tcp_fsm.c

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/**
 * @file tcp_fsm.c
 * @brief TCP finite state machine
 *
 * @section License
 *
 * SPDX-License-Identifier: GPL-2.0-or-later
 *
 * Copyright (C) 2010-2024 Oryx Embedded SARL. All rights reserved.
 *
 * This file is part of CycloneTCP Open.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 *
 * @section Description
 *
 * The TCP state machine progresses from one state to another in response to
 * events (user calls, incoming segments and timeouts). This file describes
 * the state transitions caused by incoming segments. Refer to the
 * following RFCs for complete details:
 * - RFC 793: Transmission Control Protocol
 * - RFC 1122: Requirements for Internet Hosts - Communication Layers
 *
 * @author Oryx Embedded SARL (www.oryx-embedded.com)
 * @version 2.4.0
 **/
 
//Switch to the appropriate trace level
#define TRACE_LEVEL TCP_TRACE_LEVEL
 
//Dependencies
#include "core/net.h"
#include "core/ip.h"
#include "core/socket.h"
#include "core/tcp.h"
#include "core/tcp_fsm.h"
#include "core/tcp_misc.h"
#include "core/tcp_timer.h"
#include "ipv4/ipv4.h"
#include "ipv4/ipv4_misc.h"
#include "ipv6/ipv6.h"
#include "mibs/mib2_module.h"
#include "mibs/tcp_mib_module.h"
#include "date_time.h"
#include "debug.h"
 
//Check TCP/IP stack configuration
#if (TCP_SUPPORT == ENABLED)
 
 
/**
 * @brief Incoming TCP segment processing
 * @param[in] interface Underlying network interface
 * @param[in] pseudoHeader TCP pseudo header
 * @param[in] buffer Multi-part buffer that holds the incoming TCP segment
 * @param[in] offset Offset to the first byte of the TCP header
 * @param[in] ancillary Additional options passed to the stack along with
 *   the packet
 **/
 
void tcpProcessSegment(NetInterface *interface,
   const IpPseudoHeader *pseudoHeader, const NetBuffer *buffer, size_t offset,
   const NetRxAncillary *ancillary)
{
   uint_t i;
   size_t length;
   Socket *socket;
   Socket *passiveSocket;
   TcpHeader *segment;
 
   //Total number of segments received, including those received in error
   MIB2_TCP_INC_COUNTER32(tcpInSegs, 1);
   TCP_MIB_INC_COUNTER32(tcpInSegs, 1);
   TCP_MIB_INC_COUNTER64(tcpHCInSegs, 1);
 
   //A TCP implementation must silently discard an incoming segment that is
   //addressed to a broadcast or multicast address (refer to RFC 1122, section
   //4.2.3.10)
#if (IPV4_SUPPORT == ENABLED)
   if(pseudoHeader->length == sizeof(Ipv4PseudoHeader))
   {
      //Ensure the destination address is not a broadcast address
      if(ipv4IsBroadcastAddr(interface, pseudoHeader->ipv4Data.destAddr))
         return;
 
      //Ensure the destination address is not a multicast address
      if(ipv4IsMulticastAddr(pseudoHeader->ipv4Data.destAddr))
         return;
   }
   else
#endif
#if (IPV6_SUPPORT == ENABLED)
   if(pseudoHeader->length == sizeof(Ipv6PseudoHeader))
   {
      //Ensure the destination address is not a multicast address
      if(ipv6IsMulticastAddr(&pseudoHeader->ipv6Data.destAddr))
         return;
   }
   else
#endif
   {
      //This should never occur...
      return;
   }
 
   //Retrieve the length of the TCP segment
   length = netBufferGetLength(buffer) - offset;
 
   //Point to the TCP header
   segment = netBufferAt(buffer, offset);
   //Sanity check
   if(segment == NULL)
      return;
 
   //Ensure the TCP header is valid
   if(length < sizeof(TcpHeader))
   {
      //Debug message
      TRACE_WARNING("TCP segment length is invalid!\r\n");
 
      //Total number of segments received in error
      MIB2_TCP_INC_COUNTER32(tcpInErrs, 1);
      TCP_MIB_INC_COUNTER32(tcpInErrs, 1);
 
      //Exit immediately
      return;
   }
 
   //Check header length
   if(segment->dataOffset < 5 || ((size_t) segment->dataOffset * 4) > length)
   {
      //Debug message
      TRACE_WARNING("TCP header length is invalid!\r\n");
 
      //Total number of segments received in error
      MIB2_TCP_INC_COUNTER32(tcpInErrs, 1);
      TCP_MIB_INC_COUNTER32(tcpInErrs, 1);
 
      //Exit immediately
      return;
   }
 
   //Verify TCP checksum
   if(ipCalcUpperLayerChecksumEx(pseudoHeader->data,
      pseudoHeader->length, buffer, offset, length) != 0x0000)
   {
      //Debug message
      TRACE_WARNING("Wrong TCP header checksum!\r\n");
 
      //Total number of segments received in error
      MIB2_TCP_INC_COUNTER32(tcpInErrs, 1);
      TCP_MIB_INC_COUNTER32(tcpInErrs, 1);
 
      //Exit immediately
      return;
   }
 
   //No matching socket in the LISTEN state for the moment
   passiveSocket = NULL;
 
   //Look through opened sockets
   for(i = 0; i < SOCKET_MAX_COUNT; i++)
   {
      //Point to the current socket
      socket = socketTable + i;
 
      //TCP socket found?
      if(socket->type != SOCKET_TYPE_STREAM)
         continue;
 
      //Check whether the socket is bound to a particular interface
      if(socket->interface && socket->interface != interface)
         continue;
 
      //Check destination port number
      if(socket->localPort == 0 || socket->localPort != ntohs(segment->destPort))
         continue;
 
#if (IPV4_SUPPORT == ENABLED)
      //IPv4 packet received?
      if(pseudoHeader->length == sizeof(Ipv4PseudoHeader))
      {
         //Check whether the socket is restricted to IPv6 communications only
         if((socket->options & SOCKET_OPTION_IPV6_ONLY) != 0)
            continue;
 
         //Destination IP address filtering
         if(socket->localIpAddr.length != 0)
         {
            //An IPv4 address is expected
            if(socket->localIpAddr.length != sizeof(Ipv4Addr))
               continue;
 
            //Filter out non-matching addresses
            if(socket->localIpAddr.ipv4Addr != IPV4_UNSPECIFIED_ADDR &&
               socket->localIpAddr.ipv4Addr != pseudoHeader->ipv4Data.destAddr)
            {
               continue;
            }
         }
 
         //Source IP address filtering
         if(socket->remoteIpAddr.length != 0)
         {
            //An IPv4 address is expected
            if(socket->remoteIpAddr.length != sizeof(Ipv4Addr))
               continue;
 
            //Filter out non-matching addresses
            if(socket->remoteIpAddr.ipv4Addr != IPV4_UNSPECIFIED_ADDR &&
               socket->remoteIpAddr.ipv4Addr != pseudoHeader->ipv4Data.srcAddr)
            {
               continue;
            }
         }
      }
      else
#endif
#if (IPV6_SUPPORT == ENABLED)
      //IPv6 packet received?
      if(pseudoHeader->length == sizeof(Ipv6PseudoHeader))
      {
         //Destination IP address filtering
         if(socket->localIpAddr.length != 0)
         {
            //An IPv6 address is expected
            if(socket->localIpAddr.length != sizeof(Ipv6Addr))
               continue;
 
            //Filter out non-matching addresses
            if(!ipv6CompAddr(&socket->localIpAddr.ipv6Addr, &IPV6_UNSPECIFIED_ADDR) &&
               !ipv6CompAddr(&socket->localIpAddr.ipv6Addr, &pseudoHeader->ipv6Data.destAddr))
            {
               continue;
            }
         }
 
         //Source IP address filtering
         if(socket->remoteIpAddr.length != 0)
         {
            //An IPv6 address is expected
            if(socket->remoteIpAddr.length != sizeof(Ipv6Addr))
               continue;
 
            //Filter out non-matching addresses
            if(!ipv6CompAddr(&socket->remoteIpAddr.ipv6Addr, &IPV6_UNSPECIFIED_ADDR) &&
               !ipv6CompAddr(&socket->remoteIpAddr.ipv6Addr, &pseudoHeader->ipv6Data.srcAddr))
            {
               continue;
            }
         }
      }
      else
#endif
      //Invalid packet received?
      {
         //This should never occur...
         continue;
      }
 
      //Keep track of the first matching socket in the LISTEN state
      if(socket->state == TCP_STATE_LISTEN && passiveSocket == NULL)
         passiveSocket = socket;
 
      //Source port filtering
      if(socket->remotePort != ntohs(segment->srcPort))
         continue;
 
      //A matching socket has been found
      break;
   }
 
   //If no matching socket has been found then try to use the first matching
   //socket in the LISTEN state
   if(i >= SOCKET_MAX_COUNT)
   {
      socket = passiveSocket;
   }
 
   //Offset to the first data byte
   offset += segment->dataOffset * 4;
   //Calculate the length of the data
   length -= segment->dataOffset * 4;
 
   //Debug message
   TRACE_DEBUG("%s: TCP segment received (%" PRIuSIZE " data bytes)...\r\n",
      formatSystemTime(osGetSystemTime(), NULL), length);
 
   //Dump TCP header contents for debugging purpose
   if(socket == NULL)
   {
      tcpDumpHeader(segment, length, 0, 0);
   }
   else
   {
      tcpDumpHeader(segment, length, socket->irs, socket->iss);
   }
 
   //Convert from network byte order to host byte order
   segment->srcPort = ntohs(segment->srcPort);
   segment->destPort = ntohs(segment->destPort);
   segment->seqNum = ntohl(segment->seqNum);
   segment->ackNum = ntohl(segment->ackNum);
   segment->window = ntohs(segment->window);
   segment->urgentPointer = ntohs(segment->urgentPointer);
 
   //Specified port unreachable?
   if(socket == NULL)
   {
      //An incoming segment not containing a RST causes a reset to be sent in
      //response
      if((segment->flags & TCP_FLAG_RST) == 0)
      {
         tcpRejectSegment(interface, pseudoHeader, segment, length);
      }
 
      //Return immediately
      return;
   }
 
   //Check current state
   switch(socket->state)
   {
   //Process CLOSED state
   case TCP_STATE_CLOSED:
      //This is the default state that each connection starts in before the
      //process of establishing it begins
      tcpStateClosed(interface, pseudoHeader, segment, length);
      break;
 
   //Process LISTEN state
   case TCP_STATE_LISTEN:
      //A device (normally a server) is waiting to receive a synchronize (SYN)
      //message from a client. It has not yet sent its own SYN message
      tcpStateListen(socket, interface, pseudoHeader, segment, length);
      break;
 
   //Process SYN_SENT state
   case TCP_STATE_SYN_SENT:
      //The device (normally a client) has sent a synchronize (SYN) message and
      //is waiting for a matching SYN from the other device (usually a server)
      tcpStateSynSent(socket, segment, length);
      break;
 
   //Process SYN_RECEIVED state
   case TCP_STATE_SYN_RECEIVED:
      //The device has both received a SYN from its partner and sent its own
      //SYN. It is now waiting for an ACK to its SYN to finish connection setup
      tcpStateSynReceived(socket, segment, buffer, offset, length);
      break;
 
   //Process ESTABLISHED state
   case TCP_STATE_ESTABLISHED:
      //Data can be exchanged freely once both devices in the connection enter
      //this state. This will continue until the connection is closed
      tcpStateEstablished(socket, segment, buffer, offset, length);
      break;
 
   //Process CLOSE_WAIT state
   case TCP_STATE_CLOSE_WAIT:
      //The device has received a close request (FIN) from the other device. It
      //must now wait for the application to acknowledge this request and
      //generate a matching request
      tcpStateCloseWait(socket, segment, length);
      break;
 
   //Process LAST_ACK state
   case TCP_STATE_LAST_ACK:
      //A device that has already received a close request and acknowledged it,
      //has sent its own FIN and is waiting for an ACK to this request
      tcpStateLastAck(socket, segment, length);
      break;
 
   //Process FIN_WAIT_1 state
   case TCP_STATE_FIN_WAIT_1:
      //A device in this state is waiting for an ACK for a FIN it has sent, or
      //is waiting for a connection termination request from the other device
      tcpStateFinWait1(socket, segment, buffer, offset, length);
      break;
 
   //Process FIN_WAIT_2 state
   case TCP_STATE_FIN_WAIT_2:
      //A device in this state has received an ACK for its request to terminate
      //the connection and is now waiting for a matching FIN from the other
      //device
      tcpStateFinWait2(socket, segment, buffer, offset, length);
      break;
 
   //Process CLOSING state
   case TCP_STATE_CLOSING:
      //The device has received a FIN from the other device and sent an ACK for
      //it, but not yet received an ACK for its own FIN message
      tcpStateClosing(socket, segment, length);
      break;
 
   //Process TIME_WAIT state
   case TCP_STATE_TIME_WAIT:
      //The device has now received a FIN from the other device and acknowledged
      //it, and sent its own FIN and received an ACK for it. We are done, except
      //for waiting to ensure the ACK is received and prevent potential overlap
      //with new connections
      tcpStateTimeWait(socket, segment, length);
      break;
 
   //Invalid state...
   default:
      //Back to the CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
      //Silently discard incoming packet
      break;
   }
}
 
 
/**
 * @brief CLOSED state
 *
 * This is the default state that each connection starts in before
 * the process of establishing it begins
 *
 * @param[in] interface Underlying network interface
 * @param[in] pseudoHeader TCP pseudo header
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateClosed(NetInterface *interface, const IpPseudoHeader *pseudoHeader,
   const TcpHeader *segment, size_t length)
{
   //Debug message
   TRACE_DEBUG("TCP FSM: CLOSED state\r\n");
 
   //An incoming segment not containing a RST causes a reset to be sent in
   //response
   if((segment->flags & TCP_FLAG_RST) == 0)
   {
      tcpRejectSegment(interface, pseudoHeader, segment, length);
   }
}
 
 
/**
 * @brief LISTEN state
 *
 * A device (normally a server) is waiting to receive a synchronize (SYN)
 * message from a client. It has not yet sent its own SYN message
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] interface Underlying network interface
 * @param[in] pseudoHeader TCP pseudo header
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateListen(Socket *socket, NetInterface *interface,
   const IpPseudoHeader *pseudoHeader, const TcpHeader *segment, size_t length)
{
   uint_t i;
   const TcpOption *option;
   TcpSynQueueItem *queueItem;
 
   //Debug message
   TRACE_DEBUG("TCP FSM: LISTEN state\r\n");
 
   //An incoming RST should be ignored
   if((segment->flags & TCP_FLAG_RST) != 0)
      return;
 
   //Any acknowledgment is bad if it arrives on a connection still in the
   //LISTEN state
   if((segment->flags & TCP_FLAG_ACK) != 0)
   {
      //A reset segment should be formed for any arriving ACK-bearing segment
      tcpRejectSegment(interface, pseudoHeader, segment, length);
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if((segment->flags & TCP_FLAG_SYN) != 0)
   {
      //Silently drop duplicate SYN segments
      if(tcpIsDuplicateSyn(socket, pseudoHeader, segment))
         return;
 
      //Check whether the SYN queue is empty or not
      if(socket->synQueue != NULL)
      {
         //Point to the very first item
         queueItem = socket->synQueue;
 
         //Reach the last item in the SYN queue
         for(i = 1; queueItem->next != NULL; i++)
         {
            queueItem = queueItem->next;
         }
 
         //Check whether the SYN queue is full
         if(i >= socket->synQueueSize)
         {
            //Remove the first item if the SYN queue runs out of space
            queueItem = socket->synQueue;
            socket->synQueue = queueItem->next;
            //Deallocate memory buffer
            memPoolFree(queueItem);
         }
      }
 
      //Check whether the SYN queue is empty or not
      if(socket->synQueue == NULL)
      {
         //Allocate memory to save incoming data
         queueItem = memPoolAlloc(sizeof(TcpSynQueueItem));
         //Add the newly created item to the queue
         socket->synQueue = queueItem;
      }
      else
      {
         //Point to the very first item
         queueItem = socket->synQueue;
 
         //Reach the last item in the SYN queue
         for(i = 1; queueItem->next != NULL; i++)
         {
            queueItem = queueItem->next;
         }
 
         //Allocate memory to save incoming data
         queueItem->next = memPoolAlloc(sizeof(TcpSynQueueItem));
         //Point to the newly created item
         queueItem = queueItem->next;
      }
 
      //Failed to allocate memory?
      if(queueItem == NULL)
         return;
 
#if (IPV4_SUPPORT == ENABLED)
      //IPv4 is currently used?
      if(pseudoHeader->length == sizeof(Ipv4PseudoHeader))
      {
         //Save the source IPv4 address
         queueItem->srcAddr.length = sizeof(Ipv4Addr);
         queueItem->srcAddr.ipv4Addr = pseudoHeader->ipv4Data.srcAddr;
 
         //Save the destination IPv4 address
         queueItem->destAddr.length = sizeof(Ipv4Addr);
         queueItem->destAddr.ipv4Addr = pseudoHeader->ipv4Data.destAddr;
      }
      else
#endif
#if (IPV6_SUPPORT == ENABLED)
      //IPv6 is currently used?
      if(pseudoHeader->length == sizeof(Ipv6PseudoHeader))
      {
         //Save the source IPv6 address
         queueItem->srcAddr.length = sizeof(Ipv6Addr);
         queueItem->srcAddr.ipv6Addr = pseudoHeader->ipv6Data.srcAddr;
 
         //Save the destination IPv6 address
         queueItem->destAddr.length = sizeof(Ipv6Addr);
         queueItem->destAddr.ipv6Addr = pseudoHeader->ipv6Data.destAddr;
      }
      else
#endif
      //Invalid pseudo header?
      {
         //This should never occur...
         return;
      }
 
      //Initialize next field
      queueItem->next = NULL;
      //Underlying network interface
      queueItem->interface = interface;
      //Save the port number of the client
      queueItem->srcPort = segment->srcPort;
      //Save the initial sequence number
      queueItem->isn = segment->seqNum;
 
      //Get the Maximum Segment Size option
      option = tcpGetOption(segment, TCP_OPTION_MAX_SEGMENT_SIZE);
 
      //Specified option found?
      if(option != NULL && option->length == 4)
      {
         //Retrieve MSS value
         queueItem->mss = LOAD16BE(option->value);
 
         //Debug message
         TRACE_DEBUG("Remote host MSS = %" PRIu16 "\r\n", queueItem->mss);
 
         //Make sure that the MSS advertised by the peer is acceptable
         queueItem->mss = MIN(queueItem->mss, socket->mss);
         queueItem->mss = MAX(queueItem->mss, TCP_MIN_MSS);
      }
      else
      {
         //If the option is not received, TCP must assume the default MSS
         queueItem->mss = MIN(socket->mss, TCP_DEFAULT_MSS);
      }
 
#if (TCP_SACK_SUPPORT == ENABLED)
      //Get the SACK Permitted option
      option = tcpGetOption(segment, TCP_OPTION_SACK_PERMITTED);
 
      //This option can be sent in a SYN segment to indicate that the SACK
      //option can be used once the connection is established (refer to
      //RFC 2018, section 1)
      if(option != NULL && option->length == 2)
      {
         queueItem->sackPermitted = TRUE;
      }
      else
      {
         queueItem->sackPermitted = FALSE;
      }
#endif
 
      //Notify user that a connection request is pending
      tcpUpdateEvents(socket);
 
      //The rest of the processing described in RFC 793 will be done
      //asynchronously when socketAccept() function is called
   }
}
 
 
/**
 * @brief SYN-SENT state
 *
 * The device (normally a client) has sent a synchronize (SYN) message and
 * is waiting for a matching SYN from the other device (usually a server)
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateSynSent(Socket *socket, const TcpHeader *segment, size_t length)
{
   const TcpOption *option;
 
   //Debug message
   TRACE_DEBUG("TCP FSM: SYN-SENT state\r\n");
 
   //Check the ACK bit
   if((segment->flags & TCP_FLAG_ACK) != 0)
   {
      //Make sure the acknowledgment number is valid
      if(segment->ackNum != socket->sndNxt)
      {
         //Send a reset segment unless the RST bit is set
         if((segment->flags & TCP_FLAG_RST) == 0)
         {
            tcpSendResetSegment(socket, segment->ackNum);
         }
 
         //Drop the segment and return
         return;
      }
   }
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Make sure the ACK is acceptable
      if((segment->flags & TCP_FLAG_ACK) != 0)
      {
         //Enter CLOSED state
         tcpChangeState(socket, TCP_STATE_CLOSED);
 
         //Number of times TCP connections have made a direct transition to the
         //CLOSED state from either the SYN-SENT state or the SYN-RECEIVED state
         MIB2_TCP_INC_COUNTER32(tcpAttemptFails, 1);
         TCP_MIB_INC_COUNTER32(tcpAttemptFails, 1);
      }
 
      //Drop the segment and return
      return;
   }
 
   //Check the SYN bit
   if((segment->flags & TCP_FLAG_SYN) != 0)
   {
      //Save initial receive sequence number
      socket->irs = segment->seqNum;
      //Initialize RCV.NXT pointer
      socket->rcvNxt = segment->seqNum + 1;
 
      //If there is an ACK, SND.UNA should be advanced to equal SEG.ACK
      if((segment->flags & TCP_FLAG_ACK) != 0)
      {
         socket->sndUna = segment->ackNum;
      }
 
      //Compute retransmission timeout
      tcpComputeRto(socket);
 
      //Any segments on the retransmission queue which are thereby acknowledged
      //should be removed
      tcpUpdateRetransmitQueue(socket);
 
      //Get the Maximum Segment Size option
      option = tcpGetOption(segment, TCP_OPTION_MAX_SEGMENT_SIZE);
 
      //Specified option found?
      if(option != NULL && option->length == 4)
      {
         //Retrieve MSS value
         socket->smss = LOAD16BE(option->value);
 
         //Debug message
         TRACE_DEBUG("Remote host MSS = %" PRIu16 "\r\n", socket->smss);
 
         //Make sure that the MSS advertised by the peer is acceptable
         socket->smss = MIN(socket->smss, socket->mss);
         socket->smss = MAX(socket->smss, TCP_MIN_MSS);
      }
 
#if (TCP_SACK_SUPPORT == ENABLED)
      //Get the SACK Permitted option
      option = tcpGetOption(segment, TCP_OPTION_SACK_PERMITTED);
 
      //Specified option found?
      if(option != NULL && option->length == 2)
      {
         //This option can be sent in a SYN segment to indicate that the SACK
         //option can be used once the connection is established (refer to
         //RFC 2018, section 1)
         socket->sackPermitted = TRUE;
      }
#endif
 
#if (TCP_CONGEST_CONTROL_SUPPORT == ENABLED)
      //Initial congestion window
      socket->cwnd = MIN(TCP_INITIAL_WINDOW * socket->smss,
         socket->txBufferSize);
#endif
 
      //Check whether our SYN has been acknowledged (SND.UNA > ISS)
      if(TCP_CMP_SEQ(socket->sndUna, socket->iss) > 0)
      {
         //Update the send window before entering ESTABLISHED state (refer to
         //RFC 1122, section 4.2.2.20)
         socket->sndWnd = segment->window;
         socket->sndWl1 = segment->seqNum;
         socket->sndWl2 = segment->ackNum;
 
         //Maximum send window it has seen so far on the connection
         socket->maxSndWnd = segment->window;
 
         //Form an ACK segment and send it
         tcpSendSegment(socket, TCP_FLAG_ACK, socket->sndNxt, socket->rcvNxt,
            0, FALSE);
 
         //Switch to the ESTABLISHED state
         tcpChangeState(socket, TCP_STATE_ESTABLISHED);
      }
      else
      {
         //Form an SYN ACK segment and send it
         tcpSendSegment(socket, TCP_FLAG_SYN | TCP_FLAG_ACK, socket->iss,
            socket->rcvNxt, 0, TRUE);
 
         //Enter SYN-RECEIVED state
         tcpChangeState(socket, TCP_STATE_SYN_RECEIVED);
      }
   }
}
 
 
/**
 * @brief SYN-RECEIVED state
 *
 * The device has both received a SYN from its partner and sent its own SYN.
 * It is now waiting for an ACK to its SYN to finish connection setup
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] buffer Multi-part buffer containing the incoming TCP segment
 * @param[in] offset Offset to the first data byte
 * @param[in] length Length of the segment data
 **/
 
void tcpStateSynReceived(Socket *socket, const TcpHeader *segment,
   const NetBuffer *buffer, size_t offset, size_t length)
{
   TcpHeader segment2;
 
   //Debug message
   TRACE_DEBUG("TCP FSM: SYN-RECEIVED state\r\n");
 
   //Make a copy of the TCP header
   segment2 = *segment;
   //Discard TCP options
   segment2.dataOffset = sizeof(TcpHeader) / 4;
 
   //Check the SYN bit
   if((segment->flags & TCP_FLAG_SYN) != 0 &&
      segment->seqNum == socket->irs)
   {
      //Check the ACK bit
      if((segment->flags & TCP_FLAG_ACK) != 0)
      {
         //Simultaneous open attempt
         segment2.flags &= ~TCP_FLAG_SYN;
         segment2.seqNum++;
      }
      else
      {
         //A retransmitted SYN means our SYN ACK was lost
         tcpSendSegment(socket, TCP_FLAG_SYN | TCP_FLAG_ACK, socket->iss,
            socket->rcvNxt, 0, FALSE);
 
         //We are done
         return;
      }
   }
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, &segment2, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Return to CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
 
      //Number of times TCP connections have made a direct transition to the
      //CLOSED state from either the SYN-SENT state or the SYN-RECEIVED state
      MIB2_TCP_INC_COUNTER32(tcpAttemptFails, 1);
      TCP_MIB_INC_COUNTER32(tcpAttemptFails, 1);
 
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, &segment2, length))
      return;
 
   //If the ACK bit is off drop the segment and return
   if((segment->flags & TCP_FLAG_ACK) == 0)
      return;
 
   //Make sure the acknowledgment number is valid
   if(segment->ackNum != socket->sndNxt)
   {
      //If the segment acknowledgment is not acceptable, form a reset segment
      //and send it
      tcpSendResetSegment(socket, segment->ackNum);
 
      //Drop the segment and return
      return;
   }
 
   //Update the send window before entering ESTABLISHED state (refer to
   //RFC 1122, section 4.2.2.20)
   socket->sndWnd = segment->window;
   socket->sndWl1 = segment->seqNum;
   socket->sndWl2 = segment->ackNum;
 
   //Maximum send window it has seen so far on the connection
   socket->maxSndWnd = segment->window;
 
   //Enter ESTABLISHED state
   tcpChangeState(socket, TCP_STATE_ESTABLISHED);
   //And continue processing...
   tcpStateEstablished(socket, &segment2, buffer, offset, length);
}
 
 
/**
 * @brief ESTABLISHED state
 *
 * Data can be exchanged freely once both devices in the connection enter
 * this state. This will continue until the connection is closed
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] buffer Multi-part buffer containing the incoming TCP segment
 * @param[in] offset Offset to the first data byte
 * @param[in] length Length of the segment data
 **/
 
void tcpStateEstablished(Socket *socket, const TcpHeader *segment,
   const NetBuffer *buffer, size_t offset, size_t length)
{
   uint_t flags = 0;
 
   //Debug message
   TRACE_DEBUG("TCP FSM: ESTABLISHED state\r\n");
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Switch to the CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
 
      //Number of times TCP connections have made a direct transition to the
      //CLOSED state from either the ESTABLISHED state or the CLOSE-WAIT state
      MIB2_TCP_INC_COUNTER32(tcpEstabResets, 1);
      TCP_MIB_INC_COUNTER32(tcpEstabResets, 1);
 
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //Check the ACK field
   if(tcpCheckAck(socket, segment, length))
      return;
 
   //Process the segment text
   if(length > 0)
   {
      tcpProcessSegmentData(socket, segment, buffer, offset, length);
   }
 
   //Check the FIN bit
   if((segment->flags & TCP_FLAG_FIN) != 0)
   {
      //The FIN can only be acknowledged if all the segment data has been
      //successfully transferred to the receive buffer
      if(socket->rcvNxt == (segment->seqNum + length))
      {
         //Advance RCV.NXT over the FIN
         socket->rcvNxt++;
 
         //Send an acknowledgment for the FIN
         tcpSendSegment(socket, TCP_FLAG_ACK, socket->sndNxt, socket->rcvNxt, 0,
            FALSE);
 
         //Switch to the CLOSE-WAIT state
         tcpChangeState(socket, TCP_STATE_CLOSE_WAIT);
      }
   }
 
#if (TCP_CONGEST_CONTROL_SUPPORT == ENABLED)
   //Duplicate ACK received?
   if(socket->dupAckCount > 0)
   {
      flags = SOCKET_FLAG_NO_DELAY;
   }
#endif
 
   //The Nagle algorithm should be implemented to coalesce short segments (refer
   //to RFC 1122 4.2.3.4)
   tcpNagleAlgo(socket, flags);
}
 
 
/**
 * @brief CLOSE-WAIT state
 *
 * The device has received a close request (FIN) from the other device. It
 * must now wait for the application to acknowledge this request and
 * generate a matching request
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateCloseWait(Socket *socket, const TcpHeader *segment, size_t length)
{
   uint_t flags = 0;
 
   //Debug message
   TRACE_DEBUG("TCP FSM: CLOSE-WAIT state\r\n");
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Switch to the CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
 
      //Number of times TCP connections have made a direct transition to the
      //CLOSED state from either the ESTABLISHED state or the CLOSE-WAIT state
      MIB2_TCP_INC_COUNTER32(tcpEstabResets, 1);
      TCP_MIB_INC_COUNTER32(tcpEstabResets, 1);
 
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //Check the ACK field
   if(tcpCheckAck(socket, segment, length))
      return;
 
#if (TCP_CONGEST_CONTROL_SUPPORT == ENABLED)
   //Duplicate ACK received?
   if(socket->dupAckCount > 0)
   {
      flags = SOCKET_FLAG_NO_DELAY;
   }
#endif
 
   //The Nagle algorithm should be implemented to coalesce short segments (refer
   //to RFC 1122 4.2.3.4)
   tcpNagleAlgo(socket, flags);
}
 
 
/**
 * @brief LAST-ACK state
 *
 * A device that has already received a close request and acknowledged it,
 * has sent its own FIN and is waiting for an ACK to this request
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateLastAck(Socket *socket, const TcpHeader *segment, size_t length)
{
   //Debug message
   TRACE_DEBUG("TCP FSM: LAST-ACK state\r\n");
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Enter CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //If the ACK bit is off drop the segment and return
   if((segment->flags & TCP_FLAG_ACK) == 0)
      return;
 
   //The only thing that can arrive in this state is an acknowledgment of
   //our FIN
   if(segment->ackNum == socket->sndNxt)
   {
      //Enter CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
   }
}
 
 
/**
 * @brief FIN-WAIT-1 state
 *
 * A device in this state is waiting for an ACK for a FIN it has sent, or
 * is waiting for a connection termination request from the other device
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] buffer Multi-part buffer containing the incoming TCP segment
 * @param[in] offset Offset to the first data byte
 * @param[in] length Length of the segment data
 **/
 
void tcpStateFinWait1(Socket *socket, const TcpHeader *segment,
   const NetBuffer *buffer, size_t offset, size_t length)
{
   //Debug message
   TRACE_DEBUG("TCP FSM: FIN-WAIT-1 state\r\n");
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Switch to the CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //Check the ACK field
   if(tcpCheckAck(socket, segment, length))
      return;
 
   //Check whether our FIN is now acknowledged
   if(segment->ackNum == socket->sndNxt)
   {
      //Start the FIN-WAIT-2 timer to prevent the connection from staying in
      //the FIN-WAIT-2 state forever
      netStartTimer(&socket->finWait2Timer, TCP_FIN_WAIT_2_TIMER);
 
      //enter FIN-WAIT-2 and continue processing in that state
      tcpChangeState(socket, TCP_STATE_FIN_WAIT_2);
   }
 
   //Process the segment text
   if(length > 0)
   {
      tcpProcessSegmentData(socket, segment, buffer, offset, length);
   }
 
   //Check the FIN bit
   if((segment->flags & TCP_FLAG_FIN) != 0)
   {
      //The FIN can only be acknowledged if all the segment data has been
      //successfully transferred to the receive buffer
      if(socket->rcvNxt == (segment->seqNum + length))
      {
         //Advance RCV.NXT over the FIN
         socket->rcvNxt++;
 
         //Send an acknowledgment for the FIN
         tcpSendSegment(socket, TCP_FLAG_ACK, socket->sndNxt, socket->rcvNxt, 0,
            FALSE);
 
         //Check if our FIN has been acknowledged
         if(segment->ackNum == socket->sndNxt)
         {
            //Release previously allocated resources
            tcpDeleteControlBlock(socket);
            //Start the 2MSL timer
            netStartTimer(&socket->timeWaitTimer, TCP_2MSL_TIMER);
            //Switch to the TIME-WAIT state
            tcpChangeState(socket, TCP_STATE_TIME_WAIT);
         }
         else
         {
            //If our FIN has not been acknowledged, then enter CLOSING state
            tcpChangeState(socket, TCP_STATE_CLOSING);
         }
      }
   }
}
 
 
/**
 * @brief FIN-WAIT-2 state
 *
 * A device in this state has received an ACK for its request to terminate the
 * connection and is now waiting for a matching FIN from the other device
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] buffer Multi-part buffer containing the incoming TCP segment
 * @param[in] offset Offset to the first data byte
 * @param[in] length Length of the segment data
 **/
 
void tcpStateFinWait2(Socket *socket, const TcpHeader *segment,
   const NetBuffer *buffer, size_t offset, size_t length)
{
   //Debug message
   TRACE_DEBUG("TCP FSM: FIN-WAIT-2 state\r\n");
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Switch to the CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //Check the ACK field
   if(tcpCheckAck(socket, segment, length))
      return;
 
   //Process the segment text
   if(length > 0)
   {
      tcpProcessSegmentData(socket, segment, buffer, offset, length);
   }
 
   //Check the FIN bit
   if((segment->flags & TCP_FLAG_FIN) != 0)
   {
      //The FIN can only be acknowledged if all the segment data has been
      //successfully transferred to the receive buffer
      if(socket->rcvNxt == (segment->seqNum + length))
      {
         //Advance RCV.NXT over the FIN
         socket->rcvNxt++;
 
         //Send an acknowledgment for the FIN
         tcpSendSegment(socket, TCP_FLAG_ACK, socket->sndNxt, socket->rcvNxt, 0,
            FALSE);
 
         //Release previously allocated resources
         tcpDeleteControlBlock(socket);
         //Start the 2MSL timer
         netStartTimer(&socket->timeWaitTimer, TCP_2MSL_TIMER);
         //Switch to the TIME_WAIT state
         tcpChangeState(socket, TCP_STATE_TIME_WAIT);
      }
   }
}
 
 
/**
 * @brief CLOSING state
 *
 * The device has received a FIN from the other device and sent an ACK for
 * it, but not yet received an ACK for its own FIN message
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateClosing(Socket *socket, const TcpHeader *segment, size_t length)
{
   //Debug message
   TRACE_DEBUG("TCP FSM: CLOSING state\r\n");
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the RST bit
   if((segment->flags & TCP_FLAG_RST) != 0)
   {
      //Enter CLOSED state
      tcpChangeState(socket, TCP_STATE_CLOSED);
      //Return immediately
      return;
   }
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //Check the ACK field
   if(tcpCheckAck(socket, segment, length))
      return;
 
   //If the ACK acknowledges our FIN then enter the TIME-WAIT state, otherwise
   //ignore the segment
   if(segment->ackNum == socket->sndNxt)
   {
      //Release previously allocated resources
      tcpDeleteControlBlock(socket);
      //Start the 2MSL timer
      netStartTimer(&socket->timeWaitTimer, TCP_2MSL_TIMER);
      //Switch to the TIME-WAIT state
      tcpChangeState(socket, TCP_STATE_TIME_WAIT);
   }
}
 
 
/**
 * @brief TIME-WAIT state
 *
 * The device has now received a FIN from the other device and acknowledged
 * it, and sent its own FIN and received an ACK for it. We are done, except
 * for waiting to ensure the ACK is received and prevent potential overlap
 * with new connections
 *
 * @param[in] socket Handle referencing the current socket
 * @param[in] segment Incoming TCP segment
 * @param[in] length Length of the segment data
 **/
 
void tcpStateTimeWait(Socket *socket, const TcpHeader *segment, size_t length)
{
   //Debug message
   TRACE_DEBUG("TCP FSM: TIME-WAIT state\r\n");
 
  //Ignore RST segments in TIME-WAIT state (refer to RFC 1337, section 3)
  if((segment->flags & TCP_FLAG_RST) != 0)
      return;
 
   //First check sequence number
   if(tcpCheckSeqNum(socket, segment, length))
      return;
 
   //Check the SYN bit
   if(tcpCheckSyn(socket, segment, length))
      return;
 
   //If the ACK bit is off drop the segment and return
   if((segment->flags & TCP_FLAG_ACK) == 0)
      return;
 
   //The only thing that can arrive in this state is a retransmission of the
   //remote FIN. Acknowledge it and restart the 2 MSL timeout
   if((segment->flags & TCP_FLAG_FIN) != 0)
   {
      //Send an acknowledgment for the FIN
      tcpSendSegment(socket, TCP_FLAG_ACK, socket->sndNxt, socket->rcvNxt, 0,
         FALSE);
 
      //Restart the 2MSL timer
      netStartTimer(&socket->timeWaitTimer, TCP_2MSL_TIMER);
   }
}
 
#endif