port));
   if (ipaddr != NULL) {
     pcb->remote_ip = *ipaddr;
   } else {
     return ERR_VAL;
   }
   pcb->remote_port = port;
   if (pcb->local_port == 0) {
     pcb->local_port = tcp_new_port();
   }
   iss = tcp_next_iss();
   pcb->rcv_nxt = 0;
   pcb->snd_nxt = iss;
   pcb->lastack = iss - 1;
   pcb->snd_lbb = iss - 1;
   pcb->rcv_wnd = TCP_WND;
   pcb->rcv_ann_wnd = TCP_WND;
   pcb->rcv_ann_right_edge = pcb->rcv_nxt;
   pcb->snd_wnd = TCP_WND;
   /* As initial send MSS, we use TCP_MSS but limit it to 536.
      The send MSS is updated when an MSS option is received. */
   pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS;
 #if TCP_CALCULATE_EFF_SEND_MSS
   pcb->mss = tcp_eff_send_mss(pcb->mss, ipaddr);
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
   pcb->cwnd = 1;
   pcb->ssthresh = pcb->mss * 10;
   pcb->state = SYN_SENT;
 #if LWIP_CALLBACK_API  
   pcb->connected = connected;
 #endif /* LWIP_CALLBACK_API */
   TCP_RMV(&tcp_bound_pcbs, pcb);
   TCP_REG(&tcp_active_pcbs, pcb);
 
   snmp_inc_tcpactiveopens();
   
   ret = tcp_enqueue(pcb, NULL, 0, TCP_SYN, 0, TF_SEG_OPTS_MSS
 #if LWIP_TCP_TIMESTAMPS
                     | TF_SEG_OPTS_TS
 #endif
                     );
   if (ret == ERR_OK) { 
     tcp_output(pcb);
   }
   return ret;
 } 
 
 /**
  * Called every 500 ms and implements the retransmission timer and the timer that
  * removes PCBs that have been in TIME-WAIT for enough time. It also increments
  * various timers such as the inactivity timer in each PCB.
  *
  * Automatically called from tcp_tmr().
  */
 void
 tcp_slowtmr(void)
 {
   struct tcp_pcb *pcb, *pcb2, *prev;
   u16_t eff_wnd;
   u8_t pcb_remove;      /* flag if a PCB should be removed */
   u8_t pcb_reset;       /* flag if a RST should be sent when removing */
   err_t err;
 
   err = ERR_OK;
 
   ++tcp_ticks;
 
   /* Steps through all of the active PCBs. */
   prev = NULL;
   pcb = tcp_active_pcbs;
   if (pcb == NULL) {
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: no active pcbs\n"));
   }
   while (pcb != NULL) {
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: processing active pcb\n"));
     LWIP_ASSERT("tcp_slowtmr: active pcb->state != CLOSED\n", pcb->state != CLOSED);
     LWIP_ASSERT("tcp_slowtmr: active pcb->state != LISTEN\n", pcb->state != LISTEN);
     LWIP_ASSERT("tcp_slowtmr: active pcb->state != TIME-WAIT\n", pcb->state != TIME_WAIT);
 
     pcb_remove = 0;
     pcb_reset = 0;
 
     if (pcb->state == SYN_SENT && pcb->nrtx == TCP_SYNMAXRTX) {
       ++pcb_remove;
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: max SYN retries reached\n"));
     }
     else if (pcb->nrtx == TCP_MAXRTX) {
       ++pcb_remove;
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: max DATA retries reached\n"));
     } else {
       if (pcb->persist_backoff > 0) {
         /* If snd_wnd is zero, use persist timer to send 1 byte probes
          * instead of using the standard retransmission mechanism. */
         pcb->persist_cnt++;
         if (pcb->persist_cnt >= tcp_persist_backoff[pcb->persist_backoff-1]) {
           pcb->persist_cnt = 0;
           if (pcb->persist_backoff < sizeof(tcp_persist_backoff)) {
             pcb->persist_backoff++;
           }
           tcp_zero_window_probe(pcb);
         }
       } else {
         /* Increase the retransmission timer if it is running */
         if(pcb->rtime >= 0)
           ++pcb->rtime;
 
         if (pcb->unacked != NULL && pcb->rtime >= pcb->rto) {
           /* Time for a retransmission. */
           LWIP_DEBUGF(TCP_RTO_DEBUG, ("tcp_slowtmr: rtime %"S16_F
                                       " pcb->rto %"S16_F"\n",
                                       pcb->rtime, pcb->rto));
 
           /* Double retransmission time-out unless we are trying to
            * connect to somebody (i.e., we are in SYN_SENT). */
           if (pcb->state != SYN_SENT) {
             pcb->rto = ((pcb->sa >> 3) + pcb->sv) << tcp_backoff[pcb->nrtx];
           }
 
           /* Reset the retransmission timer. */
           pcb->rtime = 0;
 
           /* Reduce congestion window and ssthresh. */
           eff_wnd = LWIP_MIN(pcb->cwnd, pcb->snd_wnd);
           pcb->ssthresh = eff_wnd >> 1;
           if (pcb->ssthresh < pcb->mss) {
             pcb->ssthresh = pcb->mss * 2;
           }
           pcb->cwnd = pcb->mss;
           LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_slowtmr: cwnd %"U16_F
                                        " ssthresh %"U16_F"\n",
                                        pcb->cwnd, pcb->ssthresh));
  
           /* The following needs to be called AFTER cwnd is set to one
              mss - STJ */
           tcp_rexmit_rto(pcb);
         }
       }
     }
     /* Check if this PCB has stayed too long in FIN-WAIT-2 */
     if (pcb->state == FIN_WAIT_2) {
       if ((u32_t)(tcp_ticks - pcb->tmr) >
           TCP_FIN_WAIT_TIMEOUT / TCP_SLOW_INTERVAL) {
         ++pcb_remove;
         LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: removing pcb stuck in FIN-WAIT-2\n"));
       }
     }
 
     /* Check if KEEPALIVE should be sent */
     if((pcb->so_options & SOF_KEEPALIVE) && 
        ((pcb->state == ESTABLISHED) || 
         (pcb->state == CLOSE_WAIT))) {
 #if LWIP_TCP_KEEPALIVE
       if((u32_t)(tcp_ticks - pcb->tmr) > 
          (pcb->keep_idle + (pcb->keep_cnt*pcb->keep_intvl))
          / TCP_SLOW_INTERVAL)
 #else      
       if((u32_t)(tcp_ticks - pcb->tmr) > 
          (pcb->keep_idle + TCP_MAXIDLE) / TCP_SLOW_INTERVAL)
 #endif /* LWIP_TCP_KEEPALIVE */
       {
         LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: KEEPALIVE timeout. Aborting connection 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)));
         
         ++pcb_remove;
         ++pcb_reset;
       }
 #if LWIP_TCP_KEEPALIVE
       else if((u32_t)(tcp_ticks - pcb->tmr) > 
               (pcb->keep_idle + pcb->keep_cnt_sent * pcb->keep_intvl)
               / TCP_SLOW_INTERVAL)
 #else
       else if((u32_t)(tcp_ticks - pcb->tmr) > 
               (pcb->keep_idle + pcb->keep_cnt_sent * TCP_KEEPINTVL_DEFAULT) 
               / TCP_SLOW_INTERVAL)
 #endif /* LWIP_TCP_KEEPALIVE */
       {
         tcp_keepalive(pcb);
         pcb->keep_cnt_sent++;
       }
     }
 
     /* If this PCB has queued out of sequence data, but has been
        inactive for too long, will drop the data (it will eventually
        be retransmitted). */
 #if TCP_QUEUE_OOSEQ    
     if (pcb->ooseq != NULL &&
         (u32_t)tcp_ticks - pcb->tmr >= pcb->rto * TCP_OOSEQ_TIMEOUT) {
       tcp_segs_free(pcb->ooseq);
       pcb->ooseq = NULL;
       LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_slowtmr: dropping OOSEQ queued data\n"));
     }
 #endif /* TCP_QUEUE_OOSEQ */
 
     /* Check if this PCB has stayed too long in SYN-RCVD */
     if (pcb->state == SYN_RCVD) {
       if ((u32_t)(tcp_ticks - pcb->tmr) >
           TCP_SYN_RCVD_TIMEOUT / TCP_SLOW_INTERVAL) {
         ++pcb_remove;
         LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: removing pcb stuck in SYN-RCVD\n"));
       }
     }
 
     /* Check if this PCB has stayed too long in LAST-ACK */
     if (pcb->state == LAST_ACK) {
       if ((u32_t)(tcp_ticks - pcb->tmr) > 2 * TCP_MSL / TCP_SLOW_INTERVAL) {
         ++pcb_remove;
         LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: removing pcb stuck in LAST-ACK\n"));
       }
     }
 
     /* If the PCB should be removed, do it. */
     if (pcb_remove) {
       tcp_pcb_purge(pcb);      
       /* Remove PCB from tcp_active_pcbs list. */
       if (prev != NULL) {
         LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_active_pcbs", pcb != tcp_active_pcbs);
         prev->next = pcb->next;
       } else {
         /* This PCB was the first. */
         LWIP_ASSERT("tcp_slowtmr: first pcb == tcp_active_pcbs", tcp_active_pcbs == pcb);
         tcp_active_pcbs = pcb->next;
       }
 
       TCP_EVENT_ERR(pcb->errf, pcb->callback_arg, ERR_ABRT);
       if (pcb_reset) {
         tcp_rst(pcb->snd_nxt, pcb->rcv_nxt, &pcb->local_ip, &pcb->remote_ip,
           pcb->local_port, pcb->remote_port);
       }
 
       pcb2 = pcb->next;
       memp_free(MEMP_TCP_PCB, pcb);
       pcb = pcb2;
     } else {
 
       /* We check if we should poll the connection. */
       ++pcb->polltmr;
       if (pcb->polltmr >= pcb->pollinterval) {
         pcb->polltmr = 0;
         LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: polling application\n"));
         TCP_EVENT_POLL(pcb, err);
         if (err == ERR_OK) {
           tcp_output(pcb);
         }
       }
       
       prev = pcb;
       pcb = pcb->next;
     }
   }
 
   
   /* Steps through all of the TIME-WAIT PCBs. */
   prev = NULL;    
   pcb = tcp_tw_pcbs;
   while (pcb != NULL) {
     LWIP_ASSERT("tcp_slowtmr: TIME-WAIT pcb->state == TIME-WAIT", pcb->state == TIME_WAIT);
     pcb_remove = 0;
 
     /* Check if this PCB has stayed long enough in TIME-WAIT */
     if ((u32_t)(tcp_ticks - pcb->tmr) > 2 * TCP_MSL / TCP_SLOW_INTERVAL) {
       ++pcb_remove;
     }
     
 
 
     /* If the PCB should be removed, do it. */
     if (pcb_remove) {
       tcp_pcb_purge(pcb);      
       /* Remove PCB from tcp_tw_pcbs list. */
       if (prev != NULL) {
         LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_tw_pcbs", pcb != tcp_tw_pcbs);
         prev->next = pcb->next;
       } else {
         /* This PCB was the first. */
         LWIP_ASSERT("tcp_slowtmr: first pcb == tcp_tw_pcbs", tcp_tw_pcbs == pcb);
         tcp_tw_pcbs = pcb->next;
       }
       pcb2 = pcb->next;
       memp_free(MEMP_TCP_PCB, pcb);
       pcb = pcb2;
     } else {
       prev = pcb;
       pcb = pcb->next;
     }
   }
 }
 
 /**
  * Is called every TCP_FAST_INTERVAL (250 ms) and process data previously
  * "refused" by upper layer (application) and sends delayed ACKs.
  *
  * Automatically called from tcp_tmr().
  */
 void
 tcp_fasttmr(void)
 {
   struct tcp_pcb *pcb;
 
   for(pcb = tcp_active_pcbs; pcb != NULL; pcb = pcb->next) {
     /* If there is data which was previously "refused" by upper layer */
     if (pcb->refused_data != NULL) {
       /* Notify again application with data previously received. */
       err_t err;
       LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_fasttmr: notify kept packet\n"));
       TCP_EVENT_RECV(pcb, pcb->refused_data, ERR_OK, err);
       if (err == ERR_OK) {
         pcb->refused_data = NULL;
       }
     }
 
     /* send delayed ACKs */  
     if (pcb->flags & TF_ACK_DELAY) {
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_fasttmr: delayed ACK\n"));
       tcp_ack_now(pcb);
       pcb->flags &= ~(TF_ACK_DELAY | TF_ACK_NOW);
     }
   }
 }
 
 /**
  * Deallocates a list of TCP segments (tcp_seg structures).
  *
  * @param seg tcp_seg list of TCP segments to free
  * @return the number of pbufs that were deallocated
  */
 u8_t
 tcp_segs_free(struct tcp_seg *seg)
 {
   u8_t count = 0;
   struct tcp_seg *next;
   while (seg != NULL) {
     next = seg->next;
     count += tcp_seg_free(seg);
     seg = next;
   }
   return count;
 }
 
 /**
  * Frees a TCP segment (tcp_seg structure).
  *
  * @param seg single tcp_seg to free
  * @return the number of pbufs that were deallocated
  */
 u8_t
 tcp_seg_free(struct tcp_seg *seg)
 {
   u8_t count = 0;
   
   if (seg != NULL) {
     if (seg->p != NULL) {
       count = pbuf_free(seg->p);
 #if TCP_DEBUG
       seg->p = NULL;
 #endif /* TCP_DEBUG */
     }
     memp_free(MEMP_TCP_SEG, seg);
   }
   return count;
 }
 
 /**
  * Sets the priority of a connection.
  *
  * @param pcb the tcp_pcb to manipulate
  * @param prio new priority
  */
 void
 tcp_setprio(struct tcp_pcb *pcb, u8_t prio)
 {
   pcb->prio = prio;
 }
 #if TCP_QUEUE_OOSEQ
 
 /**
  * Returns a copy of the given TCP segment.
  * The pbuf and data are not copied, only the pointers
  *
  * @param seg the old tcp_seg
  * @return a copy of seg
  */ 
 struct tcp_seg *
 tcp_seg_copy(struct tcp_seg *seg)
 {
   struct tcp_seg *cseg;
 
   cseg = memp_malloc(MEMP_TCP_SEG);
   if (cseg == NULL) {
     return NULL;
   }
   SMEMCPY((u8_t *)cseg, (const u8_t *)seg, sizeof(struct tcp_seg)); 
   pbuf_ref(cseg->p);
   return cseg;
 }
 #endif
 
 #if LWIP_CALLBACK_API
 /**
  * Default receive callback that is called if the user didn't register
  * a recv callback for the pcb.
  */
 err_t
 tcp_recv_null(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t err)
 {
   LWIP_UNUSED_ARG(arg);
   if (p != NULL) {
     tcp_recved(pcb, p->tot_len);
     pbuf_free(p);
   } else if (err == ERR_OK) {
     return tcp_close(pcb);
   }
   return ERR_OK;
 }
 #endif /* LWIP_CALLBACK_API */
 
 /**
  * Kills the oldest active connection that has lower priority than prio.
  *
  * @param prio minimum priority
  */
 static void
 tcp_kill_prio(u8_t prio)
 {
   struct tcp_pcb *pcb, *inactive;
   u32_t inactivity;
   u8_t mprio;
 
 
   mprio = TCP_PRIO_MAX;
   
   /* We kill the oldest active connection that has lower priority than prio. */
   inactivity = 0;
   inactive = NULL;
   for(pcb = tcp_active_pcbs; pcb != NULL; pcb = pcb->next) {
     if (pcb->prio <= prio &&
        pcb->prio <= mprio &&
        (u32_t)(tcp_ticks - pcb->tmr) >= inactivity) {
       inactivity = tcp_ticks - pcb->tmr;
       inactive = pcb;
       mprio = pcb->prio;
     }
   }
   if (inactive != NULL) {
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_kill_prio: killing oldest PCB %p (%"S32_F")\n",
            (void *)inactive, inactivity));
     tcp_abort(inactive);
   }      
 }
 
 /**
  * Kills the oldest connection that is in TIME_WAIT state.
  * Called from tcp_alloc() if no more connections are available.
  */
 static void
 tcp_kill_timewait(void)
 {
   struct tcp_pcb *pcb, *inactive;
   u32_t inactivity;
 
   inactivity = 0;
   inactive = NULL;
   /* Go through the list of TIME_WAIT pcbs and get the oldest pcb. */
   for(pcb = tcp_tw_pcbs; pcb != NULL; pcb = pcb->next) {
     if ((u32_t)(tcp_ticks - pcb->tmr) >= inactivity) {
       inactivity = tcp_ticks - pcb->tmr;
       inactive = pcb;
     }
   }
   if (inactive != NULL) {
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_kill_timewait: killing oldest TIME-WAIT PCB %p (%"S32_F")\n",
            (void *)inactive, inactivity));
     tcp_abort(inactive);
   }      
 }
 
 /**
  * Allocate a new tcp_pcb structure.
  *
  * @param prio priority for the new pcb
  * @return a new tcp_pcb that initially is in state CLOSED
  */
 struct tcp_pcb *
 tcp_alloc(u8_t prio)
 {
   struct tcp_pcb *pcb;
   u32_t iss;
   
   pcb = memp_malloc(MEMP_TCP_PCB);
   if (pcb == NULL) {
     /* Try killing oldest connection in TIME-WAIT. */
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_alloc: killing off oldest TIME-WAIT connection\n"));
     tcp_kill_timewait();
     /* Try to allocate a tcp_pcb again. */
     pcb = memp_malloc(MEMP_TCP_PCB);
     if (pcb == NULL) {
       /* Try killing active connections with lower priority than the new one. */
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_alloc: killing connection with prio lower than %d\n", prio));
       tcp_kill_prio(prio);
       /* Try to allocate a tcp_pcb again. */
       pcb = memp_malloc(MEMP_TCP_PCB);
       if (pcb != NULL) {
         /* adjust err stats: memp_malloc failed twice before */
         MEMP_STATS_DEC(err, MEMP_TCP_PCB);
       }
     }
     if (pcb != NULL) {
       /* adjust err stats: timewait PCB was freed above */
       MEMP_STATS_DEC(err, MEMP_TCP_PCB);
     }
   }
   if (pcb != NULL) {
     memset(pcb, 0, sizeof(struct tcp_pcb));
     pcb->prio = TCP_PRIO_NORMAL;
     pcb->snd_buf = TCP_SND_BUF;
     pcb->snd_queuelen = 0;
     pcb->rcv_wnd = TCP_WND;
     pcb->rcv_ann_wnd = TCP_WND;
     pcb->tos = 0;
     pcb->ttl = TCP_TTL;
     /* As initial send MSS, we use TCP_MSS but limit it to 536.
        The send MSS is updated when an MSS option is received. */
     pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS;
     pcb->rto = 3000 / TCP_SLOW_INTERVAL;
     pcb->sa = 0;
     pcb->sv = 3000 / TCP_SLOW_INTERVAL;
     pcb->rtime = -1;
     pcb->cwnd = 1;
     iss = tcp_next_iss();
     pcb->snd_wl2 = iss;
     pcb->snd_nxt = iss;
     pcb->lastack = iss;
     pcb->snd_lbb = iss;   
     pcb->tmr = tcp_ticks;
 
     pcb->polltmr = 0;
 
 #if LWIP_CALLBACK_API
     pcb->recv = tcp_recv_null;
 #endif /* LWIP_CALLBACK_API */  
     
     /* Init KEEPALIVE timer */
     pcb->keep_idle  = TCP_KEEPIDLE_DEFAULT;
     
 #if LWIP_TCP_KEEPALIVE
     pcb->keep_intvl = TCP_KEEPINTVL_DEFAULT;
     pcb->keep_cnt   = TCP_KEEPCNT_DEFAULT;
 #endif /* LWIP_TCP_KEEPALIVE */
 
     pcb->keep_cnt_sent = 0;
   }
   return pcb;
 }
 
 /**
  * Creates a new TCP protocol control block but doesn't place it on
  * any of the TCP PCB lists.
  * The pcb is not put on any list until binding using tcp_bind().
  *
  * @internal: Maybe there should be a idle TCP PCB list where these
  * PCBs are put on. Port reservation using tcp_bind() is implemented but
  * allocated pcbs that are not bound can't be killed automatically if wanting
  * to allocate a pcb with higher prio (@see tcp_kill_prio())
  *
  * @return a new tcp_pcb that initially is in state CLOSED
  */
 struct tcp_pcb *
 tcp_new(void)
 {
   return tcp_alloc(TCP_PRIO_NORMAL);
 }
 
 /**
  * Used to specify the argument that should be passed callback
  * functions.
  *
  * @param pcb tcp_pcb to set the callback argument
  * @param arg void pointer argument to pass to callback functions
  */ 
 void
 tcp_arg(struct tcp_pcb *pcb, void *arg)
 {  
   pcb->callback_arg = arg;
 }
 #if LWIP_CALLBACK_API
 
 /**
  * Used to specify the function that should be called when a TCP
  * connection receives data.
  *
  * @param pcb tcp_pcb to set the recv callback
  * @param recv callback function to call for this pcb when data is received
  */ 
 void
 tcp_recv(struct tcp_pcb *pcb,
    err_t (* recv)(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err))
 {
   pcb->recv = recv;
 }
 
 /**
  * Used to specify the function that should be called when TCP data
  * has been successfully delivered to the remote host.
  *
  * @param pcb tcp_pcb to set the sent callback
  * @param sent callback function to call for this pcb when data is successfully sent
  */ 
 void
 tcp_sent(struct tcp_pcb *pcb,
    err_t (* sent)(void *arg, struct tcp_pcb *tpcb, u16_t len))
 {
   pcb->sent = sent;
 }
 
 /**
  * Used to specify the function that should be called when a fatal error
  * has occured on the connection.
  *
  * @param pcb tcp_pcb to set the err callback
  * @param errf callback function to call for this pcb when a fatal error
  *        has occured on the connection
  */ 
 void
 tcp_err(struct tcp_pcb *pcb,
    void (* errf)(void *arg, err_t err))
 {
   pcb->errf = errf;
 }
 
 /**
  * Used for specifying the function that should be called when a
  * LISTENing connection has been connected to another host.
  *
  * @param pcb tcp_pcb to set the accept callback
  * @param accept callback function to call for this pcb when LISTENing
  *        connection has been connected to another host
  */ 
 void
 tcp_accept(struct tcp_pcb *pcb,
      err_t (* accept)(void *arg, struct tcp_pcb *newpcb, err_t err))
 {
   pcb->accept = accept;
 }
 #endif /* LWIP_CALLBACK_API */
 
 
 /**
  * Used to specify the function that should be called periodically
  * from TCP. The interval is specified in terms of the TCP coarse
  * timer interval, which is called twice a second.
  *
  */ 
 void
 tcp_poll(struct tcp_pcb *pcb,
    err_t (* poll)(void *arg, struct tcp_pcb *tpcb), u8_t interval)
 {
 #if LWIP_CALLBACK_API
   pcb->poll = poll;
 #endif /* LWIP_CALLBACK_API */  
   pcb->pollinterval = interval;
 }
 
 /**
  * Purges a TCP PCB. Removes any buffered data and frees the buffer memory
  * (pcb->ooseq, pcb->unsent and pcb->unacked are freed).
  *
  * @param pcb tcp_pcb to purge. The pcb itself is not deallocated!
  */
 void
 tcp_pcb_purge(struct tcp_pcb *pcb)
 {
   if (pcb->state != CLOSED &&
      pcb->state != TIME_WAIT &&
      pcb->state != LISTEN) {
 
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge\n"));
 
 #if TCP_LISTEN_BACKLOG
     if (pcb->state == SYN_RCVD) {
       /* Need to find the corresponding listen_pcb and decrease its accepts_pending */
       struct tcp_pcb_listen *lpcb;
       LWIP_ASSERT("tcp_pcb_purge: pcb->state == SYN_RCVD but tcp_listen_pcbs is NULL",
         tcp_listen_pcbs.listen_pcbs != NULL);
       for (lpcb = tcp_listen_pcbs.listen_pcbs; lpcb != NULL; lpcb = lpcb->next) {
         if ((lpcb->local_port == pcb->local_port) &&
             (ip_addr_isany(&lpcb->local_ip) ||
              ip_addr_cmp(&pcb->local_ip, &lpcb->local_ip))) {
             /* port and address of the listen pcb match the timed-out pcb */
             LWIP_ASSERT("tcp_pcb_purge: listen pcb does not have accepts pending",
               lpcb->accepts_pending > 0);
             lpcb->accepts_pending--;
             break;
           }
       }
     }
 #endif /* TCP_LISTEN_BACKLOG */
 
 
     if (pcb->refused_data != NULL) {
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge: data left on ->refused_data\n"));
       pbuf_free(pcb->refused_data);
       pcb->refused_data = NULL;
     }
     if (pcb->unsent != NULL) {
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge: not all data sent\n"));
     }
     if (pcb->unacked != NULL) {
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge: data left on ->unacked\n"));
     }
 #if TCP_QUEUE_OOSEQ /* LW */
     if (pcb->ooseq != NULL) {
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge: data left on ->ooseq\n"));
     }
 
     /* Stop the retransmission timer as it will expect data on unacked
        queue if it fires */
     pcb->rtime = -1;
 
     tcp_segs_free(pcb->ooseq);
     pcb->ooseq = NULL;
 #endif /* TCP_QUEUE_OOSEQ */
     tcp_segs_free(pcb->unsent);
     tcp_segs_free(pcb->unacked);
     pcb->unacked = pcb->unsent = NULL;
   }
 }
 
 /**
  * Purges the PCB and removes it from a PCB list. Any delayed ACKs are sent first.
  *
  * @param pcblist PCB list to purge.
  * @param pcb tcp_pcb to purge. The pcb itself is NOT deallocated!
  */
 void
 tcp_pcb_remove(struct tcp_pcb **pcblist, struct tcp_pcb *pcb)
 {
   TCP_RMV(pcblist, pcb);
 
   tcp_pcb_purge(pcb);
   
   /* if there is an outstanding delayed ACKs, send it */
   if (pcb->state != TIME_WAIT &&
      pcb->state != LISTEN &&
      pcb->flags & TF_ACK_DELAY) {
     pcb->flags |= TF_ACK_NOW;
     tcp_output(pcb);
   }
 
   if (pcb->state != LISTEN) {
     LWIP_ASSERT("unsent segments leaking", pcb->unsent == NULL);
     LWIP_ASSERT("unacked segments leaking", pcb->unacked == NULL);
 #if TCP_QUEUE_OOSEQ
     LWIP_ASSERT("ooseq segments leaking", pcb->ooseq == NULL);
 #endif /* TCP_QUEUE_OOSEQ */
   }
 
   pcb->state = CLOSED;
 
   LWIP_ASSERT("tcp_pcb_remove: tcp_pcbs_sane()", tcp_pcbs_sane());
 }
 
 /**
  * Calculates a new initial sequence number for new connections.
  *
  * @return u32_t pseudo random sequence number
  */
 u32_t
 tcp_next_iss(void)
 {
   static u32_t iss = 6510;
   
   iss += tcp_ticks;       /* XXX */
   return iss;
 }
 
 #if TCP_CALCULATE_EFF_SEND_MSS
 /**
  * Calcluates the effective send mss that can be used for a specific IP address
  * by using ip_route to determin the netif used to send to the address and
  * calculating the minimum of TCP_MSS and that netif's mtu (if set).
  */
 u16_t
 tcp_eff_send_mss(u16_t sendmss, struct ip_addr *addr)
 {
   u16_t mss_s;
   struct netif *outif;
 
   outif = ip_route(addr);
   if ((outif != NULL) && (outif->mtu != 0)) {
     mss_s = outif->mtu - IP_HLEN - TCP_HLEN;
     /* RFC 1122, chap 4.2.2.6:
      * Eff.snd.MSS = min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
      * We correct for TCP options in tcp_enqueue(), and don't support
      * IP options
      */
     sendmss = LWIP_MIN(sendmss, mss_s);
   }
   return sendmss;
 }
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
 
 const char*
 tcp_debug_state_str(enum tcp_state s)
 {
   return tcp_state_str[s];
 }
 
 #if TCP_DEBUG || TCP_INPUT_DEBUG || TCP_OUTPUT_DEBUG
 /**
  * Print a tcp header for debugging purposes.
  *
  * @param tcphdr pointer to a struct tcp_hdr
  */
 void
 tcp_debug_print(struct tcp_hdr *tcphdr)
 {
   LWIP_DEBUGF(TCP_DEBUG, ("TCP header:\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("+-------------------------------+\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("|    %5"U16_F"      |    %5"U16_F"      | (src port, dest port)\n",
          ntohs(tcphdr->src), ntohs(tcphdr->dest)));
   LWIP_DEBUGF(TCP_DEBUG, ("+-------------------------------+\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("|           %010"U32_F"          | (seq no)\n",
           ntohl(tcphdr->seqno)));
   LWIP_DEBUGF(TCP_DEBUG, ("+-------------------------------+\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("|           %010"U32_F"          | (ack no)\n",
          ntohl(tcphdr->ackno)));
   LWIP_DEBUGF(TCP_DEBUG, ("+-------------------------------+\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("| %2"U16_F" |   |%"U16_F"%"U16_F"%"U16_F"%"U16_F"%"U16_F"%"U16_F"|     %5"U16_F"     | (hdrlen, flags (",
        TCPH_HDRLEN(tcphdr),
          TCPH_FLAGS(tcphdr) >> 5 & 1,
          TCPH_FLAGS(tcphdr) >> 4 & 1,
          TCPH_FLAGS(tcphdr) >> 3 & 1,
          TCPH_FLAGS(tcphdr) >> 2 & 1,
          TCPH_FLAGS(tcphdr) >> 1 & 1,
          TCPH_FLAGS(tcphdr) & 1,
          ntohs(tcphdr->wnd)));
   tcp_debug_print_flags(TCPH_FLAGS(tcphdr));
   LWIP_DEBUGF(TCP_DEBUG, ("), win)\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("+-------------------------------+\n"));
   LWIP_DEBUGF(TCP_DEBUG, ("|    0x%04"X16_F"     |     %5"U16_F"     | (chksum, urgp)\n",
          ntohs(tcphdr->chksum), ntohs(tcphdr->urgp)));
   LWIP_DEBUGF(TCP_DEBUG, ("+-------------------------------+\n"));
 }
 
 /**
  * Print a tcp state for debugging purposes.
  *
  * @param s enum tcp_state to print
  */
 void
 tcp_debug_print_state(enum tcp_state s)
 {
   LWIP_DEBUGF(TCP_DEBUG, ("State: %s\n", tcp_state_str[s]));
 }
 
 /**
  * Print tcp flags for debugging purposes.
  *
  * @param flags tcp flags, all active flags are printed
  */
 void
 tcp_debug_print_flags(u8_t flags)
 {
   if (flags & TCP_FIN) {
     LWIP_DEBUGF(TCP_DEBUG, ("FIN "));
   }
   if (flags & TCP_SYN) {
     LWIP_DEBUGF(TCP_DEBUG, ("SYN "));
   }
   if (flags & TCP_RST) {
     LWIP_DEBUGF(TCP_DEBUG, ("RST "));
   }
   if (flags & TCP_PSH) {
     LWIP_DEBUGF(TCP_DEBUG, ("PSH "));
   }
   if (flags & TCP_ACK) {
     LWIP_DEBUGF(TCP_DEBUG, ("ACK "));
   }
   if (flags & TCP_URG) {
     LWIP_DEBUGF(TCP_DEBUG, ("URG "));
   }
   if (flags & TCP_ECE) {
     LWIP_DEBUGF(TCP_DEBUG, ("ECE "));
   }
   if (flags & TCP_CWR) {
     LWIP_DEBUGF(TCP_DEBUG, ("CWR "));
   }
   LWIP_DEBUGF(TCP_DEBUG, ("\n"));
 }
 
 /**
  * Print all tcp_pcbs in every list for debugging purposes.
  */
 void
 tcp_debug_print_pcbs(void)
 {
   struct tcp_pcb *pcb;
   LWIP_DEBUGF(TCP_DEBUG, ("Active PCB states:\n"));
   for(pcb = tcp_active_pcbs; pcb != NULL; pcb = pcb->next) {
     LWIP_DEBUGF(TCP_DEBUG, ("Local port %"U16_F", foreign port %"U16_F" snd_nxt %"U32_F" rcv_nxt %"U32_F" ",
                        pcb->local_port, pcb->remote_port,
                        pcb->snd_nxt, pcb->rcv_nxt));
     tcp_debug_print_state(pcb->state);
   }    
   LWIP_DEBUGF(TCP_DEBUG, ("Listen PCB states:\n"));
   for(pcb = (struct tcp_pcb *)tcp_listen_pcbs.pcbs; pcb != NULL; pcb = pcb->next) {
     LWIP_DEBUGF(TCP_DEBUG, ("Local port %"U16_F", foreign port %"U16_F" snd_nxt %"U32_F" rcv_nxt %"U32_F" ",
                        pcb->local_port, pcb->remote_port,
                        pcb->snd_nxt, pcb->rcv_nxt));
     tcp_debug_print_state(pcb->state);
   }    
   LWIP_DEBUGF(TCP_DEBUG, ("TIME-WAIT PCB states:\n"));
   for(pcb = tcp_tw_pcbs; pcb != NULL; pcb = pcb->next) {
     LWIP_DEBUGF(TCP_DEBUG, ("Local port %"U16_F", foreign port %"U16_F" snd_nxt %"U32_F" rcv_nxt %"U32_F" ",
                        pcb->local_port, pcb->remote_port,
                        pcb->snd_nxt, pcb->rcv_nxt));
     tcp_debug_print_state(pcb->state);
   }    
 }
 
 /**
  * Check state consistency of the tcp_pcb lists.
  */
 s16_t
 tcp_pcbs_sane(void)
 {
   struct tcp_pcb *pcb;
   for(pcb = tcp_active_pcbs; pcb != NULL; pcb = pcb->next) {
     LWIP_ASSERT("tcp_pcbs_sane: active pcb->state != CLOSED", pcb->state != CLOSED);
     LWIP_ASSERT("tcp_pcbs_sane: active pcb->state != LISTEN", pcb->state != LISTEN);
     LWIP_ASSERT("tcp_pcbs_sane: active pcb->state != TIME-WAIT", pcb->state != TIME_WAIT);
   }
   for(pcb = tcp_tw_pcbs; pcb != NULL; pcb = pcb->next) {
     LWIP_ASSERT("tcp_pcbs_sane: tw pcb->state == TIME-WAIT", pcb->state == TIME_WAIT);
   }
   return 1;
 }
 #endif /* TCP_DEBUG */
 
 #endif /* LWIP_TCP */